ASTROBIOLOGICAL CONSIDERATIONS FOR THE SELECTION OF THE
GEOLOGICAL FILTERS ON THE EXOMARS PANCAM INSTRUMENT
Claire R Cousins1,2., Andrew D Griffiths1,3., Ian A Crawford1,2., Bryan J Prosser4.,
Michael C Storrie-Lombardi5., Lottie E Davis1,6., Matthew Gunn7., Andrew J Coates1,3.,
Adrian P Jones1,6., John M Ward8.
1
Centre for Planetary Sciences at UCL/Birkbeck, Gower Street, London, WC1E 6BT.
2
Department of Earth and Planetary Sciences, Birkbeck College, London, WC1E 7HX
3
Mullard Space Science Laboratory, University College London, Holmbury St. Mary,
Dorking, Surrey, RH5 6NT, UK.
4
Department of Computer Science, Queen Mary College, University of London, Mile
End Road, London, E1 4NS.
5
Kinohi Institute, Pasadena, California, USA.
6
Department of Earth Sciences, University College London, Gower Street, London,
WC1E 6BT.
7
Institute of Mathematics & Physics, Aberystwyth University, Penglais, Aberystwyth,
UK, SY23 3BZ.
8
Department of Structural and Molecular Biology, University College London, Gower
Street, London, WC1E 6BT.
Corresponding author: Claire Cousins
Dept. Earth Sciences, University College London, Gower Street, London, WC1E 6BT,
Telephone: 02076792577, E-mail: c.cousins@ucl.ac.uk
1
Running title: Selecting geological filters for the ExoMars PanCam
ABSTRACT
The Panoramic Camera (PanCam) instrument will provide VIS-NIR multispectral
imaging of the ExoMars rover’s surroundings to identify regions of interest within the
nearby terrain. This multispectral capability is dependant upon the 12 pre-selected
‘geological’ filters that are integrated into two wide angle cameras. First devised by the
Imager for Mars Pathfinder team to detect iron oxides, this baseline filter set has
remained largely unchanged for subsequent missions (MER, Beagle2, Phoenix), despite
the advancing knowledge of the mineralogical diversity on Mars. Therefore, the
geological filters for the ExoMars PanCam will be re-designed to accommodate the
astrobiology focus of ExoMars, where hydrated mineral terrains (evidence of past liquid
water) will be priority targets. Here, we conduct an initial investigation into new filter
wavelengths for the ExoMars PanCam, and present results from testing these on Martian
analogue rocks. Two new filter sets were devised: one with filters spaced every 50nm
(“F1-12”), and one utilising a novel filter selection method based upon hydrated mineral
reflectance spectra (“F2-12”). These new filter sets, and the Beagle2 filter set (currently
the baseline for the ExoMars PanCam), were tested on their ability to identify hydrated
minerals and biosignatures present in Martian analogue rocks. The filter sets were found,
with varying degrees of ability, to detect the spectral features of minerals jarosite, opaline
silica, alunite, nontronite, and siderite present in these rock samples. Fossilised biomat
structures and small (<2mm) mineralogical heterogeneities present in silica sinters
however remained undetected with all the filter sets. Both the new filter sets
2
outperformed the B2 filters, with F2-12 detecting the most spectral features produced by
hydrated minerals, and providing the best discrimination between samples. Future work
involving more extensive testing on Martian analogue samples exhibiting a wider range
of mineralogies would be the next step in carefully evaluating the new filter sets.
3
1. INTRODUCTION
1.1 The ExoMars PanCam instrument
The ExoMars rover is the currently proposed Aurora flagship mission. It aims, primarily,
to find evidence for past or present life on Mars via a drill that can reach depths of 2m
into the subsurface. The rover will be equipped with a multi-purpose Panoramic Camera
(PanCam) instrument, which will provide wide angle multispectral stereoscopic
panoramic images of the rover’s surroundings (for a detailed description see Griffiths et
al. 2006). As currently implemented, multispectral imaging works by utilising a set of
filters, set at specific wavelengths and bandwidths, to record reflectance images from the
field of view. One filter yields one reflectance value, and the spread of filters produces a
spectral profile at each image pixel. The more filters available, the more detailed the
spectra will be. Two of the main scientific objectives of the PanCam that will utilise this
multispectral imaging capability are firstly the acquisition of geological information
proximal to the rover (e.g. Smith et al. 1997a; Bell et al. 2004; Morris et al. 2004;
Farrand et al. 2007), and secondly identification of the most suitable locations for further
astrobiological investigation. It is noted that the PanCam is not designed to be a lifedetection instrument in itself, but provides the initial step in identifying suitable
lithologies for further multi-level analysis with the aim to detect evidence of biological
activity (Vago 2005). This site identification will be heavily dependant upon the
surrounding geological terrain and mineralogy identified by the PanCam, and as a result,
the PanCam needs to be carefully designed to achieve this goal. As the PanCam
multispectral imaging capability is, in part, dependant upon a suitable set of ‘geological’
filters, the pre-selected wavelengths of these individual filters needs to be optimised to fit
4
the objectives of the ExoMars mission. The geological filter wavelengths for the PanCam
are required to fall in the 440 – 1000nm range (the detection limits of the CCD detector),
with 12 filter spaces available. A notional set of filter wavelengths has already been
allocated for PanCam, however this filter set is inherited from Beagle 2, which in turn
was adopted from Mars Pathfinder. Filters for the Imager for Mars Pathfinder (IMP)
device were selected with two main objectives in mind. Firstly, to identify ferric oxides
and oxyhydroxides, and secondly to determine silicate mineralogy present, particularly
pyroxenes (Smith et al. 1997b). This filter set was largely adopted for the MER PanCam,
again focusing on the detection of iron-bearing silicates, iron oxides and oxyhydroxides,
and also to provide a direct comparison to the IMP results (Bell et al. 2003). ExoMars has
a distinct astrobiological focus, and may encounter an extensive range of hydrated
mineral-bearing lithologies. As a consequence, the notional filter set for the PanCam
needs to be revised.
The ability to detect regions or outcrops of interest remotely is a fundamental aspect of
planetary rover exploration. However, whilst multispectral imaging at optical to nearinfrared wavelengths (400 – 1000nm) can reveal a lot about mineralogy and chemistry,
the majority of distinguishing spectral features occur in the infrared. As a result, the
combination of reflectance spectra with other instrumental analysis can enhance the
ability of the PanCam in identifying mineralogy and lithology. The MicrOmega
instrument can provide additional detailed spectral information in the 0.9 – 2.6µm region
(Leroi et al. 2009), helping to confirm many of the mineralogical features identified with
PanCam. Additionally, the Raman instrument will be able to provide mineral
5
identification of target samples (Wiens et al. 2005; Bazalgette et al. 2007; Sharma et al.
2007; Sharma et al. 2009), and will provide vital data to complement the PanCam’s
multispectral capabilities. These instruments are currently envisaged to work on the
micro-scale on drill samples, and as such are not remote surveying tools, potentially
leaving PanCam as the sole means of remote target selection. However, in future
missions a remote Raman system could provide an excellent way to support PanCam
data.
Here, the reflectance spectra from publically available spectral libraries were used to
formulate prospective new geology filter sets for the ExoMars PanCam. These libraries
included: the United States Geological Survey spectral library (USGS; Clark et al. 2007;
1993), the JPL spectral library (Baldridge et al. 2009), and RELAB (spectra acquired by
Bruce Fegley, Carle Pieters, Edward Cloutis, Jack Mustard, Janice Bishop, Michelle
Goryniuk, and Phoebe Hauff, with the NASA RELAB facility at Brown University)
library. Specifically, spectra of hydrated minerals already identified on Mars were used.
These alternative filter sets, plus the Beagle2 (B2) filter set (which is currently the
baseline for PanCam) were tested on their ability to successfully detect hydrated minerals
present in untreated Martian analogue rocks from Iceland. The benefit of using raw
samples instead of homogenised, size-specific particulate fractions, has been highlighted
by Harloff & Arnold (2001). Previous to that, Yon & Pieters (1988) noted that the most
appropriate analogue to outcropping rock on a planetary surface is not a powdered
sample but a rough bulk sample surface. As such, natural geological samples are used for
this work, to best simulate the nature of geological outcrops that may be encountered
6
during the ExoMars mission. Multi-instrument and spectral analysis of analogue or
astrobiologically-relevant material has previously shown the value of ground-truthing
studies (e.g. Bishop et al. 2004a; Bishop et al. 2004b; Pullan et al. 2008). Contextual
information for the samples was also gathered on mineralogy, chemistry, and
textural/morphological features using Raman spectroscopy, X-ray Diffraction (XRD),
Scanning Electron Microscopy (SEM), and Energy Dispersive X-Ray Spectrometer
(EDS) analysis.
1.2 Hydrated Minerals
Hydrated mineral terrains on Mars are of particular interest to the ExoMars mission as
they are essential in establishing the history of liquid water on the planet, which in turn is
directly related to the search for past or present Martian life. Minerals with OH or H2O as
part of their chemical structure require aqueous conditions to form, but do not necessarily
need liquid water to remain stable in their current environment after formation (Bishop
2005). It is this knowledge that has driven the need to explore terrains rich in hydrated
minerals, with the aim of identifying evidence for past habitable environments on Mars
(Murchie et al. 2009). The results of the hyper-spectrometers OMEGA and CRISM
currently orbiting Mars have revealed a large diversity in surface mineralogy and
lithology (e.g. Bibring et al. 2005; Mustard et al. 2008; Poulet et al. 2007; Murchie et al.
2009; Poulet et al. 2009). Hydrated minerals on Mars have now been identified in distinct
hydrated mineral terrains. Oldest are the phyllosilicates, which are found in early
Noachian terrains (Poulet et al. 2005; Bibring et al. 2006; Mustard et al. 2008). These are
predominantly Fe, Mg and Al – rich phyllosilicates (such as nontronite and
7
montmorillonite) and have been seen to occur as alternating lithologies (Loizeau et al.
2007). These phyllosilicates reveal a complex aqueous history of early Mars that may be
indicative of long-term aqueous alteration of basaltic/igneous material in this area (Poulet
et al. 2005; Bishop et al. 2008). Sulphates, notably gypsum, jarosite and kieserite,
amongst others, have been identified in layered terrains on Mars (Klingelhofer et al.
2004; Bibring et al. 2005; Gendrin et al. 2005). These have been seen to occur in Late
Noachian and early Hesperian terrains, and it has been suggested they indicate a change
from neutral pH conditions to more acidic environments (Bibring et al. 2006). Sulphates
are commonly formed through volcanic activity (such as alteration of volcanic rocks by
acidic fumaroles) or evaporitic processes (Martinez-Frias & Amaral 2006).
More recently discovered are deposits of opaline silica on Mars, both detected by CRISM
and by the MER Spirit at Gusev Crater (Milliken et al. 2008; Squyres et al. 2008; Rice et
al. 2010). Opaline silica commonly forms in hot spring systems, where the eruption of
silica supersaturated hot spring fluids at the surface gradually produces siliceous sinters
over time. Additionally, opaline silica can form via hydrothermal weathering of basaltic
rock, stripping the mafic minerals away leaving behind a silica-rich crust (Kraft et al.
2003). Recently Grasby et al. (2009) have described silica deposits from cold springs
believed to be unrelated to hydrothermal systems. Silica sinters are known to be excellent
bio-preservers on Earth, and it has been noted previously that the discovery of hydrated
silica on Mars could have important implications for the detection of Martian
biosignatures that may be similar to those commonly found in hot spring sinters on Earth
(Farmer & Des Marais 1999; Goryniuk et al. 2004). Likewise, the recent discovery of
8
possible carbonate deposits (Ehlmann et al. 2008; Palomba et al. 2009; Morris et al.
2010) suggest past environments on Mars were more habitable than previously thought.
The aim of this work was to carry out the preliminary exploration of potential new
geological filter wavelengths for the ExoMars PanCam, and present initial results from
testing these new filters on Martian analogue rocks. Reflectance spectra from hydrated
minerals, including gypsum, jarosite, alunite, calcite, montmorillonite, nontronite, and
opaline silica, were used to formulate an alternative filter sets for the ExoMars PanCam,
with the aim to detect these minerals within geological outcrops and deposits within the
range of the rover.
2. MATERIALS AND METHODS
2.1 Hydrated mineral reflectance spectra
Reflectance spectra between the region 440 – 1000nm of hydrated minerals from the
USGS spectral library splib06a (Clark et al. 2007) and splib04 (Clark et al. 1993), JPL
spectral library (Baldridge et al. 2009), and RELAB spectral library (Brown University,
see above), were used to identify particular wavelengths that would be optimal for
detecting the diagnostic spectral features of such minerals at the Martian surface.
Minerals were chosen based on the identification of hydrated mineral groups on Mars,
and are shown in Table 1. For filter selection (described below), the mineral spectra were
required to have evenly spaced spectral points, and as such the published spectral data
were re-sampled to produce data points every 10nm. It is noted that the mineral samples
used to generate the library spectra are not always compositionally 100% pure, and often
9
have other minor mineral fractions present. As such, we merely take these powdered
mineral reflectance spectra as an ideal spectrally pure ‘end member’. Another important
factor is the variation between different sample spectra of the same mineral within the
database. For example, the spectra for different samples of jarosite vary. Therefore,
multiple sample spectra were utilised in the formulation of the new filter set to eliminate
any bias towards one arbitrarily chosen sample. All mineral spectra used in the filter
selection are given in Table 1, together with their associated mineral sample attributes,
including impurities, and grainsize.
2.2 Martian analogue samples
Martian analogue samples (Figure 1) were collected from Iceland. They include
subglacial basaltic lavas, hydrothermally altered lavas and hot spring precipitates. The
island of Iceland is a predominantly volcanic country, formed by the surface expression
of the Mid Atlantic Ridge and an underlying mantle plume - the Icelandic Hot spot
(Sigvaldason et al. 1974; Korenaga 2004). There are numerous examples of past (mostly
Pleistocene) subglacial volcanic activity, and later Holocene lava flows, both of which
commonly have undergone hydrothermal interaction. Geologically, Iceland bears many
similarities to proposed Martian volcanism. It is dominated by basaltic volcanism that is a
result of fissure eruptions and mantle plume activity. Many volcanic features identified
on Mars are also seen in Iceland, notably widespread basaltic flows (Keszthelyi et al.
2004) and shield volcanoes. The terrestrial analogues of some features on Mars, such as
pseudocraters (rootless cones), are found predominantly in Iceland (Fagents &
Thordarson 2007), and several comparisons have been made between Martian and
10
Icelandic glaciovolcanism (Chapman & Smellie 2007). Therefore, Iceland is considered
here as an ideal analogue for volcanic environments that have existed on Mars in the past.
Additionally, as none of the Icelandic samples contained carbonate, a sample of siderite
(from UCL Earth Sciences Geology Collections) was also used for testing the new filter
wavelengths.
2.3 Multispectral imaging and analytical methods
Martian analogue samples were imaged multispectrally at the Mullard Space Science
Laboratory, UK. Samples were illuminated with a Solex solar lamp at an average
distance of 60 cm, although this varied depending on the size of the sample. Still-capture
imaging was carried out at a distance of 1 m (the minimum distance at which PanCam
will be used) from the sample by a Foculus FO432SB camera (1.4Mpixles, 8- bits/pixel
greyscale, 15° field of view lens, exposure time 1 ms to 65s). As with the ExoMars
PanCam, this camera has a 1024 x 1024 pixel CCD which responds to wavelengths
between 400 – 1000nm. The images taken at this distance typically had a spatial
resolution of between 100 – 200 µm/pixel. The camera was interfaced with one of two
CRI Varispec liquid crystal tuneable filters – one covering the visible (wavelength range
of 400 – 730nm; bandpass of 20nm) and the other covering the near infrared (wavelength
range of 650 – 1100nm; bandpass of 10nm). Images were taken with these filters in 10nm
increments. Images were processed with ImageJ software (http://rsbweb.nih.gov/ij/).
Relative reflectance spectra were calculated by dividing the brightness values averaged
across a number of pixels for the sample/target of interest (T) by that measured from a
white calibration target (C) in the background of each image. The size of the regions from
11
which spectra were extracted varied from 10 x 10 pixels to 200 x 200 pixels, depending
upon the size of the feature in question.
Mineralogical identification within the Martian analogue samples was achieved using
Raman spectroscopy and X-Ray Diffraction (XRD) to confirm spot mineralogy (within
the multispectral target area), and bulk mineralogy, respectively. Raman spectra were
gathered using a Renishaw InVia Raman spectrometer coupled with a Leica microscope
at University College London, using a 785nm laser, through either a 20x or 50x
microscope lens. XRD analysis was conducted at Aberystwyth University using a Bruker
D8 Advance XRD with a Vantec Super Speed detector. Sub-millimetre structural and
elemental data were collected using a combined Scanning Electron Microscope (SEM)
with Back Scatter Electron (BSE) detector, and Energy Dispersive X-ray Spectrometer
(EDS) from samples to provide additional context for the interpretation of the observed
reflectance spectra from the multispectral imaging data. For the SEM study, thin sections
were carbon coated and analysed using a Jeol Scanning Electron Microscope (JSM6480LV) at University College London. For associated EDS analysis, an accelerating
voltage of 15kV was used.
2.4 Selection of filter wavelengths for two possible new filter sets
Two new alternative filter sets were devised. Reflectance spectra (from spectral
databases, see section 2.1 above) of hydrated minerals nontronite, jarosite,
montmorillonite, calcite, gypsum, opaline silica, and alunite were used to devise one of
12
these two new filter sets, based on the necessity to successfully identify hydrated
minerals on Mars. The new filter sets were chosen as follows:
Filter set F1-12: This filter set had 12 evenly spaced filters, spaced every 50nm (with the
exception of the first two filters, which have a 60nm spacing) and was not biased towards
any pre-determined set of minerals.
Filter set F2-12: This filter set was generated statistically based upon hydrated mineral
spectra. Twelve optimal wavelengths that most accurately reproduced a specific mineral
spectrum were calculated for all the 70 selected hydrated minerals collectively (all
normalised to the same starting value). This filter selection used a brute force approach to
search through all the possible wavelength selections between 440 and 1000nm, using
10nm increments. Two of these 12 wavelengths were always fixed at 440 and 1000nm.
The suitability of a wavelength selection was calculated firstly by interpolating the points
between the 12 randomly chosen wavelengths to obtain a full set of data (i.e. a complete
spectrum) for each individual mineral spectrum. Then, we calculated the absolute
difference in reflectance (residual) between the actual spectrum of the mineral (Ra), and
the spectrum estimated (Re) using the current wavelength selection at each respective
10nm spaced point (λ). The summation of these absolute distances is then used as the
‘error score’ for that particular wavelength selection and mineral (σm):
m 
13
1000
R ( )  R ( )


 440
a
e
where λ increments in steps of 10nm. The lower the error score, the better the wavelength
selection, producing the optimal set of 12 wavelength points for each mineral. To find the
best wavelength selection for the whole mineral set, the error scores for each mineral
were summed into an overall error score (σ) for each combination of wavelengths
assessed:
M
   m
m 1
This generated filter set ‘F2-12’.
These two new filter sets, and the original Beagle2 (B2) filter wavelengths, were tested
on Martian analogue samples to evaluate their suitability in the detection of hydrated
minerals on Mars.
2.5 Testing filter sets on Martian analogue samples
Multispectral data gathered for the Martian analogue samples were re-sampled to match
that of the filter wavelengths in the proposed new filter sets, producing PanCam-style 12point spectra. In addition, the data were averaged within the range covered by the
bandpass for each filter, of which the B2 filter bandpasses had already been previously
assigned (Griffiths et al. 2006). These were used in the re-sampling of rock sample data
averaged across the full-width-at-half-maximum (FWHM) values. For the new filter sets,
estimated bandpasses were utilised, based on those assigned for the B2 filters. These are
shown in Table 2.
14
The Martian analogue samples contained sulphate, phyllosilicate, and opaline silica
minerals, and specific mineral species were confirmed with Raman spectroscopy and
XRD (sample details in Table 4 and Figure 4). A sample of the Fe-carbonate siderite was
also used, from which confirmatory Raman data was also acquired (Figure 4). Six
Icelandic geological samples (Figure 1; plus one siderite sample) were used, and the
spectra for each respective filter set for these samples are shown in Figure 5. Where
relevant, spectra for different coloured regions (and therefore potentially different
mineralogy) on the rock are shown, highlighting the spectral variation across a naturally
heterogeneous geological sample.
The filter sets were assessed firstly on whether or not they were able to capture spectral
features that could potentially lead to the identification of a particular mineral species
within the rocks, and secondly on their ability to distinguish between the different rock
samples. Spectral parameters, such as those used to identify spectral variability within
MER PanCam multispectral images (e.g. Farrand et al. 2006; 2008), were used, and are
listed in Table 3. Different filter sets will have different filter wavelengths covering a
particular spectral feature, such as the 440 – 700nm absorption edge, or a 900nm
absorption band. Where there is not a filter centred at the specific band in question, the
nearest filter is used, and so is representative of how spectral parameter data will appear
with the different filter sets. Finally, multivariate analysis was employed to demonstrate
the ability of the filter sets to identify and distinguish between clusters of similar mineral
targets.
15
3. RESULTS
3.1 The new geological filter sets
The new filter set F2-12 differs principally to the B2 filters in that it lacks a concentration
of filters towards the NIR end of the spectrum, with a broader range of filters within the
visible. The spectral region between 440-1000nm is dominated by Fe excitational and
charge transfer bands (Bishop 2005), and therefore is particularly sensitive to ironbearing mineralogy (Farrand et al. 2008). As a result, distinctive spectral features are
particularly evident in Fe-bearing hydrated minerals such as jarosite and nontronite, as
well as silicates pyroxene and olivine and a range of iron oxides such as haematite and
goethite. Ferric absorption bands exist at 480nm, 650nm and 950nm, whilst a ferrous
absorption band also in the 950nm region can often obscure that produced by ferric iron
(Stewart et al. 2006). Whilst nontronite and jarosite share these similar Fe-absorption
bands, nontronite has a distinctive kink around 650nm that is not present in the jarosite
spectrum, and it is features such as this that will be crucial in distinguishing PanCam
spectral data.
The filter centre wavelengths and their estimated bandpasses at FWHM are shown in
Figure 2, together with the position of ferric and ferrous iron, and water/hydroxyl bands
that are common to the hydrated minerals used in this study (also shown). It can be seen
that the B2 filters miss a Fe3+ absorption band at ~500 and 650nm, and the jarosite and
nontronite reflectance bands at 710nm and 580nm respectively. Overall, five of the B2
filters fall directly (their centre wavelength, 4 filters) or partially (coverage only by their
bandpass, 1 filter) within these spectral bands. Similarly, six of the F1-12 filters cover the
16
spectral bands (five with their centre wavelength). In comparison, filter set F2-12 has
eight filters falling within the spectral absorption/emission bands (six by their centre
wavelength). As a simple example, Figure 3 shows two hydrated mineral spectra
(nontronite and jarosite) focused on a particularly variable region, that demonstrates the
effect different filter positions have on the observed spectrum. It can be seen that the B2
filter set does not fully recover key diagnostic spectral points for the mineral nontronite,
where the Fe3+ absorption feature at 650nm falls between B2 filters 600nm and 670nm.
Likewise, the B2 filters do not capture the reflectance peak at 710nm in jarosite, whilst
filter set F1-12 partially misses it. Filter set F2-12 however closely follows the spectral
features of both nontronite and jarosite within this wavelength region.
3.2 Identification of sulphates in Martian analogue samples
Sample NAL is a subaerial basaltic lava that is covered in extensive sulphur-rich
alteration products due to acid-fumarole weathering, producing distinctive red and yellow
colour regions on the rock surface (Figure 1). EDS and BSE data show there to be
different mineralogical deposits within lava vesicles, ranging from Si-rich, to Fe-Al-rich
sulphate compositions (Figure 4, Table 4). Additionally, Raman spectroscopy shows the
surface deposits to include natroalunite, natrojarosite, and haematite (Figure 4). The
spectra of the different regions on this rock are consistent with the iron sulphate jarosite
(KFe3(SO4)2(OH)6), and the aluminium sulphate alunite (KAl3(SO4)2(OH)6), for ‘Yellow’
and ‘Red’ regions respectively (Figure 5b). The ‘Yellow’ region produces a spectrum that
has a steep ferric absorption edge between 440 – 700nm, followed by a broad absorption
centred around 950nm, which is characteristic of Fe3+. The ‘Red’ region has spectral
17
features synonymous with those often seen for alunite, including a steep gradient between
440-700nm, and particularly an absorption feature at 950nm – 1000mn, which can be
attributed to OH stretching vibrations that exist further into the NIR, the first band
existing at 1010nm (Hunt et al. 1971). Pure alunite is typically white (Hunt et al. 1971),
but in this case it exists as red deposits on the surface of the lava. This red colouration is
due to the partial replacement of Al3+ with Fe3+ (Hunt et al. 1971), which is represented
by the broad absorption feature between 800 and 900nm. Although jarosite is also a
hydrated sulphate, it doesn’t exhibit any OH bands until further into the infra-red
spectrum (Clark 1990). All these spectral features for both the ‘Yellow’ and ‘Red’
regions were best captured with the filter set F2-12, whilst both the B2 and F1-12 filter
sets miss the peak at 710nm in the ‘Yellow’ spectrum (Figure 5b).
Sample NBO, like sample NAL, is an acid-weathered basaltic lava, with orange coloured
deposits filling the vesicles that are surrounded by unaltered basalt. The spectrum of these
‘Orange’ deposits is similar to the ‘Yellow’ mineral deposit in sample NAL, exhibiting a
main peak at 720nm, and absorption features at 480 and 950nm (Figure 5a). As with the
NAL deposits, these features suggest the presence of jarosite, which is entirely consistent
with the geological setting of this lava. This is confirmed with Raman analysis (Figure 4).
Unlike the deposits in sample NAL, all filter sets are able to correctly reproduce this
spectrum, although the B2 and F1-12 filter sets only partially capture the 710nm peak,
which lies either between the 670 and 750nm filters (B2), or 650 and 700nm filters (F112).
18
3.3 Identification of phyllosilicates in Martian analogue samples
Hyaloclastite sample HH is rich in palagonite, which exists as alteration rinds around
basaltic glass clasts, as seen under BSE (Figure 4). The smectite composition of
hyaloclastite is represented by the elemental composition of the palagonite matrix
surrounding the basaltic glass fragments (Table 4). XRD data revealed the sample to
largely consist of amorphous, non-crystalline material, typical of palagonite (Stroncik &
Schmincke 2002). Despite the lack of crystalline material detected with XRD, the
PanCam spectral features for this rock suggest a nontronite component, with
characteristic ferric or ferrous iron absorption features at 470, 680, and 950nm. The
absorption features observed at ~460nm and ~950nm can be attributed to the presence of
Fe3+, whilst the shifting of the absorption from 650nm (more typical for nontronite) to
680nm is indicative of a change from Fe3+ to Fe2+ (Stewart et al. 2006). Of the three filter
sets, this spectrum is best reproduced by sets B2 and F1-12, in terms of the number of
spectral features covered (Table 5). The new filter set F2-12 misses absorption features at
680nm and 800nm. However, whilst the B2 filters only miss one spectral feature (the
kink at 680nm), this absorption is a key diagnostic feature of the nontronite spectrum
(Figure 5d).
3.4 Identification of opal and carbonate in Martian analogue samples
Silica-rich hot spring deposits are more ambiguous regarding the multispectral detection
of mineral species present. Sample KH is a hyaloclastite with an opaline silica crust (1 –
4mm thick) on the surface, as confirmed by Raman spectroscopy and EDS (Table 4,
Figure 4). XRD analysis shows this crust to be largely amorphous with minor amounts of
19
calcite. Additionally, this surface crust is inhabited by chasmolithic lichen communities.
The spectrum of this white crust is consistent with that of opal-a, exhibiting a smooth
arcing spectrum, although the hydration feature at 950nm is not present here (Figure 5f).
The additional presence of the lichens has the effect of creating a shallow, broad
absorption feature at 670nm, indicative of Chlorophyll a. As would be expected, this
670nm feature is prominent in the ‘Lichen’ spectrum, along with a steep H2O absorption
between 900 – 1000nm. All the filter sets recovered these spectral features, from both the
‘White’ and ‘Lichen’ regions (Table 5f).
Samples GY1 and GY2 are both hot spring silica sinters, again dominated by opaline
silica. GY1 is characterised by several different coloured layered regions, ranging from
black, red, yellow, and white (Figure 1b, Figure 4f), with these components ranging from
1 – 10mm in size. The spectrum of the ‘White’ region has a generally flat morphology,
with a small absorption at 950nm, comparable to the spectral features of opal-CT, where
the 950nm absorption feature can be attributed to OH-. This 950nm absorption is captured
by all the filter sets (Figure 5c). The spectra of the other coloured regions of sample GY1
however are less indicative of its true mineralogy. Raman analysis of the ‘Red’ and
‘Yellow’ sinter components identifies these regions as haematite and sulphur (Figure 4)
respectively, yet this is not revealed by the multispectral imaging. These mineral
components are 1 – 2mm in size, and are therefore well within the limits of the test
camera resolution (100 – 200 µm/pixel). Figure 6 shows the 12-point spectra for these
two coloured regions, using filter set F2-12. It can be seen that the features are more
consistent with those for the sulphate alunite, as seen in sample NAL (Figure 5b). These
20
two spectra exhibit differences in features that suggest perhaps a minor influence of their
true mineralogy, such as the flatter profile of ‘GY1 Yellow’ between 600 – 830nm
(sulphur has a flat profile between 500 – 1000nm), and the absorption between 440 560nm which is a feature typically seen for haematite. However, these features are by no
means diagnostic, and highlight potential problems of remotely identifying small
mineralogical targets within a heterogeneous rock sample. Like GY1, sample GY2 also
formed as part of a silica sinter, but it exhibits a different spectral profile to that of GY1.
GY2 is a silicified biomat with a dominantly Si composition (Table 4) and opal-a
mineralogy (Figure 4), yet this is not represented within the spectral features (Figure 5c).
As with the small sulphur and haematite mineral deposits in GY1, these spectral features
are suggestive of alunite, despite the silica-rich composition of the rock. This alunite
component can be seen to exist as a fine particulate coating on the surface of the sinter
deposit (discernable from hand specimen), therefore preventing the silica mineralogy
from influencing the observed spectrum. Likewise, the biological origin of this deposit
appears to have no impact on the observed spectra, even though morphological
microfossils can be easily identified as a significant structural component (Figure 7). As
with sample KH, all filter sets captured the key spectral features of the GY2 spectrum
(Table 5).
Finally, iron-rich carbonate siderite (FeCO3) shows clear differences between the three
filter sets. In particular, the B2 filters miss a significant peak at ~720nm (Figure 5e), a
feature that is captured with both F1-12 and F2-12 filter sets. Siderite, along with
magnesite, is one of the carbonates believed to exist on Mars (Morris et al. 2010; Bridges
21
et al. 2001), and indeed is one of the phases found in some Martian meteorites (Romanek
et al. 1994; Bridges & Grady 2000). Considering the importance of carbonate minerals to
astrobiology (e.g. Murchie et al. 2009), it is imperative PanCam is able to correctly
identify them.
3.5 Spectral parameter representation of Martian analogue samples
In addition to PanCam spectral data being used to identify putative mineral assemblages,
this information also needs to successfully distinguish between different targets. For this,
spectral parameter plots were used to group Martian analogue spectra into spectral groups
based on particular spectral features, such as the steepness of an absorption slope, or the
depth of an absorption band (listed in Table 3). These plots are often used to define
spectral variability within Martian rocks, and therefore potential lithological variability
(Farrand et al. 2006; 2008). Likewise, such parameters can be used to identify the
distribution of a particular mineralogical feature across a Martian scene (Rice et al.
2010). Therefore, the selection of particular filter wavelengths may affect the ability of
PanCam to correctly distinguish different lithologies/mineral species.
Figure 8 plots ferric iron content (steepness of the 440 – 700nm slope) against water
content (920 – 1000nm slope). The mineral spectral data used for the selection of filter
set F2-12 (Table 1) are also plotted for comparison, and it can be clearly seen that a
decrease in the ferric iron absorption slope corresponds with an increasing water
absorption slope (Figure 8a). This trend is also seen in the sulphate-containing rock
samples, with jarosite-rich samples NAL Yellow and NBO following this trend down to
22
alunite-containing samples NAL Red and GY2 (Figure 8b). There is little difference
between samples KH White, GY1 White, and HH, which cluster in a similar region to
opaline silica (Figure 8d), consistent with their mineralogy except for HH, the presence
of which is most likely explained by its amorphous composition. Iron-rich carbonate
siderite plots with a much steeper 440 – 700nm ferric absorption slope, distinct from the
rest of the samples within this plot.
Lastly, these four spectral parameters (Table 3) were used together to distinguish between
these samples using discriminant analysis (DA), and to cluster the rock sample targets
using unsupervised hierarchical cluster analysis (HCA). The DA (Figure 9a) shows the
ability of the three filter sets at distinguishing between the samples. Filter set F2-12
provides by far the best discrimination between the groups, whilst the B2 filter set
performs worst with all three groups clustering close together. Likewise, in both B2 and
F1-12 filter sets, Group 1 and Group 2 overlap, demonstrating poor discrimination
between the rock targets within these groups. Conversely F2-12 filters clearly
discriminate well between all three groups, demonstrating the benefit of optimising filter
wavelengths to putative mineral targets.
Overall, the filter testing on untreated, heterogeneous rock samples shows some variation
between the filter sets, in terms of both ability to detect potentially diagnostic spectral
features, and also to clearly discriminate between rock targets using spectral parameter
values. Of the three filter sets tested, both F1-12 and F2-12 performed better than the
baseline Beagle 2 filters, with filter set F2-12 detecting the most spectral features (Table
23
5), and also most clearly discriminating between rock samples (Figure 9). For
comparison, this filter set is given alongside the ‘Geology’ filter wavelengths for other
past and present Mars rovers/landers, including those for MER and currently proposed
for the Mars Science Laboratory (MSL; Table 6). In particular, the MSL MastCam filter
set has significantly fewer filters within the visible (<750nm). Where this wavelength
range is covered by 8 filters by filter set F2-12, it is covered by only 4 filters in the
MastCam. More extensive testing on a Martian analogue samples exhibiting a
significantly wider range of mineralogies would be the next step in identifying the value
in specific wavelength selections, or whether an entirely evenly spaced filter set would
suffice. Indeed, such work not only has relevance to the ExoMars mission, but also to
other astrobiologically focused missions, such as MSL.
4. DISCUSSION & CONCLUSIONS
This study was conducted with two aims: firstly to produce and test alternative geological
filter sets to replace the Beagle2 filters currently assigned to ExoMars, and secondly to
present initial results from the testing of these possible filter sets on Martian analogue
rock samples. It is apparent that a concentration of filters towards the NIR end of the
spectrum, as with the B2 filter set, is not optimal for the ability of the PanCam to detect
hydrated minerals as a component of heterogeneous rock samples. The first new filter set
also explored - ‘F1-12’ - has filters spaced at regular intervals every 50nm. The benefit of
this filter set is that the even spread of spectral points is not biased towards any particular
group of minerals, and so theoretically makes the PanCam equally suited to detecting any
given mineral or lithology it encounters. However, whilst much is still unknown as
24
regards to the lithology and mineralogy of the Martian surface, ExoMars will be focused
particularly on regions where hydrated minerals are likely to predominate. The aim of
positioning these points so that they favour certain mineral groups is to enhance the
detection of these specific minerals in keeping with the mission objectives. The
effectiveness of this can be seen in Figure 3.
The filter set that was generated statistically (F2-12) based on a set of seven different
hydrated minerals, proved most effective at identifying hydrated minerals in the Martian
analogue rocks, suggesting this particular method of filter wavelength assignment is
successful at selecting a suitable set of geological filters. As such, there is the potential to
utilise this method using a much wider set of mineral spectral data before the PanCam
filter wavelengths are finalised. Our knowledge of the mineralogical diversity of the
Martian surface is increasing, and future PanCam geological filter selection and testing
will need to incorporate a much wider range of minerals and analogue samples. Another
factor is the mineralogy of the selected ExoMars landing site (still to be finalised). The
availability of CRISM and OMEGA data allows the characterisation of the bulk
mineralogy of a potential landing site, and this could be used to heavily influence the
ExoMars PanCam filter wavelengths. For example, if such orbital spectrometer data
showed there to be extensive and rover accessible clay-bearing geological units, the
PanCam filters may be more useful if they were optimised to detect phyllosilicate
minerals only. Such focused target selection would benefit from potentially more
diagnostic PanCam spectra, but may significantly reduce the ability of PanCam to
undertake more opportunistic science and any subsequent unexpected discoveries.
25
The need to expand on spectral ground-truthing data sets was recently highlighted by
Poulet et al. (2009). As with natural outcrops that are likely to be encountered on Mars,
these rock samples display large and small scale heterogeneities, only some which were
clearly distinguished with PanCam-style 12-point spectra, as in sample NBO where the
spectral data were clearly able to detect the ‘Orange’ jarosite component within the
surrounding basalt lava (Figure 5a). Likewise, the lichens in sample KH were spectrally
distinct from the surrounding silica crust. However, detecting heterogeneity was a
problem for the silica sinter samples – deposits that would have a particular
astrobiological relevance. Opaline silica deposits have the potential to provide
information on past life on Mars, especially if such deposits formed through hot spring
sinter development - a process commonly seen in volcanically active regions on Earth.
There is a possibility for the discovery of morphological, mineralogical and chemical
biosignatures within these precipitates (e.g. Goryniuk et al. 2004; Schulze-Makuch et al.
2007; Preston et al. 2008), but the reflectance spectra of the silica sinter samples from
Geysir provide little evidence of their hot-spring origin, and therefore astrobiological
potential. A similar example is provided by the small mineralogical variations within
sample GY1, the spectra of which did not identify the presence of haematite or sulphur
detected by Raman spectroscopy. Haematite has been previously documented to be an
effective inorganic barrier to UV radiation (Clark 1998), and has also been shown to be
present at the Martian surface by the MER Opportunity (Klingelhofer et al. 2004), and as
such is one of the many minerals that could be expected to be found around the ExoMars
landing site.
26
In conclusion, much work still needs to be done with regards to firstly choosing and
testing an optimised geological filter set for the ExoMars PanCam instrument, and
secondly providing essential ground-truthing using Martian analogue samples.
Quantifying the limits of PanCam multispectral imaging in the remote detection of
mineral targets is especially important, as is the effect of dust coverage on an already
heterogeneous rock sample. Although the multispectral data acquired by PanCam is fairly
crude in comparison to other planetary instrumentation, it plays a crucial role in the
selection of specific targets for more detailed investigation, and as such should be
developed to its full potential.
ACKNOWLEDGEMENTS
This work was jointly funded by an EPSRC studentship award obtained through a UCL
strategic, interdisciplinary research initiative, the UK Science and Technology Facilities
Council (STFC), who are the lead funding agency for the development of the ExoMars
Panoramic Camera, and the Leverhulme Trust. Claire Cousins would also like to thank
James Davy for assistance with SEM/EDS analysis, and Dr. Steve Firth for assistance
with Raman analysis. Finally, all authors would like to acknowledge the reviewers for
their time and suggestions which led to considerable improvements to the manuscript.
REFERENCES
27
Baldridge, A. M., Hook, S. J., Grove, C. I. and G. Rivera (2009). The
ASTER Spectral Library Version 2.0. In press Remote Sensing of Environment. Remote
Sensing of Environment 113: 711-715.
Bazalgette Courrèges-Lacoste, G., Ahlers, B., Rull Pérez, F. (2007) Combined Raman
spectrometer laser-induced breakdown spectrometer for the next ESA mission to Mars.
Spectrochim. Acta, Part A: Molecular and Biomolecular Spectroscopy 68: 1023–1028.
Bell, J. F., Squyres, S. W., Arvidson, R. E., Arneson, H. M., Bass, D., Blaney, D., Cabrol,
N., Calvin, W., Farmer, J., Farrand, W. H., Goetz, W., Golombek, M., Grant, J. A.,
Greeley, R., Guinness, E., Hayes, A. G., Hubbard, M. Y. H., Herkenhoff, K. E., Johnson,
M. J., Johnson, J. R., Joseph, J., Kinch, K. M., Lemmon, M. T., Li, R., Madsen, M. B.,
Maki, J. N., Malin, M., McCartney, E., McLennan, S., McSween Jr., H. Y., Ming, D. W.,
Moersch, J. E., Morris, R. V., Noe Dobrea, E. Z., Parker, T. J., Proton, J., Rice Jr., J. W.,
Seelos, F., Soderblom, J., Soderblom, L. A., Sohl-Dickstein, J. N., Sullivan, R. J., Wolff,
M. J., Wang, A. (2004) Pancam Multispectral Imaging Results from the Spirit Rover at
Gusev Crater. Science 305: 800-806.
Bell, J. F., Squyres, S. W., Herkenhoff, K. E., Maki, J. N., Arneson, H. M., Brown, D.,
Collins, S. A., Dingizian, A., Elliot, S. T., Hagerott, E. C., Hayes, A. G., Johnson, M. J.,
Johnson, J. R., Joseph, J., Kinch, K., Lemmon, M. T., Morris, R. V., Scherr, L.,
Schwochert, M., Shepard, M. K., Smith, G. H., Sohl-Dickstein, J. N., Sullivan, R. J.,
28
Sullivan, W. T., Wadsworth, M. (2003). Mars Exploration Rover Athena Panoramic
Camera (Pancam) investigation. Journal of Geophysical Research 108 (E12): 8063.
Bibring, J.-P., Y. Langevin, J. F. Mustard, F. Poulet, R. Arvidson, A. Gendrin, B. Gondet,
N. Mangold, P. Pinet, and F. Forget. (2006) Global mineralogical and aqueous Mars
history derived from the OMEGA/Mars Express data. Science 312: 400-404.
Bibring, J-P, Langevin, Y., Gendrin, A., Gondet, B., Poulet, F., Berthe, M., Soufflot, A.,
Arvidson, R., Mangold, N., Mustard, J., Drossart, P. and the OMEGA team. (2005) Mars
Surface Diversity as Revealed by the OMEGA/Mars Express Observations. Science 307:
1576-1581.
Bishop, J. L., Dobrea, E. Z. D., McKeown, N., K., Parente, M., Ehlmann, B. L.,
Michalski, J. R., Milliken, R. E., Poulet, D., Swayze, G. A., Mustard, J. F., Murchie, S.
L., Bibring, J.-P. (2008) Phyllosilicate Diversity and Past Aqueous Activity Revealed at
Mawrth Vallis, Mars. Science 321: 830 – 833.
Bishop, J. L. (2005) Hydrated Minerals on Mars. In Water on Mars and Life, Tetsuya
Tokano (ed.), Adv. Astrobiol. Biogeophys., pp. 65-96.
Bishop, J. L., Murad, E., Lane, M., Mancinelli, R. L. (2004a) Multiple techniques for
mineral identification on Mars: a study of hydrothermal rocks as potential analogues for
astrobiology sites on Mars. Icarus 169: 311–323.
29
Bishop, J. L, Dyar, M. D., Lane, M. D. and Banfield, J. F. (2004b). Spectral identification
of hydrated sulfates on Mars and comparison with acidic environments on Earth.
International Journal of Astrobiology 3: 275–285.
Bishop, J. L., H. Fröschl, and R. L. Mancinelli. (1998) Alteration processes in volcanic
soils and identification of exobiologically important weathering products on Mars using
remote sensing. Journal of Geophysical Research 103: 31457–31476.
Bridges, J. C., Catling, D. C., Saxton, J. M., Swindle, T. D., Lyon, I. C., Grady, M. M.
(2001) Alteration Assemblages in Martian Meteorites: Implications for Near-Surface
Processes. Space Science Reviews 96: 365-392.
Bridges, J. C., Grady, M. M. (2000) Evaporite mineral assemblages in the nakhlite
(martian) meteorites. Earth and Planetary Science Letters 176: 267-279.
Chapman, M. G. and Smellie, J. L. (2007) Mars interior layered deposits and terrestrial
sub-ice volcanoes compared: observations and interpretations of similar geomorphic
characteristics. In The Geology of Mars: Evidence from Earth-Based Analog, edited by
M. G. Chapman. Published by Cambridge University Press, Cambridge, UK, 2007,
pp.178.
30
Clark, R.N., Swayze, G.A., Wise, R., Livo, E., Hoefen, T., Kokaly, R., Sutley, S.J. (2007)
USGS digital spectral library splib06a: U.S. Geological Survey, Digital Data Series 231.
Clark, B. C. (1998) Surviving the limits to life at the surface of Mars. Journal of
Geophysical Research Planets 103: 445 – 455.
Clark, R.N., Swayze, G. A., Gallagher, A. G., King, T.V.V. and Calvin, W.M. (1993)
The U. S. Geological Survey, Digital Spectral Library: Version 1: 0.2 to 3.0
microns, U.S. Geological Survey Open File Report, 93-592.
Ehlmann, B. L., Mustard, J. F., Murchie, S. L., Poulet, F., Bishop, J. L., Brown, A. J.,
Calvin, W. M., Clark, R. N., Des Marais, D. J., Milliken, R. E., Roach, L. H., Roush, T.
L., Swayze, G. A., Wray, J. J. (2008) Orbital Identification of Carbonate-Bearing Rocks
on Mars. Science 322: 1928 – 1832.
Evans, D. L., Adams, J. B. (1979) Comparison of Viking Lander multispectral images
and laboratory reflectance spectra of terrestrial samples (1979). In: Lunar and Planetary
Science Conference, 10th, Houston, Tex., March 19-23, 1979, Proceedings. Volume 2.
(A80-23617 08-91) New York, Pergamon Press, Inc. pp. 1829-1834.
Fagents, S. A. and Thordarson, T. (2007) Rootless volcanic cones in Iceland and on
Mars. In The Geology of Mars: Evidence from Earth-Based Analog edited by M. G.
Chapman Published by Cambridge University Press, Cambridge, UK, 2007, pp. 151.
31
Farmer, J. D. and Des Marais, D. J. (1999) Exploring for a record of ancient Martian life.
Journal of Geophysical Research 104: 26977-26995.
Farrand, W. H., Bell, J. F., Johnson, J. R., Arvidson, R. E., Crumpler, L. S., Hurowitz, J.
A., Schroder, C. (2008) Rock spectral classes observed by the Spirit Rover’s Pancam on
the Gusev Crater Plains and in the Columbia Hills. Journal of Geophysical Research 113:
E12S38.
Farrand, W. H., Bell, J. F., Johnson, J. R., Jolliff, B. L., Knoll, A. H., McLennan, S. M.,
Squyres, S. W., Calvin, W. M., Grotzinger, J. P., Morris, R. V., Soderblom, J.,
Thompson, S. D., Watters, W. A., Yen, A. S. (2007) Visible and near-infrared
multispectral analysis of rocks at Meridiani Planum, Mars, by the Mars Exploration
Rover Opportunity. Journal of Geophysical Research 112: E06S02.
Farrand, W. H., J. F. Bell III, J. R. Johnson, S. W. Squyres, J. Soderblom, and D. W.
Ming (2006) Spectral variability among rocks in visible and near-infrared multispectral
Pancam data collected at Gusev crater: Examinations using spectral mixture analysis and
related techniques, Journal of Geophysical Research 111: E02S15.
Gendrin, A., Mangold, N., Bibring, J-P., Langevin, Y., Gondet, B., Poulet, F., Bonello,
G., Quantin, C., Mustard, J., Arvidson, R., LeMouelic, S. (2005) Sulfates in Martian
Layered Terrains: The OMEGA/Mars Express View. Science 307: 1587-1591.
32
Goryniuk, M. C., Rivard, B. A. and Jones, B. (2004) The reflectance spectra of opal-A
(0.5–25 mm) from the Taupo Volcanic Zone: Spectra that may identify hydrothermal
systems on planetary surfaces. Geophysical Research Letters, 31,L24701.
Grasby, S. E., Bezys, R., Beauchamp, B. (2009) Silica Chimneys Formed by LowTemperature Brine Spring Discharge. Astrobiology 9: 931–941.
Griffiths, A. D., Coates, A. J., Jaumann, R., Michaelis, H., Paar, G., Barnes, D., Josset, JL. and the PanCam Team. (2006) Context for the ESA ExoMars rover: the Panoramic
Camera (PanCam) instrument. International Journal of Astrobiology 5: 269–275.
Harloff, J., Arnold, G. (2001) Near-infrared reflectance spectroscopy of bulk analog
materials for planetary crust. Planetary and Space Science 49: 191 – 211.
Hunt, G. R., Salisbury, J. W. and Lenhoff, C. J. (1971). Visible and Near-infrared spectra
of minerals and rocks: IV. Sulphides and sulphates. Modern Geology 3: 1-14.
Keszthelyi, L., Thordarson, T., McEwen, A., Haack, H., Guilbaud, M., Self, S. and Rossi,
M. J. (2004) Icelandic analogs to Martian flood lavas. Geochemistry Geophysics
Geosystems 5: 1-32.
33
Klingelhofer, G., Morris, R. V., Bernhardt, B., Schroder, C., Rodionov, D. S., de Souza
Jr., A., Yen, A., Gellert, R., Evlanov, E. N., Zubkov, B., Foh, J., Bonnes, U., Kankeleit,
E., Gutlich, P., Ming, D. W., Renz, F., Wdowiak, T., Squyres, S. W., Arvidson, R. E.
(2004) Jarosite and Hematite at Meridiani Planum from Opportunity’s Mossbauer
Spectrometer. Science 306: 1740-1745.
Korenaga, J. (2004). Mantle mixing and continental breakup magmatism. Earth and
Planetary Science Letters 218: 463-473.
Kraft, M. D., Michalski, J. R., Sharp, T. G. (2003) Effects of pure silica coatings on
thermal emission spectra of basaltic rocks: Considerations for Martian surface
mineralogy. Geophysical Research Letters 30 (24): 2288.
Loizeau, D., Mangold, N., Poulet, F., Bibring, J.-P., Gendrin, A., Ansan, V., Gomez, C,
Gondet, B., Langevin, Y., Masson, P. and Neukum, G. (2007) Phyllosilicates in the
Mawrth Vallis region of Mars. Journal of Geophysical Research 112: E08S08.
Leroi, V., Bibring, J-P., Berthe, M. (2009) Micromega/IR: Design and status of a nearinfrared spectral microscope for in situ analysis of Mars samples. Planetary and Space
Science 57: 1068 – 1075.
Malin, M. C., Caplinger M. A., Edgett, K. S., Ghaemi, F. T., Ravine, M. A., Schaffner, J.
A., Baker, J. M., Bardis, J. D., DiBiase, D. R., Maki, J. N., Willson, R. G., Bell, J. F.,
34
Dietrich, W. E., Edwards, L. J., Hallet, B., Herkenhoff, K. E., Heydari, E., Kah, L. C.,
Lemmon, M. T., Minitti, M. E., Olson, T. S., Parker, T. J., Rowland, S. K., Schieber, J.,
Sullivan, R. J., Sumner, D. Y., Thomas, P. C., Yingst, R. A. (2010) The Mars Science
Laboratory (MSL) Mast-Mounted Cameras (Mastcams) Flight Instruments. 41st Lunar
and Planetary Science Conference, held March 1-5, 2010 in The Woodlands, Texas. LPI
Contribution No. 1533, p.1123.
Martinez-Frias, J., Amaral, G., (2006) Astrobiological significance of minerals on Mars
surface environment. Reviews in Environmental Science and Biotechnology 4: 219 – 231.
Milliken, R. E., Swayze, G. G., Arvidson, R. E., Bishop, J. L., Clark, R. N., Ehlmann, B.
L., Green, R. O., Grotzinger, J. P. (2008) Opaline silica in young deposits on Mars.
Geology 36: 847 – 850.
Morris, R. V., Squyres, S., Arvidson, R. E., Bell, J. F., Christensen, P. C., Gorevan, S.,
Herkenhoff, K., Klingelhofer, G., Rieder, R., Farrand, W., Ghosh, A., Glotch, T.,
Johnson, J. R., Lemmon, M., McSween, H. Y., Ming, D. W., Schroeder, C., de Souza, P.,
Wyatt, M., and the Athena Science Team (2004). A first look at the mineralogy and
geochemistry of the MER-B landing site in Meridiani Planum. 35th Lunar and Planetary
Science Conference, March 15-19, 2004, League City, Texas, abstract no.2179.
Murchie, S. L., Mustard, J. F., Ehlmann, B. L., Milliken, R. E., Bishop, J. L., McKeown,
N. K., Noe Dobrea, E. Z., Seelos, F. P., Buczkowski, D. L., Wiseman, S. M., Arvidson,
35
R. E., Wray, J. J., Swayze, G., Clark, R. N., Des Marais, D. J., McEwen, A. S., Bibring,
J-P. (2009) A synthesis of Martian aqueous mineralogy after 1 Mars year of observations
from the Mars Reconnaissance Orbiter. Journal of Geophysical Research 114: E00D06
Mustard, J. F., Murchie, S. L., Pelkey, S. M., Ehlmann, B. L., Milliken, R. E., Grant, J.
A., Bibring, J.-P., Poulet, F., Bishop, J., Noe Dobrea, E. N., Roach, L., Seelos, F.,
Arvidson, R. E., Wiseman, S., Green, R., Hash, C., Humm, D., Malaret, E., McGovern, J.
A., Seelos, K., Clancy, T., Clark, R., Marais, D. D., Izenberg, N., Knudson, A., Langevin,
Y., Martin, T., McGuire, P., Morris, R., Robinson, M., Roush, T., Smith, M., Swayze, G.,
Taylor, H., Titus, T. and Wolff, M. (2008) Hydrated silicate minerals on Mars observed
by the Mars Reconnaissance Orbiter CRISM instrument. Nature 454: 305-309.
Palomba, E., Zinzi, A., Cloutis, E. A., D’Amore, M., Grassi, D., Maturilli, A. (2009)
Evidence for Mg-rich carbonates on Mars from a 3.0 µm absorption feature. Icarus 203:
58-65.
Poulet, F., Beaty, D. W., Bibring, J-P., Bish, D., Bishop, J. L., Dobrea, E. N., Mustard, J.
F., Petit, S., Roach, L. H. (2009). Key Scientific Questions and Key Investigations from
the First International Conference on Martian Phyllosilicates. Astrobiology 9: 257 – 267.
Poulet, F., Gomez, C., Bibring, J.-P., Langevin, Y., Gondet, B., Pinet, P., Belluci, G.,
Mustard, J. (2007) Martian surface mineralogy from Observatoire pour la Mineralogie,
36
l’Eau, les Glaces et l’Activite on board the Mars Express spacecraft (OMEGA/MEx):
Global mineral maps. Journal of Geophysical Research 112: E08S02.
Poulet, F., J.-P. Bibring, J. F. Mustard, A. Gendrin, N. Mangold, Y. Langevin, R. E.
Arvidson, B. Gondet, and C. Gomez (2005), Phyllosilicates on Mars and implications for
early Martian climate, Nature 438: 623– 627.
Preston, L. J., Benedix, G. K., Genge, M. J., Sephton, M. A. (2008) A multidisciplinary
study of silica sinter deposits with applications to silica identification and detection of
fossil life on Mars. Icarus 198: 331–350.
Pullan, D., Westall, F., Hofmann, B. A., Parnell, J., Cockell, C. S., Edwards, H. G. M.,
Jorge Villar, S. E., Schroder, C., Cressey, G., Marinangeli, L., Richter, L. and
Klingelhofer, G. (2008) Identification of Morphological Biosignatures in Martian
Analogue Field Specimens Using In Situ Planetary Instrumentation. Astrobiology 8: 119156.
Rice, M. S., Bell, J. F., Cloutis, E. A., Wang, A., Ruff, S. W. Craig, M. A., Bailey, D. R.,
Johnson, J. R., de Souza Jr., P. A., Farrand, W. H. (2010) Silica-rich deposits and
hydrated minerals at Gusev Crater, Mars: Vis-NIR spectral characterization and regional
mapping. Icarus 205: 375-395.
37
Romanek, C.S., Grady, M.M., Wright, I.P., Mittlefehldt, D.W., Socki, R.A., Pillinger,
C.T., Gibson, E.K. (1994) Record of Fluid-rock interactions on Mars from the meteorite
ALH 84001. Nature 372: 655-657.
Schulze-Makuch, D, Dohm, J. M., Fan, C., Fairen, A. G., Rodrigues, J. A. P., Baker, V.
R., Fink, W. (2007) Exploration of hydrothermal targets on Mars. Icarus 189: 308–324.
Sharma, S. K., Misra, A. K., Lucey, P. G., Wiens, R. C., Clegg, S. M. (2007) Combined
remote LIBS and Raman spectroscopy at 8.6m of sulfur-containing minerals, and
minerals coated with hematite or covered with basaltic dust. Spectrochimica Acta Part A
68: 1036–1045.
Sharma, S. K., Misra, A. K., Lucey, P. G., Lentz, R. C. F. (2009) A combined remote
Raman and LIBS instrument for characterizing minerals with 532nm laser excitation.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 73: 468 – 476.
Sigvaldason, G. E., Steinthorsson, S, Oskarsson, N. and Imsland, P. (1974)
Compositional variation in recent Icelandic tholeiites and the Kverkfjoll hotspot. Nature
251: 579-582.
Smith, P. H., Bell, J. F., Bridges, N. T., Britt, D. T., Gaddis, L., Greeley, R., Keller, H.
U., Herkenhoff, K. E., Jaumann, R., Johnson, J. R., Kirk, R. L., Lemmon, M., Maki, J.
N., Malin, M. C., Murchie, S. L., Oberst, J., Parker, T. J., Reid, R. J., Sablotny, R.,
38
Soderblom, L. A., Stoker, C., Sullivan, R., Thomas, N., Tomasko, M. G., Ward, W.,
Wegryn, E. (1997a) Results from the Mars Pathfinder Camera. Science 278: 1758 – 1764.
Smith, P. H., Tomasko, M. G., Britt, D., Crowe, D. G., Reid, R., Keller, H. U., Thomas,
N., Gliem, F., Rueffer, P., Sullivan, R., Greely, R., Knudsen, J. M., Madsen, B.,
Gunnlaugsson, H. P., Hviid, S. F., Goetz, W., Soderblom, L. A. Gaddis, L., Kirk, R.
(1997b) The imager for Mars Pathfinder experiment. Journal of Geophysical Research
102: 4003-4025.
Squyres, S. W., Arvidson, R. E., Ruff, S., Gellert, R., Morris, R. V., Ming, D. W.,
Crumpler, L., Farmer, J. D., Des Marais, D. J., Yen, A., McLennan, S. M., Calvin, W.,
Bell, J. F., Clark, B. C., Wang, A., McCoy, T. J., Schmidt, M, E., de Souza, P. A. (2008)
Detection of Si-rich Deposits on Mars. Science 320: 1063-1067.
Stewart, L., Cloutis, E., Bishop, J., Craig, M., Kaletzke. L., and McCormack, K. (2006).
Classification of iron bearing phyllosilicates based on ferric and ferrous iron absorption
bands in the 400 – 1300nm region. 37th Lunar and Planetary Science Conference, LPI
Contribution No. 2185.
Stroncik, N. A., Schmincke, H-U. (2002) Palagonite – a review. International Journal of
Earth Science 91: 680 – 697.
39
Treiman, A. H., Amundsen, H. E. F., Blake, D. F., Bunch, T. (2002) Hydrothermal origin
for carbonate globules in Martian meteorite ALH84001: a terrestrial analogue from
Spitsbergen (Norway). Earth and Planetary Science Letters 204: 323 – 332.
Vago, J.L. (2005). ExoMars Science Management Plan, EXM-MS-PLESA-00002.
Wiens, R. C., Sharma, S. K., Thompson, J., Misra, A. K, Lucey, P. G. (2005) Joint
analyses by laser-induced breakdown spectroscopy (LIBS) and Raman spectroscopy at
stand-off distances. Spectrochimica Acta Part A 61: 2324–2334.
Yon, S.A., Pieters, C.M. (1988) Interactions of light with rough dielectric surfaces spectral reectance and polarimetric properties. Proceedings of the Lunar Planetary
Science Conference 18th, pp. 581- 592.
40
TABLES
Table 1. Mineral reflectance spectra used for the selection of geological filter
wavelengths for filter set F2-12. Data on grain size and minor mineral impurities are
given where information is available.
Mineral
group
Phyllosilicates
Sulphates
41
Mineral (sample ID)
Nontronite (PS-6Ba)
Nontronite (PS-6Bb)
Nontronite (PS-6Da)
Nontronite (PS-6Aa)
Nontronite (PS-6Bc)
Nontronite (C1CY20)
Nontronite (NG-1a)
Nontronite (NG-1b)
Nontronite (Swa-1a)
Nontronite (Swa-1b)
Montmorillonite (PS-2B)
Montmorillonite (PS-2D)
Montmorillonite (CM26)
Montmorillonite (CM27)
Montmorillonite (SCa-2a)
Montmorillonite (SCa-2b)
Montmorillonite (STx-1)
Montmorillonite (CM20)
Montmorillonite (SAz-1)
Montmorillonite (SWy-1)
Jarosite (SO-7Aa)
Jarosite (SO-7Ab)
Jarosite (SO-7Ac)
Jarosite (C1CY16)
Jarosite (GDS101)
Jarosite (GDS24)
Jarosite (GDS98)
Jarosite (GDS99)
Jarosite (JR2501)
Jarosite (SJ-1)
Alunite (HS925)
Alunite (SO-4Aa)
Alunite (SO-4Ab)
Alunite (SO-4Ac)
Alunite (CBCC08)
Alunite (CBCC09)
Alunite (C1CY02)
Alunite (GDS82)
Alunite (GDS83)
Alunite (GDS85)
Gypsum (SO-2Ba)
Gypsum (SO-2Bb)
Gypsum (SO-2Bc)
Gypsum (CBCC16)
Gypsum (C1JB556)
Grainsize
(µm)
0 - 45
45 - 125
0 - 45
0 - 45
125 - 500
<500
<2
<2
0 - 45
0 - 45
0 - 45
45 - 125
125 - 500
<500
300µm
125-500 µm
45 - 125
0 - 45
25 - 75
25 - 75
<500
50
35
0 - 45
45 - 125
125 - 500
25 - 75
<63
Impurities
Database
Trace quartz
Trace quartz
Trace quartz
Trace quartz
Trace quartz
Bentonite+quartz+feldspar+bassanite
Trace quartz
Trace quattz
Trace quartz
Trace quartz
Quartz
Quartz + feldspar
-
JPL
JPL
JPL
JPL
JPL
RELAB
USGS
USGS
USGS
USGS
JPL
JPL
USGS
USGS
USGS
USGS
USGS
USGS
USGS
USGS
JPL
JPL
JPL
RELAB
USGS
USGS
USGS
USGS
USGS
USGS
USGS
JPL
JPL
JPL
RELAB
RELAB
RELAB
USGS
USGS
Pure
Pure
Pure
Pure
Trace quartz and clay
Pure
Pure
Minor quartz
-
RELAB
RELAB
RELAB
RELAB
RELAB
Opaline Silica
Carbonate
42
Gypsum (C1SF03)
Gypsum (C1CC36)
Gypsum (C2PG03)
Gypsum (HS333)
Gypsum (SU2202)
Opal (C1OP01)
Opal (C1OP02)
Opal (C1OP03)
Opal (C1OP04)
Opal (C1OP05)
Opal (C1OP06)
Opal (C1OP07)
Opal (C1OP08)
Opal (C1OP09)
Opal (TM8896)
Calcite (GDS304)
Calcite (C-3Ab)
Calcite (C-3Db)
Calcite (C-3Ec)
Calcite (C-3Eb)
Calcite (C-3Ea)
Calcite (CAL110)
Calcite (CRB109)
Calcite (CRB112)
Calcite (CO2004)
<45
<25
<250
242
133
<74
74 – 250
250 - 500
<74
74 – 250
250 - 500
<74
74 – 250
250 - 500
0 - 45
0 - 45
45 - 125
0 - 45
125 - 500
<45
<45
<45
480
Pure
Pure
Minor quartz
RELAB
RELAB
RELAB
USGS
USGS
RELAB
RELAB
RELAB
RELAB
RELAB
RELAB
RELAB
RELAB
RELAB
USGS
USGS
JPL
JPL
JPL
JPL
JPL
RELAB
RELAB
RELAB
USGS
Table 2. Filter centre wavelengths (λ) and estimated filter bandpasses at FWHM for B2
and the proposed filter sets. The ‘B2’ filter set is a duplicate of that from the Beagle2
PanCam (Griffiths et al. 2006); ‘F1-12’ has filters regularly spaced every 50nm; F2-12
was calculated statistically. All data are in nm.
λ
440
480
530
600
670
750
800
860
900
930
965
1000
43
B2
Bandpass
22
28
32
21
17
18
20
34
42
32
29
28
λ
440
500
550
600
650
700
750
800
850
900
950
1000
F1-12
Bandpass
22
30
29
21
18
17
18
20
32
42
30
28
λ
440
470
510
560
590
650
710
750
820
890
960
1000
F2-12
Bandpass
22
26
30
27
21
18
17
18
27
38
30
28
Table 3. Spectral parameters used to assess the different filter sets (adapted from Farrand
et al. 2008). All data are in nm; ‘R’ denotes reflectance.
Parameter
440 – 700 Slope
920 – 1000
Slope
900 Band Depth
470 Band Depth
44
Filter
Set
B2
F1-12
F2-12
B2
F1-12
F2-12
B2
F1-12
F2-12
B2
F1-12
F2-12
Representative
filter
wavelengths
440 – 670
440 – 700
440 – 710
930 – 1000
950 – 1000
890 – 1000
900
900
890
480
500
470
Description
(R670 – R440)/(670 – 440)
(R700 – R440)/(670 – 440)
(R710 – R440)/(710 – 440)
(R1000 – R930)/(1000 – 930)
(R1000 – R950)/(1000 – 950)
(R1000 – R890)/(1000 – 890)
1 – (R900/[(0.429*R860)+(0.571*R930)])
1 – (R900/[(0.500*R850)+(0.500*R950)])
1 – (R890/[(0.500*R820)+(0.500*R960)])
1 – (R480/[(0.556*R440)+(0.444*R530)])
1 – (R500/[(0.455*R440)+(0.545*R550)])
1 – (R470/[(0.571*R440)+(0.429*R510)])
Table 4. Mineralogical (XRD and Raman), compositional (EDS), and textural
(BSE/SEM) properties of the Martian analogue samples used to test the filter sets.
Associated data and images are given in Figure 4, also showing where EDS spot
measurements were taken. NB: EDS composition details the main elements only
(quantified as compound wt.%, normalised to 100%) – trace elements are not given. Error
values represent one standard deviation (n=10).
XRD
Raman
EDS
BSE/SEM
NBO
Jarosite
Jarosite
Mineralinfilled
vesicles
NAL
(Yellow)
Jarosite
Natrojarosite
NAL
(Red)
Alunite/
Natroalunite
Haematite
52.6 (±4.6) SiO2
23.2 (±3.8) Al2O3
17.6 (±7.2) FeO
2.0 (±0.8) TiO2
1.6 (±1.1) SO3
A) 98.9 (±0.2) SiO2
B) 86.5 (±1.2) FeO
4.7 (±0.4) SO3
4.2 (±0.7) Al2O3
C) 74.3 (±3) SiO2
16.3 (±2.7) FeO
6.6 (±0.5) Al2O3
GY1
-
GY2
Opal
Alunite
Alunite
Vesicular
glass clasts,
palagonite
Nontronite?
KH
(White)
Amorphous,
minor calcite
78.2 (±10) SiO2
8.5 (±4.3) Na2O
98.8 (±0.3) SiO2
0.4 (±0.2) Al2O3
56.0 (±1.0) SiO2
16.2 (±0.9) Al2O3
13.9 (±1.8) FeO
7.3 (1.6) MgO
4.2 (±0.4) CaO
97.8 (±0.9) SiO2
1.0 (±0.4) CaO
Amorphous
silica matrix
Microfossils
HH
Calcite
Quartz
Amorphous
Haematite
Sulphur
Opal
Amorphous
silica
overlying
hyaloclastite
Opal,
Chlorophyll a
45
Montmorillonite?
Opal
Mineralinfilled
vesicles
Multispectral
Imaging (440
– 1000nm)
Jarosite
Jarosite
Alunite
Table 5. Ability of the Beagle2 (B2), MER, and the two new filter sets to detect specific
spectral features in the Martian analogue rock spectra, displayed in Figure 5. Detection is
classed as either positive (+) or negative (-).
Sample and
description
HH
Hyaloclastite
Colour/
Region
Whole
sample
Yellow
NAL
Acid
weathered
basalt
KH
Silica
encrusted
hyaloclastite
GY1
Geysirite
GY2
Silcified
biomat
46
Red
White
Lichen
White
Whole
Siderite
Whole
NBO
Acid
weathered
basalt
Orange
Feature (nm)
B2
MER
F1-12
F2-12
440 – 700 (Ferric
absorption edge)
470 (Fe3+)
680 (Fe2+)
950 (Fe3+)
800 (Peak)
440 – 700 (Ferric
absorption edge)
710 (Peak)
950 (Fe3+)
440 – 700 (Ferric
absorption edge)
750 (Peak)
850 (Fe3+)
950 - 1000 (OH-)
670 (Chlorophyll)
800 (Peak)
670 (Chlorophyll)
900 – 1000 (H2O)
950 (OH-)
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
700 (spectral plateau)
950 - 1000 (OH-)
+
+
+
+
+
+
+
+
470 (Fe3+)
640 (Fe3+)
720 (Peak)
950 (Fe3+)
440 – 700 (Ferric
absorption edge)
480 (Fe3+)
600 (Start of Fe3+
absorption)
750 (Peak)
950 (Fe3+)
POSITIVE
NEGATIVE
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
24
4
+
+
22
6
+
+
26
2
+
+
27
1
Table 6. ‘Geology’ Filter wavelengths (in nm) for comparable Mars rover and lander
stereo cameras, given in chronological order.
Viking[I]
(1976 – 1982)
Pathfinder[II]
(1997)
Beagle[III]
(2003, Lost)
400 - 500
500 - 600
600 - 700
820 - 920
900 - 940
930 - 1100
443
479
530
599
671
752
801
858
897
931
966
1002
440
480
530
600
670
750
800
860
900
930
965
1000
[I]
Evans & Adams (1979);
Bell et al. (2003);
[IV]
47
[II]
Smith et al. (1997b);
Malin et al. (2010);
[VI]
MER[IV]
(2004 –
Present )
432
482
535
601
673
753
803
864
904
934
1009
[III]
[VII]
MSL[VI]
(Launch
2011)
440
525
675
750
800
865
905
935
1035
Griffiths et al. (2006)
This paper
‘F2-12’[VII]
(ExoMars)
(Launch 2018)
440
470
510
560
590
650
710
750
820
890
960
1000
FIGURE LEGENDS
FIG 1. Martian analogue samples (a) GY2 – silicified biomat; (b) GY1 – silica sinter; (c) KH –
subglacial hyaloclastite with opaline silica crust; (d) NAL – acid fumarole weathered lava; (e)
HH – subglacial hyaloclastite; (f) NBO – acid weathered lava. Scale bar = 1cm.
FIG 2. Plot of filters and estimated filter bandpasses for Beagle2 (B2) and new filter sets,
together with vertical spectral bands for Fe (grey, solid line) and water (blue, dashed line), and
also reflectance peaks for jarosite and nontronite (pink). Also shown are the mean spectra of each
mineral species used to statistically calculate new filter selections (only the mean is shown here
for clarity). For specific sample numbers, see Table 1.
48
FIG 3. Reflectance spectra of nontronite and jarosite (SWa-1.a and JR2501 respectively- see
Table 1) showing the capability of different filters in identifying diagnostic spectral features. For
both minerals, F2-12 provides a better fit to the original spectrum than F1-12. Both F1-12 and F212 are better than the B2 filters. Arrows highlight features that are missed by the filters.
49
FIG 4. Multispectral RGB composite image (scale bar=1cm), BSE image, and Raman
spectroscopic data for Martian analogue samples. Boxes in the RGB composites represent
multispectral targets used to acquire PanCam spectra, whilst red dots in the BSE images indicate
where EDS measurements (given in Table 4) were made. (a) NBO, showing mineral deposits
within lava vesicles, identified as jarosite by Raman; (b) NAL, showing ‘Yellow’ and ‘Red’
targets, with Raman data showing the presence of haematite and jarosite; (c) KH, showing
‘White’ and ‘Lichen’ targets, Raman identifies the white crust to be opaline silica; and the
hyaloclastite – silica crust contact can clearly be seen under BSE; (d) HH hyaloclastite showing
glass clasts surrounded by palagonite. In BSE images, ‘G’ = Glass clast; ‘GM’ = groundmass; ‘J’
= jarosite; ‘P’ = palagonite; ‘S’ = opaline silica; ‘V’ = vesicle. Associated EDS and XRD data is
given in Table 4.
50
FIG 4 Continuted. (e) GY2, showing textured biomat surface, which has a dendritic structure
under BSE. Raman data indicates the sample to be opaline silica; (f) GY1, showing variable
coloured layered regions, with Raman showing the red and yellow regions to be haematite and
sulphur respectively; (g) Siderite with confirmatory Raman data.
51
FIG 5. Martian analogue sample spectra modelled for the B2 and two new filter sets ‘F1-12’ and
‘F2-12’. (a) NBO; (b) NAL Yellow (top spectrum) and NAL Red (bottom spectrum); (c) GY1
(top spectrum) and GY2 (bottom spectrum); (d) HH; (e) Siderite; (f) KH White (top spectrum)
and KH Lichen (bottom spectrum). Where the MER filters produce a different spectrum to that
generated by the B2 filters (due to the absence of a filter at ~965nm), this is plotted on top of the
B2 spectra (plots c and d) to highlight where spectral features are missed.
52
FIG 6. GY1 Red and Yellow PanCam spectra using the F2-12 filters, and reflectance spectra for
haematite and sulphur (USGS spectral library reference WS161 and GDS94 respectively), all
normalised to a starting point of ~0.1.
FIG 7. BSE images of the GY2 silica sinter with preserved biological structures within the silica
matrix, showing branching dendritic structures that extend up from the base of the sinter (a) and
detailed cellular structures (b).
53
FIG 8. Spectral parameter plot of 920 – 1000nm slope vs. 440 – 700nm slope for all hydrated
mineral data from Table 1, showing the relationship between the two parameters depicted by the
dashed linear trendline (a); and the Martian analogue rock spectra represented by the three
different filter sets for the sulphates (b); phyllosilicates (c); and for opaline silica and carbonate
(d).
54
FIG 9. Discriminant analysis (a) and hierarchical cluster analysis (b) for the Martian analogue
samples as plotted using the three different filter sets. In (a), the further the distance between
groups, the better the discrimination between different samples. NB: colours are based on those
from the F2-12 analysis.
55