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Vidicon Observations of Mars:
Images of the
October, 1973 Dust Storm and Two-Dimensional
Narrow-Band Photometry
by
Gary Lassiter Johnson
Submitted in Partial Fulfillment
of the Requirements for the
Degree of Master of Science
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
September, 1975
Signature of Author.
.
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-
Department of Earth and Pla
j
Certified
by .
.
.---
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.
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.............
Thesis Supervisor
Accepted by. .---
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. . . .
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Chairman, Departmental Committee on Graduate Students
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page 2
Vidicon Observations of Mars: Images of the October, 1973
Dust Storm and Two-Dimensional, Narrow-Band Photometry
by
Gary Lassiter Johnson
ABSTRACT
The major Martian dust storm of October, 1973 was imaged
through twenty filters spanning the spectral region from 0.35
microns to 1.03 microns using a silicon vidicon tube. A total
of 100 images were taken, each with a resolution of about 200
kilometers. Photographs of vidicon images are presented to
show storm phenomena revealed during the 4 hour observation
period. Martian short term events are pronounced in the blue,
suggesting the presence of dissipating volatiles. Images in the
red show the main body of the storm as a dense dust cloud over
Solis Lacus. Possible evidence of sudden dust cloud growth in
the Aonis Sinus region is discussed. Projected area calculations detect no observable expansion rate in the Solis Lacus
cloud during the four hour imaging period. To assess any gross
morphological changes in the total srorm system, pictures of
artificially rotated images are displayed, each with the storm
center mapped to the sub-earth point. Procedures for obtaining
from the vidicon images reflection spectra of selected areas
on Mars have been developed. Relative reflectance spectra from
vidicon images are presented for comparison with relative reflectance spectra from photometer data taken during 3 later
nights. Similarities and discrepancies between these data are
examined with particular reference to (1) problems imposed by
the expanding dust storm and (2) weaknesses in the reduction
of the vidicon data. Further data processing of the vidicon
images is discussed.
Thesis Advisors
Titles
Thomas B. McCord
Associate Professor of Planetary Physics
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page 3
Acknowledgements
For a year and a half I have been working on the 1973
Mars vidicon data and I would never have gotten this far this
fast had it not been for the help and assistance of the people
I would like to thank. Carle Pieters provided me with my introduction to vidicon image processing and ever since she has
been a source of valuable advise. Bob Huguenin has been a fund
of ideas about the realities of the Martian world and has
offered helpful guidance during the reduction of the vidicon
data. Doug Mink is responsible for the remarkable mapping programs which have supplied this thesis with maps, adding a
color and clarity almost justifying all the late nights he
spent pouring over the computer's scrawl. Paul Kinnucan and
George Fawcett were invaluable during the desperate attempts
to debug the programs described in this thesis. Mike Gaffey
has been an eminent, unsolicited provider of sundry entertainments. And last but not least, I have Tom McCord to thank
for all the successes and rewards I have discovered in the
past year and a half for it was he who offered me this project in the first place and supported me thereafter.
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page 4
Table of Contents
Acknowledgements,,,,,,,
.
List of Figures* * ,,,,,,.
*
*..........
..
*
* ......
,,
,..
........
...
....
3
....
List of Tables*...........................................
I. Introduction..........
...........
5
7
8
...................
..11
II. The Vidicon Imaging System,,...,,........,.....
III. Images of the 1973 Mars Dust Storm....................15
IV.
Image Processing........*...................,,,,.......33
V, Comparison of Relative Spectral Reflectivity Data.......40
67
VI, Concluding Remarks....,,,,,,,.......................
References,,, ,,..,..
,,, .... ,,,,
,,0..........
.
,,,
...
69
__6m _
page 5
List of Figures
S1.The V4 .dicon Imaging System.............................12
2. A Vidicon Image of Mars..............................
3.
.14
Mars Map of Dust Storm - 9/14/73.......................16
4. Images of Mars at 0.40 Microns.........................17
5.
Disk Maps of Mars for 0.40 Micron Images...............19
6.
Images of Mars at 0.73 Microns.........................23
7.
Disk Maps of Mars for 0.73 Micron Images..............25
8. Flowchart of Rotation Program............................29
9. Rotated Images of Mars at 0.40 Microns.................30
10. Rotated Images of Mars at 0.73 Microns.................31
11. Diagram of the Image Calibration Procedure.............35
12. Flowchart of Center-Seeking Program....................37
13. Flowchart of Spot-Mapping Program......................38
14. Mars Map with Vidicon Spots & Dust Storm - 9/1 4/73.•...41
15. Mars Map with Photometer Spots & Storm - 9/15/ 73...... 42
16. Mars Map with Photometer Spots & Storm - 9/16/ 73.......43
17. Mars Map with Photometer Spots & Storm - 9/19/ 73.......44
18. Disk Maps of Mars with Vidicon Spots..........
......... 45
19. Relative Reflectance Spectra - Spot 8/Spot 20. ......... 49
20. Relative Reflectance Spectra - Spot 10/Spot 9.
.********51
21. Relative Reflectance Spectra - Spot 4/Spot 9.. ........ *53
22. Relative Reflectance Spectra - Spot 6/Spot 9.. .........54
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page 6
List of Figures
23. Relative Reflectance Spectra - Spot 7A/Spot
•.... .56
24. Relative Reflectance Spectra - Spot 11/Spot 9... S.. ••.58
25. Relative Reflectanpe Spectra - Spot 12/Spot 9... *0....
59
26. Relative Reflectance Spectra - Spot 14/Spot 9... •......60
27. Relative Reflectance Spectra - Spot 15/Spot 9... S......61
28. Relative Reflectance Spectra - Spot 17/Spot
....... 63
29. Relative Reflectance Spectra - Spot 20/Spot 9... S......65
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List of Tables
1. Centers of Spots in Latitude and Longitude..............47
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page 8
I. Introduction
The mysteries of Mars have been a prolonged and gratifying fascination. In recent years great nations have been
provoked into assembling and launching on hazardous voyages
spacecraft bound for Mars. The close-up pictures returned to
date reveal an evolved and sculptured surface. Although these
venturesome craft have unveiled the unexpected, the composition of the Martian soil has yet to be probed in situ.
Remote sensing at telescopic sites on the Earth still
provides the most useful method for determining planetary
surface compositions. The reflected light of a distant object
such as Mars is a nightly signature of the minerals that make
up its soil. In the past decade the reflectance spectra of
Mars has been studied with increasing success. A surface material of the hydrated iron oxide, limonite, was postulated by
Sagan et al (1965) based on similarities observed between the
absorption bands in limonite and the albedo curves of Mars
determined by Dollfus (1961). With data from the 1967 Mars
opposition, Adams and McCord (1969) noted that bright and dark
regions on Mars have differing geometric albedos. The surface
composition was assumed to be a mixture of oxidized basalts
and hydrated iron oxides - dissimilar albedos being indicative
of different degrees of oxidation in the same basic material.
page 9
'During the 1969 opposition McCord and Westphal (1971) using
narrow-band photometry detected apparent compositional differences amoung the seven bright and dark areas studied. At the
next opposition in 1971, Mariner 9 returned voluminous data.
The images from Martian orbit revealed several large scale
(far above the Earth-based resolution limit of about 200 km.)
geologic terrain types, see Carr et al (1973). Unraveling the
subtle compositional differences between regions of differing
albedos and terrain types will require a large sample of reflectance spectra from areas which, in some cases, are as
small as the resolution limit on Mars. There are, however, important observational difficulties in obtaining this desired
photometric data.
Photometry of restricted areas on distant objects involves using a photometer with a very small aperture (about
imm.). While data is being recorded the aperture must be maintained over the region of study by precision guiding. Problems
of drift are unavoidable and the actual area from which data
is returned tends to be larger than desired. Very small points
of interest as well as regions that are contrast obscured are,
therefore, difficult areas to observe photometricly. This is
true of such significant features on Mars as the volcanos of
Tharsis and the Coprates canyons. Furthermore, rapidly changing
events do not lend themselves well to photometric coverage
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page 10
using the photometer/aperture system. If the transient feature
is small, such as a local cloud on Mars, the guiding problem
becomes significant. On the other hand, if the temporary event
is large scale, such as the 1973 Mars dust storm, the photometer cannot study the entire feature in detail and important
events may be missed altogether. The solution to these shortcomings is narrow-band photometric coverage of the full disk
of Mars. The Vidicon Imaging System has been constructed for
this purpose. The intent of this thesis is to describe the
photometric value of this versitile imaging system by analyzing the vidicon images of Mars taken during the opposition
of 1973.
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page 11
II. The Vidicon Imaging System
The silicon vidicon imaging system has been developed at
the M.I.T. Remote Sensing Laboratory to do two-dimensional
precision photometry and low resolution spectroscopy (*). The
heart of the system is a RCA silicon, diode array vidicon tube.
The broad spectral response and high quantum efficiency of the
vidicon tube, plus its intrinsic ruggedness and simplicity,
provide the astronomer with images better formatted for data
reduction than the photographic plate and of comparable spacial
resolution. The photometric quality of the vidicon tube has
been demonstrated to within 1% of photometer data by Pieters
and McCord (1974) and McCord, Pieters and Fierberg (1975).
Photometric images are exposed by opening the shutter and
passing the light from the telescope through a filter and onto
the silicon diode array (Figure 1). Photons, striking the approximately 1000 by 1000 array of photosensitive diodes, are
converted to charge carriers, which discharge the reverse
biased diodes. When the shutter is closed an electron beam
scans the diode array and recharges clusters of 4 diodes at a
time (permitting less precision in the positioning of the
electron beam). The resulting current is proportional to the
(*) McCord and Westphal (1972)
McCord and Bosel (1973)
Mink (1974)
page 12
INCOMING LIGHT
SHUTTER
MIRROR
**i
VIEWING OPTICS
FILTER
,SILICON DIODE ARRAY
DRY ICE
COLD BO
Figure 1. A schematic diagram of the Vidicon
Imaging System.
page 13
amount of charge lost during exposure. Implicitly this current is a video signal. It is amplified, digitized and recorded on magnetic tape as a mat:ix of integer intensity
values, 250 rows: by 256 columns. A sample of a vidicon image
of Mars is displayed in Figure 2. Half the planet in its
digital format can be seen. Each number represents an intensity value derived from the video signal as the scanning
electron beam recharged four diodes in the silicon diode
array. Such images can be projected onto film for study and
display. Furthermore, with corresponding darkfield and flatfi"ld images,
ech p"lnctary image c-n be prccssed to yild
spectral reflectivity data.
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page 15
III. Images of the 1973 Mars Dust Storm
The major Martian dust storm of 1973 made its appearance
on October 13 over the usually dark area, Solis Lacus (Capen,
1973). An analysis of International Planetary Patrol photographs (Martin, 1974) captures the dramatic history of this
planet-wide disturbance. For several days following its inception, the storm expanded rapidly, ultimately obscuring the
entire Martian surface. This obscuration diminished over a
period of weeks. The images presented here refer specifically
to a four hour segment of Day 2 of the dust storm, October 14,
1973. A map of the storm as it appeared on Day 2 can be seen
in Figure 3.
The first images of the storm were taken at around 9:00
Universal Time, a few hours after Solis Lacus passed through
the dawn terminator. Successive images followed the progress
of the storm during the ensuing 4 hours. Samples from amoung
the 100 vidicon images taken comprise the photographs exhibited in Figures 4 and 6.
Figure 4 is a display of images observed through a blue
filter (0.40 microns). The contrast in the images has been
varied to enhance the features. The north polar hood crowns
the top of the planet and the smaller south polar cap appears
o
11
ISO
1
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12
o li
too
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0
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20
to
0
15
30
39
320
StO
30
20
260 2
Figure 3. Mercator projection of Mars with boundaries
of the dust storm on October 14, 1973, from
Martin (1974).
0
20
20
249
73
22
210
?
0n
0
1hI
8s53UT
9:37UT
11:24UT
11:49UT
12:17UT
c+
Figure 4. Photographs of vidicon images of Mars taken through a
0.40 micron filter. Time is Universal Time, Oct. 14,
1973 - Day 2 of the storm.
-Llli-ILWS_~_~_-P~-Y~IIIILi
-P-^_XI~L~I i- --lil~i~~~~~-LLII
page 18
at the bottom of each image. The remaining visible features in
these blue images reveal the sprawling expanse of the storm
system orly a day after it was first noted. A chronicle of
short term phenomena is clearly present. To assist in associating elements of the dust storm with place names on Mars,
disk maps of the planet are presented in Figure 5 to show the
visible face of Mars as it appeared for the first and last
images in Figure 4.
Solis Lacus (25*S, 8oW) is at the bull's eye formed by
the ring of clouds. Its bright appearnace vanishes completely
in the blue as the dust storm progresses toward the sub-solar
meridian. Even more spectacular is the brilliant cloud over
the Aonis Sinus region (47S, 107*W) southwest of Solis Lacus.
This cloud fades significantly between 9:37UT and 11s24UT
along with the Solis Lacus cloud. The sudden regression from
brilliance in the blue in both of these clouds is thought to
be evidence of volatiles (*) which, having condensed onto
suspended dust particles during the Martian night, appear
bright in the morning light and then fade away as exposure to
solar radiation causes sublimation. Furthermore, dehydroxylated
goethite has been proposed as a constituent of the Martian
(*)
Huguenin (1975)
Abramenko and Prokof'eva (1975)
Leovy et al (1972)
Smith and Smith (1972)
Baum (1974)
19
MARS
0.40 MICRONS
CT7. 14, 1973
T= 8:53 UT7
01H= 21.43 SEC
SUB-ERRTH POINT
LRT= -16.7
LONG= 43.8
SUB-SOLR POINT
LRT= -21.2
LONG= 34.3
Figure 5. Above is first, below is last of 0.40F micron
image s.
MARS
0.40 MICRONS
OC7. 14. 1973
T= 12:17 UT
DIP= 21.44 SEC
SUB-ERRTH POINT
LRT= -16.7
LONG= 93.6
SUB-SOLAR POINT
LRT= -21.2
LONG= 84.2
~ qly
~.(i~q)l-IIIP-~l)-~
-P~~UII-~-t_^ll~P.
XI-sFr' --.
-pll--i
~^w~c~-l-----c ----I---^I~--'~--
page 20
soil (Huguenin, 1974). This mineral, according to Huguenin
(1975), in a dust grain can adsorb water, increasing the
specific surface of the grain and thereby raising the reflectivity of the grain. Exposure to sunlight is sufficient to
knock off the attached water molecules.
The third large cloud visible in the blue images lies
over western Tharsis (10N, 120*W). Unlike the clouds over
Solis Lacus and Aonis Sinus, the Tharsis cloud remains bright
in the blue throughout the observation period, although it
appears to fade somewhat. Brightenings over this area of Mars
have been, as a result of observations by Mariner 9 (Leovy et
al, 1972), associated with clouds forming on the leeward side
of the four large Tharsis volcanos. The dust storm map in
Figure 3 indicates that this large, bright cloud is unconnected with the dust storm further south. The boundaries,
however, of the storm in Figure 3 were determined primarily
using photographs of Mars taken through a red filter. As will
be seen shortly (Figure 6), this technique reveals the centers
of dust concentrations. Images taken through a blue filter
apparently show the more tenuous reaches of the storm due to
condensed and adsorbed volatile brightening. The general
dustiness of the Martian atmosphere expected during dust storm
conditions, combined with reflective volatiles coating the
particles, may be enlarging and/or highlighting what might
page 21
normally be a few small clouds around the volcanos. The fact
that this large and bright cloud remains bright during the
full extent of the observation period, whereas the Solis lacus
and Aonis Sinus clouds vanish, may be (1) an indication that
it is a different kind of cloud (i.e. a leeward wave cloud
system - see Leovy et al, 1972, and Smith and Smith, 1972), or
(2) it has not yet been exposed to enough solar radiation to
volatilize all the condensates. This second possibility will
be discussed in more detail in the following analysis of the
small Araxes cloud.
The map in Figure 3 indicates that westward of Solis
Lacus there is a dust cloud over the Araxes region (200S,
120OW). It appears very clearly as a bright patch beneath the
Tharsis cloud in Figure 4 from 11:24UT onward. The Araxes dust
cloud is further west than both the Aonis Sinus and Solis
Lacus clouds. It therefore received during the imaging period
less solar radiation than the other two dust clouds. If
volatiles are indeed causing its luminescence, it is not
surprising that the Araxes cloud remains bright as its two
eastward companions vanish. The Araxes cloud can be expected
to stay bright until it has absorbed sufficient sunlight to
sublimate its condensed volatiles.
A fourth large dust cloud noted by Martin (1974) on Day 2
of the storm lies over Noachis (45S, 15oW). As the maps in
_LI_
-~l
L_
-ll_~__XLXII1I1_I~_I_-YIXI-I -L-~-~i~--~1__ FI__~
page 22
Figures 3 and 5 indicate, the Noachis cloud, were it visible,
would be seen near and on the east limb in all the images.
But since no condensed or adsorbed volatiles can be expected
to remain late into the Martian afternoon, this clouds' invisibility in the blue can be accounted for.
Finally, there are tenuous clouds directly north of Solis
Lacus which remain bright long after the Solis Lacus cloud has
been extinguished. This length of clouds is in the region of
the Coprates Canyon. The topography in this system of canyons
may be aiding the survival of adsorbed and condensed volatiles
adhering to the dust particles.
In contrast to the images exposed through the 0.40 micron
filter, images taken through a red filter tell a different
story. A 0.73 micron filter was used for the images displayed
in Figure 6. Each of these was exposed a few minutes after the
corresponding blue images in Figure 4. Contrast enhancement
has again been applied to bring out the significant features
of the dust storm at this red wavelength. Northwest of Solis
Lacus can be seen the bright Tharsis region. The rectangular
Mare Erythraeum as well as (in the later images) the protruding finger of Mare Sirenum are the dark regions to either
side of the storm. The bright knot of the main dust cloud masks
Solis Lacus. It appears in the highly contrasted images as an
9: 1i2UT
9 148UT
12t02UT
11 s 37UT
-
""- 1 "'R
12:29UT
, "
...
Ole
Figure 6. Photographs of vidicon images of Mars taken through a
0.73 micron filter. Time is Universal Time, Oct. 14,
1973 - Day 2 of the storm.
_1II__
II~.i i -~II~-~----~_ls*YI
-UI~
L--- ~^-e~Y-----LY~__X
.IXI-_----^^
-PI~~ .--
page 24
appendage at the bottom of the dome-shaped bright region to
the northwest of Solis Lacus. The disk maps in Figure 7 show
the planet as it appeared for the first and last images in
Figure 6.
The dust cloud over Solis Lacus appears bright throughout
the 4 hour observation period whereas in the blue images the
storm fades as the Martian day proceeds. The transient brightenings in the blue have been attributed to condensed and
adsorbed volatiles. The constant brightness of the storm in
the red indicates that the red dust particles themselves have
been imaged. They are bright area material that has been swept
up by the local winds. No 'dark' dust storms - composed of
dark area material - have ever been observed on Mars. Apparently the bright area material is much more fine-grained than
dark area material and hence more easily windborn (see
Hugeunin (1974) for a discussion of particle size origins and
further references).
The dust cloud over Aonis Sinus to the southwest of Solis
Lacus and the dust cloud over Araxes to the west are both
much weaker at 0.73 microns than the Solis Lacus cloud. The
Araxes cloud cannot be distinguished in the photographs in
Figure 6 although it is barely perceptible in the original
vidicon images when displayed on a viewing screen. The Aonis
Sinus cloud is faintly apparent in Figure 6 and weakens as the
MtHS
25
0.73 MICRONS
OCT.14,1973
T= 9:12 UT
ORA= 21.43 SEC
SUB-ERTH POINT
LA7= -16.7
LONG= 48.5
SUB-SOLAR POINT
LAT= -21.2
LONG= 38.9
S.
Figure 7. Above is first, below is last of 0.73 micron imagesI.
MARS
0.73 MICRONS
0C7.14,1973
T= 12:29 UT
DIR= 21.44 SEC
SUB-EARTH POINT
LA7= -16.7
LONG= 96.5
SUB-SOLAR POINT
LAT= -21.2
LONG= 87.1
III___
i_^XI
__l_
III~~LI_~^LLII
I-Y14LUI
U
LL~LC_
page 26
contrast is increased. Presumably the concentration of dust in
the disturbance over Solis Lacus was, on Day 2 of the storm,
greater than the concentration of dust particles in the other
two westward storm clouds.
In the highest contrasted images at 0.73 microns the
Aonis Sinus cloud reveals an interesting feature. A small spot
appears beneath the tail of the comma-shaped Solis Lacus cloud.
This feature is evident at 11s37UT and later but not earlier
at 9s48UT or 9:12UT. The apparent brightening of this cloud at
11:37UT may be an artifact caused by the contrast enhancement
process, or it might be a real event caused by a rapid increase
in the local concentration of dust. The area of the bright spot
is about 60,000 square kilometers.
The cloud over western Tharsis, which appeared so large
and bright in the blue, is not distinguishable in the red.
This supports the contention, already put forward, that the
Tharsis cloud is not composed primarily of dust.
The cloud over Noachis, which was not seen in the blue,
does not appear in the red either. This cloud was not evident
to Martin (1974) the next day, October 15 (see Figure 15 for a
map of the dust storm as it appeared on October 15). That the
Noachis cloud is not seen here in the red and that it vanished
altogether by the next day are indicative of its tenuous
nature. The concentration of dust over Noachis was probably
page 27
small and diminishing.
Since the bright knot of dust over Solis Lacus is both
confined and pronounced in the highly contrasted images in
Figure 6, an attempt was made to measure any change in the
area covered by this specific cloud. By calculating the projected surface area associated with each picture element
(pixel) that could be assigned to the Solis Lacus cloud, the
total area of this storm cloud can be determined for any
specified image. During the short imaging period (4 hours) no
detectable changes in the area of the Solis Lacus cloud were
discovered. This is not an unexpected result. Gierasch and
Goody (1973) and Leovy, Zurek and Pollack (1973) have described opposing mechanisms for Martian dust storm formation.
Both, however, estimate that a wind speed of 30 m/sec. is not
unreasonable during the growth of such a storm. Over a four
hour period this wind speed could move dust slightly more than
400 kilometers. Each pixel near the center of a vidicon image
of Mars represents an area about 200 km. across. Therefore,
over the 4 hour imaging period a cloud of dust expanding at
30 m/sec. would not necessarily be detected. The resolution of
the images simply is not fine enough to determine wind speeds
even in the extreme conditions a Martian dust storm imposes.
A general expansion of the total storm system as the clouds
rotate eastward is suggested by the images in Figure 4. This
iil---
--L~-~1llsl~ill~---- I_
.I-._..
page 28
apparent expansion may be accounted for by the distortion
caused by projecting the spherical planet onto the image
plane. A feature near the limb of the planet will grow in
apparent size as it is rotated toward the sub-earth point.
To equalise this distortion and search for any changes in the
gross morphology of the entire storm system, and to uncover
possible changes in cloud structure that may not effect the
integrated area of any particular cloud, the digital vidicon
images in Figures 4 and 6 were artificially rotated to place
Solis Lacus at the center in each rotated image. A flowchart
of the Rotation program is in Figure 8. The rotated images
are shown in Figure 9 (blue images) and Figure 10 (red
images).
The general expansion of the total storm that appeared
in the original images at 0.40 microns (Figure 4) is no
longer evident in the corresponding rotated images (Figure
9). In particular, the cloud over Solis Lacus, which seems to
grow in size from 8:53UT to 9:37UT in Figure 4, remains constant in size in the distortion equalized images at 8:53UT
and 9:37UT in Figure 9. The overall morphology of the dust
storm remains uniform while the rapid changes in brightness
occure.
However, possible changes in cloud structure appear in
Figure 10. In the highly contrasted versions of the rotated
images the dust cloud over Solis Lacus appears at 11s37UT,
SET UP TWO EMPTY MATRICES
& ROT
1IMG
INPUT
SUB-EARTX POINT OF VIDICON IMAGE
SUB-EARTH POINT OF ROTATED IMAGE
RADIUS OF PLANET IN PIXELSs RD
READ VIDICON IMAGE INTO ING
SCAN ROT PIXEL BY PIXEL
(xy) IS DISTANCE OF
PIXEL TO ROT CENTER
IS
ROT(x,y)=0
x,y) WITHI
RD OF CENTER OF
ROT
NO
CONVERT
(x,y) INTO
(lat,leng)
USING THE SUB-EARTH POINT OF
THE ROTATED IMAGE.
CONVERT (lat,long) TO (x',y')
USING THE SUB-EARTH POINT OF
THE VIDICON IMAGE.
ROT(, y)IMNG(x ,y')
COPY ROT ONTO MAGNETIC DISK
Figure 8. Flowchart of Rotation program.
page 29
8s53UT
9s37UT
11s24UT
11s49UT
12s17UT
Figure 9. Photographs of rotated vidicon images of Mars taken through
a 0.40 micron filter. The center of the planet for every
picture is Solis Lacus (20'S, 80*W).
9: 12UT
9 s 48UT
11 s 37UT
12sO2UT
12s29UT
CO
c+
taken through
Figure 10. Photographs of rotated vidicon images of Mars for every
a 0.73 micron filter. The center of the planet
picture is Solis Lacus (20*S, 80*W).
page 32
12s02UT and 12:29UT as a comma-shaped appendage beneath the
dome-shaped Tharsis region to the north. The shape of this
dust cloud appears slightly different at the two earlier
times, 9:s2UT and 9s48UT (see also the original images in
Figure 6). If this apparent structural change is real, it
may be associated with the appearance of the bright spot of
dust over Aonis Sinus (discussed earlier). The Aonis Sinus
spot appears as a 'period' below the 'comma' at 11:37UT and
later (Figures 6 and 10).
The vidicon images of Mars, which by luck captured the
Great Dust Storm of October, 1973, have revealed an abundance
of information about the structure and diurnal development of
the various clouds during the second day of the storm. The
broad spectral coverage of the Vidicon Imaging System and
the digital format of the images themselves aid in the analysis of rapibly changing features of both large and small
scale. Much more information remains a part of these images
and the following sections discuss the reduction and significance of this data.
_1.
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III~
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page 33
IV.
Image Processing
Since the analysis of the Mars vidicon images has, up to
now, concerned only albedo features it has not been necessary
to do any image processing (except for artificially rotating
some of the images). Intrinsically, however, these images
contain valuable photometric data from which spectral reflectivity curves can be constructed. Unlike a photographic plate,
each diode in the silicon vidicon diode array responds (ideally)
like a tiny photometer. Each picture element, i.e. each individual intensity value (as in Figure 2), can be thought of as
a photometer data point for that particular area of the planet
it covers and:for that particular wavelength at which the
image was exposed. To realize this data requires (1) the calibration of each image and (2) the derivation for each image
of an accurate coordinate grid (in latitude and longitude,
for instance) for the planetary disk. Mapping this grid onto
the image associates picture elements with regions and features on the planet.
The calibration of the- vidicon images is a two step
process utilizing the existing batch image processing system
(DIPSYS) developed at the M.I.T. Remote Sensing.Laboratory.
First, the images contain background noise caused by the dc
bias in the video circuit and the backlighting from the
page 34
electron beam filament. To eliminate this background a darkfield image is taken which is an image exposed like all other
images except that the shutter remains closed. Subtracting
the darkfield removes the background noise. Secondly, there
are irregularities in the response of the diodes and in the
filter transmissions across the face-of the silicon diode
array. Removal of these irregularities requires a flatfield,
which is an image of uniform illumination (using, for instance,
a de-focused Moon). After the darkfields have been subtracted
from the data image and flatfield image, the data image is
divided by the flatfield resulting in a calibrated image.
Both of these two fundamental steps must be performed on each
image. Figure 11 is a diagram of this calibration process.
Once the 100 data images from the October 14, 1973 Mars
observations had been calibrated and saved on magnetic tape,
a means had to be devised of producing a reflectance spectra
for any desired location on the imaged face of the planet.
Two subroutines for the DIPSYS image processing system were
formulated to accomplish this goal.
Taking the original, uncalibrated images as a data base,
a Center-Seeking program was written to find the unique row
and column location of the center of each image and the mean
radius of the planet in picture elements. In Figure 2, for
example, the center is at ROW=43.72 COLUMN=118.56 and the
page 35
Figure 11. Calibration sequence for the vidicon
images.
tli_~111_ Lil~l __1__LLLI___XI(IIJ_~--s^^l*-Llillli -YII~-_~-I~--L(
page 36
mean radius is 18.14 pixels. A flowchart of the CenterSeeking program is in Figure 12. Next a disk mapping program,
already written by Doug Mink, provided the corresponding latitude and longitude location of the center of each image (see
Figures 5 and 7 for examples). Using these two results as a
data base, a Spot-Mapping program (Figure 13) was written to
lift off of a vidicon image the average intensity value of all
picture elements falling within a specified ellipse projected
onto the image. Figure 18, for instance, shows disk maps of
Mars with such ellipses thrown around areas under investigation.
The Spot-Mapping program averages the intensity values within
each ellipse.
Intensity values returned by the Spot-Mapping program
can be treated exactly like photometer data. During an actual
observation run with a photometer, the instrument returns
data from some spot on Mars, and due to guiding problems,
that spot drifts around within some elliptical boundary. By
mapping that boundary onto calibrated vidicon images, the
Spot-Mapping program simulates the return of photometer data.
The intensity values thus derived are plotted as a
function of wavelength to produce reflectance spectra. These
spectra are generally normalized to 1.0 at 0.56 microns.
Furthermore, to reduce the adverse effects of seeing conditions and changing airmass during the observation period,
VIDICON
IMAGE
SEARCH IMAGE FOR
THE PLANET LIMB.
page 37
CREATE TABLE OF LIMB COORDINATES
IN X-Y SPACE WITH PLANET CENTER
AT THE ORIGIN.
SET DISTANCEs
NDIST=1 PIXEL
FOR EACH POINT, CALCULATE
STANDARD DEVIATION OF ALL
RADII FROQUA
PODRANT TO LIMB
COORDINATES.
SET CX,CY EQUAL TO THE
POINT WITH THE MINIMUM
STANDARD DEVIATION IN
RADIUS OF THE IMAGE.
RADIUS OF THE IMaAGE.
Figure 12. Flowchart of Center-Seeking program.
INPUT
ELLIPSE: Lat & Long of Center
Major & Minor Axes
Angle of Rotation
IMAGE: Center in Lat & Long
Center in X & Y
Mean Radius
page 38
CONVERT LAT,LONG CENTER OF
ELLIPSE TO X,Y COORDINATES
FOR VIDICON IMAGE: EX,EY
SET TOTAL INTENSITY: I=0
SET TOTAL SPOT AREA: A=0
THROW A SQUARE BOX AROUND EX,EY
WITH SIDES EQUAL TO MAJOR AXES.
VIDICON
IMAGE
SCAN BOX PIXEL BY PIXEL
NO
I
i
PIXEL WITHIN
ELLIPSE
I ==--- I + PIXEL
INTENSIY
PIXE-
VALUE
-
I
t
A = A + 1
IS
SCAN DONE
NO
AVERAGE INTENSITY VALUE
OF THIS SPOT EQUALS I/A
PRINT SPOT VALUE
Figure 13. Flowchart of Spot-Mapping program.
1__1_ ~I_~^ _~~YI__JX~Y~
page 39
relative reflectance spectra are obtained by dividing the
normalized reflectance spectra for one area of the planet by
the normalized reflectance spectra of another area on the
planet. Following these reductions a comparison can be made
between actual photometer data and the vidicon image data.
Such a comparison would verify both the accuracy of the image
processing system, particularly the Spot-Mapping program, and
the photometric quality of the vidicon images.
I ~-ly
^__~_
__/ ____IP__LII__I__^
_~l~_rLI_
page 40
V. Comparison of Relative Spectral Reflectivity Data
During the night of October 14, 1973 vidicon images of
Mars were taken using twenty narrow-band filters (4A=250 A).
The spectral region from 0.35 microns to 1.03 microns was
covered. Associated darkfields and flatfields were also
imaged. On October 15, 16, and 19 a photometer system was
used to probe numerous areas on the surface of Mars. Twenty
six filters were used spanning the spectrum from 0.30 microns
to 1.10 microns. About twenty five different areas on Mars
( refered to as 'spots') were measured by the photometer
during its three nights of observations. Figure 14 is a map
of Mars showing the extent of the dust storm on October 14,
the night the vidicon images were exposed. The ellipses define the twelve photometer spots that will be discussed.
Figures 15, 16 and 17 are maps of Mars with the extent of
the dust storm shown for October 15, 16 and 19 respectively.
The ellipses are the boundaries within which ( with 90% confidence) the photometer apeture was guided. Spot 4 and Spot 6
were taken on October 15 (Figure 15). Spots 7A through 15 were
observed on October 16 ( Figure 16). Spot 17 and Spot 20 were
measured on October 19 (Figure 17). Also, Figure 18 shows disk
maps of Mars which display the twelve photometer spots as
viewed on the first and last of the vidicon images taken on
C0 e It
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Figure 14. Mercator projection of Mars showing elliptical
photometer spot boundaries and the dust storm
as of October 14, 1973 - the same night that
the vidicon images were taken.
-
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Figure 15. Mars map with the dust storm as of October 15
1973 and two of the photometer spots (4 & 6)
taken that same night.
2
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73
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Figure 16. Mars map with the dust storm as of October 16
1973 and photometer spots 7A through 15 taken
that same night.
50
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Figure 17. Map of Mars with the dust storm as of
October 19, 1973 and photometer spots
17 and 20 taken that same night.
--
2W
260
AD20 1
70
A
20
180
1
"-"I~
.^^;~YI~'-*"~~"
CD
45
MRRS
VIOICON
1
OC7. 14. 1973
T= 8:50 UT
DIR= 21.13 SEC
SUB-EARTH POINT
LR7= -16.7
LONG= 43.1
SUB-SOLAR POINT
LA7= -21.2
LONG= 33.6
Figure 18. Photometer spots as seen on first and last image s.o
MARS
VIJOICON 100
OC7. 1, 1973
7= 12:41 UT
01R= 21.44 SEC
SUB-EARTH POINT
LAT= -16.7
LONG= 99.4
SUB-SOLR POINT
LRT= -21.2
LONG= 90.0
page 46
October 14, 1973. Furthermore, to facilitate identifying these
spots on the maps, a list of the latitude and longitude center
coordinates of the spots can be found in Table 1. For all of
these spots relative reflectance spectra have been produced
from the photometer data using reduction routines similar to
those found in the image processing system - DIPSYS. By carrying out the image processing described in the last section,
relative reflectance spectra for the same spots on Mars have
been extracted from the calibrated vidicon images. A comparison between these two sets of data follows.
A dark region on Mars appears 'dark' because of a broad
absorption band in the near infrared which is an attribute
of Fe
minerals in the soil (Adams and McCord, 1969; R.L.
Huguenin, J.B.Adams, and T.B.McCord, manuscripts in preparation, 1974). Bright areas are 'bright' because they lack this
broad absorption feature. A relative reflectance spectra for
a bright area divided by a dark area will therefore produce a
curve rising above 1.0 in the near infrared. If the opposite
case is taken (dark divided by bright), the curve will fall
below 1.0 in the near infrared.
According to the log book kept during the photometer observation period, as well as by visual and graphical interpretation of the data, photometer Spot 8 (340S, 560 W) was the
darkest of all the photometer spots. However, Spot 20 (18*S,
~1~~M'III'I1~L~III"--*-*P
"II~LI~C-.III~
IP~_II~-IPL_
page 47
Spot
Latitude
Longitude
4
38°s
46w
6
39*s
7A
200 S
3Ww
8
3L4s
560 w
9
7N
190W
to10
6N
39W
11
51's
121N
460w
4 .,
12"
14
10 N
55W
15
50N
87W
17
55"S
351'W
20
18*S
16 w
Table 1. A list of the photometer spot
center coordinates.
--~h
ll*lliI1IIPI~----
i~-~-YUI"~-'-~-^~I--r
~-~.---;_ ._ .-----
page 48
160W) was the darkest of all the spots reduced from the
vidicon image data. If both spots were equally dark in both
cases, then the photometer relatire reflectance curve of Spot 8
divided by Spot 20 (Spot 8/Spot 20) would be a straight line.
So also would be the vidicon derived relative reflectance
curve of Spot 8/Spot 20. Both of these plots should then
overlay as a single straight line equal to 1.0 at all wavelengths. Figure 19 shows the relative reflectance of Spot 8
divided by Spot 20 as reduced from both the photometer data
and the vidicon data. The 'v's represent vidicon data and the
'p's represent photometer data. Clearly the reflectance curves
for these ratioed spots are neither flat at 1.0 nor even
similar. This example highlights the interpretation problems
imposed by the expanding dust storm.
In Figure 19 the vidicon data implies that Spot 8 was
brighter than Spot 20. Yet the photometer data implies that
Spot 8 was darker than Spot 20. Either Spot 8 became darker
between October 14 and October 16 or Spot 20 became brighter
by October 19, or both. The fact that significant albedo
changes did occure between the observations of each of these
spots is outlined by the growth of the dust storm as shown in
Figures 14, 16 and 17. For the vidicon data, Spot 20 (in the
dark region - Mare Erythraeum) was far removed from any dust
clouds, as was Spot 8. Apparently Spot 20 material is intrin-
Ild~-~B1~-~_l*i_
_
S-
-i
- ----------
A
v
v
v
V.4
*
A.
,r
1.0
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V
,..
v
v
v
PP
0.8
p
A
A
AA
A
A
0.6
0.4
0.2
0,0
.0
0.2
I
I
0.3
0.4
I-~i--.-1
I
0.5
0.6
WAVELENGTH
I
0.7
(0M)
MARS SPOT 8
I
I
I
I
0.8
0.9
1.0
1.1
/ MARS SPOT 20
Figure 19
Y
I~---II-*III~
~~^C~I~I~ __*_j_ _ij IlilX-l.
-L11C-^(
4-I~~*-4C~----1----~-~a__~~
page 50
sically darker than Spot 8 material, leading to the rising
vidicon curve in Figure 19. But by October 19, Spot 20 was
almost totally engulfed by bright dust. Spot 8 in the photometer data (Figure 16) overlays part of the dust storm but it
apparently recorded enough of the dark Erythraeum material to
ultimately make it darker relative to Spot 20. Hence the
falling photometer curve in Figure 19.
To remove as much of the adverse effects of the growing
dust storm as possible, a search was made for spots that were
far removed from the dust storm in both the photometer and
vidicon image data. As Figures 14 and 16 reveal, Spot 9 (76N,
190W) and Spot 10 (66N, 39PW) both fit this criteria.
Figure 20 shows the overlay of photometer and vidicon
data for the relative reflectances of Spot 10/Spot9. Within
the error bars the two curves match, indicating that the
vidicon system does indeed return valid photometric data. As
a result, Spot 9 was chosen as the denominator for all relative reflectance spectra. Spot 10 was not chosen since as
the map in Figure 16 indicates, Spot 10 is closer to the dust
cloud. Discrepancies between photometer and vidicon data must,
therefore, be interpreted in light of (1) the rapid albedo
changes on Mars during the dust storm - as the discussion of
Spot 8/Spot 20 has revealed - and (2) possible inaccuracies
in the Spot-Mapping program.
~rm~"~
51
p
eJ..
1.0
V
V
v
~
v
A
s-
rv
A
A
0.8
0.6
0.4
0.2
0.00.
0,
2
0.3
0.4
0.7
WAVELENGTH (JuM)
0.5
F
1
I I I
0.6
0.9I1.0
0
0,9
0.8
MARS SPOT 10 / MARS
Figure 20
SPOT
9
1.0
1.1
1.1
III_*bILII__PII___jli^_
il~i-Li.IXI*_I~
~-C*ilYiCsil
-1~^L-L^p^--~
I*-*-~P-I-Y .
page 52
Spot 4 (38*S, 466W) also lies over the dark Erythraeum
region. Ratioed to a bright area, which Spot 9 is, Spot 4
should produce a curve that decreases in the near infrared.
Figure 21 shows the relative reflectance spectra for Spot 4/
Spot 9. In general the photometer data is in agreement with
this prediction. The vidicon data is not. For both sets of
data Spot 4 is removed from the dust storm, especially in
the vidicon data. However, as the analysis in Section III
has shown, the furthest, albeit tenuous, extent of the dust
storm is best revealed in the blue. The scatter in the
vidicon data for Spot 4/Spot 9, particularly in the blue,
may be caused by the thin clouds reaching down from the
Coprates Canyon area (see Figure 4).
Spot 6 (39*S, 121*W) captures the dust storm in both
sets of data. Figure 22 shows the relative reflectance of
Spot 6/Spot 9. Again there is great scatter in the vidicon
data, especially in the blue. Spot 6 lies over the Aonis
Sinus cloud, of which the extreme albedo variations in the
blue have already been discussed. This accounts for the wide
error bars around 0.40 microns. In the photometer data Spot 6
is brighter than Spot 9 as would be expected of a dust storm
composed of a concentration of bright area material. Overlooking the data scatter in the vidicon derived curve, the
spectra is contrary to the expected trend. Perhaps the
1*/
_-_YY~Y.
I
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i
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53
SS I
a
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t
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r
t A
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I
0.3
I
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I
I
I
0.7
0.5
0.6
WRVELENGTH (mm)
MARS SPOT 4
I
I
I
0.8
0.9
1.0
/ MARS SPOT
Figure 21
9
-- IC------
1.1
- IIIUII~-U~--h~g
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v
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rir
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t
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v
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t
r
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r
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r
v
A
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V
IP
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A
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0.4
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II
0.3
0.4
i --
I_
I
I
i
0.6
0.5
WAVELENGTH
L
0.7
(uM
MARS SPOT 6
[
0.8
0.9
/ MARS SPOT
9
Figure 22
i
1.0
1.1
C-PIII
---P--I_~-~~
~I~I__IL__~ Cl~i^il~.i ~~.I.-1.-ILI~~--~--~L
page 55
darkening of Spot 6 relative to Spot 9 in the 0.65 micron
region is caused by the elliptical spot boundary only partially overlaying the apparently tenuous Aonis Sinus cloud
and encompassing some of the dark gap between this cloud and
the region west of the Solis Lacus cloud (see Figure 4).
Inaccuracies caused by the image processing (using the
Center-Seeking and Spot-Mapping programs) may have caused
some of the dissimilarities discussed so far. But Spot 10/
Spot 9 in Figure 20 does not bear this out. However, Spot 10
and Spot' 9 are in close proximity, and both are bright areas.
Inaccuracies in the image processing cannot necessarily be
expected to be revealed by the ratio of Spot 10/Spot9. However, Spot 7A does clearly demonstrate the precision of the
image processing.
Spot 7A (20S05, 340 W) lies in the heart of the Erythraeum
dark region. It is out of the main body of the dust storm in
both sets of data. Spot 7A is also distant from Spot 9 as
Figure 18 in particular points out. Figure 23 displays the
relative reflectance of Spot 7A/Spot 9 for the photometer and
vidicon data. The drop below 1.0 in the red end of the
spectrum indicates that Spot 7A is indeed darker than Spot 9.
The curves match very well - revealing the accuracy of the
image reductions. The photometer curve rise around 0.90
microns may be caused by a slight dustiness from the nearby
I~--~lii- X-II_~IICXI-_-I^__II11_1__.
I~I)-Y~-iXI
~-I*UILI-*UI_
56
V
SV ,v
A
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II
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0.8
v
A
;
&
0.6
0.4
0.2
0.0 0
0.2
-
I
0.3
0.4
0.5
0.6
0.7
WRVELENGTH (WM)
MARS SPOT 7A
0.8
I
0.9
/ MAR5 SPOT
9
Figure 23
I
1.0
1.1
IY___4___11__11_IC_
(L1~~IYI*n_
I~-l-IYY*I---1I~IIIII~
CILXI
page 57
dust cloud (see Figure 16). The large error bars around 0.35
microns in the vidicon data may again be due to the blue haze
northeast of Solis Lacus ( see Figure 4).
Spot 11 (51*S, 466W) in Figure 24 shows dissimilarities
between vidicon and photometer data around 0.73 microns and
0.90 microns. These two features may have been washed out of
the photometer curve by the reddening caused by dust from the
nearby storm (Figure 16). It should be kept in mind that the
boundaries of dust clouds are certainly not as accurate as the
straight lines in Figures 14 to 17 might indicate.
Spots 12, 14 and 15 (18*N,69OW, 16N, 55*W and 5*N, 876 W,
respectively) in Figures 25, 26 and 27 all show the same basic
trend in the vidicon data, namely an apparent rise in the near
infrared. The scatter in this vidicon data is, however, severe
and may be attributed to albedo variations in the length of
clouds north of Solis Lacus in the region of the Coprates
Canyon, as well as in the cloud over Tharsis. Another cause
of the data scatter that is more fundamental to the image
processing done to the vidicon data may also be playing a
part here. The Spot-Mapping program averages all intensity
values falling within a given ellipse. In simulating photometer data, the program is therefore assuming that the photometer aperture covered the entire area within the ellipse.
This is not true. In fact, to within.90% confidence, the
r..
r
-
-T- - -
--
-
I
7
.
58
(O
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v
S
0 1.0
A
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rr
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r r.
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ct
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0,0
-
0.2
I
0.3
0.4
0.5
0.6
0.7
0.8
L
0.9
/ MARS 3POT
9
WRVELENGTH (MM
MARS SPIT 11
Figure 24
.. _
1.0
1.1
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0.3
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0.5
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WVELENGTH (AM1)
MARRS SPOT 12
0.8
0.9
/ MARS SPOT
9
Figure 25
1.0
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-
1
111
60
r
V
V
-
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'!"
1.
1.0
1I~
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I
0.5
0.6
WAVELE JGTH
0.7
1-:
A-
r
t
P+pr
r
r
-r
0.8
0.6
0.2
0.0
-
0.2
0.3
I
0.4
0.8
0.9
/ MARS SPOT
9
(AAM)
MARS SPOT 14
Figure 26
1I
1.0
1
1.1
--
i
r
7Ti
F 7T
T
7F
61 I
V
v
P
1
v
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vpr
SF
r
r
pL
L
A
1.0
ff f
V
v
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0.8
0.6
0.4
0.2
0.0 0.2
1
0.3
0.4
1
I
0.5
0.6
WRVELENGTH
0.7
MM)
MARS SPOT 15
0.8
0.9
/ MRRS SPOT
9
Figure 27
1.0
1.1
IIQCI__I_________II1YII-
~I _~L~LIICLIII~III- I.-l-C--ll-~~ll
--
page 62
photometer aperture only wandered around somewhere within the
ellipse. Spots 12, 14 and 15 lie over and near regions of
rapidly changing albedo. The Tharsis and Coprates clouds,
tenuous as they appear to be on October 14, may have been
different or may not have existed at all when the three photometer spots were measured on October 16. This design weakness
in the Spot-Mapping program may also be responsible for the
dissimilarities seen in Spot 11/Spot 9, Figure 24.
Another weakness in the vidicon data is highlighted by
the scatter in Figure 28. This shows the relative reflectance
of Spot 17/Soot 9. Figure 17 shows that the dust storm lies
partly in photometer Spot 17. The rest of Spot 17 covers the
dark area of southern Noachis. Apparently the photometer
aperture recorded mostly the dark area reflected light since
Spot 17 is darker than Spot 9 in the photometer curve of
Figure 28. The vidicon curve follows this trend but with
severe scatter. As can be seen in Figure 18, Spot 17 lies near
the east limb of Mars for the first image and most of the spot
eventually passes over the horizon. The scatter in the data
results from difficulties the Spot-Mapping program has in
locating a spot near the limb of the planet. The mean radius
returned by the Center-Seeking program may prevent the SpotMapping program from aquiring limb data or it may cause the
Spot-Mapping program to overreach the limb and return false
---a
rT
1
T
-
....
.T.
63
i
ft
ft
P
V
v
.
.v
v
1.0
VV v
y
v
V
V.
V
v
v
v
r
*
it
r
T
f
PPPP
0.8
P
-I
0.6 i
0.4
0.2
2
I
I
0.3
0.4
L
I
I
0.7
0.6
0.5
WIVELENGTH (MM)
MARS 5POT 17
i
1
1
1
0.8
0.9
1.0
1.1
/ MARS SPIT
9
Figure 28
I
I~__III__PIIL__
- L~~_~l_~jU_ ___J
page 64
data from intensity values beyond the planet. Data extracted
from any spot near the limb is suspect. This problem has
probably contributed to the vidicon data scatter not only in
Spot 17, but also in Spot 15, Spot 6 and Spot 9 (the denominator!).
Finally, just to conclude with a clear cut example,
Figure 29 shows the relative reflectance spectra for Spot 20/
Spot 9. In the vidicon data the curve indicates that Spot 20
is darker than Spot 9 as it should since both spots are out
of the dust storm (Figure 14) and Spot 20 overlays the dark
Mare Erythraeum whereas Spot 9 overlays the bright area of
eastern Chryse. In the photometer data Spot 20 appears
brighter than Spot 9, primarily because photometer Spot 20
was measured on October 19 when the dust storm covered most
of Mare Erythraeum, including most of Spot 20 (Figure 17).
Since the vidicon tube has a demonstrated photometric
responce, the final step in the reduction of the vidicon
data, which is the production of reflectance spectra normalized
to the Sun, could directly use the photometer data. Multiplying all the vidicon spectral reflectivities relative to
Spot 9 by the ratio of Spot 9/Sun - as derived from the photometer data using standard stars - will yield reflection
spectra normalized to the Sun for any area on the visible
face of Mars. An interpretation of the vidicon data could
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page 66
then begin without the ambiguities and subtleties of relative reflectance spectra. This final step awaits only an
accurate determination of Spot 9/Sun - currently in the works.
As the data presented in this section reveals, the
Vidicon Imaging System coupled with the image processing
system returns accurate, two-dimensional photometric data.
The discrepancies between the vidicon image data and the
photometer data of October, 1973 can mostly be attributed to
the obscuring caused by the expanding dust storm. Also, the
error bars in the vidicon data are in general greater than
the photometer error bars. This is probably caused by either
(1) insufficient data (100 images exposed through 20 filters
gives only five complete sets of vidicon data) or (2) small
inaccuracies in finding the spots from image to image by the
Spot-Mapping program. However, a few selected cases (Figures
20, 23 and 29 for example) clearly exhibit the power of this
data collection system. It is now possible, given the vidicon
system, to specify any area on Mars and produce a reflection
spectra for that area.
page 67
VI. Concluding Remarks
Thq observations of Mars made with the Vidicon Imaging
System during the October, 1973 opposition have produced a
fund of valuable information. The 1973 dust storm was imaged
in its early developmental stages. The diurnal evolution of
the dust clouds has been captured in explicit detail. The
rapid albedo variations in these clouds can successfully be
explained by condensed and adsorbed volatiles adhering to the
dust grains during the Martian night and then sublimating
following exposure to sunlight. The dust clouds themselves
are seen undergoing rapid alterations in structure. The digital format of the vidicon images facilitates data reductions.
The images can be artificially rotated to view interesting
features from different angles. The intensity values which
make up the images are themselves individual photometric
sensors. The reduction of the images to spectral reflectivity curves has been described, emphasizing the power of the
vidicon tube to provide two-dimensional, narrow-band photometry. Unfortunately, the presence of the dust storm has a
detrimental obscuring effect on spectral reflectivity data
which prevents a detailed compositional study of the Martian
surface. Perhaps during the next opposition Mars will present itself with an atmosphere unobscured by dust. If so,
page 68
the full disk photometric coverage of the planet provided by
the Vidicon Imaging System will yield spectral reflectivity
data for all resolvable areas on the visible face.
The Vidicon Imaging System coupled with the DIPSYS image
processing system - including especially the programs described in this thesis - has a usefulness other places than
just Mars. Elsewhere in the Solar System there are intriguing
curiosities, such as the UV clouds on Venus, transient events
on Jupiter, assorted moons with assorted surfaces, and the
several rings of Saturn - to name a few. Undoubtedly the
Vidicon Imaging System will be applied to these fascinating
objects with the same success displayed during the October,
1973 Mars observations.
page 69
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McCord,T.B.,C.Pieters and M.Fierberg (1975) paper in draft.
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