PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2015GL066146
Special Section:
First Results from the MAVEN
Mission to Mars
First measurements of composition and dynamics
of the Martian ionosphere by MAVEN’s Neutral
Gas and Ion Mass Spectrometer
M. Benna1,2, P. R. Mahaffy1, J. M. Grebowsky1, J. L. Fox3, R. V. Yelle4, and B. M. Jakosky5
1
Key Points:
• Several ionospheric species were
detected during the first 8 months of
the MAVEN mission
• The major ion density peak was
detected near the terminator at
190 km altitude
• The density profiles of all ions exhibit
day/night and dusk/dawn
asymmetries
Supporting Information:
• Figures S1–S5
Correspondence to:
M. Benna,
mehdi.benna@nasa.gov
Citation:
Benna, M., P. R. Mahaffy, J. M. Grebowsky,
J. L. Fox, R. V. Yelle, and B. M. Jakosky
(2015), First measurements of composition and dynamics of the Martian
ionosphere by MAVEN’s Neutral Gas and
Ion Mass Spectrometer, Geophys. Res.
Lett., 42, doi:10.1002/2015GL066146.
Received 9 SEP 2015
Accepted 25 SEP 2015
©2015. American Geophysical Union.
All Rights Reserved.
BENNA ET AL.
NASA Goddard Space Flight Center, Greenbelt, Maryland, USA, 2CSST, University of Maryland, Baltimore, Maryland, USA,
Department of Physics, Wright State University, Dayton, Ohio, USA, 4Department of Planetary Sciences, University of Arizona,
Tucson, Arizona, USA, 5Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, Colorado, USA
3
Abstract We report the results of the observations of the ionosphere of Mars by the Neutral Gas and Ion Mass
Spectrometer. These observations were conducted during the first 8 months of the Mars Atmosphere and Volatile
EvolutioN mission (MAVEN). These observations revealed the spatial and temporal structures in the density
distributions of 22 ions: H2+, H3+, He+, O2+, C+, CH+, N+, NH+, O+, OH+, H2O+, H3O+, N2+/CO+, HCO+/HOC+/N2H+,
NO+, HNO+, O2+, HO2+, Ar+, ArH+, CO2+, and OCOH+. Dusk/dawn and day/night asymmetries in the density
distributions were observed for nearly all ion species. Additionally, high-density fluctuations were detected on
the nightside and may reflect the effect of the partial screening of the atmosphere of Mars by the weak intrinsic
magnetic field of the planet. The two first MAVEN “deep dip” campaigns were used to investigate the location of
the primary ion peak. This peak was detected at 190 km near the terminator but was below the spacecraft altitude
of 130 km near the subsolar point.
1. Introduction
Our knowledge of the composition and dynamics of the Martian ionosphere dates from the Viking 1 and 2 landers.
The Retarding Potential Analyzers mounted on the aeroshells of both entry probes provided the first, and until
recently the only, in situ measurements of ion composition and vertical density profiles below 300 km altitude
[Hanson et al., 1977]. These direct measurements, while groundbreaking, were limited to three dominant ionospheric species (O+, O2+, and CO2+) and lacked the spatial coverage and the temporal extent required for the
understanding of the global dynamics of the ionosphere. CO2+ altitude profiles were also derived from the dayglow measurements of Mariners 6 and 7 [Krasnopolsky, 1975]. Electron density profiles of the Martian ionosphere
were collected by radio occultation and radar sounding experiments on several entry probes and orbiters, including Mariner 4, 6, 7, and 9 [Fjeldbo and Eshleman, 1968; Hogan et al., 1972; Kliore et al., 1972], Mars 2, 3, 4, and 6
[Kolosov et al., 1976; Vasilev et al., 1975], Viking 1 and 2 [Lindal et al., 1979], and more recently by the Mars
Global Surveyor [Tyler et al., 2001], and Mars Express [Pätzold et al., 2005; Gurnett et al., 2005]. While radio occultation and radar sounding data do not carry information on ion composition, when taking collectively, they provide
a global view of the spatial structures and dynamics of the ionosphere. Since the beginning of the science phase of
the MAVEN mission in mid-October 2014, the Neutral Gas and Ion Mass Spectrometer (NGIMS) has been providing
new observations of the ionosphere of Mars at altitudes ranging between 130 and 500 km. Carried by the MAVEN
spacecraft as part of its nine-instrument payload, the NGIMS instrument aims to contribute to the mission goals of
measuring the rate of atmospheric escape to space by characterizing the composition and the dynamics of neutrals and thermal ions in the upper atmosphere and ionosphere of Mars [Mahaffy et al., 2014; Jakosky et al., 2015].
The NGIMS instrument is a quadrupole mass spectrometer with a mass range of 2–150 Da and a unit mass
resolution. Because NGIMS was also designed to fully characterize the neutral upper atmosphere, ion composition measurements are typically collected every other orbit. The remaining orbits were solely dedicated to the
measurements of neutral species. The NGIMS instrument operates when the spacecraft altitude is below
500 km. During that segment of the orbit, the instrument’s narrow field of view is maintained by the spacecraft’s
Actuated Payload Platform (APP) within 2° of the spacecraft ram direction to maximize the measured signal.
Because the pointing stability of the APP is better than 0.2°, most of the measured ion fluctuations reflect either
variations of ion densities along the spacecraft track, or the presence of localized cross-track ion flows. Detailed
description of the data processing procedures of the NGIMS ion measurements is provided in the supplementary
material of Benna et al. [2015], and in Benna and Elrod [2015]. The standard deviation in individual ion
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measurements due to random uncertainties is dependent of the density
level and is ~50% at 0.1 cm 3 and
~25% above 1 cm 3. The background
level is species dependent and is typically below 0.01 cm 3 for most mass
channels. The NGIMS instrument sensitivity is derived by cross calibrating the
ion measurements against the electron
densities that are derived by the
Langmuir Probe and Waves instrument.
An initial sensitivity of 1061.1 ± 50.0
(counts/s/ions/cm3) for CO2+ was
derived by Benna et al. [2015] using
data from the first 2 weeks of operations. An energy calibration curve is
then used to scale the CO2+ sensitivity to other ion species.
In this paper we report NGIMS’ first
observations of the ionosphere of
Mars. These observations were collected
during 367 orbits and span the first
8 months of the mission (18 October
2014 to 18 May 2015). Over that time
period, the orbit of MAVEN precessed
by ~25 h in local time and ~95° in latitude, which allowed NGIMS to map
nearly two thirds of the planet’s northFigure 1. Spatial distribution of the NGIMS ion measurements for the per- ern hemisphere (Figure 1). This period
iod of 18 October 2014 to 18 May 2015. (a) The ground tracks and altitudes covered northern fall and winter with
of the MAVEN spacecraft in the Mars Solar Orbital frame during NGIMS ion
LS varying from 216° to 348°. (Solar longobservations. White area depicts the dayside, and gray area depicts the
nightside. Periapsis locations for each orbit are indicated by the black dots. itude LS specifies seasons on Mars with
+
0° and 180° at vernal and fall equinoxes,
(b) Measurements distribution of O2 as a function of solar zenith angle
and altitude. Positive solar zenith angles are located on the duskside of the respectively). Heliocentric distance of
planet, while negative solar zenith angles are on the dawnside.
Mars varied from 1.403 AU to the
minimum of 1.382 AU at 251° and then
to 1.530 AU; that is, the observations were near the perihelion. The solar activity index F10.7 varied on the
Earth from 110 to 220 with a mean value of 140. This value is 70 when scaled to the orbit of Mars, that is, between
the medium and high solar activity on Mars.
2. Composition of the Ionosphere
During the reported period, the NGIMS instrument was able to detect and measure vertical profiles of 22 ionospheric ions. These ion species are in addition to the exogenous metal ions that were detected by NGIMS from
18 October to 22 October 2014 and attributed to ablated dust from comet Siding Spring [Benna et al., 2015].
The detected ionospheric ions are H2+, H3+, He+, O2+, C+, CH+, N+, NH+, O+, OH+, H2O+, H3O+, N2+/CO+, HCO+/
HOC+/N2H+, NO+, HNO+, O2+, HO2+, Ar+, ArH+, CO2+, and OCOH+. Additionally, several isotopes such as 36Ar+,
38 +
Ar , and 18OO+ were unambiguously detected. Other isotopes such as 13C+ and 15N+ were masked by isobaric
interferences with other protonated ion species (e.g., CH+ for the case of 13C+ and NH+ for the case of 15N+) and
will require further analysis to be extracted. In order to derive vertical profiles, all observations were organized and
binned as a function of altitude and solar zenith angle (SZA) in 5° × 5 km bins. The number of measurements in
each bin ranges from 1 to 60, but on average, there are 15 measurements in each bin (Figure 1b). Figure 2 compiles the reconstructed altitude profiles at SZA = 60° for all 22 ion species. Note that each vertical profile averages
BENNA ET AL.
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Figure 2. Altitude profiles of the averaged density of ionospheric ions measured by NGIMS at SZA = 60° at altitudes
36 + 38 +
18
+
between 150 and 500 km. Other measured isotopes such as Ar , Ar , and OO are not depicted on this plot. The
vertical profile of the total ion density Ni is plotted in black.
data that were collected on both duskside and dawnside of the ionosphere and over a wide range of latitudes and
solar activities (solar wind conditions and extreme ultraviolet irradiation). These averaged density profiles reveal
that below 300 km, O2+ is the dominant ion in the ionosphere. Above this altitude, O+ becomes comparably abundant. CO2+ is the second most abundant ion below 220 km (at 150 km, 14% of the total ion density is from CO2+),
and its density decreases rapidly with altitude. One should note that above 270 km, all ions tend to converge to
the same scale height. This behavior is due to the large gradient in plasma temperature at high altitude [Fox and
Weber, 2012; Fox, 2015]. The total ion density is dictated by the vertical distribution of the two dominant ions, O+
and O2+. Based on our current calibration (see supporting information), the highest value for the average total ion
density at SZA = 60° was 2.4 × 104 cm 3 with a combined standard deviation from averaging of 3.8 × 103 cm 3.
This value is reached at 160 km. A comparable average total ion density value was also measured at SZA = 45°
(2.1 × 104 cm 3 at 160 km with a combined standard deviation from averaging of 2.8 × 103 cm 3), which is
about half of what was measured by the Viking 1 and 2 landers [Hanson et al., 1977] for similar SZA and altitude
(5.6×104 cm-3 and 4.3×104 cm-3, respectively), but lower solar activity (Viking measurements were made at the
deep solar minimum F10.7 = 27 on Mars). A thorough comparison of the NGIMS density profiles for O+, O2+,
and CO2+ with those from Viking 1 and 2, and from several models, is provided by Withers et al. [2015].
Additionally, the NGIMS measurements confirm the presence of all the protonated ions previously modeled by
Krasnopolsky [2002], Matta et al. [2013], and Fox [2015]. While the expected densities for protonated ions varied
between models, the NGIMS measurements reveal the presence of HNO+ with a substantially higher density
than previously predicted (400 cm 3 at 150 km), which makes it the third most abundant ion at 150 km, well
above HCO+. HCO+ is the terminal ion for most protonated ion species and thus was expected to be the most
abundant. The model of Fox [2015] predicted HNO+ to be present at trace levels (<0.03 cm 3) in the ionosphere
making its detection at high abundance surprising. Similarly, HO2+ is detected at much higher levels than
expected by Fox [2015]. At 150 km, HO2+ is comparable in density to OCOH+. Additionally, the detection of
H2O+ and H3O+ with peak densities of 10 and 5 cm 3, respectively, is surprising and provides strong indications
of the presence of H2O in the thermosphere. A more detailed analysis of the implications of the presence of
H2O+, OH+, and H3O+ is provided by Fox et al. [2015].
3. Primary Ion Peak
Several studies showed that the altitude at which the primary ion density peak (often labeled F1 or M2 by
various sources in the literature; it will be denoted M2 in this paper) occurs varies with SZA, planetocentric
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Figure 3. Altitude profile of the total ion density measured during the inbound leg of two MAVEN deep dip orbits
(orbit #739 on 16 February 2015 and orbit #1081 on 21 April 2015). The periapsis of orbit #739 was located over
the dusk terminator, while the periapsis of #1081 was located near the noon meridian. For orbit #739, the ion density
peak was located at an altitude of 190 km. The ion density peak was located below the periapsis altitude (130 km) for
orbit #1081 and was not observed.
longitude, and solar activity [Hantsch and Bauer, 1990; Bougher et al., 2004; Fox and Yeager, 2006; Morgan
et al., 2008; Němec et al., 2011; Fox and Weber, 2012; Peter et al., 2014; Fallows et al., 2015; Zhang et al.,
2015]. These studies place the altitude of the M2 peak between 125 and 165 km for SZA = 60°–90°. For lower
SZAs, the altitude of the M2 peak is expected to be even lower [Withers, 2009]. With a nominal periapsis
altitude of 150 km, the MAVEN spacecraft is often located above the predicted altitude of the primary ion
density peak. However, the two MAVEN deep dip campaigns of 11–17 February 2015 and 17–22 April
2015, during which the spacecraft’s periapsis altitude was lowered from 150 km to 130 km, offered good
opportunities for the detection of the primary ion density peak. Figure 3 shows the altitude profile of the
total ion density measured by NGIMS during the inbound portions of orbit #739 and #1081. Orbit #739
occurred during deep dip #1 on 16 February 2015, where the SZA of the periapsis was located at 85°.
Owing to its high SZA, deep dip #1 offered the best chance to reach the altitude of the M2 peak, which
was confirmed by its detection at altitudes between 175 and 200 km and an average altitude of 185 km.
These altitudes for the M2 peaks lay at the high end of what was observed by Mars Express from 2005 to
2009 [Němec et al., 2011]. At SZA of 85°, Mars Express data locate the M2 peak at altitudes between 120
and 190 km with an average of 150 km. The drivers that led to such an exceptionally high M2 peak altitude
near the terminator remain unclear but may find their sources in the nearly horizontal radiation, the change
in the underlying neutral atmosphere as the SZA nears and passes the terminator, and the overall topology
of the overlaying magnetosphere as it transitions between day and night. During the deep dip #2 campaign,
the SZA of the periapsis was ~7° (as illustrated by orbit #1081, which occurred on 21 April 2015), and the M2
peak was systematically located below the altitude of the spacecraft at periapsis (~130 km). This result is
expected, because the altitude of the primary ion density peak should decrease with decreasing SZA
[Withers, 2009]. It is also consistent with the observations of Mars Express of the peak altitude at
SZA < 10° being less (but close to) 130 km [Morgan et al., 2008; Němec et al., 2011]. Additional deep dip
campaigns will explore other SZAs and will further inform on the dependencies of the altitude of the M2
peak on solar zenith angles.
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Figure 4. Spatial distribution of O2 , O , CO2 , He , and H3O as function of altitude and solar zenith angle. Each spatial distribution plot combines the full data set
acquired from 18 October 2014 to 18 May 2015 by NGIMS. The observations were organized and binned as a function of altitude and solar zenith angle in 5° × 5 km
bins. Each bin combines an average of 20 measurements. All the ions that were measured by NGIMS in the ionosphere display a spatial distribution that is analogous
to one of the five species represented in this figure. The dotted area depicts the unlit portion of the ionosphere that lies in the shadow of the planet.
4. Spatial Distributions and Asymmetries
The spatial distribution of O2+, O+, CO2+, He+, and H3O+ as a function of solar SZAs and altitude is provided in
Figure 4 (see Figures S1–S5 in the supporting information for the rest of the species). These species were
chosen as the representatives of the five types of spatial distribution observed by NGIMS. The data reveal
that HCO+/HOC+/N2H+, NO+, and OCOH+ exhibit a spatial distribution similar to O2+. The findings also
show that O2+, HNO+, N2+/CO+, HO2+, Ar+, and ArH+ have distributions similar to CO2+; that H2+, C+,
CH+, and N+ have spatial distributions similar to O+; and that H3+, NH+, OH+, and H2O+ have spatial
distributions analogous to that of He+. Only H3O+ possesses a peculiar spatial distribution that is unlike
any other observed by NGIMS.
The difference in topology between the five types of spatial distributions resides mainly in their day/night
asymmetries. In this paper, we define the dayside and the nightside as the regions with SZA > 90° and
SZA < 90°, respectively. For O2+ and associated species, ion density on the dayside dominates. The nightside
densities, for both the lit and the unlit ionosphere, are noticeably high compared to their maximum value on
the dayside (no more than 2 orders of magnitude lower), and especially below 200 km altitude. For O+ and
related species, nightside density is sharply depleted below 200 km. For CO2+ and related species, the nightside
densities are many orders of magnitudes lower than those in the dayside. The terminator line forms a clear
divide between the day and the night ion distributions. For He+ and related species, there is a gradual transition
in the ion spatial distribution across the terminator. Moreover, ion densities peak at higher altitudes and are
depleted below the altitude of 180–200 km. H3O+ presents a peculiar spatial distribution. Its density peaks in
the magnetotail region (altitude < 200 km, SZA > 90°), while it remains 3 to 4 orders of magnitude lower on
the dayside. The high density of H3O+ on the nightside extends along the terminator line to higher altitudes.
The asymmetry between day and night of the ionospheric ions is not the only observable feature in the spatial
distribution of most ions. The NGIMS observations reveal that all ions exhibit a dawn/dusk asymmetry. Figure 5
depicts the ion density distribution of HNO+ as a function of SZA and altitude where the dusk and dawn regions
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Figure 5. Spatial distribution of HNO as a function of altitude and solar zenith angle. Positive solar zenith angles are
located on the duskside of the planet, while negative solar zenith angles are on the dawnside. The dotted area
depicts the unlit portion of the ionosphere that lies in the shadow of the planet. This spatial distribution reveals a clear
dawn/dusk asymmetry in which ions tend to reach higher altitude on the dawn hemisphere than on the dusk hemisphere, and the density gradient at the terminator extends more into the day on the dawnside than on the duskside.
Similar asymmetries were observed for all ion species.
are separated. The figure shows a clear dawn/dusk asymmetry in which ions tend to reach higher altitude on
the dawn hemisphere than on the dusk hemisphere. Moreover, significant ion densities extend further into
the nightside past the dusk terminator than the dawn terminator, perhaps underscoring the importance of
longitudinal transport [Cui et al., 2015]. This dawn/dusk asymmetry is common to all the ion species that were
observed (Figures S1–S5). It is important to note that all of the dawn measurements were made at +30° latitude
and above, while many of the dusk measurements were made at near-equatorial or negative latitudes
(Figure 1a). These latitudinal differences in measurement distributions could partly be responsible for the
observed asymmetry. As the orbit of the spacecraft continues precessing, the expected additional spatial coverage should shed more light on the nature of this dawn/dusk asymmetry.
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Figure 6. Variability of NO , O2 , O , CO2 , He , and H3O density during the first 8 months of the mission. This variability is expressed as a standard deviation in
percent of the average ion density in each bin. The data reveal that the highest variabilities in the measured densities are mostly localized on the nightside at
altitudes lower than 250 km.
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5. Variability
The ion density distributions that are shown in Figures 3 and 4 result from averaging measurements collected
over multiple orbits. A measure of variability of ion density for a given altitude and SZA can be derived by
operating a variance analysis on the measurement sample contained in each bin. The result of this variance
analysis reflects the spatial variability of ion density along the portion of the spacecraft track contained in one
bin. It also captures the temporal variability of measurements made at the same bin but on different orbits
and different times. As an example, Figure 6 expresses the variability of O2+, O+, CO2+, He+, NO+, and H3O+
density in terms of a standard deviation in percent of the average ion density in each bin. The plot shows that
most of the variability for all observed ions is localized in the nightside especially at altitudes below 250 km. A
similar variability has been observed by Mars Express in the nightside ionosphere [Duru et al., 2011] and has
been attributed to the partial screening of the nightside atmosphere of Mars by the weak intrinsic magnetic
field of the planet [Verigin et al., 1991].
6. Conclusions
During the first 8 months of observations of the ionosphere of Mars, NGIMS measurements revealed the spatial structures of the major and several minor ion species. The initial inventory of observed species is H2+, H3+,
He+, O2+, C+, CH+, N+, NH+, O+, OH+, H2O+, H3O+, N2+/CO+, HCO+/HOC+/N2H+, NO+, HNO+, O2+, HO2+, Ar+,
ArH+, CO2+, and OCOH+. Detections of other minor species and isotopes are likely to occur as the mission progresses and more measurements are collected. Additional chemical modeling will be required to account for
the presence of HNO, HO2+, and H3O+ with noticeably higher abundances than previously assumed. Finally,
the topology of the dawn/dusk asymmetry should get better defined as the orbit of the spacecraft continues
precessing, and additional data are collected on the dawnside.
Acknowledgments
The MAVEN/NGIMS investigation was
supported by NASA. Instrument testing
and calibrations were completed at the
Planetary Environments Laboratory of
NASA’s Goddard Space Flight Center. We
are grateful for the engineering/technical
support especially from E. Weidner, E.
Lyness, T. Navas, M. Johnson (Instrument
Operations), and E. Raaen and M. Elrod
(Calibration). We also thank E. Zubritsky
for her editing support. Information on
NGIMS data processing supporting this
article were in Benna et al. [2015]. The
NGIMS data supporting this article are
publicly available at the Planetary
Data System (http://pds-atmospheres.
nmsu.edu/data_and_services/atmospheres_data/MAVEN/ngims.html).
BENNA ET AL.
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