**MEPAG**

# 

# 

**MARS SCIENCE AND TELECOMMUNICATIONS ORBITER**

Report of the Science Analysis Group

March 2006

**This report has been approved for public release by JPL Document
Review Services (CL#**06- **1101), and may be freely circulated.**

# MSTO Science Advocacy Group Members

Barney Farmer (JPL, Chair)

Wendy Calvin (U. Nevada)

Bruce Campbell (S.Inst.)

Jane Fox (Wright State U.)

Bob Haberle (ARC)

Jim Kasting (Penn State)

Janet Luhmann (UC Berkeley)

Andy Nagy (U. Michigan)

Mark Allen (Caltech/JPL)

Daniel Winterhalter (JPL, Study Scientist)

MSTO Science Advocacy Group Members
[ii](#msto-science-advocacy-group-members)

Introduction [1](#introduction)

science goals [3](#science-goals)

Background [3](#background)

Approaches to studying Martian atmospheric evolution
[3](#approaches-to-studying-martian-atmospheric-evolution)

Atmospheric composition: implications for extant processes
[5](#atmospheric-composition-implications-for-extant-processes)

Major Questions [6](#major-questions)

Science Objectives [7](#science-objectives)

Exosphere and Atmospheric Escape [8](#exosphere-and-atmospheric-escape)

The Near-Space Environment [8](#the-near-space-environment)

Boundaries [8](#boundaries)

Escape Mechanisms [9](#escape-mechanisms)

Crustal Magnetic Fields [14](#crustal-magnetic-fields)

Upper Atmosphere Winds [15](#upper-atmosphere-winds)

Recommended Measurements [16](#recommended-measurements)

Mars lower Atmosphere [16](#mars-lower-atmosphere)

Composition [16](#composition)

Inventory of Molecular Species [16](#inventory-of-molecular-species)

Methane [17](#methane)

Circulation [20](#circulation)

recommended Measurements [21](#recommended-measurements-1)

Composition [21](#composition-1)

Circulation [22](#circulation-1)

Mission Characteristics [23](#mission-characteristics)

Launch Date [23](#launch-date)

Orbit Design Considerations [24](#orbit-design-considerations)

Summary of Measurement Requirements
[26](#summary-of-measurement-requirements)

Instrument Payload [26](#instrument-payload)

Instrument development needs [27](#instrument-development-needs)

References [29](#references)

Figures and tables [34](#figures-and-tables)

#  {#section-2 .unnumbered}

# Introduction

The Mars Science and Telecommunications Orbiter (MSTO) is a major
infrastructure component for the next decade of Mars exploration. MSTO,
a Mars Reconnaissance Orbiter (MRO) -- class spacecraft, is proposed for
launch in 2011 or 2013, depending on the findings of the Mars Program
Planning group. MSTO is to provide a combination of science and
telecommunications capabilities over nearly a full solar cycle (≈ 11
years).

This document reports the findings of the MSTO Science Advocacy Group
(SAG), which was convened by the Mars Exploration Program Analysis Group
(MEPAG) and the Mars Exploration Office at JPL to identify and
prioritize areas of Mars atmospheric and surface science objectives for
Mars that can be accomplished from orbit on a MSTO like mission.

The MSTO SAG consists of 11 members of the Mars science community with a
range of expertise in atmospheric and surface science. In addition,
mission architecture and design support was provided by the MSTO
Pre-Project group at JPL. The SAG communicated via teleconference and
electronic means from December 2005 through March 2006.

The MSTO SAG was asked to include in its deliberations consideration of
the following issues related to the Martian atmosphere and surface:

\- The evolution of the Martian atmosphere is a subject for which little
direct evidence has been found. The timing and rates of atmospheric
escape, ancient and present, are important if scientists are to
understand the habitability of Mars, particularly the potential for an
early atmosphere that was thicker and wetter than the one seen on Mars
today.

\- There are recent announcements of the possible discovery of methane
in the Martian atmosphere. The origin of the trace amounts of methane
has been variously suggested to be geologic or biologic. Investigators
from the Mars Express mission and Earth-based telescopic measurement
programs have speculated about the presence of life, gas seeps, and
active volcanism. However, there remain significant unanswered questions
about possible sources and sinks of trace gases on the surface and in
the atmosphere, as well as their spatial and temporal distributions.

\- Two radar sounders are about to return information on the unseen
subsurface of Mars: The Mars Express MARSIS is now making measurements
of the deep subsurface and MRO SHARAD radar will soon arrive at the
planet to observe the structure to depths of several hundred meters.
Undoubtedly, by 2011 or 2013 there will be observations and/or
discoveries that need to be followed up with shallow subsurface imaging.

In addition, the objectives of the MSTO mission should align with those
of MEPAG ("Mars Scientific Goals, Objectives, Investigations, and
Priorities", MEPAG, February 2006), as well as with the Decadal Study
(\"New Frontiers in the Solar System\").

MSTO would be inserted into Mars orbit in 2012 (2014) and, through a
series of aerobraking and energetic maneuvers over a period of 10 or
more years, would perform the function of a telecommunications link with
other spacecraft during this period, obtain important data on the
long-term variability of upper atmosphere density and other EDL-relevant
parameters required for subsequent missions, while satisfying the
varying orbital requirements of scientific investigations of the upper
atmosphere from altitudes as low as 100 km together with the
observational needs for temporal and spatial coverage dictated by remote
sensing composition measurements of the lower atmosphere. These
observations will add a great deal of information about the loss of
atmospheric species to space and the determination of the nature,
abundance, fluxes, and source locations for trace species. The
measure­ments are fundamental for understanding the evolution of the
atmosphere, climate, and the presence of habitable zones.

In addition to the atmospheric observations, there are distinct
advantages presented by the timing of MSTO to make high spatial
resolution measurements of the uppermost few km of the Martian crust to
map, for example, the locations of ice deposits, magmatic intrusions,
etc. The surface of Mars is covered, to varying degrees, by surficial
sediments deposited and modified by aeolian, volcanic, fluvial, impact,
or other processes. Many aspects of the geologic and climate history of
Mars are hidden beneath these sediments. A radar imaging system can
penetrate the mantling materials to reveal buried fluvial, volcanic,
impact, and perhaps glacial or ancient shoreline features. In addition,
monitoring of the backscatter properties of the near surface through the
Martian year may reveal subtle variations due to volatile cycling or
migration.

As required by its charter, the SAG considered early in its
deliberations the potential for the addition of a subsurface
imager/mapper which could make high resolution observations of regions
of special interest identified from previous and current missions. It
had been thought that a particular advantage to the MSTO atmospheric
science would be the ability to use the radar data to aid in
interpreting the nature of the sources of molecular constituents of the
lower atmosphere that were found to be spatially variable.

It was realized however that, despite the powerful advantages of SAR,
there was not sufficient evidence to suggest that the present level of
detailed knowledge of the near surface environment of the planet,
especially when combined with the refinements to be gained from ongoing
MEX and MRO observations, would be inadequate for the purpose of
correlating the spatial variability of atmospheric constituents with
variations in surface/subsurface characteristics. The SAG concluded
therefore that a strong case could not be made for attempting to
accommodate a radar system in the MSTO science payload, particularly in
view of the large fraction of the spacecraft resources likely to be
utilized by the atmospheric science instruments. This decision, of
course, does not reflect on the science value of subsurface remote
sensing studies made within this same time period.

# science goals

## Background

Mars has intrigued planetary scientists for many years, and especially
so since 1971 when Mariner 9 returned photographic evidence of fluvial
features on the Martian surface. Ever since there has been ongoing
debate about how the channels formed and what the climatic conditions
must have been to produce them. Because most of the channels are found
on heavily cratered parts of Mars\' surface, most authors agree that
they formed prior to \~3.8 billion years ago (the end of the so-called
"heavy bombardment period" of solar system history). A warm, wet,
essentially Earth-like Mars prior to that time has been favored by some
authors (*e.g.*, Mango*ld, et a*l., 2004; Polla*ck, et a*l., 1987;
Sag*an, et a*l., 1973), whereas others (*e.g.*, Clifford, 1993; Pea*le,
et a*l., 1975; Segu*ra, et a*l., 2002; Squyres and Kasting, 1994) have
argued that the fluvial features could have formed under much colder
conditions. Recent evidence from the MER rover mission (Squyr*es, et
a*l., 2004a; Squyr*es, et a*l., 2004b) supports the case for standing
liquid water, although this interpretation has already been challenged
(Knau*th, et a*l., 2005; McCollum and Hynek, 2005).

The papers cited above represent only a small fraction of the literature
on Martian atmospheric and climate history. Although progress continues
to be made with each new Mars mission, there is still no consensus view
as to how the Martian atmosphere evolved and how it helped shape the
surface that we see today. Understanding Mars' atmospheric evolution is
thus a high priority for the NASA planetary science program. This
question is of great interest to astrobiologists also, as the potential
for life on or near Mars' surface depends critically on the planet's
climate history, along with other environmental factors.

## Approaches to studying Martian atmospheric evolution

Two different approaches have proved useful in studying the evolution of
Mars' atmosphere and climate. One approach is to simply ask what is
possible from a climatic standpoint. One-dimensional, globally averaged
climate models have been used to investigate the magnitude of the
greenhouse effect on early Mars to see what, if any, type of atmospheric
composition would have been needed to warm its surface substantially. An
interesting result has emerged from these calculations: The Sun is
thought to have been only about 75 percent as bright as today at the
time when most of the channels formed, \~3.8 Ga. (billions of years
before present). Because of this, and because CO~2~ condenses at low
temperatures, a gaseous CO~2~-H~2~O atmosphere could not have produced
Earth-like conditions, regardless of the amount of CO~2~ that was
originally present (Kasting, 1991). If these calculations are correct,
such an atmosphere could have produced a mean surface temperature no
higher than 225 K at or prior to 3.8 Ga. This is probably too cold to
have allowed the formation of fluvial channels, unless transient heating
by impacts is invoked as their cause (Segu*ra, et a*l., 2002).
Additional radiative heating could have been provided by CO~2~ ice
clouds (Forget and Pierrehumbert, 1997; Misch*na, et a*l., 2000), but
near 100 percent cloud cover would be required to produce global mean
surface temperatures above freezing, and this is considered unrealistic.
Venus and Titan are completely shrouded in clouds, but in both cases the
clouds are produced photochemically. CO~2~ clouds on Mars, like H~2~O
clouds on Earth, are formed by condensation and are unlikely to cover a
planet's entire surface. So, a CO~2~-H~2~O atmosphere on early Mars
would have been cold.

One possible way to get around this problem is if the early Martian
atmosphere contained additional greenhouse gases besides CO~2~ and
H~2~O. Sagan and Mullen (1972) (see also Sagan and Chyba, 1997)
suggested NH~3~ as the additional greenhouse gas; however, NH~3~ is
known to be photochemically unstable with respect to conversion to N~2~
and H~2~ (Atreya, 1979), even in the presence of a UV-shielding organic
haze (Pavl*ov, et a*l., 2001). CH~4~ is a more plausible alternative
(Kasting, 1997). CH~4~ is itself photochemically unstable; however, as
discussed further below, its lifetime is much longer than that of NH~3~.
CH~4~ is thought to have been abundant in Earth's atmosphere prior to
about 2.4 Ga and may have provided much of the warming required to keep
the early Earth warm (Pavl*ov, et a*l., 2000). Definitive calculations
showing that CH~4~ could have kept early Mars warm have not been
published, but this remains a viable possibility. As discussed below,
the putative discovery of CH~4~ in the present Martian atmosphere lends
credence to the idea that CH~4~ could have been abundant in Mars'
distant past.

An alternative approach to studying Martian atmospheric evolution is to
start from the present atmosphere and to back-calculate what it was like
in the past. This approach itself has two components: (i) Current
isotopic ratios in different atmospheric gases can be used to show that
Mars has lost an appreciable amount of its atmosphere over time. For
example, the elevated ^15^N/^14^N ratio in Mars' atmosphere, 1.7 times
the ratio on Earth, shows that Mars has lost substantial amounts of N~2~
(Fox, 1993b). The exact amount is model dependent, but it is probably of
the order of 90 percent or more of Mars' original inventory (Jakosky and
Phillips, 2001). Similar heavy isotope enrichments are seen for H, C, O,
and non-radiogenic Ar. (ii) Theoretical models also suggest that Mars
has lost substantial amounts of atmosphere over time.

Some models suggest that the dominant loss process for molecules heavier
than H~2~ is sputtering by solar wind particles (Kass and Yung, 1995;
Luhma*nn, et a*l., 1992). Kass and Yung (1995) concluded that Mars may
have lost as much as 3 bars of CO~2~ by this sputtering mechanism during
the course of its history. This prediction is consistent with the
observed enrichments in heavy isotopes, as well as with the need for a
greenhouse gas-rich atmosphere although, as pointed out above, an
enhanced CO~2~ concentration cannot by itself explain how Mars' climate
could have been warm early in its history. Other processes such as
ionosphere initiated outflow (*Ma, et a*l., 2004) and ion-neutral
chemistry processes (Fox, 1993b) yield similar results.

Collaborative studies of Phobos and Mars Global Surveyor (MGS) data have
demonstrated that there is a thin, sharp boundary at ≈ 1.2 Mars radii
(similar in shape, but not in physics, to a magnetopause), where the
solar wind proton flux terminates and is replaced by planetary heavy
ions (Na*gy, et a*l., 2004). Thus, the removal of atmospheric components
is not via direct sputtering, but rather by a mass loading process.

In any case, all of the models for how the Martian atmosphere evolved
are at best poorly constrained by the available data. Based on the above
discussion, better answers are needed to the important questions: is
there a good reason to believe that CH~4~ was present in the early
Martian atmosphere, what processes have contributed to atmospheric
escape over Mars' history, and how much of the original volatile
inventory has been lost in this manner? These questions can be
addressed, in part, by making detailed measurements of Mars' present
atmosphere. The MSTO orbiter is ideally suited to examine both issues. A
high-resolution spectrometer could be used not only to determine
definitively whether CH~4~ is present in Mars' lower atmosphere, but
also to map its vertical profile and spatial distribution. The vertical
profile would provide a valuable test of current models of the chemistry
of the atmosphere. And from an elliptical orbit MSTO could be used to
make detailed measurements of Mars' upper atmosphere and ionosphere and
study their interaction with the solar wind. This will lead to a much
better understanding of current atmospheric escape processes and allow a
more reliable extrapolation back into Mars' past.

## Atmospheric composition: implications for extant processes

It has long been understood that the presence of life on a planet would
modify the atmosphere in such a fashion that this \"disequilibrium\"
condition could be detected by remote sensing. Moreover, active
abiogenic geological processes also will modify the environment in which
these processes exist.

Again, the earliest discussions about detection of life through remote
sensing identified methane as one example of a biosignature, say, in the
Mars atmosphere, where methane would not otherwise be present simply due
to standard atmospheric photochemistry. So the presence of methane in
the current atmosphere would be highly suggestive of extant active
processes\--geological or even biological, and most probably in the
subsurface. One line of reasoning that leads to these conclusions is
that the atmospheric lifetime is short, \~400 years, so there is no
chance that the existing atmospheric methane is an atmospheric remnant
from billions of years ago.

In fact, if active subsurface processes do exist in the current epoch,
there may be other chemical constituents in the atmosphere, albeit with
very small abundances, that may arise from processes that produce
methane or from other active geological or biological processes. Thus a
sensitive survey of the chemical composition of the Mars atmosphere will
provide important insights into the extent of active processes that
exist in the Martian subsurface and the character of these processes.
These observations should include the isotopic variants of methane and
possible species that would be co-generated in potential methanogenic
processes to seek an identification of the particular process forming
methane.

A broad understanding of the atmospheric chemistry of Mars is important
for distinguishing true \"disequilibrium\" chemical species. In
addition, the chemistry of the atmosphere controls the lifetime of
potential species that can be introduced into the atmosphere by
subsurface active processes. While the recent observations, at the
limits of their sensitivity, suggest that spatial and temporal
variability in methane has been detected, the current understanding of
the methane lifetime and the magnitude of the dynamical atmospheric
circulation suggests that methane should be well-mixed. If methane were
found to be not well-mixed, there would be significant implications for
atmospheric chemical processes that are not known at present (this is
discussed further in the following section).

In addition to trying to understand the character of subsurface active
processes forming the disequilibrium chemical species that may be
detected in the atmosphere, it is equally important to identify specific
locations on the surface where these species enter the atmosphere. Such
locations would be important sites for further exploration, and may
offer the best locales for future sample return. Depending on whether
methane is well-mixed in the atmosphere or not, methane itself may not
be the best tracer to identify these \"oases\" of subsurface activity.
Other disequilibrium species with shorter atmospheric lifetimes may form
distinct emitted plumes and therefore be easier to trace back to the
source locale. Knowledge of the chemical lifetimes of these tracer
species and the atmospheric circulation are key elements in localizing
potential source regions.

## Major Questions

The MSTO SAG identified several major science questions related to the
escape of the atmosphere over time, the detailed composition of the
lower atmosphere, the spatial variation of the composition, and what
this reveals in terms of its interaction with the near-surface
environment.

(Q1) Could the present atmospheric escape processes currently occurring
at Mars account for climatologically significant amounts of loss of
atmosphere, particularly CO~2~ and H~2~O, in the early history of the
planet - i.e., after the era of hydrodynamic escape and after impact
erosion ceased to be important?

(Q2) Could liquid water have been present on the surface of Mars at that
time?

(Q3) Do present day values for the nitrogen and argon isotopes in the
upper atmosphere, when compared with their surface values, suggest a
loss of atmosphere consistent with the CO~2~ and H~2~O escape? Are the
mechanisms responsible for atmospheric erosion consistently manifested
through all of the volatile species, C,H,O,N and Ar?

(Q4) What is the present day inventory of molecular species in the lower
atmosphere?

(Q5) Are the relative abundances of these species consistent with
contemporary models of Mars atmospheric chemistry, including the effects
of dust and aerosols?

(Q6) What are the dynamic processes (winds) that distribute and mix the
constituents on short time scales?

(Q7) Is there a clear correlation between the spatial variability of
trace species and identifiable surface or subsurface features?

(Q8) Are CH~4~ and its close derivatives present in the boundary layer
atmosphere?

(Q9) If so, do the abundance patterns of these species, and their
isotopic compositions, provide unambiguous discrimination between
geological and biological sources?

## Science Objectives

From the science questions detailed above, the MSTO SAG identified the
following prioritized science objectives for further analysis:

(O-1) Characterize the present day escape of the atmosphere in
sufficient detail, and over a period sufficiently long to account for
changes induced by seasonal and solar cycle variability, to allow
realistic backwards extrapolation of the atmospheric composition.

(O-2) Determine the relative abundances of the major, minor and trace
constituents of the lower atmosphere and their isotopic components to
provide a definitive inventory of species, in order to better understand
the current interaction between the atmosphere and the near surface
environment and, in particular, to determine whether spatially
non-uniform species are in chemical equilibrium.

(O-3) Characterize the winds, both zonal and meridional, with good
altitude resolution, and their seasonal variations.

(O-4) Map the crustal magnetic field with approximately 100 km
resolution. These data are required in part to aid in elucidating the
mechanisms contributing to atmospheric escape; they are also of
importance in advancing our understanding of the surface mineralogy and
evolution of the planetary interior.

Table 1 lists the relationship between the science goals and the
recommendations of MEPAG. The science objectives developed in this
report are also in alignment with the Decadal Study recommendations
related to investigations of the evolution of the Mars atmosphere.

# Exosphere and Atmospheric Escape 

## The Near-Space Environment

### Boundaries

To answer questions concerning the erosion of the Martian atmosphere
over time, we must have a detailed understanding of the physical
mechanisms that are involved, their variability, and the causes of this
variability.

It was realized early on that Mars' ionosphere is too thin to routinely
stand off the solar wind. Thus the effectiveness of the solar wind in
scavenging the upper atmosphere hinged on the question of whether or not
Mars possesses an Earth-like, dynamo-generated, planetary magnetic field
to protect the atmosphere from erosion by the solar wind. Only very
recently was it shown conclusively (by Mars Global Surveyor) that Mars
does not posses such a dynamo field. However, large remanent magnetic
fields were detected in localized regions on the planet, indicating that
a dynamo field did exist, probably until some 4 billion years ago
(Acu*na, et a*l., 1999; Acu*na, et a*l., 1998; Connern*ey, et a*l.,
2004). The magnetic remanents, larger than any seen on Earth, add a
dynamic character to the interaction as they rotate into view of the
solar wind every Mars rotation, periodically adding magnetic pressure to
help stand off the solar wind. They also indicate that during the early
history of Mars a global magnetic field of significant strength must
have influenced solar wind interaction-related loss processes.

Measurements by the Mars Global Surveyor (MGS) and by the Phobos-2
spacecraft led to a significant improvement in our knowledge of the
regions and boundaries surrounding Mars (e.g., Winterhalt*er, et a*l.,
2004). Figure 1, based primarily on recent insights gained from analyses
of MGS and Phobos-2 data, depicts our current understanding of the Mars
near-space environment. The main signatures include a bow shock which
reflects the size of the Martian obstacle to the solar wind, a
magnetosheath wherein the solar wind picks up planetary ions in a
process known as mass-loading, and variously named features (such as
\"Magnetic Pileup Boundary\") of the solar wind boundary (see Figure 2).

The boundary layer of the solar wind interaction, at altitudes between
the exobase (\~200 km) and the middle magnetosheath (\~ 0.5 Rm) is a key
region for characterizing the physics of solar wind interaction-related
atmosphere escape. Neither Phobos-2 nor Mars Express has had the orbit
or measurements to achieve this characterization. MGS did not carry
thermal ion and electron instruments, and so it is impossible to
determine whether the boundary layer at sites removed from the strong
remanent magnetic fields is an \"ionopause\" in the sense of that
observed at Venus on PVO. It has long been known from Viking mission
measurements, that the ionospheric thermal pressure at Mars is
frequently insufficient to balance the total incident solar wind
pressure (e.g. Zha*ng, et a*l., 1991). Thus it is expected that the
Martian ionosphere is magnetized, much like Venus\' ionosphere during
times of high solar wind dynamic pressure and/or low fluxes of ionizing
solar radiation. The Viking Landers did not carry magnetometers, but the
MGS magnetometer supports this expectation down to the altitudes sampled
by that orbiter during its aerobraking phase (Acu*na, et a*l., 1998).
The presence of this magnetization is an essential factor in the erosion
of the upper ionosphere by the solar wind interaction. Locally, crustal
magnetic fields vastly complicate the topology of the boundary layer,
and thus ionosphere escape, in as yet undetermined ways (e.g. Bra*in, et
a*l., 2005; Mitche*ll, et a*l., 2001).

Both Phobos-2 and Mars Express established the existence of significant
present-day escape of atmospheric constituents from the boundary layer
in the form of ions observed in Mars' solar wind wake (Lundin and al.,
1990, 1993; Zakharov, 1992; Verig*in, et a*l., 1991). However, the total
flux, composition, and solar cycle variability of this escaping ion
population is only roughly estimated, as is the pickup ion influx that
sputters neutral atmosphere constituents at the exobase rather than
escaping (Luhma*nn, et a*l., 1992; Luhmann and Kozyra, 1991). The lack
of a magnetometer and other supporting contextual information on Mars
Express seriously compromises that mission's ability to obtain
definitive results from its continuing ion escape measurements.

The solar wind pressure varies over the solar cycle, as does the upper
atmosphere and ionosphere-controlling solar EUV flux, providing a
changing view of the solar wind interaction that can be used to
extrapolate related atmosphere escape into the past. As a whole, the
measurements from the Viking, Phobos, MGS and Mars Express missions hint
at the complexity and potential importance of the solar wind interaction
with Mars in atmosphere escape, but no mission has had the necessary
instrument complement, orbital coverage, and duration to sufficiently
characterize this aspect of the evolution of the Martian atmosphere.

### Escape Mechanisms

Mass loading and ion pickup are intimately related by the transfer of
solar wind momentum and energy to planetary ions by the convective
electric field, their common underlying physical cause. Mass loading is
a manifestation of the replacement of solar wind protons by planetary
ions or of the addition of significant masses of planetary ions to the
solar wind. Ion pickup is the energization of planetary ions by the
penetrating solar wind convection electric field, which can occur with
or without the local penetration of the solar wind protons themselves.
Whether one refers to mass loading or ion pickup, the end result can be
removal of planetary ions if enough solar wind energy is transferred to
accelerate them beyond the escape velocity of \~5 km/s. How the solar
wind-related erosion varies with time, and on what external and/or
internal parameters the variations depend, is unclear. Measurements of
the boundary positions, and the escaping particle composition and fluxes
are crucial aspects in the understanding of the impacts of the
atmospheric erosion by the solar wind.

In addition to mass loading /ion pickup, there are other escape
processes that are occurring at Mars, but have not been measured
sufficiently to allow their overall influences to be determined. These
include: photochemical escape; sputtering (by pick-up ions); possible
other ion outflows in the wake and from the crustal field cusps; and
bulk ionospheric plasma removal.

Photochemical escape relies on ionospheric photochemistry to generate
suprathermal neutral populations above the escape velocity (e.g. by
dissociative recombination). Nagy, *et al.* (1990) and Zhang, *et al.*
(1993) estimated photochemical escape rates for Mars neutral oxygen
based on the few Viking Lander altitude profiles of ionospheric
composition. The observation of the hot oxygen corona of Mars (e.g.
Paxton and Anderson, 1991), inferred from UV spectrometer measurements
on Mariner 9, provides one way to study photochemical escape rates for
oxygen by modeling the observed neutral exosphere altitude profile.
Other photochemically escaping constituents, such as carbon, can also be
indirectly detected by this method (K*im, et a*l., 1998), which remains
unexploited. The sputtering of the neutral upper atmosphere at the
exobase level by impacting atmospheric pickup ions, which models suggest
is significant for atmosphere escape over time (Kass and Yung, 1995;
Luhma*nn, et a*l., 1992) has yet to be established by any observation.
It is difficult to observe the low energy sputtered neutrals except by
indirect means***.*** In addition, poorly characterized upper atmosphere
properties play a significant role by determining what species are
available to be ionized and removed. Nevertheless, impacting pickup ions
can be detected by an ion spectrometer at the appropriate location, and
upper atmosphere neutral particle density profiles can show evidence of
some nonthermal heating by impacting pickup ions. In a similar manner
model calculations (e.g. *Ma, et a*l., 2004) indicate that ionospheric
plasma outflow through the tail may be another escape channel, but no
measurements exist to either confirm or contradict these suggestions.
Study of special ion outflows that may be associated with the wake
structure or with crustal magnetic fields are a subject of current
analysis in the Mars Express community, but the lack of local magnetic
field measurements represents major difficulties to those efforts.
Similarly, the idea of bulk ionospheric plasma removal processes, such
as proposed shear instabilities in the solar wind/ionosphere boundary
layer (Pe*nz, et a*l., 2005), also remain to be tested observationally.

[Photochemical escape]{.underline}

Various investigators have modeled photochemical escape rates of heavy
atoms from Mars, such as N, O, and C (e.g., Fox, 1993a, 1993b; Fox and
Bakalian, 2001; Fox and Hac, 1997a, 1997b, 1999; Hodges, 2000; Jakos*ky,
et a*l., 1994; K*im, et a*l., 1998; McElr*oy, et a*l., 1977; Na*gy, et
a*l., 2001). Planetary escape processes have been reviewed, for example
by Chamberlain and Hunten (1987), Shizgal and Arkos (1996), and Hunten
(2002).

The ^15^N/^14^N ratio in the Martian atmosphere was measured as about
1.62 times the terrestrial value (Nier and McElroy, 1976, 1977). The
enhancement is presumably due to preferential escape of ^14^N. There are
several sources of escaping nitrogen, including photodissociation
(Brinkmann, 1971), photodissociative ionization and electron impact
dissociative ionization of N~2~, dissociative recombination of N~2~^+^,
and the reaction of N~2~^+^ with O. Quantifying the sources involving
N~2~ requires *in situ* measurements of the number density profiles of
N~2~ as a function of altitude in the thermosphere from about 100 km to
350 km at both low and high solar activities, and as a function of solar
zenith angle and season. The only solid information we have about
thermospheric density profiles of N~2~ comprises the measurements made
by the Vikings 1 and 2 entry probe neutral mass spectrometers, each in
one pass at one location, one solar zenith angle, and low solar activity
through the atmosphere. The altitude profiles cannot be simply
determined from the measured N~2~ mixing ratio of about 2.5 %, because
the vertical distribution depends on the diffusion coefficients, the
strength of mixing due to turbulence and convection. Usually this
distribution is modeled in 1D atmospheres with the use of an eddy
diffusion coefficient. The MTGCM's of Bougher (2000) and Bougher, *et
al.* (2004b) do not contain N~2~ as a separate species, and the altitude
profiles derived from these models are therefore only estimates.

For the contribution to escape of N from dissociative recombination of
N~2~^+^, density profiles of the ions and electrons are required from
below the F1 peak to altitudes above the exobase. Estimates of the O
densities over a small altitude range have been derived from the Viking
RPA measurements of CO~2~^+^ and O~2~^+^ at low solar activity. There is
no direct information about the density profiles of either N~2~^+^ or
any of the other minor ions, although several models have been
constructed of many major and minor ions (e.g., Fox, 1993a, 1993b, 2004;
Krasnopolsky, 2002), which have allowed estimates to be made of the
source due to dissociative recombination.

Quantifying the important source of escaping N due to reaction of
N~2~^+^ with O requires knowledge also of the density profiles of O.
There is no direct information about either the mixing ratio of O or the
altitude profiles. Estimates over a small altitude range at one altitude
have been made based on the low solar activity Viking 1 O~2~^+^/CO~2~^+^
peak ratio (Fox and Dalgarno, 1979; Hans*on, et a*l., 1977), and from
remote sensing of the moderate solar activity Mariner 9 1304A emission
(Stewa*rt, et a*l., 1992). The inferred mixing ratios range by about a
factor of 3 from about 0.7 % to 2% at 130 km. There is no *in situ*
information about the high solar activity mixing ratios or density
profiles. Experience with Venus tells us that there may be large
variations in the photochemically produced species, such as O, CO, C and
N. This is a major uncertainty in important quantities, which requires
*in situ* measurement by an (open source) mass spectrometer capable of
measuring O, C, and N.

The most recent calculations by Fox and Hac (1997b) have shown that the
presence of a dense, early atmosphere of 100 to 500 mb at 3.8 Gyr before
present reduces the isotopic fractionation of nitrogen to the observed
value. Thus quantifying the present escape rates of N is necessary to
estimate the escape rates as a function of time before present, and thus
to determine whether a dense, early atmosphere is needed to reduce the
isotope fractionation.

The largest source of photochemical escape of C is photodissociation of
CO, followed by dissociative recombination of CO^+^ Other sources of
escaping C have been found to be negligible compared to these two
mechanisms (Fox and Bakalian, 2001; Na*gy, et a*l., 2001). Quantifying
the sources of these mechanisms requires knowledge of the density
profiles of CO and CO^+^, the total electron density, and their
variations with location, solar zenith angle, season and solar activity.

The Viking 1 neutral mass spectrometer measured a CO mixing ratio of
about 0.42% at 120 km, and a density profile over the lower thermosphere
for \~45 degrees solar zenith angle and low solar activity. The only
measurements available are those from the Vikings 1 and 2 descent
modules, at only two locations and for low solar activity. Models have
been constructed by, for example, Krasnopolsky (1993) of the CO profile
in the lower thermosphere. There is no direct information available
about the solar activity variation of CO. The CO airglow emissions arise
from both CO and CO~2~, and so are not indicative of the CO mixing
ratio, with the possible exception of the (14v) series of the CO fourth
positive bands, which are produced by scattering of Lyman alpha by CO.
The CO variations have been computed by the MTGCM of Bougher (2000), but
high accuracy is not expected from this model.

Altitude profiles of CO^+^ have not been measured, but they have been
modeled by several investigators (e.g., Fox, 1993a, 1997, 2004). The
model altitude profiles of CO+ should be verified by in situ spacecraft
measurements from high altitudes down to the F1 and E peaks (\~130 and
\~105 km at low solar activity). The altitudes of the peaks are
predicted to be higher at high solar activity than at low solar activity
({Fox, 1995 #144; F*ox, et a*l., 1995; Krasnopolsky, 2002). As it is
now, it is not possible to distinguish the models, since there are no
measurements of minor ions in the Martian ionosphere, other than the
O^+^ "measurements" from the Viking RPA (e.g., Hans*on, et a*l., 1977).

Quantifying the present escape rates of O is necessary for determining
the history of water on Mars. The sources of photochemical O escape
include dissociative recombination of O2^+^, and charge transfer of H^+^
to O in the Martian exosphere. The major source is dissociative
recombination, determination of which source requires measurements of
the density profiles of O~2~^+^ and electrons. Information from the RPA
on Viking and models suggests that, since O~2~^+^ is the major ion, its
density profile is close to that of the electrons. Dissociative
recombination reactions tend to produce very energetic neutrals and
their rate coefficients are dependent on the electron temperatures.
Generally the dissociative recombination coefficients are of the form
*k=A* (300/Te) ^(0.2-0.7)^. Thus determining the magnitude of the escape
fluxes due to dissociative recombination requires knowledge of the
altitude profiles of the electron temperature, and their variation with
solar zenith angle and solar activity, for which comprehensive (e.g.
time, season, solar cycle variations) information has not been available
from either measurements or models. Estimates from the Viking RPA
indicate values of Te of 2000-3000 K above 200 km, at one solar zenith
angle and low solar activity. The values below 200 km have only been
modeled. Modeling the Viking measured electron and ion temperature
profiles has required imposing an *ad hoc* energy source at the top of
the atmosphere, the origin of which is unknown. The ion temperature is
also important in determining the high altitude density profiles of
ions, and the rate coefficients for some ion-molecule reactions. An
instrument that is capable of measuring the plasma temperatures and
their variation with solar zenith angle, local time, season and solar
activity, such as an RPA, would be necessary. Such an instrument would
also be important for determining the source of energy at high altitudes
that is necessary to model the plasma temperatures.

The computed values of the O escape rate have been compared to the
thermal escape rate of McElroy (1972). McElroy *et al.* (1977) suggested
that H and O escape in the stoichiometric proportions of water, (2:1).
If this is true it implies that the oxidation state of the atmosphere
has remained unchanged over time. Hydrogen escapes thermally mostly as H
with a small additional component due to H~2~ escape. There are also
some non-thermal mechanisms for H escape, such as the charge transfer
reaction of H^+^ with atomic H in the thermosphere and corona. These
non-thermal escape rates are more important for D than for H. Recently,
many investigators have modeled the rate of O escape due to dissociative
recombination and found that the rates are too small to balance the H
escape rate, and have thus questioned this "balanced escape hypothesis".
(We note here that this conclusion has been debated by Hodges (2000).
Thus it is possible that the atmosphere of Mars may have been more
reducing in the past, leading to larger densities of H~2~ in the
atmosphere. Fox (2003) modeled the ionosphere of Mars, for various H~2~
mixing ratios from 4 to 100 ppm. Since H~2~ reacts with N~2~^+^, CO^+^,
CO~2~^+^ and O^+^, the nonthermal escape rates of heavy atoms arising
from these species will be greatly reduced as the mixing ratio of H~2~
increases, i.e. for a more reducing atmosphere. The implications of a
more reducing atmosphere for escape rates of C and N due to dissociative
recombination of CO^+^ and N~2~^+^ are that the escape rates are reduced
by a factor of \~6 and 2.3, respectively, as the H~2~ mixing ratio in
the thermosphere increases from 10 ppm, approximately the current value
(Krasnopolsky and Feldman, 2001) to 100 ppm. The escape rates of N and C
due to photodissociation of N~2~ and CO are reduced by factors of 2-3 as
the H~2~ mixing ratio increases from 10 to 100 ppm. Since the modeled
rates of N escape seem to be too large at present, and integrating
backwards in time actually over-fractionates the N isotopes, a more
reducing state of the atmosphere in the past would decrease the escape
rates and provide a possible other or additional mechanism to that of a
dense early atmosphere for inhibiting the escape of Nitrogen.

In summary, quantifying the photochemical escape rates of heavy atoms
and the thermal escape rates of H requires *in situ* measurements of
both major and minor ions, and major and minor neutral species. The mass
range required is from 1 to 44 amu for both ions and neutrals. Also
required are the neutral, ion and electron temperatures as a function of
altitude. The escape rates are expected to be dominated by those at high
solar activity, where we have essentially no *in situ* data at present.
Thus measurements at high solar activity are essential. The only *in
situ* data we have at low solar activity is from the neutral mass
spectrometers and the retarding potential analyzers of Vikings 1 and 2,
which sampled the atmosphere at only two locations, and at low solar
activity. We have no data on the minor ion profiles, since the Viking
RPA could measure only O~2~^+^, CO~2~^+^ and O^+^.

[Ion Escape]{.underline}

The escape of ions is by traditional ion-pickup processes (McElr*oy, et
a*l., 1977 Luhmann, 1990), and by ion outflows (*Ma, et a*l., 2004). The
computed rates of traditional ion pickup are computed as the rates of
ionization of the atoms above the ionopause, followed by "pickup" by the
convection electric field of the flowing solar wind plasma. Fox (1997)
suggested, however, that theoretically all the ions above the
photochemical equilibrium region could be stripped away, rather than
only those created above the ionopause. This theory has been confirmed
by MHD (*Ma, et a*l., 2002; *Ma, et a*l., 2004) models in which the
computed escape rates are more similar to those computed by Fox than to
those computed as traditional ion pickup. On Mars, the major ion flowing
away from the planet is predicted to be O~2~^+^ (Fox, 1997), whereas the
main ion observed in the planetary wake is O^+^ (e.g., Lund*in, et a*l.,
1989). The MHD model of Ma, *et al.* (2004) suggested the major escaping
ion is O^+^ rather than O~2~^+^, but their model ion profiles showed an
F2 peak of O^+^, while other models have shown that O~2~^+^ is the major
ion over the whole ionosphere. Their model was computed with *ad hoc*
plasma temperature profiles designed to produce an F2 peak of O^+^ at
high altitudes. This is yet another reason for measuring the values of
Te and Ti in the ionosphere at several values of the solar zenith angle
and solar activity.

The escape of species as ions has the potential to be more important
than those of the photochemical processes. For example, the predicted
O^+^ escape rates are larger than the H nonthermal escape rates, and can
more than make up for the smaller predicted values of the photochemical
escape of O. H atoms will escape also as H^+^. Quantifying the escape
rates of ions requires density profiles of minor ions from 1 amu (H^+^)
to 44 amu (CO~2~^+^).

The escape rates of ions can be estimated by determining the scale
heights of the ions above the photochemical equilibrium region. This was
done for O~2~^+^ by Chen *et al.* (1978) and by Fox (1997). The altitude
profiles could only be fitted if an upward flux or velocity boundary
condition were imposed on the ions. Determining the scale height
compared to that expected for zero flux requires altitude profiles of
ions from below the peaks, i.e., from about 110 km to 350 km.
Measurements that would provide only high altitude scale heights are of
little value for the determination of escape rates of ions because
comparisons to models with zero flux upper boundary conditions would not
be possible without simultaneous measurements of the peaks. It should be
noted here that an upward flux boundary condition in a one-dimensional
model probably actually represents the divergence of the horizontal
fluxes of ions (Shinagawa and Cravens, 1989).

In summary, determining the rates of escape of ions requires *in situ*
measurements of major and minor ions from high altitudes, i.e., in the
solar wind, down to below the F1 peaks (near 110 to 140 km), as well as
measuring the fluxes of ions in the wake of the planet.

## Crustal Magnetic Fields

Present day escape processes are certainly affected by the Martian
crustal fields, in the sense that some of the upper atmosphere is
shielded from direct interaction with the solar wind and the associated
direct planetary ion scavenging. However, recent observations by Mars
Express suggest that there are some additional ion escape processes that
exist *because* of the presence of the crustal fields. It has yet to be
determined whether the associated additional losses are significant
compared to the general upper atmosphere/ionosphere removal related to
unmagnetized planet solar wind interaction physics.

Specifically, Mars Express UV observations showed evidence of aurora
occurring in the vicinity of the strong crustal field regions, which was
henceforth backed up by observations on both Mars Express and MGS of
accelerated electrons moving down the field lines into the crustal field
region-dominated upper atmosphere. These electrons probably excite the
observed UV emissions. Associated with the inflowing electrons, Lundin
et al. report (in Science, to appear shortly) outflowing \"auroral ion
beams\", and make an analogy to outflowing (oxygen) ion beams in
Earth\'s auroral zones that occur in response to electron precipitation.
Whether the ion fluxes in these beams at Mars constitute a major escape
route for Mars upper atmosphere in the form of heavy ions is not yet
determined, but it certainly indicates that understanding the magnetic
context of the observed escaping ions is important for diagnosing the
operating physical mechanisms.

Would deep observations of the Mars magnetic field make a major
difference to the above diagnosis? Certainly local (to the ion escape
sites) measurements of the magnetic field context are crucial to
understanding the nature of observed ion escape. It is important to
determine whether the crustal fields are involved and how interplanetary
conditions make a difference with or without the crustal field
involvement. Deeper atmosphere observations are only critical in this
respect if it is determined that auroral outflow mechanisms are
significant for overall planetary ion escape. Such measurements could
verify assumptions (from existing crustal field models) of the field
strength where those ion outflows or other crustal field-associated ion
outflows may be accelerated - below the region of existing spacecraft
measurements of the fields. Their importance to present ion escape is
thus dependent on how important the crustal field-related outflows turn
out to be. To some extent this will be further pinned down by Mars
Express ASPERA observations and their analysis. Within the time frame of
the next six months the general answer may be known. Where the deep
magnetic field measurements are most definitely important is in further
constraining the early history of the Martian magnetic field -
especially during the first 1.5 billion years of Mars\' history when
solar conditions should have greatly enhanced all present-day ion
scavenging-related processes.

## Upper Atmosphere Winds

A knowledge of the thermospheric winds (over the solar cycle and
seasons, and spanning the globe) is of fundamental importance to our
understanding of the Mars thermosphere-ionosphere system, and its
coupling with the Mars lower atmosphere. The Mars global thermospheric
circulation is thought to significantly modify the thermal and density
structure about the planet (e.g. Bough*er, et a*l., 1999, 2000;
Bough*er, et a*l., 1990). Strong horizontal (\~100-300 m/sec) and
vertical (\~1-10 m/sec) winds are estimated, especially during the
solstices, providing adiabatic heating/cooling and horizontal advection.
Observed winter polar warming is attributed to this inter-hemispheric
circulation (Bough*er, et a*l., 2006; Keati*ng, et a*l., 2003), and its
seasonal variations owing to changing solar and aerosol heating. The
distributions of light species (e.g. O, N, H, H~2~, He) are surely
controlled by the thermospheric circulation, resulting in winter polar
night bulges of these species and corresponding nightglow emissions
(e.g. Berta*ux, et a*l., 2005). Finally, dynamical coupling with the
Mars lower atmosphere, especially during dust storm events, helps
regulate the thermospheric structure and dynamics. These winds also
serve as a background through which upward propagating thermal tides
must propagate, on their way to driving variations in the Mars lower
thermosphere (\~100-130 km) which affect aerobraking operations (e.g.
Forb*es, et a*l., 2002; Wilson, 2002; Withe*rs, et a*l., 2003). In
short, a comprehensive characterization of the Mars upper atmosphere
must include neutral wind measurements in order to understand the
processes that regulate density, temperature and airglow distributions,
and their variations over the solar cycle and Martian seasons.

Knowledge of Mars thermospheric winds is also key to understanding some
of the processes that control Mars volatile escape. Escape processes are
mediated by the Mars upper atmosphere structure, which in turn is
regulated strongly by the global thermospheric circulation (see above).
For instance, the "thermostatic" effect of the global winds on
temperatures is calculated to regulate solar cycle variations of the
Mars thermospheric densities and temperatures (e.g. Bough*er, et a*l.,
1999, 2000); a reduced variation of dayside topside temperatures (from
that otherwise expected) results as the solar cycle advances.
Photochemical processes (e.g. O~2~^+^ production and recombination) will
vary as the background neutral structure changes. The time variable
upper atmosphere structure (near the exobase) also serves as the seed
population for O^+^ sputtering, driving both atomic and molecular escape
fluxes. Furthermore, the horizontal and vertical winds also have a
direct influence on the actual neutral escape fluxes. Hence, an
understanding of these "present day" dynamical processes and their
impact on volatile escape rates must be quantified before extrapolations
to ancient Mars conditions can be made (Bough*er, et a*l., 2004a). Mars
upper atmosphere wind measurements are thus crucial to evaluating these
dynamical feedbacks upon present day volatile escape rates.

## Recommended Measurements 

For measurements of the escape fluxes from each of these processes,
measurements of the neutral and ionized species listed in Error:
Reference source not found should be made over the altitude range from
above the bowshock to as low as possible - 100 km being suggested as the
desirable minimum altitude, especially during the solar maximum phase of
the mission. The flux measurement should map the space around Mars,
i.e., the dayside, the dawn-dusk regions and down the tail.

In addition, observations of the crustal, upper atmosphere and solar
wind vector magnetic fields, and the ion and electron 3-dimensional
energy distributions in the upper atmosphere and solar wind are
required.

# Mars lower Atmosphere

## Composition

### Inventory of Molecular Species

Measurements which would provide an inventory of the molecular
constituents of the lowest few scale-heights of the atmosphere, i.e.,
from the surface to, say, 100 km altitude, hold the key to understanding
the processes involved in interactions of the boundary layer with the
regolith, to looking for current geological activity and, most important
in the context of NASA\'s exploratory goals, to look for biological
activity in the near-surface environment. The analogy with Earth\'s
stratosphere above 20 km altitude is relevant: the pressure and
temperature are not dissimilar (6 mb, 220 K), and the major radiatively
active gases, H~2~O, CO~2~, O~3~, play similar roles, despite their
considerably smaller relative concentrations on Earth. These gases,
together with nitrogen oxides, give rise to a rich variety of transient,
reservoir and sink molecules which, in turn, provide a clear indication
of processes occurring at lower altitudes and at the surface. A
spectroscopic survey of the Earth stratosphere, which set out initially
to acquire an inventory of molecular constituents at the parts per
billion level, and to define the spectral radiative environment of this
region of the atmosphere - the Shuttle-based ATMOS experiment - revealed
the presence of more than 40 different molecular species and their
isotopologues, many of which were unexpected new detections (e.g.,
ClNO~3~, N~2~O~5~, CH~3~Cl, HNO~4~, C~2~H~2~, C~2~H~6~, SF~6~; (see
Farmer, 1987; Irion, 2002). In the case of Mars, it would be surprising
not to find an even richer inventory of constituents resulting from
contemporary geological activity, from catalytic cycles enhanced by the
surface mineralogy, or from biological processes. Thus measurements of
the spectral environment made at the high resolution achievable today
from orbit could be expected to reveal the presence of such transient
products of the major atmospheric constituents as CH~4~, CH~3~S, CH~3~X,
H~2~CO, NOx, NH~3~, SO~2~, HCN etc. lists current upper limits for a
number of other molecular species of interest for which searches have
been made from both ground based and spacecraft observations.

Whether Mars is biologically or geologically active is one of the most
important questions in the scientific exploration of the planet.
Previous observations of Mars have demonstrated that evidence of life is
neither obvious nor trivial. Nevertheless, if life ever existed on Mars,
it likely exists today, having adapted as the Martian climate evolved.
This assertion is supported by the ever-growing list of extremophile
life forms on Earth that can be found in almost any niche where even
minimally supportive conditions exist. Furthermore, geomorphologic
evidence suggests that Mars may also have been volcanically active in
its recent past.

### Methane

One of the most intriguing questions that will be addressed by MSTO is
whether or not the recent reports of methane in Mars' atmosphere are
correct. Within the past two years, three different groups have claimed
detections of Martian methane (Formisa*no, et a*l., 2004; Krasnopols*ky,
et a*l., 2004; Mum*ma, et a*l., 2003). Two of these measurements (Mumma,
Krasnopolsky) have been made using ground-based telescopes on Earth,
combined with high resolution infrared spectroscopy; the other
(Formisano) was made from ESA's Mars Express orbiter with a relatively
low-resolution spectrometer. All of them searched for absorption in the
3.3-μm absorption band of CH~4~. Krasnopolsky et al. measured a constant
10 ppbv of CH~4~ over the surface, whereas Mumma and Formisano et al.
both reported spatial gradients. Mumma's measurements suggest a CH~4~
concentration that is higher at the equator than at the poles, although
these past conclusions are currently (2/23/06) being tested by a new
series of measurements (private communication, M.Mumma). Formisano et
al. claim orbit-to-orbit CH~4~ variations that suggest longitudinal
gradients in CH~4~ concentration.

#### Implied CH~4~ source

All of these measurements have been carefully made and analyzed;
however, they are all close to or perhaps beyond, the limiting
detectivity of CH~4~ of their respective instruments. If they are
correct, the measurements may have far-reaching implications for the
nature of the Martian surface environment and possibly for astrobiology.
Consider first what is implied by a constant 10-ppbv concentration of
methane. The Martian surface pressure is about 6 mbar, and Martian
gravity is 3.73 m/s^2^, so the mass of a vertical column of atmosphere
is about 16 g/cm^2^. The corresponding column density, assuming nearly
pure CO~2~, is 2.2×10^23^ molecules/cm^2^. (Henceforth, "molecules" will
be eliminated from the units, in accordance with standard atmospheric
chemists' practice.) Hence, a 10-ppbv concentration of CH~4~ corresponds
to a column density, *N~col~*, of 2.2×10^15^ cm^-2^. The photochemical
lifetime, *τ*, of methane in Mars' atmosphere is about 340 years
(Krasnopols*ky, et a*l., 2004), or \~10^10^ s. Since CH~4~ has no
appreciable photochemical sources in the Martian atmosphere, its
concentration must be maintained by a surface flux of *N~col~*/*τ* =
2.2×10^5^ cm^-2^s^-1^. By comparison, Earth's (mostly biological)
methane flux is 535 Tg(CH~4~)/yr (Wats*on, et a*l., 1990), or
1.24×10^11^ cm^-2^s^-1^. The required Martian methane flux is thus about
2×10^-6^ times the biological methane flux on Earth.

This immediately raises the question: Could such a CH~4~ flux on Mars be
maintained abiotically, or would it necessitate a biological source?
Again, we can turn to Earth for an analogy. Could volcanism provide this
amount of methane? At first glance, it seems not. CH~4~ is neither a
predicted component of surface volcanic gases, nor one that is observed
(Holland, 1984). The magmas from which such gases are released on Earth
are relatively oxidized; they have an oxygen fugacity near the QFM
(quartz-fayalite-magnetite) synthetic buffer. Furthermore, the high
temperatures associated with surface melts, \~1200^o^C (*ibid.*), favor
molecules that are smaller than the 5-atom CH~4~ molecule. Both of these
factors conspire against emission of CH~4~ from surface volcanism.

#### CH~4~ from serpentinization reactions at midocean ridges

CH~4~ may be released, however, by submarine volcanic outgassing or by
related submarine processes. Substantial quantities of CH~4~, \~ 2
mmol/kg, are observed in vent fluids emanating from the off-axis Lost
City vent field on the Mid-Atlantic ridge (Kell*ey, et a*l., 2001;
Kell*ey, et a*l., 2005). (The original measurements of Kell*ey, et a*l.,
2001, were subsequently revised upwards by a factor of 10 by Kell*ey, et
a*l., 2005) Kelley et al. originally suggested that this CH~4~ was
produced by *serpentinization* of peridotite within the hydrothermal
circulation cells deep beneath the seafloor. Such reactions have been
studied in the laboratory (*e.g.*, Bern*dt, et a*l., 1996). Warm water
in contact with ultramafic (Fe- and Mg-rich) rocks will react with the
rock to produce various serpentine minerals. These minerals exclude
iron; thus, the iron in the rock has to find another stable phase. Over
a wide range of oxygen fugacities the stable oxide phase is magnetite,
Fe~3~O~4~. This may be considered a combination of one ferrous ion,
Fe^++^ or FeO, and two ferric ions, Fe^+3^ or Fe~2~O~3~. By contrast,
the iron in the original rock was entirely ferrous iron. Hence, for each
mole of magnetite that is produced, two moles of iron are oxidized. In
order to achieve redox balance, something else must be reduced. If H~2~O
itself is the oxidant, then H~2~ is released. If the water contains
dissolved CO~2~, then CH~4~ can be produced (Bern*dt, et a*l., 1996).
So, serpentinization is one possible source for the CH~4~ observed in
the Lost City vent fluids. Indeed, this interpretation is supported by
D/H measurements of Lost City CH~4~ and H~2~, which suggest that both
compounds are being produced by relatively high-temperature (\>110^o^C),
abiotic processes (Proskurowski et al., in press). There are, however,
methanogens (methane-producing bacteria, or Archaea) living within the
hydrothermal circulation systems. The presence of Archaeal biomarkers
within the vent fluids suggests that at least some of the methane may be
biogenic (Kell*ey, et a*l., 2005; J. Hayes, priv. comm..). Full
resolution of this question awaits C isotope measurements of a suite of
higher hydrocarbons that are also observed within the vent fluids. We
return below to the question of how carbon isotopes can be used to
discriminate between biogenic and abiogenic CH~4~ sources.

Suppose for the moment that the methane emanating from the Lost City
vent fields is abiogenic. What would this imply about the global abiotic
CH~4~ flux? Kasting and Catling (2003) (Section 4.4) have used heat flow
arguments to extrapolate the Lost City measurements to the entire globe.
Their estimates used the Kelley *et al.* (2001) CH~4~ values. If the
Lost City vents are representative of other off-axis vents worldwide,
they concluded that the global abiotic CH~4~ flux would be \~1×10^11^
mol/yr, or 1.6 Tg(CH~4~)/yr. This is about 1/300^th^ of the biological
methane flux. Ramping this up to the revised Kelley, *et al.* (2005)
CH~4~ concentrations implies an abiotic CH~4~ flux of 1/30^th^ the
biological CH­~4~ flux. In atmospheric chemists' units, this flux would
be 4×10^9^ cm^-2^s^-1^. This is larger than the flux required to sustain
10 ppbv in the Martian atmosphere by a factor of \~2×10^4^.

#### Possible subsurface sources of Martian methane

What do these arguments imply about the possible source of Martian
methane? Obviously, the answer is unclear. (That is part of why we need
a mission to confirm the CH~4~ measurements.) Liquid water probably is
present in the Martian crust at depths of one to several kilometers
(Clifford, 1993). How much water is present, and how vigorously it
circulates is unknown. Only a small fraction of Earth's midocean ridge
methane source would be required to maintain a CH~4~ concentration of 10
ppbv. The potential for generation of methane is increased by the
apparent low oxygen fugacity of the Martian crust, as indicated by the
composition of SNC meteorites (Wadhwa, 2001). It is conceivable that 10
ppbv of Martian methane could be sustained by serpentinization reactions
deep beneath the Martian surface. It is also conceivable that
methanogens could be living in the underground Martian aquifer and
metabolizing hydrogen produced by serpentinization of ultramafic rock.

#### Distinguishing between biotic and abiotic CH~4~ sources

Recent investigations of the terrestrial sources of methane suggest that
distinguishing between abiotic and biotic sources of Martian methane
would be most successful if based on simultaneous observations of the
carbon and hydrogen isotopic composition of methane and the presence of
higher alkanes. Biogenic methane on Earth is depleted in ^13^C by an
average value of about 45‰ relative to the carbon source. By comparison,
the CH~4~ in the Lost City vent fluids is only 8.8-13.6 ‰ depleted in
^13^C relative to dissolved inorganic carbon in seawater (Kell*ey, et
a*l., 2005). Martian methanogens would presumably be using CO~2~ as the
carbon source, so a simultaneous measurement of the ^13^C/^12^C ratio of
atmospheric CO~2~ and CH~4~ would be a significant contribution to
answering the question of source origin. The observed ranges of the
terrestrial ^13^C/^12^C ratio in CH~4~ (-40 to -115 ‰ for
methanogenesis, and 0 to -60 ‰ for abiotic sources) do overlap to some
extent, so that a ^13^C/^12^C ratio measurement alone becomes ambiguous
at the low depletion end where biotic and abiotic source values overlap
(Horita, 2005). Investigations of sources of terrestrial methane have
found that both biogenic and abiogenic methane can have similar values
of ^13^C depletion (Schoell, 1988 Onstott, 2006 #118}). Thus, an
observation on Mars of a ^13^C/^12^C ratio of -100‰ relative to the
carbon source would be a strong indicator of biotic origin, but one of
only -60‰ would be ambiguous. In this event, additional discrimination
is needed.

Considering D/H values, some measurements of CH~4~ derived from
low-temperature serpentinization show a depletion in deuterium about
double that of biogenically-derived CH~4~ (-400‰ vs -200‰ relative to
mean ocean water). Therefore, since water is the source of the hydrogen
in CH~4~, measurements of the isotopic composition of near-surface water
vapor might yield insights into the subsurface water fractionation and
identification of the methane source.

Measurements of the presence of higher alkanes, *e.g.*, ethane and
propane, provide important additional information. Abiotic synthesis of
organics, whether by serpentinization or by Fischer-Tropsch-type
reactions (Loll*ar, et a*l., 2002) tends to generate the entire sequence
of alkanes, with the abundance decreasing with carbon number.
Methanogenesis starting from CO~2~ (CO~2~ + 4 H~2~ → CH~4~ + 2H~2~O)
generates only methane. If methane is derived from the decomposition of
more complex organic material, higher alkanes are produced as well, but
the abundance typically *increases* with increasing carbon number
(*ibid.*). Hence, a search for ethane and propane assists in separating
different ways of generating methane. The abundance ratio of methane to
simultaneous measurements of ethane, propane, and butane when correlated
with ^13^C/^12^C values shows a clear separation between samples from
extant microbial processes and abiogenic water-rock reactions (Horita et
al.).

#### Connection with past atmospheric conditions 

The possibility that methane is present on Mars is interesting for
another reason. If it is present now, then regardless of whether it is
biogenic or abiogenic, methane is likely to have been more abundant in
the distant past. The addition of methane to a CO~2~-rich
paleoatmosphere could possibly have provided the additional greenhouse
warming needed to bring the average surface temperature closer to the
freezing point of water, and thus to explain the presence of the fluvial
features observed on the ancient, heavily cratered southern highlands of
Mars (Polla*ck, et a*l., 1987). Climate modelers have not yet produced a
self-consistent CH~4~-CO~2~-H~2~O greenhouse for early Mars, but the
possibility of doing so still looms. If we were to verify that methane
exists in the present Martian atmosphere, this would provide additional
motivation for studying its possible effect in the Martian atmosphere of
long ago.

## Circulation

There is a scarcity of observational data sufficient to characterize
Martian atmospheric dynamics. The temperature field has been the only
global measurement used to characterize the circulation (Leovy, 2001).
In fact, only Mars Global Surveyor has really been able to provide more
than sparse profile measurements (Conra*th, et a*l., 2000; Smi*th, et
a*l., 2001) although the main instrument used for this purpose (the
Thermal Emission Spectrometer, TES) was not specifically designed for
that goal.

Within that context, our current vision of the circulation is largely
based on numerical simulations performed using General Circulation
Models (see Forg*et, et a*l., 1999; Haber*le, et a*l., 1993; Hourd*in,
et a*l., 1993; Wilson and Hamilton, 1996). These studies have shown that
the global circulation is characterized by an extended Hadley
circulation modulated and modified by several kinds of waves propagating
through the atmosphere, as illustrated in Figure 3.

Of particular importance are the thermal tides of diurnal or
semi-diurnal period which are excited by the near-surface diurnal cycle
and propagate through the atmosphere with increasing amplitude as the
atmospheric density decreases with altitude (Wilson and Hamilton, 1996;
Zur*ek, et a*l., 1992). The tidal waves are thought to be the primary
phenomenon controlling the dynamics of the atmosphere above 50 km,
although very few observations are available. With regard to comparative
meteorology, the traveling planetary waves observed at high- and
mid-latitudes in winter are of primary interest since they are
counterparts of the mid-latitude low and high pressure weather systems
that control the weather in Europe and North America. Compared to Earth,
one key characteristic of the Mars atmosphere is the vertical extent of
most meteorological phenomena. On Earth, the stratosphere confines the
Hadley cell and most planetary waves to the troposphere, below 20 km. On
Mars, many structures extend vertically up to the thermosphere (120 km),
as suggested by MGS aerobraking density measurements (Keati*ng, et a*l.,
1998).

Of great importance to the specific goals of MSTO is the simultaneous
measurement of the winds and trace gases. Such measurements will allow
us to infer the time and spatial variation of various surface sources
and sinks. Methane is the obvious trace gas of interest, but CO would
also be interesting to measure and there may be others as well.

## recommended Measurements

### Composition

Measurements of the detailed molecular composition of the atmosphere
should be made over the altitude range from the surface to 100 km with
sufficient sensitivity to provide detectivity levels of parts per
billion or less, and at 1/2 scale height (4 - 5 km) vertical resolution.
These measurements should also achieve high enough spatial resolution to
reveal horizontal variability at a scale of 10 km to aid in identifying
localized sources of variable species. The measurements should be of
adequate sensitivity to allow determination of the relative isotopic
abundances ^12^C/^13^C, ^16^O/^17^O/^18^O, H/D and ^32^S/^34^S in
relevant constituent species with sufficient accuracy to permit
unambiguous discrimination between biogenic and abiogenic origins.

High sensitivity (in terms of measurable relative concentrations) in
remote sensing detection and quantification of the trace molecular
constituents of the atmosphere from orbit requires recording high
resolution, near-infrared absorption spectra in the solar occultation
mode. For such observations to be of sufficient sensitivity to allow
\"unambiguous interpretation\", (i.e., identification of species and
quantitative analysis at detection levels well below current upper
limits for the molecular species of interest) the spectral resolution
must be at least one, and preferably two, orders of magnitude higher
than has been employed at Mars to date.

This can be readily illustrated with reference to typical oscillator
strengths for molecules of interest at these wavelengths: For
\"expected\" species of interest (CH~4~, H~2~CO, H~2~S, etc.) the
*strongest* transitions in the rotation-vibration region are \~ 10^-19^
cm^-1^cm^-2^molec^-1^. In an atmosphere with a base pressure of 6mb and
scale height 12 km, the column cross section is \~2x10^23^ molecules
cm^-2^. Consider an instrument with a spectral resolution of 1cm^-1^
(FWHM) and a noise level equivalent to 1% of the background radiance
level. An absorption feature equal to the noise level (i.e., a signal to
noise of 1) would have an equivalent width (integrated absorption) of
10^-2^ cm^-1^. In this case, molecular spectral transitions of the
strongest lines would correspond (for nadir viewing) to uniform mixing
at a relative concentration of 2x10^-7^. Clearly, trace species present
at mixing ratios of parts per billion would not be detectable, and
either the spectral resolution would have to be improved by several
orders or the effective path length increased, or both. For the wide
variety of candidate species under consideration, the required
sensitivity is probably best achieved by exploiting the source intensity
and amplified path length advantages of the solar occultation technique.
The observations should be made in conjunction with nadir-looking
measurements by, for example, a tunable sub-mm radiometer at selected
frequencies and in specific locations as determined by the results of
the broad coverage, molecular trace species inventory from the infrared
instrument. This combination would allow localized sources of spatially
variable species to be identified and provide insight into their origin.

### Circulation

Except for the surface measurements performed by the Viking landers and
Pathfinder, winds on Mars have never been directly measured from
spacecraft observations. Typically, winds have been derived from the
temperature field using the thermal gradient wind approximation or
similar, more sophisticated, techniques that assume zero velocity at the
surface. However, these estimates may be far from accurate on Mars
because of the near-surface winds driven by the strong diurnal cycle,
and because of the large amplitude of the waves above 40 km and the
difficulty of accounting for the complex interactions among the
different wind components. For instance, a very limited number of
Doppler shift measurements of the CO lines have been obtained using
Earth-based radio-telescopes and interferometers (Lellou*ch, et a*l.,
1993; Lellou*ch, et a*l., 1991; More*no, et a*l., 1999; More*no, et
a*l., 2001). These measurements suggest that retrograde winds around 60
km dominate at almost all latitudes, even around equinox (Jeg*ou, et
a*l., 2000). This disagrees strongly with thermal wind estimates based
on MGS TES data as well as with theoretical GCM predictions. The enigma
raised by these remote measurements suggests that new data, obtained
from direct measurements, is essential to understand processes
controlling the dynamics of the Martian atmosphere.

The wind measurements should maximize the latitudinal coverage (0˚to ≥
\|70˚\|), in order to observe winter jets and baroclinic waves. The
horizontal resolution needs to be smaller than the scale of the free
atmosphere structures (i.e., ≤ 5˚ latitude). The vertical coverage
should be from the surface to 120 km altitude, with resolution better
than 10 km. The accuracy of the wind speed measurements should be better
than 20 m/s

# Mission Characteristics 

The investigations described above can be accomplished with a high
performance orbiter such as MRO, the most recent of the current Mars
orbiters. A long duration orbital mission, as is projected for the
decade-long telecommunication function of MSTO, is especially well
suited to the particular combination of the primary science goals
discussed here. Investigation of the solar-planetary environment at the
top of the Mars atmosphere and measurement of the escape fluxes of key
atmospheric species requires access to a range of altitudes, to allow
sampling from below the peak region of the ionosphere to above the bow
shock, in order to understand and quantify the different processes by
which the solar wind has eroded the Mars atmosphere over time. Thus an
elliptical orbit that samples this range of distances (i.e., from \~100
to 6000 km) from the surface of the planet is necessary. A second, vital
aspect of the compatibility between MSTO and investigations concerned
with the evolution of the atmosphere is the non-linear nature of the
dependence of escape processes on the intensity of the solar input. In
order to model these processes correctly, therefore, it is essential
that measurements be made at a time close to the maximum of solar
activity as well as at times between maxima. Against this, remote
sensing measurements of the detailed composition of the neutral lower
atmosphere favor near-circular orbits, with precession periods that
provide global coverage on seasonal time scales; investigation of
spatial and secular variability of the atmospheric composition again
points to the need for extended observations made at intervals spanning
several Mars years. Thus, a mission design characterized by orbital
flexibility, and an overall duration which ensures inclusion of a solar
maximum, provides the ideal opportunity for carrying out the atmospheric
investigations that are the focus of this report. Further detailed
discussion of orbit design considerations is given below.

Nine instrument types were identified by the SAG as being necessary to
make the required measurements. These are described briefly in the
preceding sections and are summarized in Table 4, which includes
examples of instruments having characteristics and performance
capability similar to the present requirements. A rough estimate of the
expected mass of this instrument complement indicates that a mass
allocation of about 100 kg would be required for the science payload.
Again, an MRO-class orbiter is appropriately suited to this purpose.

## Launch Date

The MSTO mission is proposed for the 2011 or 2013 launch opportunities.
Among the many factors that should enter into consideration in choosing
between these dates is the expected timing with respect to the maxima of
solar activity. The approximately 11-year solar cycle has nominal dates
of 2011 and 2022 for the next two maxima. Figure 4 shows the expected
variation of the solar output through this period. It can be seen that a
2011 launch, which would have an arrival date at Mars of September,
2012, may be too late to commence science activities in time to capture
the effects of the 2011 maximum. This would put the only opportunity to
observe a true maximum beyond the end of the nominal mission, with the
attendant increase in risk from component failure or depletion of fuel
margins. The 2013 launch, however, would permit a slower build up of the
science activities and give some 5 or 6 years for initial aeronomy and
lower atmosphere spectroscopy before the crucial measurements at solar
maximum are made.

MSTO will arrive at Mars (in 2012 or 2014) at a time when other relevant
measurements will have been made and will be ongoing. The Mars Science
Laboratory (MSL) will be completing two years of operations on the
surface, providing a wealth of data on atmospheric composition,
including isotope ratios. Instruments such as the Mars Climate Sounder
(MCS) on Mars Reconnaissance Orbiter (MRO) will continue to map water
vapor, dust and aerosol, and Phoenix will have completed its high
latitude observations on the surface.

Scientific investigations from MSTO will be made independently and in
context with data from MSL and MRO that can be used for validation of
remote sensing instruments. For example, measurements from the SAM suite
on MSL will include the localized abundance of methane, its ^13^C/^12^C
ratio with respect to that of CO~2~, whether hydrogen peroxide is
present in the atmosphere and soil, the presence of organics, and
several other gases. For the Martian atmosphere, MSTO will take
advantage of ground-truth opportunities with MSL, but then expand the
localized *in situ* measurements of MSL's targeted species to full
global maps that will become a subset of the full atmospheric inventory
capability of MSTO. Measurements of dust and water from MRO and initial
isotopic water abundances from MSL will be dovetailed with the MSTO
inventory to provide unprecedented capability for linking photochemical
and heterogeneous chemical models of Mars that will be vital for
assessing human habitability.

## Orbit Design Considerations 

The MSTO orbit design must accommodate the requirements of three
separate functions: lower atmosphere science, upper atmosphere science,
and command and data relay. At the outset of the SAG deliberations it
was suggested that perhaps the \"ideal\" orbits (i.e., those that would
best satisfy the individual needs of these three separate functions) are
not sufficiently advantageous, in terms of the respective returned
science value or communications coverage, to warrant the risk and
expenditure of fuel to make the frequent orbit changes required in order
to meet these idealized needs.

The preferred orbit characteristics for the individual functions are as
follows: For the remote spectroscopic observations of the lower
atmosphere, a high inclination, near-circular orbit at an altitude of a
few hundred km would provide the best spatial and seasonal coverage,
together with the required vertical resolution. The orbital motion
should provide global coverage over a period short compared to a quarter
of the Mars year and should be maintained, or at least repeated, over
two or more Mars years to allow for the observation of inter-annual and
other secular variability.

For the investigation of atmospheric escape processes at Mars the ideal
orbit is one with an eccentricity sufficient to probe, on every orbit,
regions from below the exobase (i.e., down to the ionospheric peak
region at \~110 km altitude) to above the bow shock (to, say, 5000 km
average altitude at local noon). Using the Mars-centric Solar Ecliptic
(MSE) coordinate system (*X*-axis pointing from Mars towards the sun,
*Z*-axis towards the ecliptic north pole), the line of apsides for the
ideal, high inclination orbit would rotate through 360˚ several times
per Mars year. The rotation should take place primarily in the X-Y
plane, although some rotation in the X-Z plane also has benefits. A
rapid orbit walk will allow the mapping of Mars' near-space volume on
the day-side, through the dawn and dusk regions and into the tail, over
a short enough period to allow for the detection of seasonal variations.
It is essential that the orbit configuration allow variations in escape
fluxes throughout the solar cycle to be determined (solar maxima will
occur at around 2011 +/-1 and 2022+/-1).

For the telecommunications functions it is understood that a 400 x 2000
km orbit would be ideal.

The SAG studied the spatial, seasonal and solar cycle coverage
achievable with several different orbit configurations and combinations.
These included the initial design suggested by the MSTO mission design
study team, viz. a 3-phase design (Figure 5) consisting of:

Phase 1. Post aerobraking science phase: 150 km x 6500 km; 1 (earth)
year

Phase 2. Solar occultation phase: 400 x 400 km; 1earth year

Phase 3. Telecomm. plus science phase: 400 km x 2000 km; 8 years,

as well as several other elliptical orbits which might provide good,
albeit not ideal, coverage for all three orbiter functions
simultaneously.

The SAG also took into consideration the relative advantages and
disadvantages for the lower atmosphere occultation and down-looking
observations provided by the \"ideal\" circular orbit, maintained for a
duration of one year, versus the more limited coverage of the low
altitude portions of an elliptical orbit which could be utilized over a
period of several Mars years to acquire data pertaining to inter-annual
variability and other effects, such as the occurrence of dust storms and
clouds and hazes. The long duration of the mission - essential to the
upper atmosphere and exosphere observations - is also of importance to
the scientific goals of the lower atmosphere investigations, and the
correct weighting of the many factors involved in the choice of the
\"best\" orbit, or sequence of orbits, is difficult to balance. Figure 6
illustrates, for example, the coverage achieved, for solar occultation
observations, with the 150 x 6500 km orbit over one Mars year.

The most important outcome of these considerations was the realization
that there are, in fact, many orbit strategies that will satisfy to a
large extent the key requirements of the three functions of the orbiter,
provided the activities for the science investigations can be maintained
at appropriate intervals throughout the duration of the mission, as
opposed to attempting to accommodate them within fixed \"one time only\"
science phases. This applies particularly to the measurements of
solar-planetary interactions. Further, in the context of the requirement
that *in situ* observations of the atmosphere be made as low as possible
(100 km being a desired minimum altitude), the SAG assumed that
opportunities to acquire science data at the lowest altitudes
encountered during the aerobraking maneuver can be exploited for this
purpose. The SAG concluded that orbit design and strategy should not be
an outstanding problem in the accomplishment of the science goals of the
mission.

## Summary of Measurement Requirements 

The following summarizes the required measurements:

[Lower Atmosphere]{.underline}:

-   Infrared limb spectrum: 800 to 4000 cm^-1^ x 0.02 cm^-1^; all L~s~;
    3 or more Mars years.

-   Sub-mm radiometry: nadir to limb scan; selected frequencies from IR
    spectrum; same seasonal and duration coverage.

-   Winds: surface to 130 km

[Upper Atmosphere and Exosphere]{.underline}

Measurements of the following parameters are required throughout the
solar cycle and particularly at solar maximum and solar minimum:

-   Atomic and molecular neutral and ions listed in Error: Reference
    source not found

-   Vector magnetic field: crustal, upper atmosphere and solar wind.

-   Ion and electron 3-Dim. energy distribution in the upper atmosphere
    and solar wind.

-   Atmospheric winds above 130 km

-   

## Instrument Payload

The SAG identified the following candidate instruments that would be
required to make the recommended observations described above:

-   ***High Resolution IR Spectrometer***. Measure the IR spectrum 800
    to 5000 cm^-1^. Limb measurements (solar occultation).of molecular
    composition to \<10^-10^ mrbv

-   ***Sub-mm Radiometer***; channel frequencies by command; nadir to
    limb scan. Measure the spatial distribution selected species in
    lower atmosphere. Measure zonal and longitudinal components of lower
    atmosphere winds.

-   ***High Resolution Neutral Mass Spectrometer***; mass range 1 to
    100amu, mass resolution \~3000. Measure the neutral composition and
    structure of the upper atmosphere.

-   ***Retarding Potential Analyzer***. Measure neutral and ion (thermal
    ions) species and fluxes in upper atmosphere; measure upper
    atmosphere winds.

-   ***Ion/Electron Detector***. Measure the **v**elocity distribution
    of solar wind ions and electrons, pickup ions, etc.

-   ***UV Spectrometer***. Determine the properties of the hot corona
    and the aurora.

-   ***Langmuir probe.*** Measure the electron temperature and plasma
    density.

-   ***Vector Magnetometer.*** Characterize the solar wind, the bow
    shock, and the magnetic pile-up boundary. Characterize the magnetic
    properties of the ionosphere and exosphere, and map the crustal
    magnetic field.

-   ***Context camera;*** record atmospheric phenomena, hazes, clouds,
    dust storms, polar activity; synoptic nadir observations; limb
    observations w/\<1 km vertical resolution

This information is also displayed in Table 4.

## Instrument development needs

A major characteristic of the science activities on MSTO is to carry out
measurements that span the relatively long time period of a solar cycle.
The requirement that the instrument payload be capable of functioning in
the Mars environment for this duration will necessitate a development
program that adequately ensures their survival. Previous missions, such
as Cassini, Rosetta, etc., have carried out such design-lifetime
verification programs within acceptable financial resource allocations
and a similar program will be necessary for MSTO.

Additionally, many of the proposed instrument types have technical
issues that would require early attention. Even those with a high
Technology Readiness Level (TRL) will benefit from developments
resulting in lower mass and power requirements. Upper atmosphere neutral
wind devices and mass spectrometers will need some further development
resources to enable them to fit the MSTO mission profile, for example.

Instruments needed to make critical atmospheric measurements include
Fourier Transform Infrared Spectrometers and Sub-millimeter Radiometers.
Both of these are relatively mature instrument types. However,
technology development needs arise from the constant evolution of
mission and system architectures and the needs for new, more sensitive
measurements.

Fourier Transform [Infrared Spectrometers]{.underline} require detectors
cooled to the temperature range of 80-100K in order to achieve the
required sensitivity. Instruments with detectors cooled to this
temperature range have been used in earth orbit, in sun synchronous
orbits, and in a small number of planetary instruments. For many of
these the low detector temperature was achieved by the use of radiative
coolers, designed to minimize structural and environmental heat loads
and maximize the view of a radiator to cold space. Minimizing heat loads
implies very careful structural, thermal and electronic design to
isolate the cold detectors or optics from warmer parts of the instrument
and spacecraft. Maximizing and maintaining the view from the radiator to
cold space places constraints on the spacecraft configuration, orbit
design and spacecraft pointing, to prevent solar or planetary radiation
from entering the radiator field of view.

Recently, mechanical refrigerators ([cryocoolers]{.underline}) have been
used in earth orbiting spacecraft to avoid these problems. They have
been designed to achieve multi-year lifetime and to minimize vibration
inputs to the detectors. The tradeoff is that these coolers have been
heavy and expensive. Technology approaches have recently been identified
which have the potential to provide long life, low cost, low mass
cryocoolers. These designs have been produced in response to military
requirements for cooling infrared detectors in harsh environments. They
are pulse tube coolers using non-contacting, flexure bearing compressors
that provide refrigeration in the range of 1W to 4W at 80K. The new
mechanical designs reduced their mass to just a few kg. Similarly,
recent improvements in cooler drive electronics have reduced electronics
packaging mass and volume significantly over their predecessors. 

Consideration of cooler options in the overall spacecraft design would
ensure that cooled instruments can be accommodated at reasonable cost
without overly constraining the spacecraft design or placing too great a
burden on instrument design. Provision of cooling as a spacecraft
resource is one way to accomplish that objective.

[Sub-millimeter spectrometers]{.underline} are in demand for multiple
planetary science mission applications. These instruments have in the
past used detector technology that was difficult to fabricate and
provided low device yield. To meet future demands for instrument
performance and cost, promising new detector technologies, e.g.
membrane-based sub-millimeter receiver components, must be developed to
a Technology Readiness Level appropriate to insertion into a flight
project. Even though sub-millimeter receiver technology has been infused
into flight instruments in the past (e.g. MIRO on the Rosetta Orbiter,
EOS MLS, Herschel HIFI), the new membrane-based process offers several
advantages over these previous device processing approaches, including
better yield and greater spectral coverage. Development of this
technology will provide lower cost, higher performance instruments that
can be developed in the time frames required by flight projects.

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# Figures and tables

![](media/image1.jpeg){width="5.977777777777778in"
height="4.185416666666667in"}

Figure 1. A sketch of the structure of the Martian plasma environment,
depicting the major boundaries and regions in the equatorial plane. The
scales are Mars radii.

![](media/image2.wmf){width="5.725694444444445in"
height="2.7111111111111112in"}

Figure 2. (After Mazelle et al., 2002; See also Winterhalter et al,
2001). Magnitude of the magnetic field recorded by Mars Global Surveyor
during an early elliptical orbit (October 11, 1997) around periapsis
(14:28 UT, altitude 120 km, 17.6 local time) displaying the major plasma
boundaries symmetrically on both sides: BS, MPB and CB denote the bow
shock, the magnetic pile-up boundary and the magnetic 'cavity boundary',
respectively. The horizontal axis shows both time and the spacecraft
coordinates in the Mars-centered solar orbital (MSO) system (the X-axis
points from Mars to the Sun, the Y-axis points anti-parallel to Mars\'
orbital velocity, and the Z-axis completes the right-handed coordinate
system).

![](media/image3.png){width="3.457638888888889in"
height="2.1944444444444446in"}

Figure 3. Key components of the Mars general circulation.

![](media/image4.wmf){width="4.866666666666666in"
height="3.066666666666667in"}

Figure 4. The solar wind pressure varies over the solar cycle, as does
the upper atmosphere and ionosphere-controlling solar EUV flux. The
escape rates of heavy atoms and H are expected to be significant at high
solar activity. There is no *in situ* data at present. Thus measurements
at solar max are essential (necessary, but not sufficient).

![](media/image5.wmf){width="2.9166666666666665in" height="5.1625in"}

> Figure 5. MSTO initial orbit design: Science Phase 1 (red) 150 x 6500
> km, duration 1 year. Science Phase 2 (yellow) 400 x 400 km, duration 1
> year. Telecom Infrastructure Phase (green) 400 x 2000 km, duration 8
> years.

![](media/image6.png){width="5.892361111111111in"
height="4.9430555555555555in"}

Figure 6. Solar occultation coverage for a 150 x 6500 km orbit over 2
years. Red and blue footprints correspond to 8 and 4 km vertical
resolution, respectively.

  ---------- ------------------------- ------------------- ------------------
             **Science Objective       **Applicable MSTO   **MEPAG
             Description**             SAG Questions**     Investigation
                                                           Addressed**

  **SO-1**   Characterize present-day  Q1, Q3,             II.A.1, II.A.3,
             atmospheric escape                            II.B.2

  **SO-2**   Determine relative        Q4, Q5, Q7-9        I.C.4, II.A.1,
             abundances of minor and                       II.B.1
             trace molecular species                       
             of lower atmosphere                           

  **SO-3**   Measure zonal and         Q1, Q6              II.A.1, II.A.3,
             meridional winds                              II.B.2

  **SO-4**   Map crustal magnetic      Q1                  II.B.2
             field w/100km resolution                      

  **SO-5**   Use radar or SAR to       Q7                  III.A.1, III.A.2,
             locate subsurface regions                     III.A.5
             of special interest                           
  ---------- ------------------------- ------------------- ------------------

Table 1. Relationship between MSTO Science Questions and Objectives,
MEPAG Mars Scientific Goals, (MEPAG 2006) and the recommendations of the
Decadal Study, (New Frontiers in the Solar System, NAS 2003).

.

Table 2. Upper atmosphere atomic and molecular species to be monitored.

  ---------- ------------- --------------------------- ----------------------
  Species    Upper limit   Type of observation         Reference
             (mixing                                   
             ratio)                                    

  C~2~H~2~   2x10^-9^      IRIS-Mariner 9              Maguire (1977)

  C~2~H~4~   5x10^-7^      \"                          \"

  C~2~H~6~   4x10^-7^      \"                          \"

  C~2~H~8~   4x10^-7^      \"                          \"

  N~2~O      1x10^-7^      \"                          \"

  NO~2~      1x10^-8^      \"                          \"

  NH~3~      5x10^-9^      \"                          \"

  PH~3~      1x10^-7^      \"                          \"

  SO~2~      3x10^-8^      Ground-based, mm            Encrenaz et al. (1991)

  OCS        7x10^-8^      \"                          \"

  H~2~S      2x10^-8^      Ground-based, IR            \"

  CH~2~O     3x10^-9^      \"                          Krasnopolsky et al.
                                                       (1997)

  HCl        2x10^-9^      \"                          \"
  ---------- ------------- --------------------------- ----------------------

Table 3. Upper limits for some of the plausible trace molecular
constituents of the Martian atmosphere for which spectroscopic searches
have been made. (From Ecrenaz et al., 2004.)

![](media/image7.wmf){width="5.995833333333334in"
height="7.440277777777778in"}

Table 4. Relationship between MSTO science objectives, required
measurements and proposed instrument types.