***REPORT***

***OF THE***

***NASA***

***SCIENCE DEFINITION TEAM***

***FOR THE***

***MARS SCIENCE ORBITER***

***(MSO)***

***December 15, 2007***

**CONTENTS**

**1.** **PREAMBLE 1**

**2. EXECUTIVE SUMMARY** **3**

**3. MSO MISSION OBJECTIVES 5**

**4. MSO MISSION REQUIREMENTS 7**

**5. SCIENCE OBJECTIVES 10**

> 5.1 Atmospheric Composition 10
>
> 5.2 Atmospheric State 15
>
> 5.3 Surface Change Science 22
>
> 5.4 Summary of Measurement Goals 25

**6. MSO MISSION IMPLEMENTATION 27**

> 6.1 Sample Payload 27
>
> 6.2 Mission and Spacecraft Design 28

**7. ISSUES AND CONCERNS 33**

**APPENDIX 1: MSO SCIENCE DEFINITION TEAM 35**

**APPENDIX 2: TRACING MSO REQUIREMENTS**

**TO MEPAG INVESTIGATIONS 36**

**APPENDIX 3: LIST OF ACRONYMS 39**

***REPORT OF THE NASA SCIENCE DEFINITION TEAM***

***FOR THE***

***MARS SCIENCE ORBITER***

***December 15, 2007***

**1. PREAMBLE**

NASA is considering that its Mars Exploration Program (MEP) would launch
an orbiter to Mars in the 2013 launch opportunity. To further explore
this opportunity, NASA has formed a Science Definition Team (SDT) for
this orbiter mission, provisionally called the Mars Science Orbiter
(MSO). Membership and leadership of the SDT are given in Appendix I. Dr.
Michael D. Smith chaired the SDT.

The purpose of the SDT was to define the:

-   Scientific objectives of an MSO mission to be launched to Mars no
    earlier than the 2013 launch opportunity, building on the findings
    for Plan A \[*Atmospheric Signatures and Near-Surface Change*\] of
    the Mars Exploration Program Analysis Group (MEPAG) Second Science
    Analysis Group (SAG-2).

-   Science requirements of instruments that are most likely to make
    high priority measurements from the MSO platform, giving due
    consideration to the likely mission, spacecraft and programmatic
    constraints. The possibilities and opportunities for international
    partners to provide the needed instrumentation should be considered.

-   Desired orbits and mission profile for optimal scientific return in
    support of the scientific objectives, and the likely practical
    capabilities and the potential constraints defined by the science
    requirements.

-   Potential science synergies with, or support for, future missions,
    such as a Mars Sample Return. This shall include imaging for
    evaluation and certification of future landing sites.

As a starting point, the SDT was charged to assume *spacecraft*
capabilities similar to those of the Mars Reconnaissance Orbiter (MRO).
The SDT was further charged to assume that MSO would be scoped to
support telecommunications relay of data from, and commands to, landed
assets, over a 10 Earth year period following orbit insertion. Missions
supported by MSO may include planned international missions such as
EXOMARS.

The MSO SDT study was conducted during October -- December 2007. The SDT
was directed to complete its work by December 15, 2007. This rapid
turn-around was required in order to allow time to prepare an
Announcement of Opportunity (AO) for science investigations, to be
released in early 2008.

The SDT met three times via telecon during October and early November
2007, before its main face-to-face meeting hosted at Caltech on November
12 and 13, 2007. Four additional telecons were held during November and
December 2007 to follow up on issues left undecided at the Caltech
meeting. During the telecons and face-to-face meeting, the SDT panel
members discussed the scope of scientific objectives to be accomplished
by the MSO and the resulting measurement requirements. The SDT also
considered the potential likely combination of instruments that could
credibly achieve the science goals to confirm that there were science
payloads that fit within the given MSO constraints. The JPL MSO Project
Team described a reference mission which included target payload mass,
power, and cost envelopes; mission designs, including options for
different orbital inclinations and heights; and issues and concerns
regarding various payload instrument candidates, such as pointing
requirements.

This report summarizes the activities and recommendations of the SDT.
Section 2 gives a high-level summary of the findings of the SDT. Section
3 identifies the key science objectives and programmatic areas that the
SDT believes should be addressed by MSO. Section 4 summarizes
recommendations by the SDT regarding mission requirements. Section 5
describes and justifies the scientific objectives of the MSO mission,
and lists a set of measurement goals. Section 6 describes a sample
reference mission including orbit design and a sample instrument
payload. Section 7 identifies a few concerns raised during SDT
discussions. Supporting material can be found in the Appendices. In
particular, Appendix 2 traces the MSO science questions and candidate
investigations to corresponding MEPAG investigations.

**2. EXECUTIVE SUMMARY**

*The MSO SDT recommends that NASA fly an orbiter mission in 2013 that
would largely follow the science goals and objectives of the "Plan A"
mission (Atmospheric Signatures and Near-Surface Change) as described by
the MEPAG SAG-2 report.*

The SDT recommends the following three main science drivers for the MSO
mission: 1) A new and comprehensive view of **Atmospheric Composition**
to seek evidence for the present habitability of Mars; 2) A vastly
improved characterization of the present **Atmospheric State** to
provide new insight into processes that control the martian weather and
climate; and 3) An in-depth study of **Surface Change Science** to
better understand the crucial interactions at the surface-atmosphere
interface. In addition to the above science objectives, NASA may choose
to include a very high (sub-meter) spatial resolution imager necessary
for landing site certification and the telecommunications equipment
necessary for MSO to serve as a long-term (10 Earth years) asset for the
relay of data from, and commands to, future landed spacecraft.
Scientifically, the inclusion of the sub-meter resolution imager would
augment the range of surface change science that could be addressed,
while the long-term telecommunications capability holds promise for an
extended period of science operations to cover interannual variability.
Both, however, may be beyond the funding envelope described for the
mission without a major international contribution. Even without
sub-meter scale imaging and extended scientific operations, the MSO
mission defined below, with one-meter scale imaging and operating for at
least one Mars year, would make major advances in the areas of climate
and global habitability.

To achieve the atmospheric science objectives would require a capable
suite of instruments, some with capabilities not previously flown to
Mars. *The SDT recommends inclusion of remote sensing instrumentation
with extremely high sensitivity to a broad suite of important trace
gases combined with nearly continuous spatial mapping of key minor
constituents and of atmospheric state.* As an existence proof that such
measurements can be made, the SDT notes, based on instrumentation
already flown to study the Earth's upper atmosphere, that a combination
of solar occultation, limb sounding, and nadir mapping observations
could provide the required two-tiered approach.

*To best achieve all science goals, the SDT recommends a near-circular,
high-inclination orbit at an altitude of 300 km with an orbital
inclination of 82.5°.* This relatively low altitude would allow the
highest possible spatial resolution for imaging and limb-sounding while
the inclination would strike a good balance between a rapid change of
observed local time during the course of the mission (favored by lower
inclination) and the ability to adequately observe the poles (favored by
higher inclination).

*The SDT also notes that it would be necessary that the spacecraft be
able to point accurately and with sufficient stability to regularly
acquire solar occultation and limb-geometry observations as often as
possible.* Furthermore, the continuity and global coverage of
atmospheric observations on a repeated, daily basis necessary to fully
realize the potential of the mapping portions of the investigations
envisioned here would require that observations be able to be taken in a
nearly continuous fashion (goal of \~85% coverage along the orbit
track).

*The SDT endorses the planned Science Emphasis Phase, with one Mars year
of observations with the science payload, but strongly recommends the
goal of extending the phase with science observations to cover
additional Mars years to fully leverage the scientific capabilities of
MSO.* Presently, the Science Emphasis Phase would be followed by a
transition to a near-circular, high-inclination orbit at a higher
altitude of 400 km for the Telecom Emphasis Phase. The higher altitude
satisfies planetary protection requirements, and would also be more
desirable for telecommunications relay between Earth and future landed
missions. MSO science observations should continue in this higher orbit.

The SDT recognizes that the full instrument suite given as a sample
payload in Section 6 may exceed the baseline cost allocated to MSO
science instruments. Cost could be reduced to fit within budget by the
potential foreign contribution of an instrument or by descope of the
instruments. In particular, most *science* goals could be met within
budget if the proposed high-resolution camera were descoped from 30 cm
to 1 meter per pixel resolution, although site certification
requirements would not.

A concern sometimes expressed about MSO is that it would be too focused
on a gas or suite of gases that might not be there. Such concerns arise
in part from the continuing controversy about the detection (or not) of
methane by ground-based or Mars Express observations. The SDT notes the
following: 1) the atmospheric objectives of MSO as defined here would
encompass a much more comprehensive atmospheric survey designed to
characterize the variations of known gases (e.g., water vapor, peroxide,
and carbon monoxide), as well as to improve *by an order of magnitude or
more* the detection limits of gases not yet seen; 2) a measurement which
could definitively state that methane is, or is not, present with a
detectability threshold orders of magnitude more sensitive than the
presently debated values *would be a major finding whether or not
methane is detected*; and 3) the first direct, globally distributed
measurements of wind and the measurements of temperature and water vapor
even in a dusty atmosphere would yield a *major advance in our ability
to understand and to simulate (for science and engineering*) the Mars
atmosphere, its dynamic processes and transport.

**3. MSO MISSION OBJECTIVES**

Following the recommendations put forth by the MSO SAG-2 Report "Plan A"
and the charge of MSO SDT Charter, the SDT identified five major
objectives for the MSO mission:

**3.1 Atmospheric Composition**

The overarching goal of the Atmospheric Composition objective would be
to seek atmospheric evidence for present habitability and life through a
sensitive and comprehensive survey of the abundance and temporal and
seasonal distribution of atmospheric species and isotopologues. It has
long been understood that the presence of life on a planet could 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 occur. The atmospheric signatures of active processes that
might be present and at what abundances they exist are largely unknown.
Thus, the recommended approach would be to sensitively search for a
diversity of signature molecules so that, in addition to characterizing
the variations of known minor gases over a broad range of temporal and
spatial scales, the detection limits of key gases not yet
(unambiguously) detected would be improved *by an order of magnitude or
more*.

**3.2 Atmospheric State**

The Atmospheric State objective seeks to provide new insight into
climate processes responsible for seasonal and interannual change. This
would be accomplished by both providing new observations that constrain
and validate models of atmospheric dynamics and state, and by extending
the present record of martian climatology to characterize interannual
variability and long-term trends of the atmospheric state, circulation,
and cycles of dust, water, and carbon dioxide. New observations would
include the first-ever direct observations of vertically resolved wind
velocity over the globe on a daily basis, and broad coverage of the
diurnal cycle of temperatures, winds, aerosol optical depth, and gas
abundances. With these observations and the improved transport and
climate models that would result from them, it would be possible to
better describe surface-atmospheric interactions key to many climate
processes, and may be possible to trace spatially varying minor
atmospheric constituents (including water vapor) to localized source
areas. Real-time monitoring of the atmospheric state would also play an
important role in supporting the implementation of future spacecraft
arrival and operation.

**3.3 Surface Change Science**

The surface science objectives of the MSO mission would be focused on
surface change as recorded in surface properties and morphologies due to
seasonal cycling of polar layered deposits, aeolian movement of fine
material globally and locally, mass wasting, the slow accumulation of
small impact craters and possibly the action of water even today in
special regions of the planet. Observations with good signal to noise
and a combination of spatial-temporal resolutions, from periodic global
survey to high resolution imaging of particular areas of activity would
continue to improve our understanding of modern processes of
surface-atmospheric interaction and surface change and our ability to
extrapolate them back into the recent geologic past.

**3.4 Site Certification Imaging**

Images taken by the HiRISE instrument on-board the Mars Reconnaissance
Orbiter have demonstrated the value of high-resolution (\~30 cm
resolution) imaging for the purpose of site certification for future
landed missions. The inclusion of a high-resolution camera with similar
capability would allow the certification of new locations for potential
landing sites that may be identified by new MSO observations. The SDT
concurs that (sub-meter) very high resolution imaging would be required
for landing site certification.

**3.5 Telecommunications**

The SDT endorses the plan for MSO to provide key telecommunications
infrastructure over its planned 10 Earth year lifetime supporting the
relay of science data from, and commands to, landed assets. MSO would
also provide telecommunications coverage of future planned critical
events such as EDL, Mars ascent vehicle launches, and MOI for other
missions. This capability would add significantly to the science return
and robustness of all future missions to Mars during MSO operations.
Because it is likely that science and relay would frequently occur
during the same mission phases, special care should be given to ensuring
that the science payload and telecommunications packages could be
operated concurrently with satisfactory results to both.

**4. MSO MISSION REQUIREMENTS**

***The SDT fully recommends use of the 2013 opportunity for launch of
MSO to Mars***. The favorable energetics of this launch opportunity
means that MSO would have sufficient mass margin to accommodate an
ambitious payload (mass could also be used to reduce cost and cost risk)
and could carry the fuel needed for a long-lived mission. There would
also be the potential for considerable synergy with the presently
planned 2011 Mars Scout aeronomy mission operating in an extended phase,
with simultaneous measurements in the upper and lower atmosphere nicely
complementing one another. The provision of a robust telecommunications
capability by 2014 could support a still working MSL and/or a newly
arrived EXOMARS.

The following discusses other high-level requirements for the mission:

**4.1 Operational Phases and Mission Life**

MSO would perform its mission during two main operation phases:

-   Phase One (Science Emphasis), consisting of a low (300 km),
    near-circular, high-inclination orbit compatible with science
    requirements, for a duration of at least 1 Martian year.

-   Phase Two (Telecom Emphasis), consisting of a low (400 km),
    near-circular, high-inclination orbit compatible with planetary
    protection, as well as telecommunications support and long-term
    station keeping, for a duration of 7 Earth years.

In the context above, the term "emphasis" is used to highlight the idea
that while priority is given to one kind of activity, the other would be
executed as well, but on a best-effort basis. Thus, during the Science
Emphasis phase, telecom tasks would be performed also, and during the
Telecom Emphasis phase, science observations would also take place.

The nominal end of the MSO mission would be 10 Earth years after Mars
Orbit Insertion (MOI).

**4.2 Orbit Characteristics**

Remote sensing measurements of the detailed composition and dynamics of
the atmosphere favor near-circular orbits, with precession periods that
provide full diurnal coverage on a time scale of one Martian season
(about 6 Earth months) or less. A high-inclination (not
Sun-synchronous), near-circular orbit at an altitude of a few hundred km
would provide the best spatial, seasonal, and diurnal coverage, together
with the required vertical resolution.

The SDT spent some time debating the optimal inclination and altitude of
the MSO orbit. A study of candidate orbits by the JPL MSO Project Team
showed that an inclination of 82.5° would provide a good compromise
allowing for sufficiently fast cycling through local time, acceptable
latitude distribution of solar occultation points (including
pole-to-pole coverage), and the ability to image key parts of the polar
regions (see also Section 6). The driving science requirement here would
be that the local time of solar occultation points at each latitude
cycle through a complete diurnal cycle in one Martian season or less. A
lower inclination (e.g. 74°) would allow both a faster precession of
local time and a more uniform latitude distribution of solar occultation
points, but would not allow the atmosphere above the rotational pole to
be imaged and limits nadir-viewing mapping of the polar residual ice
caps. A higher inclination (e.g. 85°) would be more favorable to polar
surface imaging, but would have a poor latitude distribution of solar
occultation points and would not cycle through local time quickly enough
to distinguish between diurnal and seasonal variations in observed
quantities. After much discussion, the SDT settled on a compromise
inclination of 82.5˚.

An orbital altitude of 400 km would have the advantage of giving
limb-to-limb coverage for wide-angle global imaging of atmospheric
phenomena, whereas an altitude of 300 km would tend to leave small gaps
near the equator in such coverage, but would provide higher spatial
resolution. The SDT recommended the lower altitude pioneered by MRO.

*The SDT recommends this combination (\~300 km, \~82.5˚) for the orbit
to be advertised in the AO*, but recognizes that there will surely be
some discussion on these parameters once the payload has been selected.

**4.3 Science Observing Capabilities**

The very high sensitivity required to allow the unambiguous
interpretation for trace gas species (i.e. the identification of species
and quantitative analysis at detection levels well below current upper
limits) would likely be best achieved by exploiting the enormous source
intensity and long path length of the solar occultation technique.
*Therefore, the SDT highly recommends that the MSO mission plan maximize
the number of solar occultation measurements possible, ideally two per
orbit.*

A mapping capability complementary to the solar occultations would be a
crucial element of the science plans endorsed by the SDT. Mapping
requires the ability of instruments to be able to view both the limb
(along-track and cross-track) and nadir at all points along the orbit.
While specific operations, such as a pitch/roll to position for solar
occultation or high-resolution imaging of landing sites, may sometimes
require movement of the spacecraft that precludes the ability of mapping
instruments to view nadir or the limb, *the SDT recommends a goal that
such mapping be possible 85% of the time.* If possible, the gaps in
mapping operations that do occur should be planned to minimize repeated
loss of mapping at any particular latitude.

**4.4 Archival of Science Data**

Science investigations will provide to the project calibrated data
within six months after its receipt on earth. The project will archive
copies of all verified, validated, and calibrated data acquired by the
mission to the Planetary Data System.

**4.5 Infrastructure Objectives**

In addition to its scientific objectives, the MSO would provide several
infrastructure services to support future Mars missions. First, it is
planned that MSO would establish a key telecommunications infrastructure
component for future Mars missions, providing coverage of critical
events, significantly increasing the robustness and overall science
return of the Mars Exploration Program, and enabling greater flexibility
of Mars program planning. Using high-performance proximity and deep
space links, MSO would offer increased amounts of science data return to
Earth for all Mars missions flown during its operational period. By
providing access to energy-efficient relay links, MSO would allow user
missions to reduce their telecom power and hardware requirements,
leaving more delivered mass and power for science instruments. *The SDT
endorses MSO's planned capability to provide telecommunications
infrastructure.*

The SDT recommends that MSO provides atmospheric data that would support
the design of aero-assist and EDL activities of future missions.
Principally, the MSO science investigations would study the state, and
the spatial and temporal variations (diurnal and seasonal) of Mars'
atmosphere. The resulting increase of our knowledge of the atmosphere,
and the increased fidelity of the numerical models (GCMs), would
directly benefit the design of aero-assist and EDL activities of future
missions. In addition, it is envisioned that near-real time monitoring
of relevant atmospheric parameters (density, dust optical depth, etc)
would provide essential information to MSO-concurrent spacecraft
arriving at Mars. *The SDT recommends that MSO be capable of providing
the relevant information at regular intervals as needed*, so that
aerobraking or aero-entry trajectory parameters could be adjusted if
necessary.

**4.6 Planetary Protection**

MSO will be compliant with NPD 8020.7 (Biological Contamination Control
for Outbound and Inbound Planetary Spacecraft), NPG 8020.12 (Planetary
Protection Provisions for Robotic Extraterrestrial Missions) and
subsidiary documentation (COSPAR PPP) for planetary protection purposes.

**4.7 Public Outreach**

*MSO science investigations will provide to the MSO project
science/technology data suitable for release to the public via the
Internet*. *The MSO project will assure regular releases for public
information purposes in a timely manner.* The public releases are to
convey the excitement and wonder of space exploration to the US
taxpayer.

**5. SCIENCE OBJECTIVES**

As briefly described in Section 2, the SDT divided the scientific
objectives of the MSO mission into three main categories: 1)
**Atmospheric Composition**, 2) **Atmospheric State**, and 3) **Surface
Change Science**. Below are details of the justification and measurement
goals for each science objective.

**5.1 Atmospheric Composition**

Measurements of atmospheric composition constitute a primary objective
of the MSO mission, thereby enabling comprehensive study of the coupled
volatile environment of the Mars atmosphere, surface, and subsurface.
Time and spatially dependent exchanges among the photochemically active
atmosphere, surface ice of the polar caps, and potential subsurface
sources (water vapor in adsorbed or permafrost state, methane from
biogenic or geochemical sources) remain poorly constrained due to the
limited breadth and sensitivity of existing measurements. Hence, MSO
atmospheric observations should include a baseline set of molecular
species necessary to isolate the key photochemical, transport,
condensation, and biogenic-geochemical processes that control the
current chemical state of the Mars atmosphere. In many cases these
observations would require exceptional sensitivities relative to prior
mission capabilities. Most notably, detection-mapping capability for
atmospheric methane presents a core requirement due to its potential as
a biogenic marker. Key atmospheric priorities are identified for
photochemistry (H2O2, O3, CO, H2O), transport (CO, H2O), isotopic
fractionation (isotopomers of H2O and CO2), and surface-subsurface
sources (CH4 and H2O). Additional targets of interest include HO2, NOx,
and sulfur/carbon/ chlorine components associated with potential
subsurface sources.

**[Photochemistry]{.underline}**

Mars photochemistry exhibits similarities to the terrestrial mesosphere
in that HOx radicals (H2O2, HO2, OH, and H) associated with water vapor
photolysis exert catalytic control over the trace specie families of Ox
(O2, O3, and O), NOx (NO2, NO), and CO. In the case of Mars, these trace
oxygen, nitrogen and carbon constituents are produced through
photo-dissociation of the bulk CO2/N2 atmosphere. Loss and partitioning
rates are dominated by reactions with the HOx radicals HO2, OH, and H;
for which H2O2 serves as a short-lived (1000 seconds) reservoir (i.e.,
2xOH). This link with HOx chemistry leads to a fairly direct
correspondence of short-lived trace gas species with the variable
distribution of atmospheric water vapor in space and time. Long-lived
photochemical products (CO, O2, H2) should exhibit much weaker spatial
and temporal variations in the lower atmosphere, associated with
transport and condensation effects.

Existing observations of Mars CO, O2, O3, and H2O2 indicate the activity
of homogeneous (gas-phase) photochemistry on Mars, but the data are
insufficient to define the roles of heterogeneous, transport and
non-equilibrium processes. These include heterogeneous loss of HOx on
ice clouds, HOx production on charged dust particles, condensation and
transport enrichment of species, and subsurface sources/reservoirs. A
much more detailed characterization of the spatial/temporal
distributions of key photochemical constituents than currently supported
by existing measurements is required to separate and so identify these
processes.

***Measurement Goals:*** A baseline set of MSO photochemical measurement
objectives includes water vapor, H2O2, O3, CO, water ice and dust
aerosols. Other targets of photochemical interest are HO2, O2, H2, NO2
or NO, and SO2. Baseline vertical profile ranges are 0--60 km for water
vapor, CO, and dust and ice aerosol opacities; and 0--40 km for H2O2 and
O3, with 5 km vertical resolution. The sensitivity of the measurements
(at 20 km altitude) should be 10 ppbv for water vapor, 100 pptv for
H2O2,, 1 ppmv for CO, and 1 ppbv for O3. Nearly continuous global
mapping capability is needed to characterize the dynamic variation of
Mars atmospheric water vapor (and hence, the short-lived H2O2 and O3
species), associated with surface and cloud ice formation and
sublimation. Such mapping capability would also enable optimum
observational constraints on potential heterogeneous chemistry
associated with dust and ice aerosol variability, as well as potential
subsurface out-gassing and adsorption.

**[Methane and Other Subsurface Source Gases]{.underline}**

In the earliest discussion of the detection of life on Mars, Hitchcock &
Lovelock suggested that the presence of reduced gases, such as methane
(CH4), in an oxidizing atmosphere would be indirect evidence of life.
Indeed, terrestrial microorganisms produce a wide variety of reduced
gases as products of both energy-yielding oxidation-reduction (redox)
reactions and synthesis and decomposition of organic matter. However,
several geological processes may also inject reduced gases into the
atmosphere, including direct degassing from magma rising from the
subsurface storage regions through the crust, magma degassing into
shallow hydrothermal systems, and interaction of rocks with hydrothermal
solutions or ground waters. The molecular composition of released gases
likely differs from that on Earth and will depend on several variables,
including temperature of equilibration, pressure of degassing, and
oxidation state.

The discovery of either extant geothermal or biological processes and
their source locations would have profound implications for astrobiology
and the Mars Exploration Program. The case involving active microbiology
is clear. Identifying the locations of active geothermal processes also
would be profound. Such places would be obvious targets for future
surface exploration.

A key objective of the MSO investigation would be to follow up on the
reported observations of methane in the Martian atmosphere. Reports have
suggested a seasonal variability in the CH4 abundance and meridional
and longitudinal variability, implying that the distribution of CH4
could reveal the location of its source. However, current Martian
atmospheric photochemical models indicate that its lifetime is \~250
Mars years, which is so much longer than atmospheric transport
timescales that the expectation is that CH4 should be well-mixed
throughout the atmosphere. If the reported spatial variability is true,
then some atmospheric chemical processes (possibly dust-related), or
exchange with the surface/cryosphere have been seriously underestimated
in the current models. On the other hand, if observations
with significantly higher precision show that CH4 is well-mixed, then
its distribution will not be a useful guide to locations of active
processes. Thus, it is necessary to map co-generated species that
have much shorter atmospheric lifetimes and therefore could be tracers
leading back to source zones. 

The recent putative detections of Martian atmospheric CH4 have
stimulated numerous hypotheses about the nature of the methane sources,
their magnitude, and locales. Efforts to identify the sources of
terrestrial methane have found that measurements of CH4 isotopologues do
not necessarily distinguish between possible abiogenic and biogenic
sources. However, it has been found that the abundances of other
co-generated species, such as ethane (C2H6), relative to CH4 can
distinguish between a source from active biology and other potential
sources; the C2H6/CH4 abundance ratio is much less than unity for the
former, while other sources produce more equivalent amounts of CH4 and
C2H6.  Likewise, precise measurements of the carbon and hydrogen
isotopic abundance (\~10 per mil) of methane could provide insight into
its putative source.

With respect to trace gas disequilibria, this investigation has four
specific objectives: 

-   Identify chemical constituents in the atmosphere that cannot be
    formed by atmospheric photochemical processes (signature species).
    Besides the tentative CH4 detections, other chemical signatures
    arising from possible active subsurface processes may be present
    in the atmosphere (e.g., SO2). 

-   Locate source regions on the surface for these gases. Knowledge of a
    source region would provide direction for later missions.

-   Determine atmospheric lifetimes of signature species. Knowledge of
    lifetimes is necessary for indirectly locating a source region and
    for characterizing the magnitude of the source process. 

-   Determine character of processes producing signature species. To the
    extent possible, distinguish between abiogenic and biogenic origins.
    It may be feasible to identify the existence of active biological
    processes without waiting for in situ analysis or sample return. 

***Measurement Goals:*** What atmospheric signatures of active processes
that might be present and at what abundances is unknown -- this is both
a challenge and a major exploration opportunity. Therefore, the
measurement requirement is to sensitively detect a diversity of
molecules over broad temporal and spatial scales -- not only
methane. Molecules diagnostic of active geological and biogenic
processes include sulfur, nitrogen, and reduced carbon species. These
gases will have very low abundances in the Martian atmosphere, and thus
state-of-the-art instrumentation approaches are required for their
detection.

The detection of an atmospheric constituent in extremely low abundance
is made secure only when its presence is confirmed by simultaneous
measurement of multiple spectral features. In turn, simultaneous
detection of different species can serve as important evidence for
the identity of potential source processes; co-generated species may
span several orders of magnitude in abundance. There is generally a
trade between detection sensitivity and spatial and temporal resolution.
For the purpose of maximum understanding of signatures present in the
atmosphere and their seasonal abundance, a detection threshold of at
least a few parts per trillion for a zonal average over 5° of latitude
is necessary to significantly exceed current observations.

The search for atmospheric signatures must be global. While long-lived
species will be widely distributed, many trace signature molecules have
short chemical lifetimes and, consequently, will be detectable only near
their sources. These sources will generate plumes, the dynamics of which
suggest spatial scales of a few tens of kilometers to hundreds of
kilometers. However, actual source vents (as on Earth) are likely to
occur at scales from tens of meters to \~1 km. The detection of such
plumes, therefore, would require sampling of the full Martian surface
with individual measurements having a resolution of better than 104 km2
and a sensitivity at least \~1 ppbv, and preferably tens of pptv. 

To tie an observed plume of chemicals to its surface source in an
optimal fashion, both measurements that would resolve the plume
structure and knowledge of the wind field are needed. The latter
requires knowledge of the atmospheric state---both temperature fields
and the distribution of aerosols---on a global scale with a vertical
resolution of less than one scale height. Observation of a detectable
signature gas with an appropriate lifetime (e.g., SO2), could be used to
identify interesting source regions, particularly in conjunction with
transport modeling. Direct observations of wind with current
technologies are inadequate to track species back to localized sources,
but they provide the validation needed to have confidence in
using circulation models to track the signature species back to its
source region (see Section 5.2). 

Knowledge of the atmospheric lifetime of a signature species is required
to estimate the flux emanating from the surface from observations of an
atmospheric plume and to facilitate application of inverse modeling
techniques for source location. This requires an understanding of the
background atmospheric chemistry, which would be improved by
observations of the vertical distribution of composition, temperature,
and dust (at half scale height, or 5 km, resolution). In addition,
observations would be required under all atmospheric conditions, in
particular over the range of dust loading, to assess the potential
impact of heterogeneous chemistry, including electrification-related
processes; the requisite measurements of species and
temperature distribution must be unaffected by the degree of atmospheric
dust content. 

Besides episodic events, both climatic and biological phenomena may
introduce a seasonal signal into the atmospheric composition. Low
volatility molecular species may be depleted as ice caps form and
reappear as they sublime. When the latter occurs, resulting
atmospheric concentrations may be particularly elevated. As on Earth,
any biosphere may introduce a distinct atmospheric seasonal cycle.
Therefore, it is necessary to monitor the atmosphere over at least
one Martian year over a broad latitude range and with observations with
full latitudinal coverage at least once per season to optimize detection
of seasonal variability and more frequently to optimize detection of
episodic events.  

**[Isotopic Ratios]{.underline}**

Measurements of hydrogen, oxygen, and carbon isotopic abundance in the
Martian atmosphere would allow new and important constraints to be
placed on atmospheric chemistry, meteorology and interactions between
the atmosphere and surface. Measurements of these isotopologues of CO2
and water vapor in the lower atmosphere of Mars would provide a boundary
condition for aiding in the interpretation of the aeronomy measurements
in the upper atmosphere planned for the proposed 2011 Scout aeronomy
mission. Currently, knowledge of the isotopic composition of present day
Mars is limited to low-precision Viking analyses and Earth-based
spectroscopic observations of D/H ratio of Martian atmospheric water
vapor and the ratio of D to H atom abundance in the exosphere.
Additional, though less direct constraints are provided by measurements
of ancient Martian volatiles in the SNC meteorites. While these
measurements are significant constraints on models of long-term
atmospheric evolution, the generally poor precision and lack of
knowledge of the vertical and latitudinal isotopic variations of
atmospheric gases limits their usefulness for answering specific
questions about the internal dynamics of the Martian atmosphere and its
interactions with the surface. Likewise for carbon, measurement of
13C/12C in atmospheric CO2 would allow the anomalous observations of
this ratio in SNC meteorites to be placed in the context of a well-mixed
and large Martian carbon pool. Finally, latitudinally resolved
observations of oxygen isotopes in CO2 when combined with the carbon
isotope measurements would provide new insight into the sublimation and
condensation of CO2 in polar regions.

***Measurement Goals: ***To address these goals, spatially resolved
profiles of the isotopologues of the major and minor gases are needed.
Precision requirements depend on the isotope. Deuterium content of water
must be measured with at least 5% precision and 10% accuracy, while the
carbon and oxygen isotopes of CO2 and CO must be measured with 0.5%
precision and 2% accuracy. Isotopic abundances need to be measured over
the lowest 20 km of the atmosphere (or 40 km for deuterium) with a
vertical resolution of 5 km.

**[Trace Gas Inventory]{.underline}**

Primary measurement requirements for MSO trace gases are summarized in
Table 1. The desired sensitivities reflect science objectives that may
be attained by a combination of infrared and sub-millimeter (or
millimeter) spectral approaches. In general, optimum trace gas
detections (methane and isotope ratios) are best supported by infrared
solar occultation spectroscopy. Mapping dust and ice aerosol
measurements would be best supported by passive infrared limb radiometry
or spectroscopy. Optimum mapping and vertical coverage of photochemical
species, including aerosol-independent sensitivity below 20 km
altitudes, would be best supported by passive sub-millimeter limb
spectroscopy. Global mapping of species would provide
latitudinal-longitudinal-temporal coverage and resolution sufficient to
resolve the key spatial and temporal correlations among the
photochemical, aerosol and surface/sub-surface constituents of the Mars
atmosphere.

***Measurement Goals:*** Very little is known about the trace gas
composition of the Martian atmosphere. Generally, previous attempts to
quantify many compounds of interest for studies of photochemistry and
surface exchange have been unsuccessful and yielded only very high upper
limits. Earth's atmosphere contains many of these compounds due to the
existence of biology and volcanism. The table below lists a number of
species that might possibly be detected. Given is the current upper
limit from existing measurements along with the predicted abundance due
only to gas phase photochemistry. A significant improvement in current
knowledge would require a sensitivity of about 1--10 pptv.

  ----------------- ----------------------------- ------------------------
  Molecule          Predicted Abundance due only  Upper Limit (ppbv) of
                    to gas phase photochemistry   Existing Measurements
                    (ppbv)                        

  HO2               \~1                           \-

  NO2               \~0.1                         10

  N2O               \~0.001                       7

  C2H2              0                             2

  C2H4              0                             500

  C2H6              0                             400

  H2CO              0                             3

  HCN               0                             \-

  H2S               0                             20

  OCS               0                             10

  SO2               0                             1

  HCl               0                             2
  ----------------- ----------------------------- ------------------------

**Table 1**. Current upper limit and predicted abundances of a number of
trace atmospheric species for Mars.

**5.2 Atmospheric State**

**[Dynamics, Transport, and Winds]{.underline}**

Measurements of atmospheric dynamics, winds, and transport would be a
primary objective of the MSO mission. Mars is a very windy planet, owing
fundamentally to its thin, mostly CO2, atmosphere. Despite the low
density of the atmosphere, the winds are strong enough to raise large
amounts of dust from the surface. At times, the planet becomes almost
completely enshrouded in a near-global veil of dust in the atmosphere.
The atmospheric dust has an enormous impact on the thermal structure of
the atmosphere and thus on the winds (and therefore on the dust raising
itself) due to the fact that it strongly absorbs solar radiation and
heats the thin atmosphere very strongly. The dust also produces strong
effects via its interaction with IR radiation. The winds on Mars
transport water vapor, water ice clouds, and CO2 ice clouds, along with
CO2 itself to the condensing and subliming polar caps. The winds also
transport and mix all of the minor atmospheric gaseous constituents
(e.g., ozone, carbon monoxide, H2O2, etc.), and they are a primary
player in all surface-atmosphere interactions and exchanges on the
planet. If there are local and/or transient sources of gases such as
methane and SO2 on Mars, then the winds are critical in determining the
distribution of these gases in the atmosphere. Finally, the winds are of
crucial importance for spacecraft EDL on Mars.

Despite the huge importance of winds on Mars, at present there are
remarkably very few direct observations of them. Only the Viking Landers
and Mars Pathfinder have obtained in-situ wind measurements, at the
surface. The only direct wind measurements above the surface have been
made from the Earth, and both the spatial resolution and the accuracy
(as well as the coverage) of these are very limited. Some winds have
been derived from cloud tracking imagery, but these tend to be
problematic -- most strongly because of the difficulty of determining
what altitudes they are relevant to. A primary objective of MSO would be
to obtain good direct wind measurements for the first time, with both
complete global and local time-of-day coverage, from near the surface to
altitudes of \~80 km or more. When combined with good measurements of
the atmospheric temperatures and the dust and water abundances this
would enable, for the first time, a complete characterization of the
general circulation of Mars. The latter is the central player in the
climate of Mars, which is dominated by the global cycles of dust, water,
and CO2. Most importantly, wind measurements throughout the bulk of the
lower atmosphere would enable the transport of atmospheric constituents
to be directly investigated. This would enable the sources and sinks of
these to be determined much better than has previously been possible.
The wind measurements would also greatly enhance our ability to design
and engineer spacecraft EDL, and other spacecraft maneuvers in the lower
atmosphere (e.g., aerocapture). Ultimately, much better knowledge of the
Mars winds would most certainly be required for human missions.

Mars atmospheric dynamics have many fundamental similarities to the
Earth, as well as a number of major differences. Both planets are
relatively small ones with shallow atmospheres that are strongly heated
by the Sun. Both planets are rapidly rotating, such that the effects of
the planetary rotation are of dominant importance for large-scale
motions. Both planets have very similar obliquities at present. These
very basic similarities lead to many similarities in the atmospheric
dynamics, at both large and smaller scales. Mars has traveling
mid-latitude weather systems fundamentally similar to those on Earth.
Mars has forced quasi-stationary waves fundamentally similar to those on
Earth. Mars has a Hadley cell circulation, which is of tremendous
importance for the tropical and subtropical atmospheric circulation.
There are also some very large differences between the two planets. Mars
has no oceans that strongly buffer seasonal and diurnal temperature
changes. One consequence of this is that the summer hemisphere
atmospheric dynamics on Mars differs much more greatly from that in the
winter hemisphere than it does on Earth. Mars has a very eccentric
orbit, which plays a critical role in the "seasonality" on Mars. In
particular, the dust storm season occurs during the portion of the
annual cycle when Mars is closest to the Sun and the atmospheric heating
is strongest. The thinness and CO2 composition of the Mars atmosphere
cause it to respond much more strongly to the diurnal solar forcing,
such that Mars experiences very large thermal tides by comparison with
Earth. The smaller size of Mars causes the Hadley cell to be much larger
in planetary extent than on Earth; the stronger atmospheric heating
causes it to be much stronger and thus much more effective in producing
cross-equatorial transport. The Mars topography is huge and
characterized by a unique hemispheric asymmetry. This plays a key role
in causing the Hadley cell to be much stronger in southern summer than
in northern summer, which has a crucial impact on the key climate
cycles. The topography also is of critical importance for the weather
systems in producing storm zones, and for the quasi-stationary waves
that can produce regional climates. Finally, the lack of a tropopause
and the lack of substantial ozone on Mars contribute to yield an
essentially continuous lower atmosphere that essentially extends from
the ground up to the homopause (\~100 km).

While existing measurements of the Mars atmosphere from orbit have now
enabled a first-order characterization of temperatures, dust, and water
vapor, they have not enabled a characterization of winds and transport.
Some indirect inference of winds from temperatures can be made, but this
is generally much more difficult to do for Mars than for Earth because
of the basic fact that motions are much less geostrophic on Mars
(largely because Mars is a much windier planet). Direct measurements of
winds from orbit would be required in order to determine the true global
distribution of winds in the Mars atmosphere. Near-surface winds, which
are absolutely critical for atmosphere-surface interactions, can only be
determined by direct observations. The planetary-scale winds on Mars are
considerably more two-dimensional than those in the Earth's atmosphere
because of the very strong atmospheric heating and the much stronger
Hadley cell. The best current Mars circulation models show that
meridional jets exist in the Mars atmosphere under dusty conditions,
with speeds of \~40--80 m/s and more. Thus two-dimensional (vector)
measurements of the horizontal winds would be critically important, in
general, for Mars. They are especially so in the context of north-south
transport. Such measurements would also be required in order to define
the traveling weather systems, the quasi-stationary waves, and the
thermal tides, which all have strong zonal and meridional components.
The characteristic vertical scale for many atmospheric motions is of the
order of the pressure scale height, about 10 km for Mars. Many motions
have larger vertical scales than this, but there are many examples of
motions that have smaller vertical scales -- e.g., gravity waves, slope
winds, and western boundary currents. Thus the winds need to be measured
with a minimum vertical resolution of \~10 km, but any possible
improvement in this resolution is highly desirable. A vertical
resolution of \~5 km would be much better than one of \~10 km,
especially so in the context of transport. It would also be commensurate
with the MSO measurements of temperatures, dust, and water. The MSO
orbit, which would be high-inclination, essentially limits the
longitudinal resolution of the wind measurements. At an altitude of
\~300 km this means that there are about 13 orbits per sol, which limits
the longitude resolution to \~30°. For large-scale circulations, this is
not a primary limitation because of the fact that the major components
of the general circulation are much closer to planetary scale than on
Earth. Finally, the desired latitude resolution is \~5°; 10° is a
minimum resolution. Extremely sharp latitudinal gradients of wind exist
on Mars, especially in association with the winter polar vortexes, and
in these regions even higher resolutions (\~2--3° or better) are very
desirable, at least at times. In the core of the polar vortexes the
winds tend to be predominantly zonal, so the highest latitude-resolution
wind measurements there could be one-dimensional and still be highly
useful.

Transport is the movement of energy, momentum, and material both
vertically and horizontally from place to place on the planet by
atmospheric winds. The effects of such winds are reflected in
atmospheric temperature structure, in the mass distributions of
non-uniformly mixed gases, and in the structure and intensity of the
wind field itself. The transports themselves are key to understanding
the cycles of dust and water vapor and the redistribution of heat and
momentum. Despite the short radiative time constant of the thin CO2
Martian atmosphere, transports of heat and momentum are sufficient to
affect the polar heat balance, the seasonal polar caps and their
interannual variation. Furthermore, the transports of material from
surface sources to atmospheric or surface sinks are important to
explaining the distribution of water vapor and aerosol and potentially
of regional sources of trace gases which have subsurface origins, either
geochemical or biochemical in nature. Vertical mixing and transport in
the atmosphere also affect photochemically important gases (e.g., CO).

In principle transports of energy, momentum or material can be derived
from direct measurements of winds and of the transported quantity (e.g.,
temperature or water vapor). In practice, remote sensing techniques from
orbital platforms have adequate temporal and spatial coverage to address
directly only regional and global components of the transport and even
then (and even for Earth) remotely sensed winds seldom have the
precision to directly derive atmospheric transports. In part this is due
to the fact that it is not just the flux of material or energy that is
important, but its flux-divergence, which requires spatial derivatives
that tend to amplify measurement noise and error.

Thus, the determination of transport and its inverse, in which trace
species are tracked back to their sources, is done using atmospheric
circulation models. One approach is to validate the model predictions by
comparison with observed winds and with the transported trace gases
themselves. This requires having a good description of atmospheric
heating which for Mars depends upon the distributions of dust and ice,
surface albedo and thermal inertia. A second approach is to use methods
of data assimilation in which a blend of model predictions and direct
observations is produced, essentially applying systematic corrections to
the model predictions as a function of time. Data assimilation methods
have the advantage that the modeled quantities can be analyzed more
objectively (than trial and error methods) to diagnose both the model
limitations and observational systematic errors.

In either case, whether using assimilation or simulation alone, the
analysis works best when a suite of tracers with different life cycles
and spatial variations is used. Gases with lifetimes of months to a few
years are most diagnostic; shorter lifetimes will localize the tracer,
while long lifetimes will produce uniformly mixed distributions that are
no longer independent. One exception to the latter is that
non-condensable inert gases (e.g., argon or CO) can reveal seasonal
transport given the massive condensation/sublimation of up to 30% of the
Martian atmosphere in the polar regions. For transport purposes, water
vapor, carbon dioxide, ozone, methane and their isotopes, together with
some shorter lived, higher order hydrocarbons, can provide the variety
of trace gases needed to test and improve model predictions and our
basic understanding of the physical processes involved in their origin,
loss and transport.

The value of wind measurements should not be underestimated in this
analysis of transport and the ability to define surface (and thus
subsurface) sources and sinks. While temperature measurements alone
provide powerful constraints on model simulations, particularly when
coupled with observations of aerosol loading, the system is not wholly
constrained without the additional observations of winds and/or
pressure.

***Measurement Goals:*** The baseline set of wind measurement objectives
for MSO would be composed of two-dimensional (vector) horizontal winds,
observed over the whole globe each sol, on both the day and night sides.
The minimum precision that is needed is \~10 meters/sec, but the goal is
\~5 meters/sec over at least a good portion of the lower atmosphere. The
minimum vertical resolution for the wind measurements is \~10 km, but
doing better than this is an extremely important goal. The minimum
latitude resolution of the wind measurements is \~10°, but a resolution
of \~5° is much better. Even higher latitudinal resolutions are very
desirable at least at times, in certain regions of the planet (e.g., the
winter polar vortexes). The wind measurements need to be obtained
throughout the lower atmosphere, extending from within the lowest scale
height to \~80 km. Even higher altitude measurements would be very
valuable, especially in relation to the Mars Scout aeronomy mission
proposed for launch in 2011. Good wind measurements in the lowest scale
height would be very valuable for the scientific objectives of the MSO
Mission, but it is recognized that these would be difficult to obtain.
It is fully recognized that a wind instrument would have to make a
number of trade-offs between the various measurement specifications
(e.g., precision, vertical resolution, and latitudinal resolution) in
order to be feasible in the basic contexts of cost, volume, power, and
schedule. A wind instrument that would be flexible enough to allow some
of these trade-offs to be made during the mission (and even changed from
sol to sol) is viewed as distinctly advantageous.

An understanding of transport and the ability to invert tracer
distributions to define regional sources and sinks (including
photochemical loss higher in the atmosphere) essentially requires the
same wind, temperature and opacity measurements as described previously
plus observations of the time-varying concentrations of trace gases
(including water vapor). Optimally, a suite of trace gases having a
range of lifetimes and/or spatially varying sources should be measured.
Requirements include planetary-scale coverage throughout at least one
Martian year and preferably for several diurnal cycles. Measuring both
horizontal components of the wind is much preferred. Various
combinations of precision and vertical resolution of the wind components
could produce comparable results, but terrestrial experience has shown
that the value of wind measurements on model simulations and transport
inversion is significantly greater when the vertical resolution of wind
observations is comparable to an atmospheric scale height or better and
the latitude resolution is 10˚ or better, particularly in the vicinity
of the stronger jets.

**[Climate Characterization]{.underline}**

An initial assessment of the current Martian climatology has been
obtained by data returned from Viking, Mars Global Surveyor (MGS), Mars
Odyssey, Mars Express, and Mars Reconnaissance Orbiter (MRO). The key
atmospheric state parameters observed to date have been the atmospheric
temperature profile as a function of height, the column-integrated
optical depth of dust and water ice aerosol, and the column abundance of
water vapor. These observations show a distinct and generally repeatable
dependence on season, latitude, and longitude, although significant
variations from one Martian year to the next are also observed, most
notably associated with the intermittent occurrence of global-scale dust
storm events.

Martian seasons are generally similar to those of Earth because of the
similar obliquities of the two planets. Differences between Martian
seasons are more extreme, however, because of the lack of moderating
oceans and because of the large Martian orbital eccentricity. Mars'
perihelion is currently near southern summer solstice; this, coupled
with inter-hemispheric topography differences, enhances the differences
in climate between the two hemispheres. TES on MGS created an invaluable
dataset showing geographical and seasonal variations in dust,
temperature, clouds and water vapor for three Mars years. MOC wide-angle
daily global maps in red and blue wavelengths left a similar legacy in
the visible portion of the spectrum for four Mars years, and MARCI on
MRO is adding to this continuous record. Although many of the weather
phenomena observed are fairly repeatable from year to year, there are
notable exceptions -- especially for the dust cycle. An early dust
storm, similar to the first 1977 Viking storm, was observed in Mars Year
(MY) 25 (2001), and a perihelic storm similar to the second 1977 Viking
storm was observed in the most recent year (2007, MY 28). There were no
such "global-scale events" in MY 24, 26, or 27. Large regional storms
moved from the northern hemisphere to the South Pole in MY 26 and MY 27.
Although valuable data on these events have been obtained by MGS and
MRO, the reasons for the large interannual variations in dust activity
are still obscure. They may be related to changes in surface albedo due
to deposition and removal of dust that can be monitored by a wide-angle
camera similar to MOC and MARCI.

The inter-hemispheric asymmetry extends to the Martian water cycle: the
northern hemisphere is moister than the southern hemisphere. This is
partly related to the fact that the perennial north polar cap is a major
source for atmospheric vapor, but asymmetric transport of the vapor also
plays a major role. The aphelion cloud belt that forms in the ascending
branch of the Hadley Cell during northern summer appears to act as a
block, preventing the transport of water to the southern hemisphere. No
such impediment exists to transfer in the opposite direction during
southern spring and summer, and so there is in effect a pump moving
water from south to north at this time.

Although much progress has been made, knowledge of the current
climatology is limited by lack of observations in several key areas.
Most importantly, there are currently no missions planned after Mars
Express and MRO capable of global-scale monitoring of the atmospheric
state. This is a serious shortfall. A long baseline of observations is
crucial to understanding the current climate and the physical mechanisms
that govern it, both to be able to model past climates, and to
successfully predict the current atmospheric state for future robotic
and manned landings. Given that global-scale dust storm events occur
intermittently only about once every three martian years, it is likely
that at least ten Mars years of observations would be needed to
adequately define the variability associated with that important
process. A continuous record of at least that length would be preferred
to better understand the feedbacks and cause-and-effect relations
between processes on seasonal and year-to-year time scales, but
minimizing the gap in observations after MRO and Mars Express is the
next best option. Therefore, it is important to include atmospheric
monitoring on MSO, which represents the next possible chance for such
observations.

Other aspects of the current climate not well sampled so far by
observations are diurnal variations (for example, both MGS and MRO
observe only afternoon and early morning local time), direct
measurements of wind velocity, and the vertical distribution of aerosols
and water vapor. Full coverage of the diurnal cycle would greatly help
the identification of wavemodes and the modeling of the water cycle
including condensate clouds. The addition of wind observations would be
an essentially new observation that would provide important new
constraints for circulation models. Information on the vertical
distribution of aerosols would greatly improve both the modeling of
heating rates and the knowledge of circulation patterns through tracer
motions.

***Measurement Goals:*** The most important observations to make for
climate monitoring are:

-   Atmospheric temperature

-   Optical depth of dust and water ice aerosols

-   Wind velocity

-   Water vapor abundance

-   Surface pressure, temperature, and albedo

Each of the above quantities (except for the surface values) should be
determined as a function of height with a vertical resolution of at
least a pressure scale height (10 km minimum, 5 km preferred) over a
broad vertical range (from the surface to at least 60 km). The desired
spatial coverage and resolution is driven by the size and extent of
features important to climate characterization. Global coverage,
obtained with near-continuous (\~85% or better) observations at least
every 5°--10° of latitude along the orbit track is required so that
phenomena such as regional-scale dust storms and the polar vortex are
adequately resolved. Climate phenomena change on time scales ranging
from a day or less (e.g. waves, evolution of dust storms) to seasonal
(e.g. formation and decay of polar hoods and aphelion season water ice
cloud belt), to interannual (e.g. occurrence of global-scale dust storm
events and redistribution of surface dust). Therefore, observations with
global coverage should be taken on a nearly daily basis for at least one
Martian year, and preferably for several Martian years.

**[Mesoscale Atmospheric Processes]{.underline}**

Observations of Martian atmospheric phenomena at the mesoscale level
(i.e., from several to hundreds of kilometers) were first obtained by
the camera systems onboard the Mariner 9 orbiter. Although the Viking
orbiter cameras expanded significantly upon both the number of
observations and the scope of phenomena imaged, it was only with the
arrival of MGS that such data were obtained with both global and
systematic coverage. Through the use of discrete dust and water ice
aerosol clouds (or lack thereof) and their subsequent evolution,
researchers have been able to examine a range of mesoscale processes
that include frontal systems, transient eddies, boundary layer stresses,
volatile transport, and aerosol microphysics. Although such studies have
revealed repeatability for some of the observed phenomena (e.g.,
cross-equatorial storms, polar storm fronts), significant spatial and
interannual variability is also seen. This includes the poorly
understood transition of common localized lifting of dust to the less
frequent global-scale events.

Despite the insight provided by the MGS record from 1997 through 2006, a
longer temporal baseline is critical for further characterizing the
annual and the interannual behavior of mesoscale processes in the
Martian atmosphere. Fortunately, the Mars Reconnaissance Orbiter (MRO)
has been able to fill the void left by MGS in terms of instrument
capability and coverage. However, the unexpected loss of MGS raises the
possibility of a lack of observing capability in the post-MRO era.
Neither the other current missions (Mars Odyssey, Mars Express) nor
those planned (Mars Scout 2011) possesses the same capability for global
monitoring. A gap in the continuous record will have the particular
consequence of greatly hampering efforts to study the interannual
variability of processes that only occur ever few years, such as the
aforementioned development of global dust events. As a result, MSO would
offer the first and best opportunity to continue the observational
record of atmospheric mesoscale processes.

A missing facet of mesoscale studies enabled by MGS and MRO global
observations is that of diurnal variations; the orbits of MGS and MRO
allow for the (nadir) viewing of only two local times. Because a variety
of mesoscale phenomena are sensitive to time-of-day effects (e.g., solar
insolation, downwelling radiance, and abundance of water vapor), the
ability of MSO to sample the diurnal cycle would offer a significant
enhancement to the MGS-MRO record by adding another dimension of
information. In addition, the value of diurnal coverage by MSO would be
greatly amplified by any overlap in operations with MRO. By providing
direct comparison of the same location within the same Martian day (as
opposed to doing so on a climatological basis), one could much better
isolate the diurnal variation of the observed phenomena (e.g., dust
lifting center, eddy activity) from the sol-to-sol evolution.

***Measurement Goals:*** The key aspects of the MGS-MRO atmospheric
mesoscale observations that should be included in an MSO capability are:

-   Ability to observe continuously

-   Limb-to-limb coverage with a resolution of \~1--2 km at nadir (5--10
    km on the limb)

-   Ability to discriminate between condensate and dust aerosols

The above requirements would produce a global map of Mars, and limb
observations at 26 longitudes every day.

**5.3 Surface Change Science**

The surface science objectives of the MSO mission are focused on the
surface properties and morphologies associated with surface-atmosphere
interactions and surface change. Experience has shown that these changes
occur on both local and regional scales, and can occur on intermittent,
seasonal, and year-to-year time scales. Some of the most dynamic changes
occur in the polar regions, but numerous changes occur in the equatorial
and mid-latitudes, associated with aeolian, mass wasting, impact, and
possibly, volatile release, activity.

**[Regional and Local Changes]{.underline}**

A variety of changes have been observed that involve process occurring
at regional scales. The most conspicuous of these are widespread albedo
changes associated with regional and global dust storms. These albedo
changes are caused by the deposition and removal of thin layers of
surface dust. This process can, however, have significant implications
for the long-term evolution of the Martian surface, acting as the major
element of contemporary erosion and geologic change. Observed changes in
surface albedo also provide the best indication of Martian wind
transport on a global scale. These changes directly influence surface
temperatures, which in turn influence the global circulation. Wind
stress is enhanced in darkened areas and decreased in brightened areas,
which can produce a positive feedback mechanism by which the albedo
changes strengthen the winds that generate the changes. Recent models
predict a year-to-year global change in surface air temperatures as a
result of these changes in surface heating that may play an important
role in climate fluctuations on yearly, decadal, or longer timescales.

A variety of small (10's to 100's of meter)-scale features, including
dust devil tracks and slope streaks, exhibit regional differences in
their spatial occurrence and temporal variability. These features are
readily observed at 2--5 meters/pixel resolution, and their occurrence
has been well documented using MOC imaging. Convective vortices - dust
devils -- may form an important component of the dust lifting processes
on Mars. These features have high tangential winds and strong vertical
velocities, and are very efficient at lifting dust from the surface and
raising it to high altitudes. This process is less size-dependent than
near-surface wind stress lifting. Dust devil tracks provide an excellent
means of observing the surface-atmosphere interface across the planet.
Dust devils may play an important role in lifting surface dust and
raising the global opacity, which can then lead to enhanced global
heating and wind velocities, which may then trigger regional dust
lifting, leading to large-scale dust storms. Dust devils may also be an
important component of the long-term global dust cycle, acting to lift
dust and prevent its long-term accumulation. Previous mapping has
demonstrated that dust devils, as observed through imaging of dust devil
tracks, vary in occurrence on both seasonal and year-to-year timescales.
A long-term record of these features, acquired through meter-scale
imaging, is critical to developing models of the current climate and its
interannual variability.

Slope streaks on Mars are a class of active surface process that were
first observed by the Viking Orbiters, and have been shown to have wide
distributions using MOC imaging. These features are 10's to 100's of
meters wide and are strongly correlated with regions of low thermal
inertia. Two types of models have been suggested: dry mass movement in
the form of dust avalanches and wet liquid flow that transports or
stains the surface material. Recent HiRISE imaging shows that these
features transport significant amounts of materials, with some having
meter-thick deposits, and are often associated with localized
disturbances (i.e. rock falls). Long-term observations of these active
features would provide a means of studying active processes on Mars, and
determining the nature of the role of volatiles in this activity.

A final example of small-scale processes that occur on regional and
global scales is the formation of small impact craters over time. The
current impact flux has been observed using MOC high-resolution and
wide-angle images, which revealed the formation of 20 new impact craters
2 to 150 meters in diameter that formed during the decade of MGS
observations. The impact flux rate is currently only weakly constrained
by the MOC data; MRO CTX and MARCI observations will extend this record,
but the capability for a decade-long survey of impacts from wide-angle
imaging, together with the ability to inspect the morphology of these
fresh craters at meter-scales, would significantly improve the
statistical determination of the current crater flux. Of particular
interest would be the formation of modest sized (5--50 m diameter)
craters in the mid- and high-latitude regions to use as probes into the
martian sub-surface in regions of potential near-surface ground ice.

In addition to small-scale features that occur on regional scales, a
variety of small features are of interest in determining the nature of
the current Martian surface and the role of volatiles in highly
localized environments. Examples of these features include gullies, with
the potential for isolated, current activity, avalanches and rock falls,
and dune motions. These processes occur very infrequently or at rates
that require long-term monitoring at meter-scales to detect changes at
the expected rate of motion on Mars. When these changes occur however,
they have been observed, or are expected, to occur at spatial scales of
meters to tens of meters, allowing their occurrence to be detected and
studied with meter-scale imaging. While rare, the occurrence of these
changes is critical to our understanding of the occurrence and magnitude
of volatile release from the sub-surface and the current rate of surface
evolution. For example, repeated MOC imaging of young gully features
revealed the occurrence of several cases where changes, potentially due
to the release of volatiles, were detected of the course of several
years. Several models have been proposed for gully formation, including
discharge of liquid water from sub-surface aquifers, the melting of pore
ice that diffused inward from the atmosphere during periods of colder
temperatures, or melting of a snow layer deposited during periods of
higher obliquity when surface ice was stable at these latitudes are the
most plausible. Long-term monitoring of gullies is a high-priority for
determining their current activity and for testing these models of gully
formation.

A wide range of surface morphologies have been observed with meter-scale
imaging, including intracrater and polar layered deposits, fluvial
features ranging from chaotic outflows, streamlined islands, eroded
terraces, to deltaic deposits, mid-latitude mantles and morphologies
potentially related to glacial process, and a full range of aeolian
features. MOC imaged approximately 5% of the surface at 3--6 m/pixel
resolution. The MRO HiRISE will image far less. Clearly, a vast fraction
of Mars remains to be observed at meter-scale resolution. Given the
suite of discoveries made to date by MOC and HiRISE, it is likely that
additional discoveries and insights would be obtained from meter-scale
imaging of the remainder of Mars.

***Measurement Goals:*** For the observation of albedo changes,
wide-angle imaging at 50--100 meters/pixel is required along with
calibrated broadband solar reflectance from multiple emission angles to
separate surface and atmospheric components. For the observation of
morphology changes, the scale of features to be observed is typically
5--100 meters for gullies, layers, slope streaks, dust devil tracks, and
small impact craters. These features could be detected and their
occurrence and distribution mapped using imaging at 1 meter/pixel.

**[Polar Regions]{.underline}**

The polar regions of planets are especially sensitive to processes that
modify climate patterns because of the strong seasonal modulation of
insolation in those areas. For example, the effects of global warming on
Earth are strongest in the polar-regions. Following discovery of layered
terrains in the Martian polar-regions by space missions in the 1970s the
principal conjecture concerning their significance is that they are
associated with changes in climate induced by astronomically induced
changes in orbital parameters. Perturbations of Mars' orbit by the
gravitational forces from other planets cause the inclination of its
rotational axis, the eccentricity of its orbit, and the longitude of
perihelion to vary with periods of tens of thousands to millions of
years. These in turn affect the distribution of insolation on the
planet. The effects are largest at the poles where phase changes occur,
and the Martian volatile cycles therefore respond strongly. In
particular, the near vicinities of the two perennial caps -- the
portions of the caps that survive their respective summer seasons --
contain clues as to climate variations on Mars.

The region surrounding the north polar perennial cap is of special
interest because this cap is a major source of atmospheric water on Mars
at the present time. It is presumed that the layers that underlie the
perennial cap consist of mixtures of water ice with varying amounts of
dust or sand that were deposited in the past during favorable
combinations of orbital parameters. However, it is difficult to
extrapolate to previous combinations of orbital parameters until the
processes that are currently active are fully understood. The mass
balance of the perennial north polar cap (i.e. whether it is gaining or
losing water currently) has still not been definitively determined from
the data that are currently available. Profiles of wind speed and water
vapor concentration as functions of latitude and altitude would help
resolve the issue by allowing estimates of the flux of water returned to
the cap in late summer and autumn. Circulation models that are
constrained by observations of the current north perennial cap water
cycle could then be used with other combinations of orbital parameters
to extrapolate the Martian water cycle to past and future climates.

The south perennial cap unlike its northern counterpart consists of a
veneer of CO2 ice perhaps 10--20 meters thick set on a basement that
consists mostly of water ice. The residual cap CO2 ice remains
relatively dust free through the summer, and the resulting high albedo
prevents its sublimation. The frost surface in the south cap consists of
a bewildering range of morphologies (popularly referred to as "Swiss
cheese") including often-complex collapse features surrounded by flat
mesas as well as complex ridges and valleys resembling fingerprints. Up
to six individual layers can be identified in the various structures.
Observations from MY 24--27 reveal that the pits grew larger during the
four-MY MGS mission. For a large subclass of the features the rate of
increase in diameter is about 4 meters / pixel / MY, requiring a minimum
resolution of 2 meters / pixel for measurement. These results indicate
that there are components to the CO2 cycle that have periods of decades
or centuries and suggest that there is a net transfer of CO2 from the
Residual South Polar Cap to the atmosphere averaged over the year.

There is strong evidence that we have not just happened to observe the
residual south cap at the exact time of its disappearance. Comparisons
of images of the residual cap obtained by Mariner 9 (MY 10) and MGS (MY
24) clearly show that a significant area that is currently covered with
residual CO2 frost was unfrosted in the former year. Therefore some
process, currently unknown, led to net deposition of CO2. Taken
together, the current erosion of the "Swiss cheese" mentioned earlier
and the net deposition of frost in the past suggest a component of
climate change with a time scale of decades that is not understood. This
cycle has strong implications for the Martian water cycle because
exposure and sublimation of the water ice beneath an area from which CO2
frost has been removed could be an important factor. Deciphering these
clues would require continuous monitoring of the residual south cap for
decades both at high resolution (\>2 meters/pixel) and lower resolution
(\> 500 meters/pixel).

***Measurement Goals:*** High-resolution imaging of the residual polar
caps at 2 meters per pixel or better is required. This could be
accomplished with an imager with 1 meter per pixel resolution if the air
mass to the pole is 2. Wide-angle imaging of the residual caps at
resolution of 500 meters per pixel with a minimum of red and blue
filters is also desired for monitoring larger areas.

**5.4 Summary of Science Measurement Goals**

Below is a summary of the measurement goals for all quantities as
recommended by the SDT.

+------------------+----------------------+-----------+--------------+
| **               |                      |           |              |
| Photochemistry** |                      |           |              |
+------------------+----------------------+-----------+--------------+
| *Quantity*       | *Sensitivity*        | *Vertical | *Vertical    |
|                  |                      | range*    | resolution*  |
+------------------+----------------------+-----------+--------------+
| H2O              | 10 ppbv              | 0--60 km  | 5 km         |
+------------------+----------------------+-----------+--------------+
| H2O2             | 100 pptv             | 0--40 km  | 5 km         |
+------------------+----------------------+-----------+--------------+
| CO               | 1 ppmv               | 0--60 km  | 5 km         |
+------------------+----------------------+-----------+--------------+
| O3               | 1 ppbv               | 0--40 km  | 5 km         |
+------------------+----------------------+-----------+--------------+
| **Isotopic       |                      |           |              |
| Measurements**   |                      |           |              |
+------------------+----------------------+-----------+--------------+
| *Quantity*       | *Sensitivity*        | *Vertical | *Vertical    |
|                  |                      | range*    | resolution*  |
+------------------+----------------------+-----------+--------------+
| D/H (in H2O)     | 5% precision, 10%    | 0--40 km  | 5 km         |
|                  | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| 13C/14C (in CO2) | 0.5% precision, 2%   | 0--20 km  | 5 km         |
|                  | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| 16O/18O (in CO2, | 0.5% precision, 2%   | 0--20 km  | 5 km         |
| H2O)             | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| 17O/18O (in CO2, | 0.5% precision, 2%   | 0--20 km  | 5 km         |
| H2O)             | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| **Trace Gases**  |                      |           |              |
+------------------+----------------------+-----------+--------------+
| *Quantity*       | *Sensitivity*        | *Vertical | *Vertical    |
|                  |                      | range*    | resolution*  |
+------------------+----------------------+-----------+--------------+
| Methane          | 50 pptv              | 0--60 km  | 5 km         |
+------------------+----------------------+-----------+--------------+
| Other species    | 1 pptv               | ------    | ------       |
+------------------+----------------------+-----------+--------------+
| **Atmospheric    |                      |           |              |
| State**          |                      |           |              |
+------------------+----------------------+-----------+--------------+
| *Quantity*       | *Sensitivity*        | *Vertical | *Vertical    |
|                  |                      | range*    | resolution*  |
+------------------+----------------------+-----------+--------------+
| Temperature      | 2 K precision, 2 K   | 0--80 km  | 5 km         |
|                  | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| Wind Velocity    | 10 m/sec precision   | 5--80 km  | 10 km        |
| (2-D)            |                      |           |              |
+------------------+----------------------+-----------+--------------+
| Aerosol Optical  | 0.05 precision at    | 0--60 km  | 5 km         |
| Depth            | visible wavelength   |           |              |
|                  |                      |           |              |
| (dust and water  |                      |           |              |
| ice)             |                      |           |              |
+------------------+----------------------+-----------+--------------+
| Water Vapor      | 10% precision, 10%   | 0--60 km  | 5 km         |
|                  | accuracy             |           |              |
+------------------+----------------------+-----------+--------------+
| **Surface        |                      |           |              |
| Properties**     |                      |           |              |
+------------------+----------------------+-----------+--------------+
| *Quantity*       | *Sensitivity*        | *Vertical | *Vertical    |
|                  |                      | range*    | resolution*  |
+------------------+----------------------+-----------+--------------+
| Surface          | 1 K precision (at    | ------    | ------       |
| Temperature      | 240 K)               |           |              |
+------------------+----------------------+-----------+--------------+
| Solar-band       | 5% precision         | ------    | ------       |
| integrated       |                      |           |              |
| albedo           |                      |           |              |
+------------------+----------------------+-----------+--------------+

**Table 2**. Recommended measurement goals for MSO.

In addition to the above, mapped quantities should be obtained with at
least 90% coverage along the orbit track, amounting to at least every 5°
of latitude along-track. Mapped quantities include all photochemical
products, methane, atmospheric state quantities, and surface properties.

Imaging capability should include a high-resolution camera with 1 meter
per pixel resolution and a wide-angle camera with 500 meters per pixel
resolution and at least red and blue color channels.

**6. MSO MISSION IMPLEMENTATION**

**6.1 Sample Payload**

In order to gain an understanding of the technical and resource
requirements of MSO, the MSO team has been working with a set of
strawman instruments that represent one possible response to the Plan A
requirements detailed by the MEPAG SAG-2 report. This strawman payload,
based on capabilities demonstrated either at Mars or in studies of the
Earth's upper atmosphere, is comprised of five instruments:

**Solar Occultation FTIR spectrometer**

A solar occultation FTIR spectrometer would characterize the composition
of the Martian atmosphere from the surface to an altitude of 100 km. The
global coverage of this instrument is controlled by the inclination of
the spacecraft orbit. As the inclination increases poleward of 74^o^,
frequency of sampling, particularly at low and mid latitudes, is
compromised. The orbit trades have been studied by the project and are
discussed below. The utility of combining a bright source and very high
spectral resolution for surveying atmospheric trace gases is well
demonstrated in observations of the upper atmosphere of Earth

**Sub-millimeter spectrometer**

A sub-millimeter spectrometer would have a dual purpose: 1) Determine
the wind velocity by measuring the Doppler shift of spectral features
along the line of sight to the limb, and 2) It would measure the
concentrations, locations and dispersal characteristics of specific
atmospheric signatures, including water, as well as the temperature
distribution of the atmosphere. The global atmosphere would be sampled
on a daily basis, through several seasonal cycles. Observations over at
least 1 Mars year are required to enable the diurnal and seasonal cycles
to be distinguished.

**Wide-angle camera (MRO/MARCI-like)**

The wide-angle camera would provide daily global monitoring of the
distribution of atmospheric dust and ices, together with surveys of the
surface albedo. This would continue the synoptic monitoring of the
surface and atmosphere carried out by MGS and MRO.

**Thermal IR spectrometer system**

A visual/infrared spectrometer with moderate spectral resolution would
provide daily global measurements of the atmospheric dust and ices for
correlation with the species and temperature measurements from the
sub-millimeter spectrometer. This instrument would also carry out
mineralogical studies and provide inputs for global atmospheric
simulation models.

**High-resolution camera (HiRISE-class or TBD)**

A camera system with similar capability and mass to the MRO-HiRISE
instrument, would provide additional surface coverage beyond that
achievable with MRO, and allow for continued and longer-term monitoring
of active processes. A HiRISE-class (sub-meter resolution) imager is
desired for the continued certification (beyond MRO) of future landing
sites.

**6.2 Mission and Spacecraft Design**

The MSO mission would be a 10 Earth year on-orbit mission. After a
nine-month cruise, MSO would be propulsively inserted into a 1-sol Mars
orbit and then aerobrake for approximately 6 to 9 months, followed by
about 1 month of spacecraft and instrument commissioning. Accelerometer
measurements made during aerobraking would complement the
characterization of upper atmospheric structure carried out by previous
Mars orbiters (MGS, ODY, MRO) during their aerobraking phases and, most
likely, of the high-altitude observations by a still operating Mars
Scout aeronomy mission. Observations by the 2011 Mars Scout would aid
the monitoring of the Mars atmosphere during the MSO aerobraking phase,
which occurs during that part of the Mars year when great dust storms
can occur, increasing densities at aerobraking altitudes (\> 100 km).

MSO would enter a circular orbit of approximately 300 km altitude and
82.5^o^ inclination, and spend approximately 1 Mars year in a Science
Emphasis phase, before increasing its altitude to approximately 400 km.
It would stay in this higher (circular) orbit for the next seven Earth
years, in a Telecom Emphasis phase. Atmospheric science and landing site
selection observations would continue throughout this latter phase, but
with potentially lesser frequency than in the science phase. MSO could
provide relay to a 2013 lander, e.g. EXOMARS (ESA), and 2016
lander/orbiter (NASA and/or ESA - tentative). The mission timeline
(Figure 1) depicts the key mission events.

![](media/image1.png){width="8.0in" height="3.6680555555555556in"}

**[Orbit Tradeoffs]{.underline}**

Orbit selection is driven by a combination of science and relay
communication requirements. Like MRO, science data would be
preferentially taken from a low altitude. At 300 km, a near-circular
orbit results in approximately 13 revolutions per Earth day. The
high-resolution camera and the FTIR trace gas instrument prefer
significantly different orbit inclinations. In order to allow the camera
to have global access and to have robust relay capability and adequate
critical event coverage, high (high-inclination) inclinations are
desired. On the other hand, the FTIR collects its best results, with
most comprehensive latitude coverage, at 74^o^. An orbit trade study was
performed to ensure the primary objectives of both could be met. It was
concluded that the optimal orbit inclination to satisfy all mission
requirements would be 82.5^o^. The selected orbit, which is a
non-Sun-synchronous walking orbit, would have several advantages for
science gathering, but has some impacts on the flight system design.
These consist of:

-   To obtain optimized high-resolution camera coverage, the orbit need
    only be slightly adjusted in altitude to achieve the required
    overlaps of swaths for mapping extended regions (Figure 2).

-   The orbit enables the trace gas and atmospheric state instruments to
    view a full diurnal cycle each season, including solar occultations
    as needed, over a wide range of latitudes in multiple Mars seasons
    (Figure 3).

-   However, the continuously varying solar beta angle of the orbit
    (Figure 4) becomes a significant driver for the mounting of the
    instruments, the trace gas instrument radiator design and placement,
    and the operational scenarios.

After one Mars year, the orbit would be boosted to a near 400 km
altitude for the relay Telecom Emphasis phase. In this orbit, the
camera's spatial resolution is slightly reduced and the trace gas
observations are marginally degraded. However, this phase constitutes an
additional seven Earth years of observing opportunity, extending the
mission to almost a full solar cycle, so the cumulative science return
is expected to be significant. The telecommunications orbit also meets
the safe orbit requirement for planetary protection considerations.

![](media/image2.png){width="4.0in" height="3.201968503937008in"}

![](media/image3.pct){width="3.5722222222222224in"
height="2.8541666666666665in"}

![](media/image4.jpeg){width="4.0in" height="2.9618110236220474in"}

**[Instrument Accommodation]{.underline}**

The MSO Project continues to investigate several approaches to
accommodate the strawman payload. One promising configuration is
described here.

The spacecraft would nominally maintain pointing with a fixed
orientation relative to both nadir and the velocity vector, or
ground-track. This pointing strategy results in widely varying Sun
positions and any solar occultation experiment would have to be mounted
on a two-axis gimbal in order to align with the Sun. If the payload
requires a cold, passive radiator, then the spacecraft would need to
change its flight direction (alternating between flying forward and
flying backwards) on a seasonal basis in order to keep the Sun off the
radiator. Nadir pointing instruments such as the cameras would need to
be able to handle flying in both directions.

High-resolution imaging would require the spacecraft to temporarily roll
off-nadir, as the MRO has been doing routinely during its primary
science phase.

The spacecraft would include 2-axis gimbaled solar arrays and a 2-axis
gimbaled high-gain antenna. Earth communications would be nominally
carried out on a non-interference basis with science observations.

The instruments would be mounted to the sides of the spacecraft,
distributed around the circumference of a cylindrical structure.

**[Observing Environment]{.underline}**

(1) Trace gas observations require a quiet spacecraft. It is anticipated
    that articulations or mechanical actions on the spacecraft may
    introduce vibrations and therefore may not be permitted during
    occultation measurements.

(2) The trace gas experiment requires cooling. The passive thermal
    radiator assumed in the MSO configuration would prohibit viewing by
    this radiator of the Sun, Mars or any spacecraft surface. In this
    concept, the instrument should be gimbaled to align with the Sun
    during occultation measurements and to keep the radiator pointing
    towards the cold sky.

(3) High-resolution imaging also requires a quiet spacecraft as well as
    alignment of the camera with the ground-track to allow push-broom
    images.

(4) To accommodate these very different viewing requirements, instrument
    operations would be time-multiplexed.

(5) The wind instrument requires the spacecraft to keep a constant
    position angle with respect to the velocity vector and the limb
    during its observation.

**[Schedule]{.underline}**

The proposed MSO development schedule is shown below (Figure 5). The
development phase durations and associated products are based on NASA
project management requirements and lessons learned from recent
planetary missions. Early schedule milestones include the Instrument
Announcement of Opportunity release in early 2008 and the Mission
Concept Review by the Fall of 2008.

![](media/image5.pct){width="6.68125in" height="3.2888888888888888in"}

**7. ISSUES AND CONCERNS**

**7.1 Scope and Payload Affordability**

The SDT was told by the MSO Project that the main constraint on the
payload was not mass or power, but cost of the payload. The
instrumentation and subsystems that would be required to carry out the
five major objectives of MSO (atmospheric composition, atmospheric
state, surface change, landing site certification, and
telecommunications) is significant in scope and requires use in some
cases of newly developed technology, with the need for increased
reserves. The MSO SAG-2 had also noted that the desired (called core)
payload was beyond the guidelines that had been provided to the project.
There was hope that a contributed instrument would enable all key
instruments to be accommodated. Certainly, there are international
partners who have the technology to contribute key instruments to MSO,
including any of the three major instruments identified below. While the
SDT hopes that NASA would seek out and pursue contributions to the
payload from potential partners worldwide, the SDT was uncomfortable
relying on the hope that such a contribution would materialize. In
addition, there was some concern that the temptation to invite
participation on a noncompetitive basis might lead to an instrument that
did not have the required measurement capabilities. Thus the SDT
recommends that the required capabilities for the core payload are well
specified, and that all instrument proposals, including proposed
contributions, are competitively evaluated.

Three instruments tend to dominate the strawman payload: 1) a highly
sensitive solar-occultation instrument driven by the need to map spatial
variations in known trace gases and to detect the suspected, but not yet
proven, presence of trace gases whose source could be geochemical or
biochemical in nature; 2) a sub-millimeter spectrometer driven by the
need to map key trace gases, even in the presence of dust, and to
measure winds; and 3) a very high resolution (sub-meter) camera desired
for landing site certification. The SDT notes two points: 1) a high
resolution (one-meter class) camera could address the MSO surface change
science objectives; and 2) the observations of trace gas concentrations
and winds are a critical pairing for understanding transport itself and
the trace gas distribution, including potential regional sources of
bio/geochemically important gases. The SDT thus suggests that NASA
either support the use of MSO for future landing site certification by
providing the funding for the more capable camera or remove the
requirement, with the attendant reliance on imaging for site
certification necessarily provided by current orbiters like MRO. The SDT
emphasizes that accommodation of a more capable imager is a cost, and
not a mass, issue for a 2013 MSO.

**7.2 Flight System Complexity**

The viewing requirements needed to accomplish the MSO objectives likely
involve widespread solar occultation observations, nearly continuous
limb viewing measurements, and surface imaging. These lead naturally to
a spacecraft that would need to return a large volume of data, that
would need to accommodate competing fields of view (possibly including
radiators) of its various instruments, and whose orbit would not be
fixed in local time. Such requirements could lead to a very complicated
operations scenario and possibly a situation in which the spacecraft is
overconstrained, resulting in a loss of key science data or in other
undesirable trade-offs in data acquisition. The SDT recommends that the
implementation for MSO avoid these pitfalls and that the MSO Project
continue to investigate the more promising concepts in this regard. NASA
may wish to consider trade-offs between instrument and spacecraft
capabilities (e.g., providing mass and dollars to include a small
pointing mirror in an instrument). The group did not have time to
investigate how this might be described in an AO or, for that matter, to
investigate in-depth alternate spacecraft/mission configurations.
However, selection of an instrument payload in advance of choosing a
spacecraft implementation would help avoid major problems later.

**APPENDIX 1. MSO Science Definition Team (SDT)**

+-----------------------------+----------------------------------------+
| # SDT Members               | # Affiliations                         |
+-----------------------------+----------------------------------------+
| Michael Smith (Chair)       | ## NASA Goddard Space Flight Center    |
+-----------------------------+----------------------------------------+
| Don Banfield                | Cornell University                     |
+-----------------------------+----------------------------------------+
| Jeffrey Barnes              | Oregon State University                |
+-----------------------------+----------------------------------------+
| Philip Christensen          | Arizona State University               |
+-----------------------------+----------------------------------------+
| R. Todd Clancy              | Space Science Institute                |
+-----------------------------+----------------------------------------+
| Philip James                | University of Toledo (retired)         |
+-----------------------------+----------------------------------------+
| James Kasting               | Pennsylvania State University          |
+-----------------------------+----------------------------------------+
| Paul Wennberg               | California Institute of Technology     |
+-----------------------------+----------------------------------------+
| Daniel Winterhalter         | JPL                                    |
+-----------------------------+----------------------------------------+
| Michael Wolff               | Space Science Institute                |
+-----------------------------+----------------------------------------+
|                             |                                        |
+-----------------------------+----------------------------------------+
| # Ex Officio Members        | ### Affiliations                       |
+-----------------------------+----------------------------------------+
| Michael Meyer               | Mars Program Lead Scientist, NASA      |
|                             | Headquarters                           |
+-----------------------------+----------------------------------------+
| John Mustard                | MEPAG Chair, Brown University          |
+-----------------------------+----------------------------------------+
| Richard Zurek               | JPL                                    |
+-----------------------------+----------------------------------------+
|                             |                                        |
+-----------------------------+----------------------------------------+
| Janis Chodas                | MSO Project Manager, JPL               |
+-----------------------------+----------------------------------------+
| Tomas Komarek               | MSO Mission Concept Manager, JPL       |
+-----------------------------+----------------------------------------+
|                             |                                        |
+-----------------------------+----------------------------------------+

**APPENDIX 2. TRACING MSO REQUIREMENTS TO MEPAG INVESTIGATIONS**

Listed below are the main science objectives proposed for MSO as
determined by the SDT. Beside each science objective is listed the MEPAG
investigations that would be addressed by that objective. The numbering
of MEPAG investigations is taken from the February 2006 report from
MEPAG \[*Mars Science Goals, Objectives, Investigations, and Priorities:
2006*\].

**Atmospheric Composition**

*Photochemistry:* IA3, IA4, IB2, IB3, IB4, IIA3, IIB1

*Methane and Other Subsurface Source Gases:* IB1, IB3, IB4, IC2, IC4,
IIB1

*Isotopic Ratios:* IB2, IB3, IC2, IIB1, IIB4

*Trace Gas Inventory:* IA3, IB1, IB3, IC2, IIB1

**Atmospheric State**

*Dynamics, Transport, and Wind:* IA1, IA4, IIA1, IIA3, IIB2, IIC2

*Climate Characterization:* IA1, IA4, IIA1, IIA3, IIB2, IIC2, IIIA1

*Mesoscale Atmospheric Processes:* IA1, IIA1, IIA2, IIC2

**Surface Change Science**

*Regional and Local Changes:* IA1, IIB5, IIIA2, IIIA5

*Polar Science:* IA1, IA2, IIB5, IIIA1, IIIA5

The table below gives the reverse mapping. Under each MEPAG
investigation listed are the MSO observations that would provide direct,
new information. Note that only those MEPAG investigations that would be
directly address by MSO science observations are listed.

**Goal I: Determine if life ever arose on Mars**

*Investigation IA1 (Current distribution of water in all its forms on
Mars)*

Water vapor abundance as a function of height, location, season, local
time

Water ice cloud optical depth as a function of height, location, season,
local time

Wind velocity as a function of height, location, season, and local time

Daily global imaging

Imaging of gullies and other possible sites of water release

Imaging of swiss cheese terrain and other polar morphologies

*Investigation IA2 (Geologic history of water on Mars)*

Imaging of swiss cheese terrain and other polar morphologies

*Investigation IA3 (Phases containing C, H, O, N, P, and S)*

Trace gas inventory

Photochemistry species mapping

*Investigation IA4 (Potential energy sources available to sustain
biology)*

Climate characterization

Photochemistry species mapping

*Investigation IB1 (Distribution of organic carbon)*

Trace gas inventory

*Investigation IB2 (Distribution of inorganic carbon)*

Photochemistry species mapping

Isotope characterization

*Investigation IB3 (Links between C and H, O, N, P, and S)*

Photochemistry species mapping

Trace gas inventory and mapping

Isotope characterization

*Investigation IB4 (Preservation of reduced compounds on near-surface
through time)*

Photochemistry species mapping

*Investigation IC2 (Spatial distribution of chemical and/or isotopic
signatures)*

Trace gas inventory and mapping

Isotope characterization

*Investigation IC4 (Temporal chemical variations requiring life)*

Trace gas inventory

**Goal II: Understanding the processes and history of climate on Mars**

*Investigation IIA1 (Processes controlling the present distribution of
water, CO2, dust)*

Climate characterization

Wind velocity as a function of height, location, season, and local time

Water vapor abundance as a function of height, location, season, local
time

Daily global imaging

*Investigation IIA2 (Search for microclimates)*

Daily global imaging

*Investigation IIA3 (Production/loss, reaction rates, distribution of
photochemical species)*

Photochemistry species mapping

Climate characterization

Wind velocity as a function of height, location, season, and local time

*Investigation IIB1 (Isotopic, noble gas, trace gas composition of
present atmosphere)*

Photochemistry species mapping

Trace gas inventory

Isotope characterization

*Investigation IIB2 (Escape rates of key species)*

Climate characterization

Wind velocity as a function of height, location, season, and local time

*Investigation IIB4 (Physical and chemical records of past climates)*

Isotope characterization

*Investigation IIB5 (Stratigraphic record of climate change)*

Imaging of swiss cheese terrain and other polar morphologies

*Investigation IIC2 (Monitor lower atmosphere, 0-80 km, on synoptic
scales)*

Climate characterization

Wind velocity as a function of height, location, season, and local time

Daily global imaging

**Goal III: Determine the evolution of the surface and interior of
Mars**

*Investigation IIIA1 (Present state, distribution, and cycling of water,
volatiles)*

Climate characterization

Imaging of swiss cheese terrain and other polar morphologies

*Investigation IIIA2 (Fluvial, subaqueous, other sedimentary processes)*

Daily global imaging

Imaging of dunes, ripples, drifts

*Investigation IIIA5 (Surface-atmosphere interactions on Mars)*

Monitoring changes in albedo patterns

Imaging of dunes, ripples, drifts

**APPENDIX 3. LIST OF ACRONYMS**

AO NASA Announcement of Opportunity for MSO science selection

CTX Context Camera, an instrument on MRO

EDL Entry, Descent, and Landing portion of a landed mission to Mars

ESA European Space Agency

EXOMARS European Space Agency rover mission to Mars for 2013

FBO Fed Biz Opps

FTIR Fourier Transform Infrared

GCM General Circulation Model

HiRISE High-Resolution Imaging Science Experiment, an instrument on MRO

JPL Jet Propulsion Laboratory, California Institute of Technology

MARCI Mars Color Imager, an instrument on MRO

MEP Mars Exploration Program

MEPAG Mars Exploration Program Analysis Group

MGS Mars Global Surveyor, in orbit around Mars 1997--2006

MOC Mars Orbiter Camera, an instrument on MGS

MOI Mars Orbit Insertion

MRO Mars Reconnaissance Orbiter, currently in orbit around Mars

MSO Mars Science Orbiter, to be launched in the 2013 Mars launch
opportunity

MY Mars Year, where Mars Year 1 began on 1 April 1955

NASA National Aeronautics and Space Administration

ODY Mars Odyssey, currently in orbit around Mars

SAG Science Analysis Group

SAG-2 MSO Second Science Analysis Group (report issued May 2007)

SNC A class of meteorites (named for shergottite, nakhlite, and
chassigny) thought to have originated from Mars

SDT Science Definition Team

TES Thermal Emission Spectrometer, an instrument on MGS