1. Martian Atmosphere and Climate Investigations

This article, Martian Atmosphere and Climate Investigations, is Part 3 of the results of the Human Exploration of Mars Science Analysis Group (HEM-SAG) which are presented in five papers in this issue of the Journal of Cosmology (Levine et al., 2010a,b,c,d,e).

HEM polar cap objectives are included as of high scientific value, but are understood to be challenging due to polar night considerations.

A third class of activity is associated with the early evolution of climate and would benefit from the return of samples containing gas inclusions to Earth.

In the following sections two nominal reference missions are identified: an atmospheric reference mission and a reference mission to the north polar dome for deep drilling, in order to define the more site-specific human-enabled mission activities necessary to sample the critical volatile records contained within the polar ice caps. Atmospheric reference mission activities are anticipated to be included in all human missions.

2. Human Science Reference Missions (HSRM): Atmosphere/Climate Science Goals And Approach

The nominal atmospheric reference mission would address the goals outlined in "Quantitative understanding of atmospheric processes" and "Microclimates." These goals would require similar investigations, however a microclimate objective would be more specific, requiring additional planning to optimize site selection for meteorological stations, and timephasing of investigations relative to relevant seasonal cycles. Site selection considerations are described under "location" below.

A proposed baseline is one central station (could be close to the Mars surface habitat) plus remote stations either to broadly characterize the region (co-sited with major geology/biology investigations) or arranged to give three-dimensional information on specific flows associated with microclimate. The microclimate objective would also require (a) reference meteorological station(s) to provide regional context.

The HSRM atmosphere/climate goals and objectives are summarized in Table 2, and the climate instrumentation is summarized in Table 3.

In the human era of exploration, atmospheric measurements at all sites will be seen as important not only to the understanding Mars atmosphere and climate, but also as environmental characterization essential to the interpretation of many biology and geology objectives. The trend towards system science called out in MEPAG 2006 Goal II Objective A "ground-to-exosphere approach to monitoring the Martian atmospheric structure and dynamics," will continue with more emphasis on the mass, heat and momentum fluxes between the three Mars climate components: atmosphere, cryosphere, and planetary surface.

This systems approach will be enabled by advances in Mars Global Circulation Models (GCMs), a doubling in length of global time-series derived from monitoring Mars surface and atmosphere from orbit, new atmospheric vertical structure information from Mars Express and MRO, new anticipated global data sets on aeronomy, atmospheric composition and winds, and by network science and coordinated lander-orbiter campaigns. In 2007, trends in Mars GCM development are towards coupling of upper and lower atmosphere (e.g., Angelats i Coll et al 2005), coupling with regolith models, integrating models of atmospheric chemistry and dynamics (Lefevre et al, 2004; Moudden and McConnell 2007), multiscale, nested models — where small scale surface-atmosphere interactions could be studied within the context of global transport (Richardson et al 2007, Moudden and McConnell 2005), and data assimilation (Montabone et al 2007). Models have not yet been successful in reproducing the observed Martian dust cycle with active dust transport (Newman et al. 200₂a,b; Kahre et al. (2006). Temperature and wind profile information from heights between the top of instrumented masts and the free atmosphere would likely remain sparse or non-existent.

Understanding of Mars past climate (MEPAG 2006 Goal II Objective B), will benefit from anticipated new knowledge of current atmospheric escape rates from the 2013 Mars Scout Aeronomy Orbiter, MAVEN. However, significant advance in the key area of access to the polar stratigraphic record is not expected in the decades before human exploration. In 2030, this will remain one of the highest priorities for MEPAG Goal II. On the other hand, the study of the paleoclimatic parameters imprinted in the ancient geological record (e.g. Noachian to Amazonian) also concerns the high priorities of the MEPAG Goal II Objective B, which directly relates to unlock the ancient climatic conditions of Mars through the physical (e.g., geomorphic and/or sedimentary), petrological, mineral and geochemical (including isotopic) material characterization.

While recognizing that the MEPAG 2006 Goal II objectives are sufficiently general that they will all remain largely valid, some updating relevant to 2030 is captured in the following four subsections.

3. Quantitative Understanding of Mars Atmospheric Processes

Characterizing the basic state and critical processes of the current Martian atmosphere constitutes 2006 MEPAG Goal IIA. Here we describe globally active physical processes that determine the basic state and variability of the Mars atmosphere, and so are most important to resolve. These processes are inherently global in character such that relevant measurements might be obtained from HEM activities at all sites visited, There are, however, large scale atmospheric provinces which exhibit distinctive dynamical, aerosol (dust and clouds), surface and potential subsurface, volatile conditions. Consequently, although site selection is unlikely to be driven by atmospheric science, the specific complement of atmospheric experiments and measurement goals is likely to vary according to site selection.

The emphasis of HEM atmospheric science would likely focus on processes within the planetary boundary layer (PBL, surface to ~2 km), where surface-atmosphere interactions impart fundamental influences on the dynamical, chemical, and aerosol characters of the global Mars atmosphere, Orbital remote sensing for this region remains difficult and lander/rover atmospheric payloads limited such that sufficiently detailed measurements of the PBL may not be returned from Mars science missions prior to 2030. HEM atmospheric observations can provide optimum in situ and remote access to the PBL and, in turn, characterize local environmental conditions in support of HEM operations.

Atmospheric dynamics, in concert with radiative forcing, determines the basic thermal structure of the Mars atmosphere, the global transport of volatiles (CO₂, water, dust), and the maintenance of Mars polar ice caps, all of which vary on seasonal and interannual timescales. Current understanding of Mars atmospheric dynamics is based to a large extent on remotely sounded atmospheric temperature profiles, analyzed in the context of Mars general circulation models (MGCM). Recent Mars missions (MGS, MER, Mars Express) have extended the vertical, global, and temporal coverage of atmospheric temperature and aerosol (cloud and dust) distributions towards enhanced constraints on MGCM dynamical simulations. The dynamical state of the upper Mars atmosphere (altitudes above 80 km), which carries additional significance in terms of spacecraft aerobraking and atmospheric escape rates, has been inferred from in-situ density measurements associated with aerobraking (e.g., Withers 2006). Dedicated, global observations from the 2013 Mars Aeronomy Scout Mission, MAVEN, would greatly expand our understanding of Mars upper atmospheric dynamics. Within the near-surface atmosphere, atmospheric observational constraints remain sparse. This reflects both the limitations of orbital remote sensing and the geological focus of lander/rover operations to date. Viking lander in situ observations of surface pressure and winds reflect active planetary wave systems and storm fronts (e.g., Barnes 1980 and Murphy et al 1990). MER-based thermal and dust-aerosol profiling within the lower (<5 km) atmosphere also indicate strong PBL variability over local turbulent to diurnal to seasonal timescales (Smith et al. 2006).

Human Exploration: Dedicated observations of surface pressure and temperature-wind-dust profiles of the PBL from distributed surface stations constitutes a key priority for HEM investigations of Mars atmospheric dynamics.

4. Atmospheric Dust

Radiative forcing of the Mars atmosphere may be represented roughly as an energy balance between cooling through CO₂ thermal infrared emission and heating through absorption of solar flux by suspended dust particles. Atmospheric heating associated with atmospheric dust intensifies global atmospheric circulation and near surface winds, which in turn increases lifting of surface dust into the atmosphere. A dramatic result of this dust radiative-dynamic feedback is ubiquitous aeolian activity on Mars, with significant dust lofting and transport occurring over a wide range of spatial and temporal scales. These range from nearly continuous dust devil activity, to regional dust storms in every Mars year, to global dust storms that may occur once every three or four Mars years (Cantor et al 2001). As a consequence, atmospheric dust plays a major role in the spatial, seasonal, and interannual variability of Mars atmospheric thermal structure and circulation. Global imaging and thermal IR dust abundance observations of Mars atmospheric dust extend from the Mariner 9 mission to Viking, MGS, and current MER, and Mars Express, MRO missions, providing an accumulating timeline of Mars dust storm activity. Current mission observations have also substantially advanced vertical profile and dust radiative property definitions (McCleese et al 2006; Wolff and Clancy 2003). Both of these factors are critical to understanding the radiative-dynamical relationships associated with Mars dust storm activity. A key element yet to be addressed regards the particle size dependent flux of dust at the surface-atmosphere boundary as a function of atmospheric and surface conditions. Hence, our understanding of dust lifting rates from the Mars surface is characterized by relatively simple surface wind parameterizations, and it remains uncertain as to whether global surface dust distributions limit or are influenced by atmospheric dust transport.

Human Exploration: In-situ observations of dust surface flux (lifting and deposition), particle sizes, radiative properties, and vertical profiles within the PBL constitute primary objectives for HEM atmospheric dust studies.

5. Atmospheric Water

Atmospheric water, in the form of vapor and ice clouds, plays significant roles in atmospheric chemistry, dust radiative forcing, and climate balance. The photolysis products of atmospheric water vapor determine Mars trace specie abundances (e.g., Nair et al 1994) and regulate current escape rates for the atmosphere (Liu and Donahue 1976). Water ice clouds have long been associated with major topographic features, autumnal polar hoods, and a variety of cloud wave structures (Kahn 1984). The existence of an aphelion, low latitude cloud belt is identified as a significant influence on the vertical distribution of atmospheric dust and water vapor, as well as meridional transport of atmospheric water (Clancy and Nair 1996). Atmospheric exchange with polar cap water ice deposits dominates the seasonal variation of atmospheric water vapor (Jakosky and Farmer 1983), whereas atmospheric exchange with subsurface ice and adsorbed water at lower latitudes remains uncertain, and polar cap water ice (Langevin et al 2005) from MGS, Odyssesy and Mars Express have begun to illuminate surface-atmospheric exchanges of Mars water over seasonal, interannual, and possibly longer timescales. MRO supports dedicated vertical profiling of atmospheric water vapor and ice clouds (McCleese et al 2006), which are likely to be augmented by high sensitivity MSO limb profiling observations.

Human Exploration Investigations: HEM studies of atmospheric water are likely to focus on vertical profile measurements within the PBL, which are not easily addressed from orbital remote sensing. Subsurface core sampling of adsorbed water and water ice water deposits (sitedependent in this case) also constitutes a key Mars water objective that is uniquely facilitated by HEM operations.

6. Atmospheric Chemistry

The trace chemical composition of the current Mars atmosphere reflects photochemical cycles associated with the major atmospheric constituents CO₂, H₂O, and N₂; and perhaps nonequilibrium chemistry associated with potential subsurface sources-sinks of methane (CH₄), sulfur dioxide (SO₂), and hydrogen peroxide (H₂O₂). Some of these compounds can be essential to sustain a Mars cryptic biosphere through direct or indirect (bio)chemical pathways (e.g., atmospheric oxidants can be used as electron acceptors for microbial metabolism, whereas reducing gases, (e.g., -CH_4-) can be electron donors). Existing measurements of the Mars trace species CO, O₂, O₃, and H₂O₂ appear to confirm the dominant HO_xcatalytic cycle proposed to prevent buildup of large CO and O₂ concentrations from photolysis of the primary CO₂ constituent (Parkinson and Hunten 1972, McElroy and Donahue 1972). Hence, atmospheric water vapor, as the primary photolytic source of atmospheric HO_x species, plays a dominant role in Mars atmospheric chemistry. Definitions of spatial and seasonal variations in atmospheric trace composition remain tentative, with the exception of Mars ozone which exhibits large increases towards winter high latitudes (Barth, 1985). The detailed seasonal variation of Mars ozone also suggests that heterogeneous HO_x chemistry may occur on the surface of Mars water ice clouds (Lefevre et al 2004). Vertical gradients in trace specie abundances, associated with a saturation-dependent water mixing profile (Clancy and Nair 1996) or vertical variations in photolysis rates (Nair et al 1994), are inferred but not definitively measured. The most problematic trace specie measurements, on both observational and modeling grounds, are the recent reported detections of significant atmospheric methane abundances (Formisano et al 2004, Krasnopolsky et al 2004, Mumma et al 2007, Mumma et al 2009). Methane is not photochemically produced and is not stable in the current Mars atmosphere such that detectable amounts (parts per billion) require a source from the subsurface (Krasnopolsky et al 2004). Reported variations in methane abundance versus time and space (Mumma et al 2007, Mumma et al 2009) place further requirements on atmospheric loss rates for methane, which remain extremely challenging. Subsurface sources for sulfur bearing gases such as SO₂ and triboelectric sources for enhanced production of peroxide remain unsubstantiated by observations and so unconstrained. MSL and the Mars Aeronomy Scout Mission, MAVEN should address many of the above questions regarding Mars atmospheric chemistry, including the degree to which subsurface sources of non-equilibrium gases are significant globally.

Human Exploration Investigations: HEM observations of atmospheric chemistry are likely to focus on detections of locally enhanced methane, SO₂, H₂S, HCN, or peroxide concentrations associated with confined source regions specific to the geophysics (or biology) of the HEM site.

7. Electrical Effects

Experimental and theoretical investigations of frictional charging mechanisms in both smalland large-scale meteorological phenomena suggest that Mars very likely possesses an electrically active atmosphere as a result of dust-lifting processes of all scales, including dust devils and dust storms. Naturally occurring dust activity is nearly always associated with significant electrification via the process of triboelectricity — the frictional charging of dust grains in contact with one another or the surface as they are transported by wind or convective circulations. Based on the results of terrestrial experiments and their implications for the presence of electrification processes on Mars, it has been shown that electric fields up to the breakdown potential of 25 kV/m can easily occur near the martian surface. A large-scale electric dipole moment can be generated by nearly any process with a vertical lifting component, as the smaller, negatively charged grains are transported to higher altitudes than the heavier, positively charged grains. In dust devils and dust storms, the vertical stratification of grains based on size and mass will create a stratification of charge, which creates an electric dipole moment with a spatial scale on the order of the storm size.

Electrical effects have impact on human exploration and on the environment of Mars as a source of both continual and episodic energy. Differential charging between separate objects in the presence of electrified dust that then come into contact and cause a discharge, directly damaging electronics or interfering with radio communications. Suspended electrified dust presents a hazard for launch operations (an example is the Apollo 12 launch, struck by lightning due to the short-to-ground caused by the vehicle exhaust trail). Dust adhesion may also be dominated by electrical effects — with implications in terms of its transport into the hab/human environment where other effects may take over (toxicity, friction in seals/machinery, etc.) Currently, measurements of electric charging within the Mars atmosphere do not exist. For operational safety concerns alone, basic measurements of martian surface charging conditions should be obtained prior to HEM activities.

Human Exploration Investigations: HEM measurements of atmospheric charging within active dust devils are especially relevant to the dynamic response times associated with dust devil occurrences and motions.

8. Microclimates

Microclimates are defined in 2006 MEPAG Goal II as "exceptionally or recently wet or warm locales, exceptionally cold localities, and areas of significant change in surface accumulations of volatiles or dust" "identified through local surface properties (e.g., geomorphic evidence, topography, local thermal properties, albedo) or changes in volatile (especially H₂O) distributions." Definitions for Earth typically also include a spatial scale varying from centimeters to hundreds of metres (e.g., Geiger et al, 2003). Microclimates, by definition regions of extremes and exceptions, are fascinating and important targets for study. An exceptionally warm and wet locale could correspond to the proposed new definition for a Mars Special Region (Beaty et al, 2006) and hence be of great interest for extant biology. A region of change in surface accumulation of volatiles or dust identifies a source or sink region for global atmospheric volatile and dust transport. The climate objective relevant to these sites is to understand local heat, mass and momentum balance and transport of dust and volatiles. Whereas the 2006 MEPAG Goal II investigation focused on detecting these locales, a 2030 objective will be to carry out in situ studies to understand the processes responsible for generation of the microclimate. Several locales that would qualify as interesting microclimates have already been detected and some examples are given here.