Planning for the Scientific Exploration of Mars by Humans By the mepag human Exploration of Mars Science Analysis Group (hem-sag)
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Appendix B. Goal II (Climate): Objectives and Investigations for Human Exploration of MarsOverview of Updated Goals and ObjectivesIn 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, such as that planned with Phoenix-MRO. 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 [Bottger et al (2005)], 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, 2002b); Basu et al. (2004); 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 2011 Mars Aeronomy Scout. 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. Quantitative Understanding of Mars Atmospheric ProcessesCharacterizing 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, MCO) have extended the vertical, global, and temporal coverages of atmospheric temperature and aerosol (cloud and dust) distributions towards enhanced constraints on MGCM dynamical simulations. The planned MSO mission would further complement improved thermal and aerosol profiling with direct wind profile measurements for the first time. 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 2011 Mars Aeronomy Scout mission.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]). MSL and Phoenix will conduct limited meteorological measurements as constrained by the primary surface science objectives of these missions. 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. Atmospheric DustRadiative 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 (Martin and Zurek [1993], 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, Mars Express, and MCO 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 [2007], Wolff and Clancy [2003]), with further progress in these areas would be contributed by MSO. 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. 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. Atmospheric WaterAtmospheric 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 et al [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 (e.g., Montmessin et al [2004]). Recent spacecraft observations of atmospheric water vapor (Smith [2002]), subsurface water ice (Feldman et al [2004]), 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. MCO supports dedicated vertical profiling of atmospheric water vapor and ice clouds (McCleese et al [2007]), which are likely to be augmented by high sensitivity MSO limb profiling observations. The Phoenix lander (Smith [2006]) will excavate and analyze subsurface water ice on Mars for the first time, and MSL will provide measurements of surface humidity and the water content of surface materials over the course of a Martian year. 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 (site-dependent in this case) also constitutes a key Mars water objective that is uniquely facilitated by HEM operations. Atmospheric ChemistryThe trace chemical composition of the current Mars atmosphere reflects photochemical cycles associated with the major atmospheric constituents CO2, H2O, and N2; and perhaps non-equilibrium chemistry associated with potential subsurface sources-sinks of methane (), SO2 (), and 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 –CH4- can be electron donors). Existing measurements of the Mars trace species CO, O2, O3, and H2O2 appear to confirm the dominant HOx catalytic 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 HOx 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, ). The detailed seasonal variation of Mars ozone also suggests that heterogeneous HOx 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 (Formasano et al [2004], Krasnopolsky et al [2004]). 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]) place further requirements on atmospheric loss rates for methane, which remain extremely challenging. Subsurface sources for sulfur bearing gases such as SO2 (Krasnopolsky [2005]), and triboelectric sources for enhanced production of peroxide (Atreya et al [2007])) remain unsubstantiated by observations and so unconstrained. MSL, the Mars Aeronomy Scout mission, and MSO 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. 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. Existing measurements of the Mars trace species CO, O2, O3, and H2O2 appear to confirm the dominant HOx catalytic 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 HOx 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, ). The detailed seasonal variation of Mars ozone also suggests that heterogeneous HOx 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 (Formasano et al [2004], Krasnopolsky et al [2004]). 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]) place further requirements on atmospheric loss rates for methane, which remain extremely challenging. Subsurface sources for sulfur bearing gases such as SO2 (Krasnopolsky [2005]), and triboelectric sources for enhanced production of peroxide (Atreya et al [2007])) remain unsubstantiated by observations and so unconstrained. MSL, the Mars Aeronomy Scout mission, and MSO 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. 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. Electrical EffectsExperimental and theoretical investigations of frictional charging mechanisms in both small- and 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, Melnick and Parrot (1998) used a particle-in-cell numerical model to show 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 (Figure 1). 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 and experiments necessary for such measurements are not incorporated in the Phoenix lander or MSL missions. For operational safety concerns alone, basic measurements of martian surface charging conditions should be obtained prior to HEM activities. HEM measurements of atmospheric charging within active dust devils are especially relevant to the dynamic response times associated with dust devil occurrences and motions.
Desired Precursor MeasurementsDynamics — global direct wind measurements, network measurements of near-surface pressure, temperature and winds, improved definitions of upper atmospheric thermal tides, circulation, and aerosol loading Dust Storms — The vertical distributions of dust radiative properties, particle sizes, and number densities versus time and space; improved characterizations of dust lifting rates and localities, and dust deposition localities, dust charging levels Water Vapor and Ice Clouds — Improved water vapor, cloud opacity and particle size vertical profiling (in vertical and global coverage), lander/rover based measurements of surface ice fogs and frosts, diurnal coverages for water vapour and cloud vertical profiles Atmospheric Chemistry — Further definition of Mars atmospheric methane, in abundance and spatial/temporal variability. Global, temporal, and vertical variations in CO2 ozone, and H2O2 abundances. Dependence of trace gas species with seasonal ice cap growth and recession. Electrical Effects — Identification of dust charging in dust devils and storms. Investigations for the Human Era of ExplorationSurface-atmosphere interactions: dynamics, heat and mass balance.
Ground heat and moisture flux (heat and conductivity probes)
Search for sources of volatiles and trace gases
MicroclimatesMicroclimates 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 (eg. 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. Topographically controlled microclimates (small scale to large scale)
Locales where changes in volatile distributions are observed (both spatial and temporal change; small scale to large scale)
Locales where significant changes in dust distribution are observed (e.g., Szwast et al, 2006)
Exceptionally cold locales
Desired Precursor MeasurementsIdeally, meteorological stations would be emplaced at each of the HEM sites by precursor missions for long term (multi-annual) monitoring to record in situ climate variables over diurnal and seasonal cycles.
Continual monitoring from orbit for mulitannual variability and seasonal change
Investigations for the human era of explorationSurface-atmosphere interactions: dynamics, heat and mass balance. In situ investigations to meet microclimate objectives require micrometeorology measurements at the microclimate site, preferably in conjunction with upwind reference stations outside its boundary to give regional context. Emphasis on instrumentation may vary depending on characteristics and focus of site. Search for sources of volatiles and trace gasesA search would involve characterizing the chemical environment (surface and atmospheric) and using portable instrumentation to follow local gradients in trace and volatile gases. |
