Supplementary material to “Determining Priorities for Future Mars Polar Research”

Published 6 October 2009

Timothy N. Titus, U.S. Geological Survey, Flagstaff, Arizona

Timothy I. Michaels, Southwest Research Institute, Boulder, Colorado

Citation:

Titus, T. N., and T. I. Michaels (2009), Determining priorities for future Mars polar research, Eos Trans. AGU, 90(40), 351. [Full Article (pdf)]

The seasonal polar caps of Mars consist primarily of CO₂ that condenses from the atmosphere to form surface ice at high latitudes following the autumnal equinox in both hemispheres. The seasonal caps are prominent features on Mars that were first viewed by Herschel in 1784. They extend equatorward as far as 40° S in the southern hemisphere and 55° N in the northern hemisphere. Approximately 25% of the Martian atmosphere is cycled annually into and out of the seasonal caps. Consequently, the seasonal CO₂ cycle plays an important role in the planet's atmospheric general circulation. Questions about the seasonal caps that remain unresolved concern local cap properties (e.g. column abundance, volumetric density, geometric thickness, dust and water ice fraction, albedo and emissivity), energy-balance terms and CO₂ condensation mechanisms. The rate of seasonal deposition and sublimation of CO₂ ice is determined by the local energy balance, which depends on solar insolation, atmospheric properties (such as dust optical depth), emissivity and albedo of the surface, advection of energy by the atmosphere and energy storage within the regolith.

Since 1997, five spacecraft have observed Mars polar processes, resulting in an unprecedented amount of data with ranges of spatial and spectral resolutions not available from prior missions in the 1960s and 1970s. These datasets provide a view of the seasonal caps in five dimensions; three spatial, one temporal, and one spectral (composition). The vast amount of new data and the complex nature of the Mars polar processes necessitated the gathering of Mars polar scientists from around the world into a small group environment where the data, models, and emerging theories could be discussed.

The Third International Workshop on Mars Polar Energy Balance and the CO₂ Cycle was convened 21–24 July 2009 in Seattle, WA. A total of 36 scientists from five countries (spanning two continents) attended. The expertise of attendees included remote sensing, laboratory experimentation and climate modeling. Nearly all spacecraft instruments that have been or are currently being used to monitor Mars’ CO₂ cycle were represented. Even with this diverse and eclectic group, we were able to reach consensus on several important research goals for the next decade. Listed below are the five highest priority programmatic and research recommendations to increase our understanding of Mars polar processes. We also list an additional three moderately high-priority recommendations, which are not specific to polar research, but have an impact on Mars science in general.

Recommendation 1: NASA should establish a new research program under the ROSES solicitation for proposals called the Mars Polar Research Program (MPRP). Mars polar research is particularly an interdisciplinary endeavor, often requiring equal portions of data analysis, laboratory work, and/or numerical modeling; thus polar-research proposals are often not well-suited for current NASA ROSES programs.

Recommendation 2: Increased emphasis must be placed on laboratory experiments to measure the spectral and physical properties of CO₂ ice, and CO₂ ice mixed with H2O ice and dust. A great deal of our current interpretations of remote sensing observations is based on inadequate or incomplete laboratory data.

Recommendation 3: It has been repeatedly suggested that Mars is currently undergoing climate change, based on observations of the south polar residual cap (SPRC). Observations of hectometer-scale CO₂ ice topographic features that resemble Swiss cheese show that the “holes” are growing larger, suggesting that the residual CO₂ ice may be experiencing net sublimation. Where the subliming CO₂ ice goes is a question of significant debate. If the CO₂ ice component of the SPRC is subliming away, then the CO₂ is going back into the atmosphere, thus raising the overall surface pressure by a few Pa per Mars decade (Haberle, 2009). However, other studies suggest that much of that CO₂ may be recondensing along the SPRC edge (e.g. Winfree & Titus.2006, Winfree & Titus, 2007). Solving for this question of SPRC stability and Mars climate change could be as simple as long term monitoring of the surface air pressure. We recommend that all future landers and rovers destined for the surface of Mars be equipped with accurate, precise, and stable pressure sensors that operate as long and as often as possible.

Recommendation 4: Surface CO₂ ice emplacement can occur either as direct deposition onto the surface or as precipitation (snow) from the atmosphere aloft. The time evolution of these two modes of ice emplacement may primarily determine the seasonal cap density. Spatial and temporal density variations of the seasonal CO₂ ice are expected, but cannot be easily measured with present day observations. To determine CO₂ ice density as a function of space and time, NASA should send a spacecraft to Mars that is capable of two specific measurements: vertical changes in the cap height (and thus depth, given the substrate topography) during the fall, winter, and spring seasons, and a simultaneous determination of the CO₂ ice column abundance. The changes in elevation could be monitored by either a laser altimeter or by interferometric synthetic-aperture radar. The second measurement could be accomplished with a collimated thermal-neutron detector. Since thermal neutrons are highly sensitive to the column abundance of CO₂ ice on the surface, and since thermal neutrons are readily absorbed by thin layers of material (e.g. Cd or Gd sheets), it would be possible to build a compact CO₂ ice imaging system with high spatial resolution (e.g., able to resolve spatial variations in the cap on the scale of 50-100 km).

Recommendation 5: Mars Odyssey Gamma Ray Spectrometer (GRS) and Neutron Spectrometer (NS) data have shown that the wintertime atmosphere in the polar regions can become strongly enhanced with non-condensable gases (and are depleted in springtime). This affects CO₂ condensation on the ground and in the atmosphere by changing the frost point, thus affecting the basic thermal structure of the atmosphere (and thereby affecting atmospheric circulation on a global scale). Because noncondensable gases are passive tracers, their time-dependent distribution can provide a great deal of information about general circulation. It is thus very important that improved measurements of the enhancement/depletion of these noncondensable gases be made by future spacecraft. The GRS and NS Argon data have very low resolution in both space and time. Because higher resolution and more sensitive GRS and NS instruments may not be feasible, observations of other trace gases with very long lifetimes may be the best option. Carbon monoxide is an obvious candidate because it can be measured very accurately at microwave wavelengths.

Additional high priority recommendations which are not strictly polar include the following:

Recommendation 6: Continued monitoring of the polar regions with orbital assets. The instrumentation necessary to continue the observational record started by the Mars Global Surveyor spacecraft (1997) must include the ability to determine surface and atmospheric temperatures, the ability to differentiate between ices of different composition, and to measure surface albedo and emissivity. These measurements will enable researchers to search for interannual variations of the Mars climate.

Recommendation 7: With the notable exception of the Mars Express spacecraft, most data since 1997 have been acquired during the early to late afternoon and the pre-dawn night. Observations at other times (e.g., mid-morning) might provide useful insights into the diurnal cycle of Mars.

Recommendation 8: The final recommendation revolves around the concept of an imaging LIDAR. While not one of the top five highest priorities, an imaging LIDAR system with lasers tuned to the continuum and spectral features of ices could provide new insights into polar processes deep inside the polar night — both surface and atmospheric. Nighttime cloud studies would also be possible with such an instrument. Acknowledgements: We would like to acknowledge the following people for their insightful suggestions that have greatly improved this article.

Jeffery Barnes, Oregon State University, Corvallis, OR 97331–4501 (barnes@coas.oregonstate.edu)
Sylvain Piqueux, Arizona State University, Tempe, AZ (sylvain.piqueux@asu.edu)
Thomas H. Prettyman, Planetary Science Institute, 1700 E Fort Lowell, Suite 106, Tucson, AZ 85719 (prettyman@psi.edu)

References:

Haberle, R. M. et al., 2009, The Disappearing South Residual Cap on Mars: Where is the CO₂ Going?, Third International Workshop on Mars Polar Energy Balance and the CO₂ Cycle, held July 21–24, 2009 in Seattle, WA., http://www.lpi.usra.edu/meetings/mpeb2009/

Winfree, K. N. and T.N. Titus, 2006, Estimation of CO₂ Coverage on Mars' South Pole: An Interannual Assessment, 37th Annual Lunar and Planetary Science Conference, March 13–17, 2006, League City, Texas, abstract no.2283, http://www.lpi.usra.edu/meetings/lpsc2006/

Winfree, K. N. and T.N. Titus, 2007, Trends in the South Polar Cap of Mars, Seventh International Conference on Mars, held July 9–13, 2007 in Pasadena, California, LPI Contribution No. 1353, p.3373, http://www.lpi.usra.edu/meetings/7thmars2007/