Glaciers on Mars

A summary of research on areoglaciology in Hellas basin

for Portland State University Glacier Field Study, Fall 2012

The Viking spacecraft which orbited Mars in the 1970s revealed several massifs on the eastern slopes of Hellas basin. These massifs are surrounded by lobate debris aprons morphologically suggestive of gelifluction, indicating the presence of some quantity of subsurface water ice (Squyres, 1979). Further observations by the Mars Global Surveyor revealed a knobby terrain atop these aprons consistent with significant thermokarstic processes associated with glaciers (Pierce & Crown, 2003). These features prompted inquiry into the possibility of atmospheric conditions by which moisture might have been directed to this region, and climate simulations for high-obliquity periods of Mars’s past indicated that atmospheric moisture from the southern pole may have precipitated in eastern Hellas during narrow time periods of oblique summers. The Mars Reconnaissance Orbiter’s Shallow Subsurface Radar was used to analyse these features, and the radar data collected suggests massive subsurface glaciers beneath a thin layer of debris. These observations in Hellas unveiled a significant portion of the martian cryosphere and indicated other probable locations where martian ground ice might be found (Holt et al., 2008).

Initial observations of the east Hellas massifs’ lobate debris aprons came from Viking orbiter data which began being taken in 1976. These features were hypothesized even at that time to be likely candidate sites for subsurface water deposits (Squyres, 1979). More detailed observations became available from the Mars Global Surveyor, which began its observations in 1999. The massifs of east Hellas were identified as geometrically similar to those mountainous formations known on Earth to form glacial cirques where snowfall might accumulate and compact. Knobby terrain and evidence of postemplacement debris flow on these aprons indicate an ongoing process of thermokarstic degradation (Pierce & Crown, 2003). Though not related to melt processes which form marshy terrain as in terrestrial thermokarst, the knobs atop these lobate debris aprons could be the result of wasting behavior associated with the loss of ice near the surface to sublimation due to fluctuations in local temperature and pressure. However, while the Viking and Mars Global Surveyor orbiter findings were both sufficient to morphologically suggest that Hellas contained subsurface water, morphology alone cannot verify or disprove the presence of subsurface water. Before more detailed physical observations were made, a model was devised to explain the possibility of high water ice concentrations that might have led to glaciation in Hellas basin.

As any great volume of trapped water ice on Mars is likely a result of past climate behavior, historical climate simulations became invaluable tools in assessing moisture distribution and concentration hypotheses. Mars’s current spin-axis obliquity of 25.1o (Mutch, 1976) does not indicate a clear climatic cause for moisture to have accumulated in Hellas. The water cycle of Mars as it is currently understood involves an annual water cycle wherein a great mass of water is sublimed from one hemispheric pole during its summer and precipitated on the other. The Laboratoire de Météorologie Dynamique’s martian global climate model was used to simulate the conditions of prior epochs in order to find a climatic model for accumulation of moisture in Hellas. When periods of 45o obliquity were simulated with an annual water cycle similar to the present one, it emerged that periods of ~90 days during southern summer would have experienced an interruption in northward travel of sublimed southern polar moisture by a “midlatitude westward summer vortex” in all southern mid-latitudes excepting the eastern portion of Hellas basin. This would have resulted in a “stationary planetary wave” involving transport, condensation, and precipitation over the region on an annual basis in past martian epochs: a robust model for the origin of the hypothetical subsurface water under this part of Hellas (Forget, 2006).

With climatic simulations supportive of past moisture precipitation in the Hellas basin, its morphological features were perceived with greater weight in assessing the region as a potential glacial site. Relative crater densities reveal the lobate debris aprons to be the youngest features of the region. The shape of the lobate debris aprons on the east Hellas massifs suggest the presence of some viscosity-lowering agent, and craters superposed atop these lobate debris aprons possess wide central peaks and moat-shaped borders, which indicate that they most likely formed in a substrate rich in water ice. On the basis of morphology and surface topology, it was estimated that between 10% and 50% of the interior composition of the lobate debris aprons could be comprised of water (Holt, 2008). These physical observations and their supporting climatic  models were still insufficient to verify or discount the presence of water, however. The physical properties of these features required more subtle analysis.

Thus, the Mars Reconnaissance Orbiter’s team recruited its shallow radar to scan for reflectors beneath the surface of these lobate debris aprons. Strong radar reflection signals would indicate a predominantly rocky composition beneath the lobate debris aprons, while less reflection could be interpreted to indicate a water-rich makeup. These scans did not produce the volume scattering patterns that would indicate a majority composition of reflective rocky material. In fact, those reflection signals which were produced appeared to originate either from well beneath the features, or from debris at their bases. The radar signal attenuated at a rate of about 10 decibels per kilometer, which is consistent with attenuation by water ice. The attenuation interpretation was checked by using the known velocity of electromagnetic waves through water ice to convert the time-delay radar data to a depth graph. The resultant depth model matched the time delay model very closely. All told, the radar data indicated that the material is homogeneous, of negligible rocky composition, and attenuates signal the way water does. This left little doubt that the vast majority of the material beneath these lobate debris aprons is in fact water ice (Holt, 2008).

The thickness of the superficial debris covering these subsurface glaciers could not be determined due to vertical resolution limitations on the radar. However, the unobservability on the basis of this limitation at least indicates that the surface covering is on the order of 10 meters or less (Holt, 2008). Such thin self-burying behavior on the part of glaciers is not unheard of in terrestrial analogs. Elliot Glacier on Mount Hood is partially buried by a process of plucking rocks from its accumulating end and bringing them via plastic ice flow to the surface near its bottom. If a glacier continues this plastic sedimentary conveyance for a sufficiently long time, its terminal moraine can eventually convey debris so as to merge with its headwall and be fully buried. Such processes may have also caused the lobate debris aprons surrounding the eastern Hellas massifs to bury these martian glaciers.

From this data, it was estimated that the aprons surrounding the massifs of the east Hellas basin contain approximately 28,000 cubic kilometers of relatively pure water ice, or about 1% of the total average water ice volume of the polar caps (Holt, 2008). It has been suggested that as much as ~90% of the total initial outgassed water of Mars is unaccounted for by surface ice or exospheric escape. The remaining explanation appears to be that most of the water outgassed by Mars during its planetary accretion and differentiation is frozen in a subsurface cryosphere. In addition to those areoglaciological features observed in eastern Hellas, similar observations and inferences from morphology have placed significant cryospheric ground ice beneath many regions of Mars: most prominently the northern provinces of Tharsis and Elysium. Many of these regions beyond Hellas exhibit thermokarstic pits, patterned terrain, seemingly aqueous crater morphologies, and debris flows associated with ground ice. It is possible that some of the water held beneath the surface in ice formations may exist in liquid form (Rossbacher, 1981).

The lobate debris aprons on the massifs of the eastern Hellas basin were revealed to 

be massive covered water glaciers of great purity. If other lobate debris aprons on Mars are interpreted to be similar in composition, it can be inferred that they represent the largest nonpolar water reservoirs on Mars. These and similar cryospheric features are crucial to a proper hydrological understanding of Mars, and may be the primary locations of concern regarding most martian water. These subsurface glaciers represent vital sites of interest to the continuing search for martian life. Should manned exploration or settlement of Mars take place, such thinly-buried, pure glacial deposits will prove invaluable to in-situ resource utilization and sustainable civilization.

Works Cited

Forget, F. et al. "Formation of Glaciers on Mars by Atmospheric Precipitation at High Obliquity." Science 311.5759 (2006): 368-71.

Holt, J. W. et al.. “Radar sounding evidence for buried glaciers in the southern mid-latitudes of Mars.” Science, 322 (2008) 1235-1238.

Mutch, Thomas A. The Geology of Mars. Princeton, NJ: Princeton University Press, 1976.

Pierce, T., and David A. Crown. "Morphologic and Topographic Analyses of Debris Aprons in the Eastern Hellas Region, Mars." Icarus 163.1 (2003): 46-65.

Rossbacher, L. "Ground Ice on Mars: Inventory, Distribution, and Resulting Landforms." Icarus 45.1 (1981): 39-59.

Squyres, Steven W. "The Distribution of Lobate Debris Aprons and Similar Flows on Mars." Journal of Geophysical Research, 84.B14 (1979): 8087.