Challenges associated with investigations into the geomorphology of other worlds: A case study examining glacier-like forms in the mid-latitudes of Mars
Shortcut URL: https://serc.carleton.edu/75361
Location
Planet: Mars
Continent: N/A
Country: N/A
State/Province:N/A
City/Town: N/A
UTM coordinates and datum: none
Setting
Climate Setting: Polar
Tectonic setting: N/A
Type: Process
Description
Ever since global imagery of Mars was first gathered and transmitted back to Earth in 1971 by the Mariner 9 orbiter, geomorphologists have remarked upon the abundance of landforms which seem to suggest the action of both liquid water and, with more detailed imagery following in later years, moving ice. Now, with the availability of very high resolution images, a whole suite of forms has been observed which appear to share many morphological and contextual characteristics with terrestrial glaciers (Figure 1). These glacier-like forms (GLFs) appear to be composed of a deformable material that has undergone motion through a mechanism of flow or creep. The morphology of many such features suggests either that they could, in fact, still be active, or only became inactive in Mars' recent geological history.
Work is currently underway to develop our understanding of these Martian GLFs, for if they are indeed composed of water ice (as available evidence suggests to be the case) a fuller awareness of where that ice came from and how it behaves on Mars could prove extremely valuable in the event of a future manned mission to that planet. Also, if Mars (a planet that no longer supports an atmosphere capable of precipitating water through rain or snowfall) does indeed have large deposits of water ice on its surface then it seems possible that Mars has undergone dramatic climate change during a relatively recent epoch. Therefore, a fuller understanding of the origins of remaining H2O deposits may help us to understand that change better, and thus expand our knowledge of climate on the planetary scale, learning lessons we could well apply to our own planet.
Different Base Conditions
However, there are some problems associated with research into landform origins on other planets, especially where most of the available data is in the form of remotely sensed imagery and not from direct observation or sampling. Many of these problems revolve around some of geomorphology's most basic scientific principles and the issue of how those basic principles can be applied to other worlds. Can extra-terrestrial landscapes always be interpreted within the context of terrestrial knowledge?
All processes in geomorphology operate within a context determined by several basic parameters. These parameters are often known as base conditions and include environmentally fundamental factors such as local gravity, temperature and atmospheric density. Alterations to these base conditions can change many aspects of the ways in which processes occur.
Mars has an average surface gravity little more than one third of Earth's. The average surface temperature is around -65°C, and the mean atmospheric pressure is 0.6% that of Earth's. These factors combine to ensure that processes operate very differently on the two planets. For geomorphologists, this means that very often the processes and principles that we have studied and modelled on Earth cannot always be applied directly to Mars. For example, although glaciers on Earth can flow and move as much as several decimetres per day in some areas, on Mars it is likely that icy landforms move at speeds as low as centimetres per thousands of years, or perhaps even slower! This is mainly because of low gravity and very low temperatures. Therefore, developing an understanding of how Mars' surface has developed can be a very challenging task. Due attention must be paid to the effects of base conditions that are often hugely removed from our own.
A good example of how Mars' base conditions may have influenced glacial activity can be seen in the spacing of suspected post-glacial valleys around the edges of the Argyre and Hellas impact basins in Mars southern hemisphere (Pelletier et al., 2010). Here, valleys are spaced 10 – 30 km apart. On Earth glacial valleys in mountainous areas tend to be separated by only 1 – 3 km. This difference in scale could be due to Mars' low temperatures, extreme cold increasing the viscosity of ice and reducing its susceptibility to deformation. This reduced deformability would increase friction at the bed of any glacier and increase drag along valley walls. Therefore, glaciers would essentially get 'stuck' in narrow valleys. This means that when glaciation first began on Mars, ice masses in wider alcoves may have flowed better than in narrower basins. Therefore over time these wider flows would most likely have become the dominant landscapers, slowly carving valleys on a scale much larger than we see on Earth.
Equifinality
Another principle of geomorphology that can be problematic in planetary landscape analyses is equifinality. This refers to the phenomenon of multiple distinct, possibly starkly dissimilar, processes generating landforms that appear to be (or may even be) similar - different events having a similar landforming outcome. This can sometimes make it difficult to infer the origins of particular landscapes by observation alone, presenting problems for the use of satellite imagery as a means of studying Martian glacier-like forms.
Ideally, to maximise landform discrimination interpretations should be based on the analysis of multiple, independent properties. Unfortunately it has not yet been possible to touch or sample many Marian landforms, making visual analysis the best available option. This method alone is a fairly blunt instrument and is particularly susceptible to equifinality.
An example of one case of equifinality is the observed similarity between channels incised by running water and those cut by flowing lava. Lava channels of this kind exist on the flanks of many terrestrial volcanoes (such as in Hawaii), and it is possible that the Martian river-like channels that have often been interpreted as evidence for plentiful flowing water are in fact remnants of Mars' past volcanic episodes. Similar suggestions have been made for the glacier-like flows observed on Mars. Could they in fact be long-since cooled rivers of molten rock rather than slowly creeping ice masses (Figure 2)? Most of the available evidence, including data from orbital radar, suggests that this is unlikely. However, as it has not yet been possible to sample the fabric of Martian GLFs directly, uncertainty persists and all possibilities should be considered, demanding a full appreciation of equifinality throughout the extra-terrestrial geomorphological community.
Associated References
- Pelletier, J. D., Comeau, D. and Kargel, J. 2010. Controls of glacial valley spacing on Earth and Mars. Geomorphology, 116, p. 189-201.
- Howard, A. 2009. Planetary morphodynamics: Scaling and interpreting sedimentary processes from Earth to Mars and Titan. In Vionnet, C. A., Garcia, M. H., Latrubesse, E. M., and Perillo, G. M. E. (eds), River, Coastal and Esturine Morphodynamics: RCEM 09, CRC Press, Boca Raton, Volume 1, p. 207-216.
- Milliken, R. E, Mustard, J. F. and Goldsby, D. L. 2003. Viscous flow features on the surface of Mars: Observations from high-resolution Mars Orbiter Camera (MOC) images. Journal of Geophysical Research, 108. No. E6, 5057. http://hirise.lpl.arizona.edu/
- http://viewer.mars.asu.edu/planetview/inst/ctx#start
Additional references:
http://m.ppg.sagepub.com/content/36/2/238
http://www.sciencedirect.com/science/article/pii/S0019103513001462