Mars On Earth

The Great Kobuk Sand Dunes in Alaska provide an Earth analog for Martian geology

Cynthia L. Dinwiddie, Ph.D.     image of PDF button

photo of Dinwiddie

Dr. Cynthia L. Dinwiddie is a principal engineer in the Earth, Material and Planetary Sciences Department of the Geosciences and Engineering Division. She is a hydrogeologist who develops integrated geophysical and remote-sensing characterization methodologies to investigate hydrologic processes on Earth and Mars.


image of the surface of Gale Crater, Mars

Credit: NASA/JPL-Caltech/MSSS

A photographic image of the surface of Gale Crater, Mars, taken by the Curiosity rover reveals a field of dark sand in the foreground, with the foothills of Mt. Sharp in the distance.


image of 100 MHz ground-penetrating radar antennas towed on a sled by a snowmobile

100 MHz ground-penetrating radar antennas towed on a sled by a snowmobile.

On Aug. 5, 2012, at 9:32 p.m. Alaska time, the Mars Science Laboratory on NASA's Curiosity rover descended to the Martian surface in a place called Gale Crater. Between the rover and a mountainous peak in the center of the impact crater lies a field of dark sand dunes. Planetary scientists have recently discovered that sand dunes on Mars are actively moving by using a satellite remote-sensing method that was first developed at Southwest Research Institute (SwRI) by Dr. Marius Necsoiu to estimate the speed at which the Great Kobuk Sand Dunes are moving in Kobuk Valley National Park, Alaska. Because good repeat photographic images of the Martian surface are relatively few, scientists are just beginning to have enough data to compare images of a single planetary scene at multiple times. Some people talk about Mars as if it were a dead planet, but if one looks closely at the planet long enough, it becomes apparent it is alive with many physical processes. In the recent past, planetary scientists thought sand dunes on Mars were frozen in space and time, but since 2008 they’ve known that it isn’t true. Geologically speaking, Mars is very much alive.

To help prepare humankind for exploration of other worlds and expand understanding of extraterrestrial geologic processes, planetary scientists study the extreme landscapes of Earth that are most similar to other planets or their moons. In the scientific discipline of comparative planetology, the features and processes that are observed on extraterrestrial planetary bodies in our solar system are compared to similar features and processes on Earth because our own landscapes are more easily accessible for detailed study and analysis — we call these places “planetary analogs” because they are reasonably comparable to planetary landscapes. This is why a team of SwRI researchers began conducting satellite remote-sensing investigations of the Great Kobuk Sand Dunes in 2008, and then traveled to this planetary analog site to perform geophysical, meteorological and geomorphological field research in 2010.

Sand dunes in Kobuk Valley National Park are excruciatingly slow-moving, just like dunes on Mars. Sand dunes near Earth’s equator don’t move slowly like this, and the smaller the dune, the faster it moves. Strangely, however, remote-sensing data analyses suggest that the largest dunes in Kobuk Valley may actually move faster than the smallest ones. Why do Arctic dunes behave differently than warm-climate dunes, and do any Martian dunes behave like Earth’s Arctic dunes? To better understand why cold dunes move so slowly, the SwRI team used tools including shallow boreholes, ground temperature sensors and ground-penetrating radar and capacitively coupled resistivity surveys to peer inside the sand dunes in Kobuk Valley in late March 2010, when the weather was cold. During March in Alaska, "cold" means an average daily temperature of 6° Fahrenheit. This is cold enough that the seasonally frozen active layer was at its maximum annual thickness, and it was assumed that the dunes likely would be frozen to their base. The annual average temperature in Kobuk Valley is 25° Fahrenheit, which is cold enough for the dunes to be surrounded by widespread, discontinuous permafrost.

At the Great Kobuk Sand Dunes, lowland areas between dunes (called interdunes), small dunes, downwind lee slopes of large dunes, and most of the upwind stoss slopes of large dunes are snow-covered for approximately two-thirds of the year, and only the elevated crests of the largest dunes remain exposed to wind throughout much of the winter season. It is thought that this seasonal disconnect between the dune sand and winds may be why the large dunes in Kobuk Valley move faster than the small ones. Polar dunes on Mars experience a similar disconnection during Martian winter, when they are covered with carbon dioxide and water frost.

Electrical resistivity surveys of the sand dunes showed that the seasonally frozen active layer was approximately 13 feet thick beneath dune crests, and less than 7 feet thick beneath interdunes. The dunes are composed of fine sand, through which liquid water should permeate and drain rapidly. However, the team found groundwater in boreholes below the frozen interdunes, and no permafrost. So, despite an average annual temperature that is 7° colder than water’s freezing point, liquid water persists year-round beneath the dunes.

Water where ice should be

Geophysical data suggest that the regional groundwater beneath the dunes is relatively flat-lying, like a table top, and approaches the surface within the interdunes; however, the data also strongly suggest that there is a thin layer of liquid water just below the frozen active layer, which mirrors dune elevation and relief (i.e., topography). This liquid water perched high in the dune system was curious and unexpected, leading the SwRI research team to look into possible explanations. The presence and topographic mirroring behavior of the near-surface liquid water layer suggests that it is perched on a thermally controlled, low-permeability barrier to downward water flow. This barrier would have had to have developed in dynamic equilibrium with slow dune migration, and eroding remnants of it may be visible on upwind stoss slopes when not covered by snow.

image of series of 2.4-kilometer-long radargrams

Dr. Cynthia L. Dinwiddie is a principal engineer in the Earth, Material and Planetary Sciences Department of the Geosciences and Engineering Division. She is a hydrogeologist who develops integrated geophysical and remote-sensing characterization methodologies to investigate hydrologic processes on Earth and Mars.


image of Resistivity data from three sites on the Great Kobuk Sand Dunes

Resistivity data from three sites on the Great Kobuk Sand Dunes indicate transitions between the frozen active layer (hot colors) and the regional groundwater aquifer (cool colors). Intermediate colors beneath the elevated dune at the third site probably indicate a thick ice- and water-free vadose zone between the active layer and the regional water table aquifer.

image of Polar dunes on Mars

Credit: NASA/JPL/University of Arizona

Polar dunes on Mars are shown during early spring when covered with carbon dioxide and water frost. Dark sand cascades down the lee slopes as the frost begins to warm and sublimate.


image of Snow-covered sand dunes in the Kobuk Valley National Park, Alaska

Snow-covered sand dunes in the Kobuk Valley National Park, Alaska, provide planetary geologists with a planetary analog for similar structures observed on the surface of Mars. A surprising observation at the Great Kobuk Sand Dunes was the discovery of liquid water emerging from the dunes, despite subfreezing temperatures that correspond to some measured on Mars.

The data suggest that this low-permeability barrier develops throughout the Great Kobuk Sand Dunes by freeze-drying, which can produce both ice lenses and calcium-carbonate cements, called calcrete, at the base of the active layer, where downward freezing from the land surface occurs. A cryogenic barrier could be composed of an ice-rich layer that lies perpendicular to the direction of heat flow at the base of the active layer. It also may be composed of cryogenic cement or other clay-sized particles preferentially deposited through cryogenic processes during annual freeze-up. Supporting the calcrete hypotheses, carbonate grains comprise 7 percent of the dune sand, and widespread calcrete has been observed by others when snow cover is absent.

The SwRI team believes these cold-climate sand dunes move slowly because seasonal snow cover acts like a windshield above the sand, and seasonally frozen water in the active layer immobilizes most of the sand during the winter. Warm-season rains also play a role in minimizing the sand that is available to be lofted by wind. Finally, the regional aquifer beneath the interdunes and the perched water high in the dune uplands both make the sand sticky, like wet beach sand that can be molded when one builds a sand castle. The Great Kobuk Sand Dunes are a “wet” sand dune system. Although they are influenced by a semi-arid climate, there is a lot of near-surface water trapped above the near-continuous permafrost in this region. These dunes provide an excellent planetary analog site for studying how the water cycle influences sand transport under conditions similar to those of Martian polar deserts, especially ancient Mars, which was a bit warmer and wetter, and subject to higher atmospheric pressure, than the planet is today.

Gully erosion parallels

While conducting this planetary analog study at the Great Kobuk Sand Dunes, SwRI scientist Dr. Don Hooper noticed that several meltwater debris flows were forming on some west-facing slopes of the dunes. Debris flows with gully or erosion tracks also appear on the slopes of several dune fields on Mars. This new observation was important because it indicated that yet another planetary analog process was occurring at the dunes, and only a few minutes of above-freezing temperatures are needed to melt water and mobilize sand transport down steep slopes. Small debris flows originate near dune crests, become channelized down lee slopes, and terminate with a fan-shaped deposit. New surveys will be needed to measure debris flow rates and gully morphologies, slope angle, solar radiation, subsurface temperature and moisture profiles and other variables to validate a conceptual model of the processes that control debris flow formation on the Great Kobuk Sand Dunes.

Conclusion

Liquid water, solid ice and water vapor can coexist in stable equilibrium at what is called the “triple point of water.” Recent measurements of air temperature and pressure in Gale Crater on Mars suggest that liquid water potentially would be stable there during the warmest portion of each day under current environmental conditions. Late-winter to early-spring conditions and processes at the Great Kobuk Sand Dunes are sufficiently similar that they can serve as an informative analog to near-equatorial processes in Gale Crater, Mars. Consequently, information from SwRI’s studies at the Great Kobuk Sand Dunes can be directly applied to mission results from Curiosity. Effective use of this Earth analog can give important clues to the search for water on Mars.

Questions about this article? Contact Dinwiddie at (210) 522-6085 or cynthia.dinwiddie@swri.org.

Acknowledgements

This work was supported by NASA Mars Fundamental Research Program grant NNX08AN65G and by Southwest Research Institute’s internal research and development program. Colleagues who have contributed to these studies include Dr. Marius Necsoiu, Dr. David E. Stillman, Ronald N. McGinnis, Dr. Donald M. Hooper, Dr. Gary R. Walter, Dr. Stuart A. Stothoff and Dr. Robert E. Grimm, all of SwRI; and also Timothy I. Michaels, Kevin J. Bjella and Sébastien Leprince.

The author also thanks Seth Kantner for his invaluable field knowledge and support, Clarence Wood for use of his private allotment, Jim Kincaid and Alvin Williams for their logistical support, and the National Park Service (NPS) for research permit KOVA-2010-SCI-0001.

Any opinions, findings, and conclusions or recommendations expressed in this article are those of the author and do not necessarily reflect the official positions or views of the National Aeronautics and Space Administration or of the U.S. National Park Service.

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04/15/14