Stratigraphy of the Martian North Polar Ice Cap

Post by Kathryn Fishbaugh.

At the north pole of Mars lies Planum Boreum, a dome of layered, icy materials similar in some ways to the large ice caps in Greenland and Antarctica and comparable in size to the former. The dome itself consists of the polar layered deposits, consisting of over 90% ice with a little bit of dust, and the basal unit, consisting of ice, dust, and sand.

Mars Polar Deposits

An enhanced color image from the Mars Reconnaissance Orbiter (MRO) High Resolution Imaging Science Experiment (HiRISE) shows a portion of the martian north polar layered deposits, the basal unit, and the Olympia Undae dune field. The image is 1.2 km (0.75 mi) across. This image is best understood if you imagine yourself flying over a cliff in a plane. Note that the colors in an enhanced color image do not re-create what it would look like to the naked, human eye, but rather bring out the compositional differences between the materials.

The image shows a cliff in the Polar Cap deposits. The upper portion of the cliff consists, for the most part, of fractured portions of the polar layered deposits and has a reddish appearance due to dust both coating and entrained within the ice (red arrow). Below that is the basal unit, with more flat-lying layers of blueish material that is basaltic sand (blue arrow) (like the black sand beaches in Hawaii). You might also notice some lighter colored layers. Those are also fractured and composed of ice and dust, like the polar layers above them. And at the bottom of the image, sand eroding from the basal unit is collecting into dunes (white arrow). The entire cliff is about 700 m (2300 ft.) tall (comparable to the depth of the Grand Canyon).

Scientists study past climates and trends in global warming on Earth by examining the air bubbles trapped within ice cores (long, cylindrical samples of ice, extracted with a drill) taken from Greenland and Antarctica. These ice cores contain ice created from last year’s snowfall to many hundreds of thousands of years ago and have trapped bubbles with the same atmospheric composition as existed when the snow fell. From this composition, scientists can figure out what was the contemporary temperature and hence how the climate has changed over time. Similarly, the ice in the polar layers and basal unit on Mars must also have recorded how the martian climate has changed. If we can decipher the climate record stored in those deposits, then we can better determine whether and when the planet was ever habitable for life (e.g., bacteria). As an example, since the basal unit has intervening sandy layers between the ice-rich layers, we know that something, at least locally, was different when the basal unit formed as opposed to when the upper icy polar layered deposits formed.

Further Reading:

Byrne, S. and B. Murray (2002). North polar stratigraphy and the paleo-erg of Mars. J. Geophys. Res. 108 (E11), doi:10.1029/2004JE002267. [Abstract]

Edgett, K., R. Williams, M. Malin, B. Cantor, and P. Thomas (2003). Mars landscape evolution: Influence of stratigraphy on geomorphology in the north polar region. Geomorph. 52, 289-297. [Abstract]

Fishbaugh, K. and J. Head (2005). Origin and characteristics of the Mars north polar basal unit and implications for polar geologic history. Icarus 174 (2), 444-474. [Abstract]

Fishbaugh, K., C. Hvidberg, D. Beaty, S. Clifford, D. Fisher, A. Haldeman, J. Head, M. Hecht, M. Koutnik, K. Tanaka, and W. Ammann (2008). Introduction to the 4th Mars Polar Science and Exploration Conference special issue: Five top questions in Mars polar science. Icarus 196 (2), 305-317. [Abstract]

Phillips, R. and 26 colleagues (2008). Mars North Polar Deposits: Stratigraphy, age, and geodynamical response. Science 320, 1182-1185. [Abstract]

Russell, P., S. Byrne, K. Herkenhoff, K. Fishbaugh, N. Thomas, A. McEwen, and the HiRISE Team (in press). Active mass wasting processes on Mars’ north polar scarps discovered by HiRISE. Geophys. Res. Lett.

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