Posted by Dr. Matt Balme, Open University, UK.
(Re-posted from IAG Image of the Month, October, 2007)
In these images huge ‘plates’ of solid material have been mobile over a fluid or plastic substrate, as demonstrated by ‘lanes’ in the plates (bottom right of b) and how the plates can be ‘jigsawed’ back together (b,c). While it is clear that the image in (c) is of ice rafts over liquid water, it is unclear as to what process formed such plates on Mars; debate is ongoing as to whether these landforms represent the fractured surface of huge, low viscosity flood lavas, or show the dust covered remains of a frozen ocean or sea, formed from a catastrophic flood. The images show only a small area of such ‘platy terrain’ and similar surfaces can in fact be traced continuously for over 3500km across the equatorial Elysium Planitia and Amazonis Planitia regions of Mars.
The origin of these terrains is controversial. Early interpretations from NASA Viking Orbiter images in the ’70s and ’80s were of a volcanic or fluvial/lacustrine/volcanic emplacement [1,2], but this region’s similarity to terrestrial shelf-ice was noticed as early as 1993 . In the late 90’s, higher resolution images from the Mars Orbiter Camera (MOC) onboard the NASA Mars Global Surveyor mission, combined with new observations of platy lavas in Iceland and elsewhere, led most researchers to discard any thoughts of the ‘platy terrain’ being ice-related and instead interpretations based on voluminous, extremely low viscosity flood lavas dominated [4,5]. The debate was reopened in late 2005 when images from the European High Resolution Stereo Camera (HRSC) from the ESA Mars Express mission allowed large, continuous sections of the region to be viewed at a medium resolution, and again a sea-ice style formation was proposed . The HRSC images allowed the morphology and topography to be studied over a very large area, giving the regional picture alongside the details. European researchers presented several new lines of evidence for a sea-ice genesis for the platy terrain, including ‘draping’ of the plates over topographic obstacles, lowering of surface levels within topographically closed impact craters, and observations of ‘tide cracks’ where the platy material had retreated from ‘shorelines’.
Many researchers are uncomfortable with the sea-ice hypothesis though, often because it is difficult to imagine how such a large reservoir of water ice could exist at low latitudes on Mars without subliming away in the thin atmosphere. Many also cite observations of the sometimes lobate and steep margins of the platy terrain as being more consistent with the edge of lava flow than the edge of a sea, and the absence of any spectral signature of water ice in this region. Nevertheless, there are strong morphological and topographic arguments for the sea ice hypothesis and, while astrobiology and the hunt for extraterrestrial water remains a strong focus of planetary research, this region will remain an important region for study.
Image credits (a,b) ESA/DLR/FU Berlin (G. Neukum), (c) ESA
 Plescia, J.B., Recent flood lavas in the Elysium region of Mars, Icarus 88, 465-490, 1990.
 Scott, D.H. and Chapman, M.G., Geologic and topographic maps of the Elysium Palaeolake Basin, Mars, USGS Map I-2397, USGS, Flagstaff USA, 1995.
 Brakenridge, G.R. Modern shelf ice, equatorial Aeolis Quadrangle, Mars, Lunar and Planetary Science Conference XXIV, pp175-176, 1993.
 Keszthelyi, L., McEwen, A.S., and Thordarson, T., Terrestrial analogs and thermal models for Martian flood lavas, J. Geophys. Res. 105, 15,027-15,050, 2000.
 Berman, D.C., and Hartman, W.K., Recent fluvial, volcanic and tectonic activity on the Cerberus plain of mars, Icarus, 159, 1-17, 2002.
 Murray, J.B., Muller, J.-P., Neukum, G., Werner, S.C., van Gasselt, S., Hauber, E., Markiewicz, Head, J.W., Foing, B.H., Page, D., Mitchell, K.L., Portyankina, G. and the HRSC CoI team, Evidence from the Mars Express High Resolution Stereo Camera for a frozen sea close to Mars’ equator, Nature, 434: 352-355, 2005.