Friday 25 April 2014

Ice in Martian impact craters.

The surface of Mars has been observed continuously by the Mars Orbiter Camera from 1997 to 2006 and the Mars Reconnaissance Orbiter since 2006. During the time that these observations have been occurring around 200 new impact craters have been observed on the surface of the planet; most of them in dusty areas, where they are easily detected due to the dark blast patterns that surround fresh impacts in these regions. Many of the impacts at mid-to-high latitudes show ice formation within these craters, therefore providing useful data in the study of Martian ground ice.

In a paper published in the Journal of Geophysical Research: Planets on 27 January 2014, a team of scientists led by Colin Dundas of the Astrogeology Science Center of the U. S. Geological Survey discuss the implications of this crater ice for our understanding of Martian ground ice in general.

HiRISE image of an ice-exposing impact crater. Dundas et al. (2014).

The atmosphere of Mars is too thin to support liquid water; therefore sublimates directly to water vapour when it is warmed, and water vapour precipitates as ice when it is cooled. It is thoughts that the equatorial region of Mars is permanently too cool for the formation of ground ice (although seldom reaching as warm as 0˚C; water has a lower freezing point in Mars’s thin atmosphere), but around 40˚-50˚ latitude ground ice begins to appear seasonally at a depth of about one metre in the soil, becoming shallower and more permanent towards the poles, where visible ice caps exist above the soil surface.

Twenty new impact sites with visible ice inside the craters have been seen since permanent observations of the Martian surface commenced. The ice in these craters appears to be relatively clean, suggesting that it has formed after the impact; exposed soil-cementing ice would be predicted to have a dirty appearance. 

New impacts on Mars are generally detected first by their ejecta, and are therefore preferentially observed in dusty regions, where large, dark, blast zones are produced around new craters. Where ice is exposed at such sites it appears as a bright white spot within the larger darker patch. Such observations are typically followed up by a more detailed observation by the High Resolution Imaging Science Experiment (HiRISE) camera on the Mars Reconnaissance Orbiter.

Example of a discovery image by the Context Camera on the Mars Reconnaissance Orbiter . Note the dark ejecta and bright point at the centre. Dundas et al. (2014).

Eighteen of the newly discovered craters are in the Martian Northern Hemisphere, all above 39˚ North. Only two of the new impact craters are in the Martian Southern Hemisphere, two of which are in the Southern Polar Region. This accords well with the predicted distribution for the discovery of new craters, not because more craters are formed in the Northern Hemisphere, but because the Southern Hemisphere lacks the large dust fields of the Northern Hemisphere.

Global distribution of new impacts. White symbols indicate ice-exposing impacts; black indicates no visible ice. Background warm colours indicate dusty areas. New craters are preferentially detected in dusty areas. Dundas et al. (2014).

Some of the newly discovered craters in the Northern Hemisphere are at latitudes where ice exposure would be predicted, but do not appear to show any ice. This suggests that either the craters are not deep enough to hit the ice layer, or that exposed ice has subsequently sublimated from the crater prior to its discovery. It is also possible that the presence of the ice layer at these latitudes is discontinuous, but this is thought unlikely, as there is no known mechanism for such a discontinuous ice distribution. Several craters have been shown to have flattened floors, suggesting that a more solid layer of material was reached that the impact did not have the energy to disrupt; this could conceivably be the ground ice layer. It is also possible that some of the exposed bright patches are lighter coloured rock or regolith, particularly in craters at lower latitudes and where there is a significant proportion of lighter material in the ejecta.

Most of the new craters are in areas where older craters are uncommon, suggesting that enough reworking of the Martian surface is going on to slowly remove such craters. Most of the craters that have been observed for any period of time slowly faded, with the ice disappearing more rapidly than the surrounding dust apron, which suggests that more is going on than simple burial of the ice by fresh, wind-blown, dust. In some cases the ice appeared to brighten temporarily, suggesting seasonality was affecting the extent of the ice in the crater. This is particularly true at sites with latitudes of over 50˚.

Changes at a site at 63.92˚N over time. Over the initial summer after impact, the ice (a) fades and (b) shrinks somewhat. It is then essentially invisible at the start of the following (c and d) spring before (e) rebrightening and (f) fading again. Dundas et al. (2014).

At latitudes of above 50˚ the darker ejecta zone was also prone to seasonality. Below 50˚ these darker zones simply faded progressively over time, but above 50˚ they often disappeared completely during their first winter, or in some cases even became brighter than their surroundings. This is probably due to reworking by a seasonal ice cap. On Mars this is made up of a mixture of dust, water ice and carbon dioxide ice (seasonal water frost appears above 45˚ on Mars, and carbon dioxide frost above 50˚).

The formation of ice in the lowest latitude craters where ice is seen would require the water content on the Martian atmosphere to be about twice as high as it is currently thought to be, or the concentration of water vapour within the atmosphere closer to the ground than in the well mixed atmosphere of Earth. This later explanation is consistent with humidity measurements made by the Phoenix Lander. Such a concentration of humidity close to the ground requires an explanation, though it could be caused by the presence of liquid brine (water with a very high salt content) or deliquescent perchlorate salts (salts with a very high water content) beneath the Martian surface.

The ice in the craters appears to be clean ice, rather than ice mixed with regolith, and the time taken for it to disappear suggests that it is probably at least several millimetres thick. Excavations by thr Phoenix Lander have found ice mixed with regolith but not pure ice. It is possible that in some of the larger craters melting led to the formation of liquid (if briny) water in the crater, which then refroze to produce an ice layer in the crater. However such melting would be expected to produce mud rather than pure water, and this would need to remain liquid for long enough for the regolith particles to precipitate out in order to form an ice layer, which seems unlikely on the low-pressure Martian surface, and most predictions about thermal conditions within impact craters suggest that vaporization of sub-surface water-ice leading to a layer of dried regolith would be a more likely outcome. 

Dundas et al. tentatively suggest that this ice may be the result of migration of thin films of water within the craters as the best explanation for the formation of this clean ice, but not that this explanation would require excess ice (ice at higher levels than predicted) beneath the Martian surface, which would also require explanation, the most probable cause being a significant Martian Ice Age within the last million years that led to the formation of extensive ice sheets.

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