Craters embedded on pedestals that tower above the Martian landscape like giant egg cups could be used to trace the planet鈥檚 climate history, a new study suggests.
鈥楶edestal鈥� craters were gouged out by impacts, like other craters, but stand out because they sit atop plateaus that loom an average of 50 metres above the Martian surface.
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It鈥檚 not clear exactly how the pedestals formed. One idea is that wind simply eroded the area around the plateaus, but critics say the wind would have had to have acted very symmetrically to produce the structures.
Now, a comprehensive catalogue of the objects is lending weight to the idea that the pedestals may conceal ice-rich soil from previous eras, when the planet鈥檚 spin axis tilted at a different angle than it does today.
Seth Kadish of Brown University in Providence, Rhode Island, and colleagues identified 2696 pedestal craters in the planet鈥檚 mid- and low-latitudes from images taken primarily by the thermal imager aboard NASA鈥檚 Mars Odyssey spacecraft.
Barren equator
The craters seem to be concentrated at the mid-latitudes, with very few found at the planet鈥檚 equator. About 3 per cent of them have depressions around their bases that resemble areas in Antarctica where permafrost ice vaporises, creating pits in the soil left behind (see image).
The team says that strengthens the hypothesis that the pedestals were created from soil that was enriched in ice during a period when the Martian poles pointed more towards the sun and its mid-latitudes were colder. Because Mars does not have a massive satellite that stabilises it, like Earth鈥檚 moon, the tilt of its axis is thought to change regularly on scales of tens of thousands of years.
When the planet is tilted most drastically on its side, the planet鈥檚 poles receive a lot of sunshine. Any water locked in ice there is thought to vaporise and move towards the equator, where it falls as snow. Tens of metres of snow are thought to be deposited on the planet鈥檚 mid-latitudes during these episodes.
Cookie-cutter section
Pedestal craters may preserve regions with this ancient snow. The researchers suspect the impact of the meteorite that created each pedestal crater could somehow 鈥榓rmour鈥� the ground in the area, producing a top layer that protected ice from sublimating into gas during warmer periods.
But it is unclear exactly how this layer might form. 鈥淥ur best guess is that when the impact occurs there鈥檚 basically a shock wave, a combination of an atmospheric blast with really high-speed winds and a thermal pulse that travels through the upper layer of soil,鈥� Kadish told 快猫短视频.
Although the heat of the impact could cause the ice in the upper centimetres of soil to sublimate, winds from the violent event could then compact the remaining material. That could protect underlying ice from the sun, since the compressed soil would block sunlight and make it difficult for water vapour to find pores to escape to the surface.
The unprotected ice surrounding the armoured area, however, would eventually disappear when the planet鈥檚 tilt changed and the area warmed. That would leave behind the modern-day, ice-laden pedestals that can be more than 100 metres thick. 鈥淭hese pedestals represent almost like a cookie-cutter section of past icy, dust-rich layers,鈥� Kadish says.
Wobbly planet
Because pedestal craters are so numerous, they probably formed over different tilting episodes in the planet鈥檚 past. Images of the Martian surface have also revealed evidence of pedestals stacked on top of pedestals, suggesting multiple rounds of egg cup crater formation.
Using the high-resolution camera aboard the Mars Reconnaissance Orbiter could help count the craters on top of pedestals to date when the pedestals may have formed, since pedestals with fewer craters likely formed more recently. That could help trace out the planet鈥檚 past wobbles, Kadish says.
鈥淚t鈥檚 an interesting part of the puzzle, but the quest [to understand pedestal craters] continues,鈥� says of the University of Hawaii in Honolulu.
He says the study raises a number of unanswered questions, including whether armouring actually occurs. If the ground around 鈥榦rdinary鈥� craters also shows evidence of soil that was altered by impacts, it would show that armouring is a universal process that only produces pedestals in ice-rich areas, he says.
Journal reference: (DOI:10.1029/2008JE003318, in press)
