A dust layer sandwiched between layers of ice near Mars鈥檚 north pole suggests the planet鈥檚 climate has shifted dramatically in the past 100,000 years or so, reveal images recently obtained by NASA鈥檚 Mars Reconnaissance Orbiter (MRO).
Previous research has suggested that the tilt of Mars鈥檚 axis of rotation fluctuates from 10掳 to 40掳 with time, leading to major climate shifts (see Wrecking ball could break the ice on Mars).
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Now MRO, which started returning close-up images of the Red Planet in late September 2006, has provided new evidence that Mars鈥檚 climate has undergone major shifts relatively recently.
The image was made with the spacecraft鈥檚 Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) and shows part of a valley called Chasma Boreale that cuts into Mars鈥檚 north polar ice cap.
Three layers are visible in the cap. The one on top corresponds to the most recent period in Mars鈥檚 climate history, during which ice has built up around the north pole. The bottom layer is also made of ice and appears to represent a period with a similar climate.
In between is a layer of dust. Its presence suggests an era with a very different climate, when ice was no longer being deposited in the north polar region.
鈥淲hat this is telling us is that at the north polar cap, over the past 100,000 years, there鈥檚 been a really dynamic change of climate,鈥� says CRISM chief scientist Scott Murchie of the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, US.
Braided structure
MRO has also sent back images taken by its High Resolution Imaging Science Experiment (HiRISE) that show braided streambeds and other signs of past water on Mars (scroll down for image, on right).
In the southern hemisphere, a high-resolution image of gullies carved into the sides of a crater reinforces the view that some of these features, which are seen in various places on the planet, may have been caused by liquid water.
Some researchers had previously suggested these sinuous channels had been created by avalanches of carbon dioxide or rivers of dust.
The image shows that some of the gullies have a braided structure, which is characteristic of sediment-laden streams on Earth. The sediment builds up in slower-moving parts of the stream, eventually blocking it and forcing the stream to weave around those areas.
Multiple events
Water appears to have flowed more than once in the area of the image, since some smaller gullies cut into larger and presumably older ones, says Alfred McEwan of the University of Arizona in Tucson, US. 鈥淚t didn鈥檛 just form in one day,鈥� he says. 鈥淭his is something that formed over a period of time with multiple events.鈥�
Since the images were taken in the first week of October, Mars has moved very close to the Sun from Earth鈥檚 perspective, preventing reliable communication with MRO for about a month.
During the communications gap, the spacecraft will take images with its Mars Color Imager and make measurements of the planet鈥檚 atmosphere with its Mars Climate Sounder. Data from the two instruments will be stored on the spacecraft until regular communication resumes in early November.
