* Astronomy

Clouds scoot across the Martian sky in a movie clip consisting of 10 frames taken by the Surface Stereo Imager on NASA's Phoenix Mars Lander.



Image NASA/JPL-Caltech/University of Arizona/Texas A&M University

This clip accelerates the motion. The camera took these 10 frames over a 10-minute period from 2:52 p.m. to 3:02 p.m. local solar time at the Phoenix site during Sol 94 (Aug. 29), the 94th Martian day since landing.
Particles of water-ice make up these clouds, like ice-crystal cirrus clouds on Earth. Ice hazes have been common at the Phoenix site in recent days.
The camera took these images as part of a campaign by the Phoenix team to see clouds and track winds. The view is toward slightly west of due south, so the clouds are moving westward or west-northwestward.
The clouds are a dramatic visualisation of the Martian water cycle. The water vapour comes off the north pole during the peak of summer. The northern-Mars summer has just passed its peak water-vapour abundance at the Phoenix site. The atmospheric water is available to form into clouds, fog and frost, such as the lander has been observing recently.

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Until now, Mars has generally been regarded as a desert world, where a visiting astronaut would be surprised to see clouds scudding across the orange sky. However, new results show that the arid planet possesses high-level clouds that are sufficiently dense to cast a shadow on the surface.
 The results were obtained by the OMEGA Visible and Infrared Mineralogical Mapping Spectrometer instrument on board ESAs Mars Express.
Mars is not entirely a haven for Sun worshippers. Clouds of water ice particles do occur, for example on the flanks of the giant Martian volcanoes. There have also been hints of much higher, wispy clouds made up of carbon dioxide (CO2) ice crystals. This is not too surprising, since the thin Martian atmosphere is mostly made of carbon dioxide, and temperatures on the fourth planet from the Sun often plunge well below the freezing point of carbon dioxide.
Now, a team of French scientists has shown that such clouds of dry ice do, indeed, exist. Furthermore, they are sometimes so large and dense that they throw quite dark shadows on the dusty surface.

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The case for an ancient ocean on Mars just got stronger.
For years scientists have been baffled by what look like shorelines on Mars. But, impossibly, sea level appears to have been 2.5 kilometres higher in some parts than in others - so many have doubted the ocean really existed. Now Taylor Perron of the University of California, Berkeley, and colleagues think that massive wobbles in the planet's rotation may explain the mystery.

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Strong evidence that Mars once had an ocean
A paper in this week's issue of Nature by University of California, Berkeley, geophysicists demolishes one of the key arguments against the past presence of large oceans on Mars.

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