Polar explorer: Phoenix landed at 68.2 degrees north latitude, 234.2 degrees east longitude. The far-northern location of the site is indicated on this global view from the Mars Orbiter Camera on NASA's Mars Global Surveyor spacecraft.

Credit: NASA/JPL-Caltech/University of Arizona/MSSS

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SYDNEY: A laser instrument aboard NASA's Phoenix lander, has detected snow falling from Martian clouds. Found at the Red Planet's north pole, falling light levels now threaten to deprive the probe of power within months.

Designed to gather knowledge of how the atmosphere and surface interact on Mars, Phoenix's laser instrument detected snow from clouds about four kilometres above the spacecraft's landing site. Data show the snow vaporizing before reaching the ground.

Good progress

"Nothing like this view has ever been seen on Mars," said Jim Whiteway, of York University in Toronto, Canada, lead scientist for Phoenix's Canada-supplied meteorological station. "We'll be looking for signs that the snow may even reach the ground."

Since landing on 25 May, Phoenix has confirmed that a hard subsurface layer at its far-northern site contains water-ice. Determining whether that ice ever thaws would help answer whether the environment there has been favourable for life, a key aim of the mission.

"We are still collecting data and have lots of analysis ahead, but we are making good progress on the big questions we set out for ourselves," said Phoenix Principal Investigator Peter Smith of the University of Arizona, Tucson.

New soil test experiments now also provide evidence of past interaction between minerals and liquid water, processes that occur on Earth. The clues point to calcium carbonate, the main composition of chalk, and particles that could be clay. Most carbonates and clays on Earth form only in the presence of liquid water.

Interaction with water

The evidence for calcium carbonate in soil samples dug by Phoenix's robotic arm comes from two laboratory instruments called the Thermal and Evolved Gas Analyzer (TEGA) and the wet chemistry laboratory of the Microscopy, Electrochemistry and Conductivity Analyzer (MECA).

"We have found carbonate," said William Boynton of the University of Arizona, lead scientist for the TEGA. "This points toward episodes of interaction with water in the past."

The TEGA evidence for calcium carbonate came from a high-temperature release of carbon dioxide from soil samples. The temperature of the release matches a temperature known to decompose calcium carbonate and release carbon dioxide gas, which was identified by the instrument's mass spectrometer.