BY DR EMILY BALDWIN
Posted: 22 December, 2008
NASA’s Phoenix Mars Lander could have visited the Red Planet during a particularly dry phase of climate, say scientists who presented their work at the American Geophysical Union meeting in San Francisco last week.
Although Phoenix tasted water ice just a few centimetres below the frozen surface, scientists predict that when long-term climate cycles related to the tilt of Mars’ axis make the site warmer, the soil may get moist enough to modify the chemistry, producing effects that persist through the colder times.
The Snow White trench, where overnight temperatures were dipping to minus 89 Celsius by the end of the mission. Image NASA/JPL-Caltech/University of Arizona/Texas A&M University.
"We have snowfall from the clouds and frost at the surface, with ice just a few inches below, and dry soil in between," says Phoenix Principal Investigator Peter Smith. "During a warmer climate several million years ago, the ice would have been deeper, but frost on the surface could have melted and wet the soil."
The tilt of Earth on its axis varies between 22.1 and 24.5 degrees over a 41,000 year period, and is also responsible for our seasons. With no large moon like Earth's to stabilise Mars, the Red Planet experiences much more dramatic variations in its tilt, or obliquity, on the order of tens of degrees. When the obliquity is low, the poles are the coldest places on the planet while the Sun is located above the equator all of the time, and when the obliquity is high the polar regions will experience warmer summers than Phoenix did.
"The ice under the soil around Phoenix is not a sealed-off deposit left from some ancient ocean," says Ray Arvidson, lead scientist for the lander's robotic arm. "It is in equilibrium with the environment, and the environment changes with the obliquity cycles on scales from hundreds of thousands of years to a few million years. There have probably been dozens of times in the past 10 million years when thin films of water were active in the soil, and probably there will be dozens more times in the next 10 million years."
This frosty image was one of the last to be returned by Phoenix before communications ceased. Image: NASA/JPL-Caltech/University of Arizona/Texas A&M University.
The somewhat clumpy texture of the soil scooped up by Phoenix provides one clue to the effects of water. Microscopic examination of the soil revealed that individual particles were characteristic of windblown dust and sand, but that clumps of the soil held together more cohesively than expected for unaltered dust and sand. "It's not strongly cemented," says Arvidson. "It would break up in your hand, but the cloddiness tells us that something is taking the windblown material and mildly cementing it."
That cementing effect could result from water molecules adhering to the surfaces of soil particles, or it could be from water mobilising and redepositing salts that Phoenix identified in the soil, such as magnesium perchlorate and calcium carbonate. The Thermal and Electrical Conductivity Probe on Phoenix detected electrical property changes consistent with accumulation of water molecules on surfaces of soil grains during daily cycles of water vapour moving through the soil.
"There's exchange between the atmosphere and the subsurface ice," says Aaron Zent, lead scientist for the probe. "A film of water molecules accumulates on the surfaces of mineral particles. It's not enough right now to transform the chemistry, but the measurements are providing verification that these molecular films are occurring when you would expect them to, and this gives us more confidence in predicting the way they would behave in other parts of the obliquity cycles."
Phoenix returned spectacular images and science results from the surface of Mars for five months, two months longer than was expected, but the Phoenix science team will continue to analyse data for months and years to come.
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