New research in the journal Science offers new details about the history of water on Mars, gathered from the 2008 NASA Phoenix Mars Mission.
Peter H. Smith, a scientist with the University of Arizona Lunar and Planetary Laboratory and the mission's principal investigator, is the first author of "H2O at the Phoenix Landing Site" in Science. Smith and his group of scientists and students used the lander to investigate the role of water and ice on Mars, as well as the changing weather patterns.
The mission launched in August 2007, and 10 months later, it successfully landed on Martian soil. The landing site was an ejecta field. A comet or asteroid that crashed into the surface melted the ice below.
For the next five months, the UA Science Operations Center clattered with researchers gearing themselves to follow the Martian diurnal phases. The phases are about 40 minutes longer than day and night on Earth.
Smith and his group found patterns in the ground near the lander, multi-sided shapes about three to ten meters in size. The shapes are created when the surface contracts and the ice cracks. Sand fills in the cracks before the ice expands and buckles the surface to make the distinctive patterns.
Smith and his team used the robotic arms to dig a series of trenches to expose subsurface ice and found that the ice in the centers of the polygons was shallow, about a few inches deep.
"But in the troughs in between, we went down as much as eight inches and never found ice. We weren't able to dig further down because the robot arm was hitting against the side of the lander." he said.
"We wanted to know the origin of the ice," Smith said. "It could have been the remnant of a larger polar ice cap that shrank; could have been a frozen ocean; could have been a snowfall frozen into the ground," he said.
"The most likely theory is that water vapor from the atmosphere slowly diffused into the surface and froze at the level where the temperature matches the frost point. We expected that was probably the source of the ice, but some of what we found was surprising."
One of the surprises was finding perchlorate, a salt found naturally on Earth, as well as manufacturing processes.
"Perchlorate was not predicted at this landing site and nobody had it on their list of likely chemicals. There was a very high concentration of it, higher than the salts we might have expected like sodium chloride (table salt). As an oxidized state of chlorine, it has properties such as a strong affinity for water. On Earth, microbes use it as a chemical energy source."
During the mission, Mars moved from summer to winter, giving Smith and others an unprecedented look at the planet's changing weather patterns, including frost and snow.
"Frost was predicted, but snowfall was quite a welcome surprise," Smith said. "In summer there was a lot of dust in the atmosphere. As we neared fall, the dust cleared, and all of a sudden there were water ice clouds forming at about 4 km (2.5 mi.) above the surface. We could see the clouds scud by, moving through the camera field, and we saw snow coming out of the bottom of a cloud. It was very exciting to watch the daily weather changes. No one has ever had this experience."
Smith said there are clues that thin films of water modified the soil chemistry. Unlike Earth, Mars has an unstable spin axis, which currently is tilted at about 25 degrees from vertical. Perhaps five millions years ago, he said, it was tilted much more, which would have exposed the north pole to larger amounts of sunlight creating warmer, wetter conditions during summer.
"During that previous climate, you would expect huge increase in the amounts of water vapor coming off the polar cap. If the cap goes unstable, you can have up to three hundred times as much water in the atmosphere," Smith said.
It would have been enough for snowdrifts. During the summer, melting snow could have formed thin films of water.
Not enough for a lake or a river, but this could have been a time when damp soil provided a growth period for any microbes that learned to survive those long periods of dryness.
"Who knows? Evolution is a powerful force. If life ever started on Mars, there are niches where still it could survive."
Source: University of Arizona