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Bumpy Road to Mars, Part 2
May 22, 2008by Edna DeVore, Director of Education and Public Outreach
On Sunday, May 25, the Phoenix Lab lands on Mars to look for evidence of water and organics in the soil. Like its namesake mythological bird, NASA's Phoenix Mission rises from remnants of its predecessors. It will use many components of a spacecraft originally built for a 2001 Mars lander mission, that were kept in careful storage after that mission was cancelled.
Long-time scientist in SETI Institute's Carl Sagan Center, geologist John Marshall studies dust here on Earth. As a science team member of Phoenix, he will apply his skills to understanding the soil and dust on Mars. Don't think of him as the "dustman," rather, he's a geologist who works at the microscopic scale. He studies dust to understand how water and wind have altered the surface of the tiny bits of rock to learn about the geological history of materials here on Earth, and soon, on Mars.
Marshall works on the Microscopy, Electrochemistry and Conductivity Analyzer (MECA), which has several components. Optical and atomic-force microscopes will examine samples' mineral grains. Marshall is lead scientist for interpreting data from the optical microscope. Last week, this column profiled Richard Quinn, who is a scientist on the team responsible for the four electrochemistry cells that will measure a wide range of chemical properties, such as the presence of dissolved salts and the level of acidity or alkalinity. The third component of MECA is a conductivity probe mounted on the robotic arm that will check the soil's thermal and electrical properties.
Marshall describes himself as a scientist for the optical microscope; he's the only geological microscopist working with this instrument. The remainder of the team are primarily technical microscopists who made sure the instrument was designed and tested successfully, and will be working correctly to do science. The optical microscope is set up to image at least 10 samples. He anticipates that will be sufficient, but if the onboard cameras reveal other intriguing targets within reach, the group can re-use the sample equipment to do further studies.
I asked Marshall what he'd like to see, and he replied "I'll be looking for evidence of water activity in the particles. Water rounds particles as they tumble about, smoothing off the edges and corners. Water can also etch particles that simply sit submerged for a length of time. Clay particles are direct evidence of water. I'll also look for crystallization and hydrated minerals. All of these can be interpreted as evidence of water."
Water is essential for life as we know it. Understanding the history of water at this Arctic-like site will reveal whether it is habitable, either in the past or present. In the tiny bits of dust, Marshall expects to seek evidence of water on Mars. There's ice only centimeters below the surface at the landing site, which leaves little doubt that water exists on Mars. The core question is whether there were lakes and rivers on Mars in the past. Marshall thinks that the physical shape, size and condition of dust particles can help answer this question.
But, what might the Phoenix lander actually sample? Marshall described the landing site as an ejecta blanket. He anticipates seeing volcanic particles (basalts), weathered aeolian dust deposited by the major dust storms that envelop Mars periodically, and sand-sized particles. He also expects to find concrete-hard ice just below the surface. One of the planned activities for the mission is to dig a trench to profile the soil. If the ice is shallow, the profiles will be limited. It's hard to predict what will actually occur.
The Phoenix Lab uses pulsed engines to slow descent to the surface. In preparation for this, Marshall was also part of a team that tested the impact of the landing system at NASA Ames Research Center, working with Lockheed Martin, Jet Propulsion Laboratory, and University of Michigan. They modeled the impact of the pulsed engines on soil disturbance, and field-tested an engine in a mock-Mars landscape to understand what might happen when Phoenix lands. The team anticipates that the pulsed engines could excavate dust from beneath the lander and erode particles from the bottom of the lander out onto the surrounding area. The scattered dust might include contaminants from the undersurface of the lander. I asked whether this could confuse results, and Marshall was confident that it would not because the likelihood of actually picking up a tiny bit from the lander was very low.
On Saturday, Marshall heads to Tucson, Arizona to be present for the landing and celebrate with the whole team as Phoenix sets down on Mars. Bon Voyage, John, and best wishes for a happy landing.