Olivine Makes the Scene, with Solar Blasts In Between
Mars Odyssey scientists announced new findings Thursday about the geology and radiation environment of Mars. While Mars has evidence of more water ice than anticipated in the polar regions, other areas of Mars have remained dry for billions of years.
Mars also experiences different amounts of solar radiation than Earth, so future explorers to the Red Planet may face higher risks of cancer than astronauts on the International Space Station.
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The Martian Radiation Environment Experiment (MARIE) on Odyssey measures space radiation in the Mars orbital environment. MARIE monitors both galactic cosmic rays - a continuous background radiation that impinges on the solar system at all times from every direction - and the radiation given off by the sun. Solar particle events are unpredictable, and they can sometimes produce very intense flows of particles out into the solar system.
The galactic cosmic rays hitting Mars are not much different from those hitting Earth. Solar radiation also can be uniform, but sometimes it goes out in a specific direction. These solar particle events therefore can be experienced differently by the two planets as they travel their different orbits around the sun.
"If people go out venturing deep into the solar system, they're not necessarily going to be on the same side of the sun as the Earth," says Cary Zeitlin of the National Space Biomedical Research Institute, the principal investigator for MARIE. "We'll have to have radiation monitors around the inner solar system to watch out for these sorts of events, and send out alarms so that astronauts can take appropriate shelter as needed."
Both galactic cosmic radiation and solar radiation present a health hazard for humans. Earth's atmosphere and magnetic field provides us protection against this radiation, and presumably the martian atmosphere and magnetic field would also provide some protection to astronauts exploring the planet's surface. However, Mars only has a trace magnetic field, and the martian atmosphere is much thinner than Earth's. It is not currently known how much radiation actually makes it to the martian surface.
The GOES-8 satellite measures space radiation in Earth's orbit, among other things. Comparing GOES-8 data with data gathered by MARIE, the scientists found that solar radiation is 2.5 times higher at Mars. Zeitlin says this level of exposure is still manageable, but long-term missions to Mars might be endangered.
Three years to Mars
"If you take our one year of data and extrapolate it to three years, which is the sort of time that people talk about for a Mars exploration mission, you would be within the career limits for astronauts," says Zeitlin.
For manned missions to Mars, planners have suggested that any water ice on Mars could be used for a water supply, or for the production of fuel. The Odyssey's gamma ray spectrometer suite has found new evidence for such ice in the north polar region of Mars
The spectrometer actually found the signature for hydrogen, which indicates the presence of water ice (H2O). Last year, the spectrometer found evidence for hydrogen in the southern polar region as soon as it was turned on. At that time, the north pole was experiencing winter and covered by a thick layer of carbon dioxide frost.
University of Arizona's William Boynton, team leader for the Odyssey's gamma ray spectrometer, says there's much more water ice in the north pole than in the south.
"It might be in the order of 75 percent or more by volume," says Boynton. "It's really a huge amount of ice. We're talking about ice with a little bit of dirt mixed in with it, not the other way around."
Water Dissolved Potassium?
The geology of Mars also is providing some clues about the presence of water on the planet. The gamma ray spectrometer has found indications of potassium and thorium on the surface of Mars. Potassium and thorium both tend to be enriched in magmas, so they are often found together. However, at least one place on Mars has an abundance of thorium but not much potassium. Boynton says that water could explain this difference.
|Mars makes its closest approach this year in the last 50 millenia
Credit: NASA/JPL Viking
"Thorium and potassium do behave very differently in terms of their ability to be dissolved slowly over time by water," says Boynton. The presence of thorium but no potassium may point to water leeching in the past.
The Thermal Emission Imaging System (THEMIS) has produced infrared images of Mars that indicate the presence of the green mineral olivine in the large Valles Marineris canyon.
Arizona State University's Philip Christensen, principal investigator for THEMIS, says that olivine weathers into other minerals in the presence of warm water. The presence of olivine in billion year old rocks on Mars suggests that particular region of Mars has not seen liquid water for a long time.
THEMIS also produced an image of a crater located at 40 degrees north latitude - the equivalent latitude of Chicago. Bands of material seem to have flowed down toward the center of this crater, suggesting the presence of ice or glacial-type processes.
"I think what THEMIS is telling us is that Mars has evidence for ice and snow features, and yet we see minerals that argue that Mars hasn't been a warm wet climate," says Christensen. "I think THEMIS is starting to put pieces together that says Mars has a lot of water, but it's cold and frozen - it's been a cold wet planet for much of its history."
One of the goals for the Mars Odyssey over the coming year will be to completely map the surface in the thermal infrared. The global maps will have the highest resolution of any map that's ever been made of Mars, with 100 meters per pixel element.
The Mars Odyssey mission is planned to continue until August of 2004, but the scientists hope to extend the mission beyond that date. In less than a year, three lander vehicles will arrive at the surface of Mars. Odyssey will support these missions by observing the landing sites, monitoring the weather on Mars, and providing an important data relay link for the landers and rovers so that they can send back scientific data and images from the surface.