Volume XXVII Number 1
Rippling dunes on the floor of the Endurance Crater on Mars are captured in an image taken by the Mars Exploration Rover.
David Bish examines a model of the crystal structure of a zeolite. Zeolites are natural hydrated minerals that occur in lava flows and in volcanic deposits that have been altered by groundwater. Zeolites contain large amounts of water, making them important possible hydrous minerals on Mars.
Photo courtesy David Bish
The Red Stuff
As a bright, curiously colored object in the night skies, Mars must have caught human attention early. Whether or not they knew what they were seeing, our ancestors named the planet. Most of these names are lost to prehistory, but we know some of them--the Babylonians called it Nergal; for the Greeks, it was Ares; the Egyptians called it Har Decher. The Romans, who conquered all, called it Mars, after their champion of war.
For two millennia, night watchers learned little of Mars beyond its name and its movement across the sky. But as human science and technology began its powerful ascent in the 16th century, Mars attracted the attentions of early scholars--Brahe, Copernicus, Kepler, Huygens, Newton, Galileo. Thousands of astronomers, cosmologists, and geologists followed the lead of these early visionaries, scrutinizing cool, dry, mysterious Mars.
Yet, despite 500 years of rigorous study, important discoveries from Earth's surface, recent revelations by robotic Martian probes, and our unique cultural familiarity with the Red Planet, we still don't know--right stuff or wrong stuff--just what stuff Mars is made of.
It may seem strange that after all this time we know so little. But while scientists have been able to detect elements and a few basic molecules at or near the Martian surface, substantive information about the rocks and minerals that comprise Mars is as scarce as desert water. When asked what complex molecules or rocks make up Mars, scientists must begin, "We think . . . ."
This dearth of reliable information about the planet's constitution nags scientists and government policy types alike. Space agencies such as the National Aeronautics and Space Administration (NASA) and the European Space Agency are actively discussing how best to solve Mars' most substantial mystery.
"One of the best ways to solve this mystery is to send up an X-ray diffraction instrument," says Indiana University Bloomington geologist David Bish, who happens to be working with a team on an X-ray diffractometer prototype he hopes NASA will use. "Right now we know very little about the mineralogy of the surface of Mars. A diffractometer would give us unambiguous data. It would tell us what minerals make up the rocks, and this in turn will tell us how rocks were formed and what kinds of geological processes shaped them."
X-ray diffractometers irradiate a sample (for example, the surface and near-subsurface of Mars) with X-rays. Part of the device records the pattern of radiation scattered by the sample. Every mineral responds differently to X-ray bombardment, so the diffractometers are able to distinguish between even closely allied minerals, such as iron pyrite ("fool's gold") and marcasite, a mineral composed of the same elements as iron pyrite but organized differently at the molecular level.
Such differences are crucial, especially as NASA devotes more of its resources to looking for evidence of past or present life on Mars. A mineral's atoms and its crystalline structure can make it water-friendly or as inhospitable to water as oil. Bish thinks water may be trapped in the structures of minerals inside rocks at or below the Martian surface. If future missions to Mars reveal minerals capable of trapping moisture, geologists will learn something new and exciting about the planet. Of course, such a discovery would also spur scientists to wonder whether a presently hypothetical organism might be using that water to live.
Recent discoveries by satellites orbiting Mars suggest that water exists around the Martian equator, where scientists least expected to find it.
"Previously, Mars had been thought to be a very dry planet with water only at the poles," says Bish, who is also studying the distribution of water around the equator of Mars. "If there was water, it was thought that it would be thinly and homogeneously distributed. But probes have discovered two water-rich areas near the equator, separated on opposite sides of the planet."
Scientists had known for some time that the planet's ultra-frigid poles are capped with frozen carbon dioxide and water, but they'd figured air at the Mars equator was too dry and too thin to abide the existence of water in liquid or solid form.
Could these water deposits be former north-south poles from Mars's distant past? Bish says the question provides added impetus for continued probing with diffractometers and other highly precise devices.
Bish and colleagues at NASA's Ames Laboratory and Los Alamos National Laboratory submitted a proposal to NASA this year that would put an X-ray diffractometer the size of a soda can on the Mars Science Lab, an unmanned Mars mission currently scheduled for 2009.
When it comes to NASA unmanned rovers, size matters. A conventional X-ray diffractometer is about as big as a full-size refrigerator. Not only would such an instrument crush the lander, it would be pretty hard to get into space in the first place.
Bish is cautiously optimistic that NASA will give his group the go-ahead to put a tiny X-ray diffractometer on Mars Science Lab, with good reason. The space agency is already giving the scientists funding to engineer a shrunken version.
"So far, we're down to toaster-size," says Bish, who joined the IUB geological sciences faculty in 2003 as the Haydn Murray Chair in Applied Clay Mineralogy. "And the data we're getting from the smaller instrument are of high quality, very close to what we get from large laboratory diffractometers. We've made a lot of progress in 14 years."
David Bricker is a media relations specialist in the IU Office of Media Relations and a freelance writer in Bloomington.