Astronomers have long attributed Mars's global orange-brown color to oxidized iron rust in the dust that coats its surface. The source of the rust was always assumed to be water, whether from Percival Lowell's canals of the 19th century or the torrential outflow channels seen by the Viking orbiters in 1976.
However, recent observations have put a damper on the notion that Mars was once awash, at least for long. The surface lacks deposits of carbonates (such as limestones) that would have formed from the interaction of water with Mars's carbon-dioxide atmosphere. Nor are there clays from weathered materials. And Mars shows widespread surface deposits of minerals such as olivine that don't survive long in the presence of water.
At the American Astronomical Society's Division of Planetary Sciences meeting, held this week in Monterey, California, Albert Yen (NASA/Jet Propulsion Laboratory) offered a dry source for the rust. Perhaps, Yen proposes, Mars is covered by iron-rich meteorites.
The idea isn't far-fetched. Astronomers know that some 20 to 60 tons of meteoritic debris falls onto Mars every year. Over a billion years this would make a global layer up to 5 centimeters thick, if it all stayed on the surface. Yen points out that an atom of meteoritic iron releases an electron when hit with ultraviolet radiation. If that electron is captured by oxygen in the Martian atmosphere before it can return to its source, the iron atom becomes oxidized, or rusted. And it happens without water. As additional circumstantial evidence, the crusts of meteorites that fall to Earth are magnetic; so is Martian dirt.
If Yen is correct, about 0.1 percent of the planet's ruddy dust should be meteoritic nickel. Neither the Viking nor the Pathfinder landers were sensitive enough to discern so scant an abundance. But the Mars Exploration Rovers, now en route to the red planet and due to land in January, should be able to detect the nickel if it is there.
"We'll have the answer in February," says Yen. "Stay tuned."