Informers from Mars

Bursts of energy from the gases churning on the surface of the sun have long offered important clues about the atmosphere of Earth. Now, thanks to research at BU’s Center for Space Physics, solar flares are helping to explain the atmosphere on Mars.

Michael Mendillo (GRS’68,’71), a College of Arts and Sciences professor of astronomy, and Paul Withers, a research associate at the center, worked with three scientists, from Stanford, UMass Lowell, and the University of Southampton, UK, to analyze data surrounding the moments after the solar flares of April 15 and 26, 2001. Their report, “Effects of Solar Flares on the Ionosphere of Mars,” was published Friday, February 24, in the journal Science.

Mendillo, who was the lead author of the study, says looking at the way a planet responds to solar flares is a bit like studying the way a person responds to stress: both reveal a lot about the thing that is acted upon. “You want to see how an object reacts,” he says. “If it’s a person, you may want to see how they react to stress; or [you may want to] poke an elephant with a needle. You disrupt a big system and see what happens.”

The recent report offers a way to test some theories about what the Martian atmosphere is like. “You develop a theory based on the data,” says Withers, “and you hope those theories are close enough to the way nature works that you can take them to a different environment and they will still give a good description of what’s going on there.”

In fact, much more may come from the scientists’ discovery than the ability to test theories about the atmospheres of other planets. Because we know, for example, that GPS (global positioning system) devices on Earth are disrupted when the ionosphere is affected by X-rays released by solar flares, understanding the behavior of Mars’ atmosphere when struck by those X-rays could influence a long-term NASA plan to put GPS-type satellites around Mars.

“It is odd for astronomy [studies] to have an actual practical application,” Mendillo notes, adding that he expects that the usefulness of his report will be applied on a molecular level, if not a galactic one.

The scientists also looked at how the X-rays were absorbed, or not absorbed, by the atmosphere of Mars. What they saw was a dramatic enhancement of the ionosphere when X-rays from the sun hit molecules in the Martian atmosphere.

On Earth, says Mendillo, X-rays from the sun are absorbed by nitrogen and oxygen, but the atmosphere on Mars has more carbon dioxide. When an X-ray hits a carbon dioxide molecule, electrons shoot out, hit other molecules, and dislodge more electrons, ionizing the atmosphere.

“After each solar flare,” Withers says, “each X-ray photon absorbed creates 5 to 10 ionized particles from these careening pinballs of an electron zooming around at high speeds.”

The study marked the first time that scientists examined the consequences of X-rays striking the atmosphere of another planet, says Mendillo, who has already prepared a research proposal to follow up on these findings.

“That’s where the science yield will be,” he says. “That’s what we’re really interested in.”