NASA’s next mission to Mars aims to answer one question: What happened to the air that once made the surface habitable?

That is the latest piece in the scientific exploration of whether Mars could have been, perhaps four billion years ago, a place friendly for life.

The answer may come from a space probe known as Mars Atmosphere and Volatile Evolution, or Maven for short, which is ready for the launching pad at Cape Canaveral, Florida, and poised to lift off on Monday. After a 10-month journey, the spacecraft is to enter orbit around Mars and spend at least a year observing Mars’s atmosphere.

“It’s clear that major questions about the history of Mars centre on the history of its climate and atmosphere, and how that’s influenced the surface, the geology and on the possibility for life,” Bruce M. Jakosky, a professor of geological sciences at the University of Colorado who is Maven’s principal investigator, said at a NASA news conference last month.

Planetary scientists believe that young Mars was blanketed with a thick layer of air — heat-trapping carbon dioxide, in particular — that kept it warmer and wetter. Ancient channels on Mars look as if they were carved by flowing water.

Atmospheric transformation

Sometime between then and now, the atmosphere went away, and Mars today is an airless, frigid desert with average surface temperatures of — 64° Fahrenheit, or — 53° Celsius.

The once-bountiful air molecules must have either gone up, escaping into space, or down, transformed by chemical reactions into rock. Hydrogen, the lightest of gases, can simply float away from gravity’s grasp. Heavier molecules like oxygen and carbon dioxide might have been knocked out by particles and radiation streaming from the sun. “As the solar wind sweeps by, it is able to strip off the atmospheric gas,” Dr. Jakosky said. “It’s actually stripped away, molecule by molecule, atom by atom.”

To figure out the puzzle of the missing atmosphere, Maven will carefully measure the wisps that remain.

The spacecraft — which will widen to the length of a school bus after it fans out its solar arrays — will loop Mars in an elliptical, 4.5-hour orbit, climbing 3,860 miles above the planet then swooping down to within 93 miles of the surface. It will also make some particularly deep dips, to within 77.6 miles of the surface. Maven’s eight instruments will take stock of what is in Mars’s upper atmosphere as well as catalogue the solar wind particles bombarding Mars. That will allow the scientists to determine not only the rate at which the atmosphere is disappearing, but also the particulars of how it is disappearing.

“There are a lot of processes that we think may have played a role, and we don’t have the measurements to understand them today,” Dr. Jakosky said.

James F. Kasting, a professor of geosciences at Pennsylvania State University who is not involved with the Maven mission, said it would be useful to know how quickly Mars was losing its atmosphere today. He was less sure about what it would say about Mars in the ancient past. “Conditions would have been, I think, very different,” he said.

The young sun, for example, was about 30 per cent dimmer, but emitted more ultraviolet light.

But Dr. Kasting also said the data could help in understanding whether distant Earth-size planets that are being discovered around other stars would be likely to have significant atmospheres. “That’s interesting for us who are interested in exo-Earths,” he said.

If weather or technical problems keep Maven on the ground, NASA will still have three more weeks to launch it before Mars and Earth move too far out of alignment.

The $671 million mission was almost derailed by the federal government shutdown last month, with work halted when almost all of NASA’s employees and contractors were furloughed. Within a couple of days, however, top NASA officials decided that Maven’s launching fell into the category of “essential,” not because of an urgent need to study the Martian atmosphere but because Maven is also to serve as a communication relay for the Curiosity and Opportunity rovers on Mars.

NASA has two other orbiters circling Mars — Mars Odyssey and Mars Reconnaissance Orbiter — but both are ageing. If Maven does not launch this year, the next chance would be in 2016, and a less favourable configuration of orbits means that it would have to expend more fuel to get to Mars, diminishing its worth as a communication relay. At present, NASA does not have any plans for orbiters to follow Maven.

But Maven is back on schedule, Dr. Jakosky said, and once it completes its primary work, it should be able to stay in orbit for almost another decade. “We’re hoping for a very long mission,” he said. — New York Times News Service

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