The question seems simple: How did Mars, initially a warm world with ample water flowing across its surface, lose the atmosphere that made those life-encouraging conditions possible?
With NASA's latest mission to the red planet, set for launch Monday afternoon, scientists hope to find answers.
The mission is more than a $671 million effort to solve a riddle for one planet in one planetary system, offers Bruce Jakosky, the mission's lead scientist, or principal investigator.
As astronomers hunt for other Earths and evidence of life beyond the solar system, lessons from Mars can help researchers understand the processes that turn a once-habitable planet into a barren wasteland inhospitable to life, at least on its surface, Dr. Jakosky explains.
By some estimates, Mars orbits within the sun's habitable zone, although at the outer fringes. Over its 4.6 billion-year history, it's remained there. Yet something fundamental changed to alter the planet's potential for hosting at least microbial life.
“I see this mission at its core as an astrobiology mission,” Jakosky said during a prelaunch briefing Sunday. “As we're starting to discover more and more planets outside our solar system and see Earth-like planets ... we want to understand what makes a planet habitable and what makes a planet go from being habitable to not being habitable.”
MAVEN, short for Mars Atmosphere and Volatile Evolution, is slated for launch from the National Aeronautics and Space Administration's Kennedy Space Center in Florida at 1:28 p.m., Eastern Standard Time, on Monday.
After MAVEN's 10-month journey to Mars, scientists will use a suite of eight instruments on the craft to take measurements throughout the planet's upper atmosphere, where the Martian atmosphere's vanishing act largely takes place.
The main culprit in the mystery of the missing atmosphere is the sun. It constantly sends charged particles and their associated magnetic fields streaming into the solar system. This flow of solar wind is punctuated by powerful storms – flares and larger coronal-mass ejections – that can send billions of tons of matter hurtling through the solar system at speeds of up to several million miles an hour.
Earth's gravity and global magnetic field significantly reduce the rate at which the sun's activity strips the atmosphere from the planet, although some leakage occurs at Earth's poles, researchers say. Indeed, the third rock from the sun lost more of its early atmospheric water vapor and carbon dioxide to the formation of oceans and carbonate rocks than it did to space.
A decade of measurements taken by a series of orbiters, landers, and rovers has revealed that Mars also has sequestered CO2 in carbonate rocks. But the planet doesn't have enough carbonate rocks to account for the CO2 the planet needed to lose in order to make the climate transition it underwent after the first 500 million to 1 billion years of its 4.5 billion-year history.
Mars' gravity is only about 40 percent as strong as Earth's gravity. And while Mars once had a global magnetic field, it didn't last long. All that remains are small areas of regional magnetic fields that arch over patches of the surface, dotting the planet like so many mushroom caps.
Observations during the past decade have demonstrated that “Mars bristles with loss processes,” said Janet Luhmann, a planetary scientist at the University of California at Berkeley and the mission's deputy principal investigator, during the briefing on Sunday.
Random collisions between atoms and molecules in the uppermost reaches of the atmosphere can occur with enough energy to fling some of the atoms or molecules into space. Ultraviolet light from the sun can knock electrons off atoms and molecules, which can recombine with other loose electrons with enough energy to kick them into space. Magnetic fields associated with the solar wind or with pulses of charged particles during solar storms can accelerate ionized molecules and atoms as the solar material sweeps past the planet. Some of the accelerated ions head into space. Others can hurtle toward the surface with enough energy to send atoms or molecules lower in the atmosphere ricocheting into space like billiard balls.
Mars' relatively weak gravity compared with Earth means particles can travel more slowly and still escape the planet's grip.
MAVEN carries eight instruments grouped in three packages to help scientists gauge the relative influence each of these processes has on atmospheric losses.
One package is designed to measure various aspects of the sun's influence, including: the energy carried by charged particles from the sun; the density, temperature, and speed of the solar wind; and levels of extreme ultraviolet light that the sun emits. The mission is taking place during an active period in the sun's 11-year sunspot cycle, where its relatively high levels of ultraviolet radiation and stormy outbursts most closely resemble the level of activity thought to have been the norm during a young Mars.
A second package will measure properties of the planet's ionosphere, made up of atoms and molecules that have lost electrons to the sun's radiation.
The third package measures the structure and chemical composition of the upper atmosphere. Researchers are particularly interested in how the distribution of various atoms and molecules, as well as the ratios of their variants, or isotopes, changes with altitude, says Paul Mahaffy, a researcher at NASA's Goddard Space Flight Center in Greenbelt, Md.
He's the lead investigator for one of the craft's instruments, MAVEN's version of the “Major Mass Spec” from the TV show "NCIS." It's a mass spectrometer, an instrument that will help identify the abundance of various gases and the isotopes they contain.
The instrument has a counterpart on NASA's Mars rover Curiosity, which is measuring similar features in the atmosphere at the surface.
Measurements involving argon isotopes will be particularly instructive, Jakosky says. Unlike other chemicals in the atmosphere, argon doesn't react with its surroundings. So argon is likely to prove the most simple, straightforward indicator of atmospheric losses today, he says. Since lighter isotopes escape the planet more readily than heavier isotopes, the losses will appear as a change in the relative abundance of heavy to light isotopes of argon with altitude.
At ground level, Curiosity has measured the relative abundance of argon 38 to the lighter argon 36. The abundance of the heavier isotope is higher than “anyplace else we know of in the solar system,” Dr. Mahaffy said at the briefing. “That's really a signature of the lighter argon having escaped.”
By apportioning today's losses among the various loss mechanisms and the rates of loss they provide, plus some clever modeling of the sun's behavior during the past 4 billion years, the researchers anticipate solving the mystery of Mars' missing atmosphere.
“We can't go back and study what happened over 4 billion years” of Martian atmospheric history, Jakosky said. “But we can go and look at how these processes are operating today and how they might have changed over time and what the integrated effect would have been.”