NASA's MAVEN spacecraft is hoisted toward the top of an Atlas V rocket… (Kim Shiflett / NASA )
Four billion years ago, rivers and lakes dotted the surface of Mars, their waters reflecting puffy clouds drifting in a blue sky, scientists believe.
Now, it's a dry, rusty rock that's subject to fierce sandstorms, withering blasts of radiation and freezing temperatures that have frozen carbon dioxide to the planet's poles.
That's the question NASA seeks to answer with the scheduled launch Monday of the MAVEN spacecraft.
Planetary scientists believe the answer lies high in the Martian atmosphere. Today, it's a thin layer of mostly carbon dioxide gas. But long ago, it may have been thick enough to host a life-friendly, even Earth-like, environment. If so, the Mars Atmosphere and Volatile EvolutioN mission may reveal clues about where it all went.
Previous missions, from rovers to orbiters, have tried to see into the Red Planet's past by examining its surface. But that only tells part of the story, said Bruce Jakosky, a planetary scientist at the University of Colorado-Boulder and principal investigator for the MAVEN mission.
"If we want to understand Mars as a system, we need to include the role of the upper atmosphere," he said. "We can no longer just look at the geology and understand it in isolation."
Earth is flanked by two alternative worlds. On the side closer to the sun is Venus, a planet with too much atmosphere, boiling away under thick layers of carbon dioxide. On the other side lies Mars, a planet with too little air shielding its cold, dead deserts.
"We think all three of those planets, when they formed, were not all that different," said Steven Lee, curator of planetary science at the Denver Museum of Nature and Science. Life arose on Earth because it's a "Goldilocks" case, protected by just the right amount of air.
Robotic explorers sent to Mars by NASA and other space agencies have already picked up many signs that water once flowed across the surface. Scientists have identified craters that they think were once filled with lakes, along with clay minerals that must have been formed by exposure to water.
"I wouldn't be surprised if, for many tens of millions of years, Mars was a pretty friendly place with natural water, wind, waves and rain," said Paul Mahaffy, an atmospheric scientist at NASA's Goddard Space Flight Center in Greenbelt, Md., who leads instruments on both MAVEN and the Mars rover Curiosity. "But I think we just don't know."
Liquid water can't exist on the Martian surface today — it would quickly freeze in the coldest spots and boil off elsewhere, because the low-pressure atmosphere is far too thin to keep it in.
Understanding why Mars' atmosphere vanished and Earth's did not is key to understanding Earth's ancient history. Someday, it may even help scientists study the atmospheres of planets orbiting distant stars.
There are several theories about why the Martian atmosphere has been reduced to a wisp. Some researchers think a giant catastrophic impact blew out much of the atmosphere. Others suspect that much of the carbon dioxide may have been sunk into Mars' rocks in the form of carbonates. But neither of these explanations can fully account for the massive loss of water vapor and gas.
The sun, many experts believe, is the most likely culprit — and it has been whittling away at the outer edge of the Martian atmosphere for billions of years. Unlike Earth, which is protected from the sun's radiation by a strong magnetic field, Mars has little defense against solar erosion. This may be because the planet's inner dynamo cooled long ago, while Earth's molten core keeps churning.
MAVEN will study solar erosion in the Martian atmosphere with a suite of sensitive instruments that can pluck ions and electrons from the solar wind, analyze energetic particles from the sun and examine faded magnetic fields.
MAVEN also has tools to analyze the gas near the top of the Martian atmosphere. Elements, like hydrogen or nitrogen or argon, typically come in slightly heavier and slightly lighter versions, called isotopes. As an atmosphere slowly escapes over time, the lighter isotopes tend to flee faster than the heavier ones. So if the scientists find that many of the lighter isotopes are missing, it will give them a clear sense of how much of the atmosphere has escaped into space — and thus, how much more of it there must have been in the past.
The scientists hope they'll be able to catch a Martian aurora in action, which could provide insight into weak patches of magnetic field that still exist on parts of Mars. They've also got their fingers crossed that a few solar storms will hit the planet during the mission, so that MAVEN can witness the type of solar pummeling they think would have been common in the past, when the sun was far more active.