A planet's atmosphere can up and disappear. Now a satellite is approaching Mars to tell us how. The satellite is MAVEN: Mars Atmosphere and Volatile EvolutioN (MAVEN). As NASA's first spacecraft devoted to exploring the Red Planet's upper atmosphere, MAVEN is on a 52-week mission to find out what exactly happened to Mars' thick and protective atmosphere, which could have sustained life many eons ago.

MAVEN's expected arrival builds on decades of Mars research. In 1975, the Viking 1 lander revealed Mars' cold, thin atmosphere of Carbon dioxide. At the same time, it sent back pictures of what appeared to be dry river beds. Decades later, the rover Opportunity discovered minerals known to form in the presence of water, and rock formations suggesting that water may still intermittently flow on the surface.

Chasing the water, orbiters such as the European Space Agency's Mars Express has found that the solar wind penetrates deep into the Martian atmosphere, leading to loss of water and carbon dioxide to space. In 2013, the NASA rover Curiosity sent back definitive evidence of clay formations and the former presence of water.

All signs point to a past for Mars that was warmer and wetter. Four billion years ago there was probably a thick atmosphere. Mars, much like Earth, had the factors to sustain life forms amenable to carbon dioxide.

"We see evidence that liquid water flowed over the surface early in history in a way that doesn't happen today," said Bruce Jakosky, MAVEN's principle investigator. "However, it's an open question as to whether there were, for example, global oceans, or whether the atmosphere was thick enough that it would have appeared blue. That's really part of the question that MAVEN is trying to answer."

At an earlier time, Earth and Mars seemed to be heading in similar directions. For reasons we are about to uncover, our planet swam in an ever-thickening milieu of oxygen and water, while Mars' prospects of a sustainable atmosphere thinned.

Evidence of this long-extinct atmosphere lingers today in the form of heavy carbon and oxygen isotopes. These sank below the surface of Mars and were captured in the soil. Lighter molecules such as carbon dioxide and nitrogen (the most abundant molecule in Earth's atmosphere) were blown away, or so we believe. While our planet swam in an ever-thickening milieu that came to include the oxygen and water vapor we all depend on for life, Mars' prospects of a sustainable atmosphere somehow thinned. This phenomenon of an "atmospheric escape" is the subject of much inquiry. How does a planet up and lose the bulk of its atmosphere?

The answer seems to lie somewhere between the relentless solar wind, large-scale events like coronal mass ejections and a missing magnetosphere. This grey area, where the upper atmosphere blurs into outer space, is where NASA's latest Martian satellite is headed.

MAVEN will probe the top layers of the Red Planet's remaining air supply. A mere 11.3 meters long and 3 meters wide, MAVEN is on target to enter an eccentric Martian orbit on September 22, 2014. The extended ellipse of MAVEN's orbit will bring the satellite to within 150 kilometers (93 miles) of the surface into what would be the thermosphere on Earth, where the bulk of solar radiation is absorbed.

It will then spin out to 6,000 kilometers (3,728 miles), far beyond the boundary of the exosphere. Over the course of the year, seven "deep dips" to 125 km (77 miles) over the surface will bring MAVEN into the upper boundary of the lower atmosphere. This will give MAVEN's onboard instrumentation the opportunity to sample, observe and analyze all atmospheric points in between.

The satellite will be tracing the path that particles themselves take as they leave the planet, noting how the particles change as they move from near ground level into space. Along the way, MAVEN will also observe the abundance of oxygen, nitrogen and water vapor. These are the most active in the upper atmosphere in terms of absorbing incoming energy and changing state, which has earned them the title, "volatiles".

In addition to observing the volatiles in the upper atmosphere, the presence or absence of the magnetosphere will be detected, as will the force and direction of the solar wind and the fluctuations of the ionosphere. Armed with this data, MAVEN may act as a bookend to the Curiosity mission, filling in the knowledge gaps from the ground to the corona of the atmosphere over perhaps billions of years of planetary evolution.

"The assumption is that what we observe today gives us a pretty good idea of what processes have been going on for the last three billion years," said Janet Luhmann, MAVEN's deputy principal investigator. "If Curiosity does the lower atmosphere, and we [MAVEN] can say something about escape into space, I'm hoping between the two we could fill in the puzzle."

Current thinking, according to Luhmann, is that 90 percent of the Martian atmosphere has escaped. A combination of solar activity, which was more pronounced when the Sun was younger, ionization in the upper atmosphere, and the loss of the magnetosphere have all contributed to the loss of the atmosphere over time. These are the same processes that have molded Earth over the past four billion years, and presumably that of the ever-increasing number of Earth-like planets NASA has identified. Such processes would guide planetary evolution near all Sun-like stars. Solving the mystery of Mars' lost atmosphere would also expand our understanding of other inner planets near suns.

MAVEN's ultimate scientific goal is a model of Martian planetary evolution covering the last three billion years.

"Filling in three to four billion years of history is not an easy thing," said Luhmann, "If Curiosity does the lower atmosphere, and we [MAVEN] can say something about escape into space. I'm hoping between the two we could fill in the puzzle."

Teams of scientists around the globe will undertake this mission using MAVEN's eight onboard instruments, which are organized into three instrument groups, each with a mission that addresses one piece of the puzzle.

The first group powered up on December 4. NGIMS, the Neutral Gas and Ion Mass Spectrometer is destined to measure the minute components of the Martian atmosphere at various altitudes, checked out as working perfectly. It has already begun to take readings of gases, some of which we hope to also find once MAVEN arrives in September.

NGIMS partner in examining neutral particles is, the Imaging Ultraviolet Spectrograph (IUVS). The IUVS will measure atoms and molecules over the entire planet's atmosphere and corona, where the gases from the atmosphere are actively expanding into space.

MAVEN's second group of instruments has also begun its work. One of the four sensors in the Sun, Solar Wind and Storms package caught a whiff of a Coronal Mass Ejection as it sped by MAVEN in early December. The Solar Energetic Particles (SEP) instrument had to cover part of its sensor to avoid being overwhelmed by the high-energy photons, ions and atoms in the supercharged solar wind.

Starting in September, when fast-moving ions like these arrive, MAVEN will watch them interact with the lingering magnetic fields around Mars. Measurements from the thrid instrument suite, which contains a magnetometer (MAG) and a ion-composition detector (STATIC) will tell us about how the solar wind leads to the atmosphere is escaping from Mars.

The energy of incoming solar particles, the speed with which they pummel the remaining atmosphere away, the basic structure of the atmosphere itself each piece of information plays a key roll in unlocking the mystery of how our neighboring planet transformed from a world of oceans to a desert sea. With MAVEN's help, by this time next year we may have answers to the evolution of this once blue, now red, planet next door.