Before MAVEN, our hypothesis as to the demise of Mars's habitability was as follows: several billion years ago, when the Solar System and its planets were still young, Mars had, like Earth, a molten core.
Earth's molten core consists of metals such as iron that conduct electricity. The circulation of these metals throughout the core, induced by the rotation of the Earth, generates a magnetic field (as in the illustration above). This magnetic field, in turn, protects us from the charged solar wind by deflecting it towards the poles. Mars is theorized to have had a similar core and magnetic field. However, being farther from the Sun and smaller, Mars's core cooled over time, and eventually solidified. The planet's magnetic field then weakened, and solar wind blew away a majority of Mars's atmosphere, leaving it with a rarefied layer of mostly carbon dioxide. Such a thin atmosphere would have been inhospitable for liquid water, and thus any bodies of water dried up, likely killing any life, if it had developed there.
MAVEN's observations did not alter this basic understanding, but will provided much more detailed information. For example, by observing how solar wind interacts with Mars's current atmosphere, the spacecraft provided data concerning the exact mechanisms involved in atmospheric loss (see below). To this end, MAVEN carried several instruments that measure solar wind and its impact on Mars. Also, by observing the current rate of gas loss, the mission allowed scientists to extrapolate backward and infer a more precise timeline of Mars's climate evolution. In order to accomplish this goal, MAVEN included instruments that detect gaseous ions escaping from the Martian atmosphere. These are the "volatiles" to which the name refers. Finally, the probe also had a device known as a mass spectrometer, which can measure the abundance of different isotopes of certain chemical elements. Since heavier isotopes are less likely to be ejected from Mars simply due to their slightly greater mass, the ratio of isotope abundance in Mars's atmosphere versus that in Earth's, for example, can indicate exactly how much has been lost and thus how dense the atmosphere was in the past.
|Artist's rendering of MAVEN|
MAVEN launched on November 18, 2013. After about a ten month cruise, the probe entered orbit of Mars on September 21, 2014. A few weeks after, the spacecraft assumed its science orbit, a highly elliptical orbit. The low point of its orbit brings MAVEN within 100 miles of the surface, allowing it to easily sample the atmosphere, and the high point brings it to more than 3000 miles from Mars's surface, so that the spacecraft can also take global observations.
Late in 2014, MAVEN discovered a mechanism by which ions in the solar wind penetrated deeper into the atmosphere than previously thought possible. Also, to extend its range further, MAVEN periodically dipped its orbit even farther down toward the surface, reaching a minimum altitude of 78 miles instead of 93 miles. The first of these "deep-dip" campaigns took place in February 2015.
During March, MAVEN encountered multiple unexpected atmospheric phenomena. The first was the presence of an aurora in ultraviolet wavelengths at a relatively low altitude over Mars's northern hemisphere (the geographic locations are indicated in the image above). Also, the orbiter detected a dust cloud between 93 and 190 miles above the surface of Mars, which could not be explained by any atmospheric mechanism known at the time. On April 3, MAVEN completed its 1000th orbit of Mars.
MAVEN achieved one of the basic goals of its mission in November 2015; in that month, NASA announced that the solar wind rips gas away from the Martian atmosphere at a rate of about 100 grams per second. This rate varies significantly with solar activity and is believed to have been greater billions of years ago. This result, when combined with other Mars missions, finally allowed a comprehensive understanding of Mars's loss of carbon dioxide (the main component of the Martian atmosphere).
Mars used to have a great deal of carbon dioxide in a thick atmosphere, far more than in the rarefied blanket of gas surrounding the red planet today. Some of this carbon dioxide was trapped in mineral carbonates (as shown) or is cycled between the atmosphere and ice caps as they melt and refreeze. In addition, MAVEN's atmospheric loss measurement shows how a process called "sputtering" allows some gas to escape. However, orbital spacecraft indicate that the amount trapped in carbonates is not enough to explain the loss. Further, while the "sputtering" process favors the escape of the heavier isotope carbon 13 over carbon 12, the preference is only slight: this process alone cannot explain the higher isotope ratio measured on the ground by the Curiosity rover. Instead, the discoveries of MAVEN suggest that another mechanism is primarily responsible, namely the interaction of solar ultraviolet radiation (denoted "hv" in the image) with carbon dioxide molecules that causes dissociation. After this process, carbon 12 has a much higher chance of escaping the atmosphere than carbon 13, explaining the observations.
In October 2016, after observing Mars for more than a total Martian year, MAVEN had a fairly fleshed out account of how water escapes Mars. More precisely, it measured escaping hydrogen in the upper atmosphere that resulted from disassociation of water vapor (H2O) in the lower atmosphere. Before MAVEN, it was believed that this loss of water vapor occurred at a relatively constant rate. However, the spacecraft found that the rate of hydrogen escape is larger by a factor of 10 between when Mars is closest to the Sun compared to when it is farthest. This suggested that the amount of water vapor in the Martian atmosphere varies significantly throughout its year and helped to elucidate the process of its escape.
Sources: http://www.nasa.gov/mission_pages/maven/main/index.html#.Uf03vZIsmG0, http://www.nasa.gov/sites/default/files/files/MAVENFactSheet_Final20130610.pdf, http://lasp.colorado.edu/home/maven/science/, http://www.ipgp.fr/~aubert/Julien_Aubert,_Geodynamo,_IPG_Paris/Home.html, http://mars.nasa.gov/news/whatsnew/index.cfm?FuseAction=ShowNews&NewsID=1869,