The Red Planet is Losing its Air

Mars atmospheric loss
An artist's conception of Mars losing its upper atmosphere to a solar storm. NASA/GSFC. NASA/GSFC

The fate of planet Mars is one that planetary scientists have studied for years. It seems that the Red Planet started out early in its history with water and a warmer atmosphere. But, unlike Earth — which started out in a similar way — Mars cooled and its water disappeared. It also lost much of its atmosphere, which continues to slip away to this day. How could this have happened to a place where surface features show clear and unmistakeable signs that water once flowed freely across its surface? 

What Happened to Mars?

To find out why the fourth rock from the Sun has suffered such a strange fate (for a rocky planet in the habitable zone of its star), scientists sent the MAVEN mission to Mars to measure its atmosphere. MAVEN, which stands for "Mars Atmosphere and Volatile Evolution Mission" is purely an atmospheric probe, looking at all characteristics of Mars's remaining blanke of air. Data from its instruments have pinpointed a process that very likely has played a role in drying out Mars and sending its atmosphere to space. 

It's called "solar wind stripping" and it happens because Mars doesn't have a very strong magnetic field to protect itself. Earth, on the other hand, has a very strong magnetic field (compared to Mars) that diverts the solar wind around our planet, sparing it from the worst of the radiation emitted from the Sun. Mars does not have a strong global magnetic field, although it has smaller regional ones. Without such a field, Mars is bombarded by radiation from the Sun driven by the solar wind.

Gone with the (Solar) Wind

MAVEN measurements taken since it arrived at the planet show that the ongoing action of the solar wind strips away molecules of atmospheric gases from the planet at a rate of 1/4 pound per second. The actual measurement is 100 grams per second. That doesn't sound like much, but it adds up over time.  It gets even worse when the Sun acts up and sends strong gusts of solar wind out through the solar system. Then, it strips away even more gas. Since the Sun was much more active earlier in its existence, it very likely robbed the planet of even more of its atmosphere. And, that would have been enough to contribute to Mars's dry and dusty desert existence today. 

The tale that MAVEN is uncovering is unfolding in one of atmospheric loss in three regions above and behind Mars. The first is down the “tail,” where the solar wind flows behind Mars. The second region that shows evidence of atmospheric loss is above the Martian poles in a “polar plume.” Finally, MAVEN detected an extended cloud of gas surrounding Mars. Almost 75 percent of the escaping material it studied come from the tail region, and nearly 25 percent are from the plume region, with just a minor contribution from the extended cloud.

The Long-gone Wet History of Mars

Planetary scientists have long seen evidence that water once existed on Mars, several billion years ago. Riverbeds, dry lakebeds, and carved rocky regions tell a tale of what looks like flowing water, even as the planet underwent volcanism and tectonic changes. The evidence for water also likes in the soil. 

For example, the Mars Reconnaissance Orbiter observed the seasonal appearance of hydrated salts (salts that had been in contact with water). They are evidence of briny liquid water on Mars. However, the current Martian atmosphere is far too cold and thin to support long-lived or extensive amounts of liquid water on the planet’s surface.

With increased solar activity in the past and the lack of a magnetic field, the Red Planet began to lose its atmosphere and its water. MAVEN is telling the story of that ongoing loss through its long-term study of Mars's atmosphere

MAVEN was built to determine how much of the planet’s atmosphere and water have been lost to space, and its recent reports are part of that mission. It is the first mission devoted solely to understanding how the Sun 's activity could have played a role in changing ancient Mars from a watery, warm refuge welcoming to life to a dry, frozen, desert world where no life has yet been found.