杏吧原创

Note: This story is an updated version of 鈥�Mars probe to sniff atmosphere and scout safer landings鈥�, published 14 November.

After decades of robots scouring the surface, a Mars explorer is about to get its head in the clouds. NASA鈥檚 new mission to the Red Planet will be the first to extensively explore Mars鈥檚 upper atmosphere. The (MAVEN) mission lifted off today at 1328 EST from Kennedy Space Center in Florida.

When it reaches Mars in late 2014, the orbiter will search for clues to when and why the planet went from a warm, wet world to the cold, dry desert we see today. The probe could also fill a looming break in our ability to communicate with rovers on the planet鈥檚 surface, and it could provide valuable data for future missions hoping to land humans on Mars.

MAVEN will swoop through the atmosphere to take deep breaths and analyse the alien gases. It is part of a growing international effort to understand the Martian climate and to use its atmosphere to determine whether the planet is or ever was friendly to life.

Surface distraction

To date, most missions to Mars have been trained on its surface, seeking geological and chemical evidence of habitability. Only a few missions have probed the planet鈥檚 atmosphere, and most of those were confined to the lower layers. MAVEN will be dedicated to aeronomy, the study of a planet鈥檚 upper atmosphere and how gases there are altered or lost to space.

鈥淭he first mission we sent to Venus did extensive aeronomy things, quite like what we鈥檙e doing with MAVEN now,鈥� says , MAVEN project scientist at NASA鈥檚 Goddard Spaceflight Center in Greenbelt, Maryland. 鈥淏ut we can鈥檛 see the surface of Venus. With Mars you can see the surface, and that distracted people.鈥�

The distraction has proved fruitful. Rovers and landers have gathered mountains of evidence that Mars once hosted flowing liquid water with the right chemistry to support life. That鈥檚 a strong sign that the planet also once had a thicker, warmer atmosphere. But today, the Martian atmosphere is just 1 per cent as dense as Earth鈥檚 鈥� so where did it all go?

Sneeze guard

The leading theory is that early Mars had a global magnetic field that for some reason switched off about 4 billion years ago. Without this protective field, the upper atmosphere was vulnerable to being stripped away by radiation constantly streaming from the sun, known as the solar wind.

鈥淎 magnetic field protects an atmosphere from the sun, because it turns charged particles away,鈥� says team member at the University of Colorado in Boulder. 鈥淚t鈥檚 like a big sneeze guard. If you take the sneeze guard away, the solar wind comes crashing into the atmosphere, and the atmosphere comes splashing out.鈥�

But there is a host of other possibilities. A global magnetic field might have funnelled enough energy into the atmosphere to push lighter particles escape into space, like steam from a pot of boiling water, for example. Or large amounts of gas were lost during a catastrophic impact on early Mars, or chemically trapped in rocks over millions of years. Each option speaks to the chances that Mars had the right conditions for life to emerge and evolve.

Red Planet rewind

In a bid to find out what happened, MAVEN will examine how the solar wind interacts with Mars鈥檚 upper atmosphere today. Once at Mars, the spacecraft will enter a highly elliptical orbit that will let it dip directly into the Red Planet鈥檚 atmosphere. It will also bend and twist in its orbit to gather simultaneous data on the planet and the particles streaming from the sun. Teams on Earth can use those measurements to try and piece together what the planet was like in the past.

鈥淲e want to try to rewind the movie and see what happened as Mars aged,鈥� says MAVEN scientist Mehdi Benna, also at NASA Goddard.

Although MAVEN has its eyes on the sky, it comes at an important time for robots on the ground. The Mars rovers Opportunity and Curiosity need orbiters to relay communications with Earth. The currently available spacecraft 鈥� Mars Odyssey, Mars Express and the Mars Reconnaissance Orbiter 鈥� are each roughly a decade old and may soon be out of operation.

滨苍诲颈补鈥檚 Mars Orbiter Mission (MOM), which launched on 5 November, should also arrive at Mars in late 2014. But that mission is primarily a technology demonstration and won鈥檛 be relied upon as an international communications relay. After MAVEN, the next potential relay to launch will be the European Space Agency鈥檚 ExoMars Trace Gas Orbiter, which won鈥檛 arrive at the Red Planet until late 2016. That means MAVEN will be the only new probe available during a critical period for Mars exploration.

Soft landing

Like MAVEN, both the Indian and European missions will be studying Mars鈥檚 atmosphere and searching for signs of past or present habitability. In particular, those probes will be scanning for methane gas, a potential sign of microbial life buried in the Martian subsurface.

MAVEN and ExoMars could also offer key data for helping future astronauts reach the Martian surface. One of the most difficult aspects of landing on Mars is dealing with the thin atmosphere 鈥� there is just enough to create dangerous heat during the descent, but not enough to slow down a spacecraft the same way we do on Earth.

This challenge led to the ambitious 鈥渟ky crane鈥� manoeuver used to lower Curiosity 鈥� the heaviest mission yet sent to Mars 鈥� to the surface. But it is unclear how to safely land crewed missions, predicted to be about 40 times heavier. Better understanding Mars鈥檚 atmosphere and how its density changes from year to year could be critical to gently landing crewed spacecraft.

鈥淎s we start a major campaign looking toward getting humans there, the atmosphere will become a major focus,鈥� says , lead scientist for NASA鈥檚 Mars Exploration Program in Washington DC.