A mission to Mars: Aerosniff AN ALIEN astronomer, observing Earth's solar system from afar, might conclude that two or even three of its planets were clement enough for life. The worlds that humans call Earth and Mars lie within the sun's habitable zone, being far enough from it for liquid water to exist on their surfaces. Venus, which skirts the zone's inner edge, would be a marginal case. In reality, only Earth is inhabited. Venus is the devil's greenhouse, wrapped in a thick layer of planet-heating carbon dioxide that gives it a surface temperature of more than 450°C. Mars is the opposite: a frigid desert almost bereft of atmosphere, with all its water locked away as ice. But it was not always thus. Ancient Martian river beds and sedimentary rocks suggest that, early in its history, around four billion years ago, the planet was wet, with the pressure and warmth provided by a thicker atmosphere allowing water to run freely across its surface. On November 18th, if all goes well, a NASA spacecraft called MAVEN - short for "Mars Atmosphere and Volatile Evolution" - will blast off in an attempt to work out where that early atmosphere went. It will arrive next September and adopt a highly elliptical orbit that will let it take regular sniffs of the planet's upper atmosphere without air resistance degrading its orbit too quickly. Researchers have several theories about how Mars might have lost its air. One prominent one involves the planet's lack of a magnetic field. Magnetised minerals on the surface suggest that, like modern Earth, ancient Mars had such a field. This would have helped protect its atmosphere from the full force of the solar wind, a stream of charged particles that radiates from the sun, by deflecting them away. At some point between then and now, however, Mars lost its field. That loss let the solar wind thin its atmosphere to the wraith which remains today. If this theory is correct, that process should still be happening. It is this loss of air (among other things) that MAVEN is designed to measure. MAVEN will go into orbit around an increasingly crowded planet. Dozens of missions to Mars have been attempted in the past half century (see chart). On November 5th the Indian Space Research Organisation launched one. It, too, should arrive next September. Three working orbiters - NASA's Mars Odyssey and Mars Reconnaissance Orbiter, and the European Space Agency's Mars Express - will be waiting to greet it. Four rovers, all American, have landed on the planet. Two, Opportunity and Curiosity, are still working. Russia, though it was the first spacefaring country to send a mission to Mars, has no working craft there at the moment. Its last attempt, Fobos-Grunt (aimed at Phobos, one of Mars's moons), was marooned in orbit around Earth in 2011 along with a Chinese vehicle supposed to accompany it. And there are more to come. NASA intends to send a seismic probe called InSight in 2016, as well as a second version of Curiosity in 2020. ExoMars - a European combination of orbiter, lander and rover - will arrive on two flights, one in 2016 and one in 2018. Mars specialists are delighted. Those whose interests extend to other planets might be forgiven for feeling jealous.