<!--intro-->If it were possible to magically transport a cup of water from Earth to the surface of Mars, the liquid would instantly vaporize. Mars's atmosphere is so vacuous (it's less than 1% as dense as Earth's) that liquid water simply can't exist for very long on the Red Planet. That's a puzzle to planetary scientists, because Mars's surface is littered with signs of liquid water. Dried up valley networks, sedimentary deposits, and chaotic flood plains hint that billions of years ago Martian water flowed freely and that the atmosphere there must have been substantially thicker than it is now. But where did it all that Martian air go?<!--/intro--> New evidence from NASA's Mars Global Surveyor (MGS) spacecraft supports a long-held suspicion that much of the Red Planet's atmosphere was simply blown away -- by the solar wind. The solar wind is a fast-moving part of the Sun's outer atmosphere. The solar corona, with a temperature greater than one million degrees C, is so hot that the Sun's gravity can't hold it down. It flows away in all directions traveling 400 to 800 km/s. Every planet in the solar system is immersed in this gusty breeze of charged particles. Here on Earth we're protected from the solar wind by a global magnetic field (the same one that causes compass needles to point north). Our planet's magnetosphere, which extends far out into space, deflects solar wind ions before they penetrate to the atmosphere below. Mars isn't so fortunate. Lacking a planet-wide magnetic field, most of the Red Planet is exposed to the full force of the incoming solar wind. "The Martian atmosphere extends hundreds of kilometers above the surface where it's ionized by solar ultraviolet radiation," says Dave Mitchell, a space scientist at the University of California at Berkeley. "The magnetized solar wind simply picks up these ions and sweeps them away." "In 1989 the Soviet Phobos probe made direct measurements of the atmospheric erosion," he continued. When the spacecraft passed through the solar wind wake behind Mars, onboard instruments detected ions that had been stripped from Mars's atmosphere and were flowing downstream with the solar wind. "If we extrapolate those Phobos measurements 4 billion years backwards in time, solar wind erosion can account for most of the planet's lost atmosphere." "To calculate the total loss of atmosphere," he added, "we must take into account how the Sun has changed during the past four billion years. The Sun's ultraviolet output was larger in the past, and the solar wind was probably much stronger. This means that solar wind erosion was likely much more effective in the past than it is today." Although Mars no longer has a substantial magnetosphere, scientists think it once did and that the remnants of it still exist. In 1998 magnetometers on MGS discovered a network of magnetic loops arrayed across Mars's southern hemisphere. Locally, the magnetic fields arch over the surface like umbrellas, hundreds of km high. "If you were standing on Mars in one of these areas," says Mitchell, "you would measure a magnetic field about as strong as Earth's -- a few tenths of a gauss." Elsewhere on the planet the magnetic field is 100 to 1000 times weaker. Indeed, it appears that Mars's magnetic umbrellas act like miniature magnetospheres. They ward off the solar wind in their vicinity and harbor pockets of gas ionized by solar UV radiation that would otherwise be blown away. At a recent meeting of the American Geophysical Union, Mitchell and colleagues unveiled the first-ever global map of the Red Planet's ionosphere (the ionized part of the atmosphere), based on data from the Mars Global Surveyor electron reflectometer. "The ionosphere nicely traces the distribution of the surface magnetic field, and there seems to be a 1-to-1 correspondence," noted Mitchell. Places where magnetic umbrellas deflect the solar wind are also spots where the ionosphere is retained high above the surface of the planet. Mitchell cautions that beneath these magnetic umbrellas the neutral atmosphere at Martian "sea level" isn't particularly dense -- they are not oases of air for future colonists! Rather, the mini-magnetospheres are simply places where high-altitude atmospheric losses are relatively low. Most of Mars is still subjected to the full force of the solar wind. To retain a thick atmosphere, a planet-wide magnetic field would be needed. Earth's global magnetic field comes from an active dynamo -- that is, circulating currents at the planet's liquid metallic core. A similar dynamo once churned inside Mars, but for reasons unknown it stopped working four billion years ago. The patchwork fields we see now are remnants of that original magnetic field. How do scientists know when the dynamo turned off? "Mars has been kind to us," explains Mitchell. "There are two large impact basins, Hellas and Argyre, about four billion years old that are demagnetized. If the dynamo was still operating when those impact features formed, the crust would have re-magnetized as they cooled. The dynamo must have stopped before then." Earth also has an ionosphere maintained by solar UV, but on our world --unlike Mars-- the ionosphere envelops the entire planet. It begins at an altitude of about 90 km and stretches thousands of km into space. Because the ionosphere fits safely inside our planet's much-larger magnetosphere, solar wind erosion is not a problem. That's good news for ham radio operators who depend on the radio-reflective ionosphere for over-the-horizon shortwave communications. Living on a magnetized planet has its advantages! The advantages might be even bigger than amateur radio, though. Planetary magnetic fields could be an essential ingredient for life-bearing worlds circling stars with strong solar winds, worlds that need to retain a substantial atmosphere and liquid water. Indeed, if the Martian dynamo hadn't shut down billions of years ago, the Red Planet might be teeming with Martians today. Instead Mars is a frigid desert, apparently as barren of life as it is of its long-gone magnetic personality!