Mercury, the closest planet to the Sun in our solar system, is regarded by many as a dead rock. Not many people know that the small planet houses a very thin atmosphere, which shouldn't be there at all, if we are to believe theories. With surface temperatures of about 450 degrees Celsius (800 degrees Fahrenheit) during the day, and a gravitational pull of only 38 percent that of our planet's, the tiny celestial body should, by all accounts and purposes, be unable to keep its atmosphere around it. Unless, that is, something keeps replenishing it daily. One proposed explanation is the fact that solar wind keeps pounding the surface of the planet.

In 1974 and 1975, the space probe Mariner 10 discovered the fact that the planet has a magnetic field around it, similar, but weaker than our own. However, this posed an interesting dilemma, astrophysicists say, because it's a known fact that the thin gas of electrically charged particles, known as plasma, which is emitted by the Sun constantly, in fact “feel” the magnetosphere, and are repealed by it. This natural property of the magnetic field is one of the main reasons why life exists on Earth to this day. But, in the case of Mercury, the shield would have made it impossible to explain that action of solar wind on the atmosphere, which is invisible to the human eye.

However, the sensitive instruments aboard NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) space probe can see it, and they've discovered the only thing that could help bring the magnetic field theory and the solar winds one to common ground, namely the fact that magnetic tornadoes in the shield make it “leak,” as in get holes in it. Through these holes, the solar plasma can easily permeate the magnetic field, and replenish the atmosphere below, despite large temperatures. The way it does that is by crashing into the planet's surface with such high speeds that it generates a bounce-back effect of atoms launching into ballistic trajectories above the surface, creating the atmosphere.

“Mercury's atmosphere is so thin, it would have vanished long ago unless something was replenishing it/ These 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as flux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface,” explains NASA Goddard Space Flight Center expert Dr. James A. Slavin, who is also a co-investigator on NASA's MESSENGER mission to Mercury.

“Mercury's proximity to the sun only accounts for about a third of the reconnection rate we see. It will be exciting to see what's special about Mercury to explain the rest. We'll get more clues from MESSENGER's third flyby on September 29, 2009, and when we get into orbit in March 2011,” the expert concludes.