Scientists from Boston University’s Center for Space Physics reported today that NASA satellites designed to view the escaping atmosphere of the Sun have also recorded evidence of escaping gas from the planet Mercury.

The STEREO mission has two satellites placed in the same orbit around the Sun that the Earth has, but at locations ahead and behind it. This configuration offers multi-directional views of the electrons and ions that make up the escaping solar wind. On occasion, the planet Mercury appears in the field of view of one or both satellites. In addition to its appearance as a bright disk of reflected sunlight, a “tail” of emission can be seen in some of the images. Announcing this new method of observing Mercury and trying to understand the nature of the gases that might make up this tail feature were the topics presented at the European Planetary Science Congress meeting in Rome yesterday.

It has been known that Mercury exhibits comet-like features, with a coma of tenuous gas surrounding the planet and a very long tail extending in the anti-sunward direction. From Earth, observations of both of these features can be done using light from sodium gas sputtered off the surface of Mercury. The Sun’s radiation pressure then pushes many of the sodium atoms in the anti-solar direction creating a tail that extends many hundreds of times the physical size of Mercury.

“We have observed this extended sodium tail to great distances using our telescope at the McDonald Observatory ,” Boston University graduate student Carl Schmidt explained, “and now the tail can also be seen from satellites near Earth.”  Much closer to Mercury, several smaller tails composed of other gases, both neutral and ionized, have been found by NASA’s MESSENGER satellite as it flew by Mercury in its long approach to entering into a stable orbit there.

“What makes the STEREO detections so interesting is that the brightness levels seem to be too strong to be from sodium,” commented Schmidt, lead author on the paper presented at EPSC.

Of special interest is the way the tail feature was spotted in the STEREO data. It was not found by the Boston University team, but by an Australian amateur astronomer Ian Musgrave.

Source: Europlanet