Scientists from Boston University's Center for Space Physics have reported that NASA satellites designed to view the escaping atmosphere of the Sun have also recorded evidence of gas escaping from the planet Mercury. The scientists reported these findings at the European Planetary Science Congress (EPSC) meeting in Rome, Italy this week.

The STEREO mission has two spacecraft, in orbits just inside and outside the earth's orbit around the Sun, and thus increasingly ahead and behind the earth (STEREO, or Solar TErrestrial RElations Observatory, is the third mission in NASA's Solar Terrestrial Probes program).

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. The announcement of this new method of observing Mercury and possible explanations for the nature of the gases that make up this tail were presented at the EPSC.

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 in Texas," 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, were 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 was spotted in the STEREO data by Ian Musgrave, a medical researcher in Australia who has a strong interest in astronomy. Viewing the on-line database of STEREO images and movies, Dr. Musgrave recognized the tail and sent news of it to Boston asking the BU team to compare it with their observations.

"A joint study was started and now we have found several cases, with detections by both STEREO satellites," explained Jeffrey Baumgartner, senior research associate in the Center for Space Physics at Boston University. Baumgartner designed of the optical instruments that discovered the exceptionally long sodium tail.

The current focus of the team is to sort out all of the possibilities for the gases that make up the tail. Christopher Davis, a researcher at the Rutherford Appleton Laboratory in Chilton, England and member of the STEREO team responsible for the camera systems on both satellites, is working closely with the Boston University group on refining brightness calibration methods and determining the precise wavelengths of light that would get through the cameras' filters.

"The combination of our ground-based data with the new STEREO data is an exciting way to learn as much as possible about the sources and fates of gases escaping from Mercury," said Michael Mendillo, professor of astronomy at Boston University and director of the Imaging Science Lab where the work is being done.

"This is precisely the type of research that makes for a terrific Ph.D. dissertation," Mendillo added.