Mercury's Comet-like Appearance Spotted by Satellites Looking at the Sun
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Figure 1 shows examples of the very long tails of sodium escaping from Mercury. These images were taken at the Boston
University Station at the McDonald Observatory, run by the University of Texas in Austin.
Reference: Orbital Effects on Mercury’s Escaping Sodium Exosphere, Carl Schmidt, Jody
Wilson, Jeffrey Baumgardner and Michael Mendillo, ICARUS, 2010.
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,” BU graduate student Carl Schmidt explained, “and now the tail can also be seen from satellites near Earth.”
Figure 2: gives a schematic representation of the viewing geometry that allows the STEREO camera systems to make observations of Mercury’s tail.
Of special interest is the way the tail feature was spotted in the STEREO data. It was not found by the BU team, but by Ian Musgrave, a medical researcher in Australia who has a strong interest in astronomy. Viewing the on-line data base 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 Baumgardner, Senior Research Associate in the Center for Space Physics at BU, and the designer of the optical instruments that discovered the exceptionally long sodium tail.
Figure 3: An image of Mercury’s tail obtained from combining a full day of data from a camera aboard the STEREO-A spacecraft. The reflected sunlight off the planet's surface results in a type of over-exposure that causes Mercury to appear much larger than its actual size. The tail-like structure extending anti-sunward from the planet is visible over several days and spans an angular size exceeding that of a full Moon in the night sky.
A movie showing a 4 day period when Mercury tail was visible from the STEREO A spacecraft on 6-9 February 2008. (click image to open 5.5MB avi file; click here to open 1.4MB animated gif)
This work was sponsored by grants from NASA to Boston University and by research funds from its Center for Space Physics, and was conducted in collaboration with colleagues at the Rutherford Appleton Laboratory in England and the University of Adelaide in Australia.
- Carl Schmidt, (617) 353-5990, email@example.com
- Jeffrey Baumgardner, (617) 353-5639, firstname.lastname@example.org
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- Prof. Michael Mendillo, The Opera Hotel, Via Nazionale, [Tel: +39 06 4891 3093], firstname.lastname@example.org
- Patrick Farrell, Communications Director, email@example.com (617) 358-1185; cell: 617-543-6480
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