News Release

Boston University astronomers map full extent of Mercury's comet-like tail

New observations show tail can extend 2.5 million kilometers

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Boston University

Boston – Boston University astronomers released today new images of Mercury that capture both the source regions of and, for the first time, the extraordinary length of the planet’s comet-like tail.Earlier research had mapped Mercury’s sodium gas tail out to approximately 40,000 kilometers, but planetary scientists from BU’s Center for Space Physics (CSP) have found that the tail can extend more than 2.5 million kilometers, or 1.5 million miles, from the planet.A paper detailing these observations will appear in the February 2, 2008issue of Geophysical Research Letters, published by the American Geophysical Union.

“We were surprised to find the tail extending that long,” said Jeffrey Baumgardner, the study team leader and CSP senior research associate.“Another group had mapped the tail out to about 15 times the radius of the planet, but by using several observation techniques we were able to produce images of the entire length and see that it was about 1,500 times the radius.While the length of the tail can fluctuate, this clearly establishes a new domain for us to understand.”

By observing the bright light emitted from sodium atoms ejected from Mercury’s surface, the researchers were able to generate images of the sources of other harder-to-see gases escaping from the planet’s surface.Once theses gases are released, Mercury’s weak gravity is not able to retain all of them and pressure from sunlight pushes the gases with the highest initial speeds away from the Sun.The result is a long comet-like tail that trails behind the planet.

“It’s similar to evaporation – when atoms or molecules receive enough energy to escape as vapors from a substance,” said Michael Mendillo, a study co-author and professor of astronomy at BU. “It’s called surface-sputtering, and it can be instigated by sunlight, the impacts of micrometeors, or by the ions and electrons in the solar wind.”

While sodium atoms are not the major component of Mercury’s dusty soil, they serve as particularly bright “tracers” of other more abundant gases that are difficult to see using ground-based telescopes.

“Sodium atoms can be detected because the light they emit is in the visible part of the spectrum – the same familiar orange glow from sodium street lights that are in most American cities,” Mendillo explains.“Therefore, sodium becomes an indicator of other elements that might be more abundant, but less easy to see.”

While comets often have tails visible to the naked eye, Mercury’s is too faint.In order to produce the new images, the BU team used four telescopes – two on Mt. Haleakala in Hawaii and two at the McDonald Observatory in Texas – to relate the large-scale tail to the much smaller gas-emitting source regions on the planet.

“By observing the length and breadth of the tail, we can determine the characteristics of the various surface-sputtering sources that produce it,” Baumgardner said.“If we see changes in the tail, these can be related to variability of the sources.”

The first image, obtained by the BU team in June 2006 from a 3.7 meter telescope operated by the U.S. Air Force on Mt. Haleakala, was of the source regions on the surface.The telescope employs a sophisticated system that uses adaptive optics to image targets through the Earth’s turbulent atmosphere.Baumgardner and Mendillo used this capability to capture details of the source locations.The resulting image shows the highest spatial resolution obtained to date of the gas-emitting regions at high latitudes.

To capture the full extent of the tail produced from those regions, the BU group used three smaller telescopes – one on Haleakala and two at the McDonald Observatory – that imaged progressively larger portions of the sky.A telescope with a four inch lens was used to obtain the largest field of view.That image was taken in May 2007 by Jody Wilson, CSP senior research associate, and Carl Schmidt, a graduate student in Astronomy.

While all three telescopes showed Mercury to have a long tail of sodium gas, the one with the largest field of view gave the surprising result of a tail that extends millions of kilometers from the planet.

“Seeing a tail more than one thousand times the radius of the planet, we immediately realized that a new record had been set for the extent of Mercury’s influence within the Solar System,” said Wilson.

“We have been developing the instrumentation and observing strategies over the past ten years to study these effects, and our results from 2006 and 2007 have now pulled it all together,” said Baumgardner.“It is certainly rewarding to see all of this effort reach the point of providing such a wonderful set of results.”

The BU team was taking part in the International Mercury Watch (IMW) program coordinated by the group in Boston.The IMW efforts involve scientists from Japan, Italy, France and the USA.The BU findings were made possible by grants from the Office of Naval Research and NASA to BU to develop the low light level detection systems required for such work and to apply them at several observatories.The observations made in Hawaii in 2006 were funded by a joint Air Force – National Science Foundation (NSF) program and by collaboration between BU and a new observatory on Maui built by Tohoku University in Japan.The 2007 observations at the McDonald Observatory were funded by NASA and the Center for Space Physics.


Research in BU’s Center for Space Physics involves interdisciplinary projects between members of the Astronomy Department in the College of Arts and Sciences and faculty, staff and students in the College of Engineering. Research areas include observational and theoretical studies in atmospheric, ionospheric and magnetospheric physics, planetary and cometary atmospheres, solar and heliospheric physics, and space weather.

Founded in 1839, Boston University is an internationally recognized institution of higher education and research. With more than 30,000 students, it is the fourth largest independent university in the United States. It contains 17 colleges and schools along with a number of multi-disciplinary centers and institutes which are central to the university’s research and teaching mission.

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