(Boston, Mass.) ó Ever since Galileo first used a telescope in 1609,
astronomers have tried to capture images of the surface of Mercury with
a ground-based telescope. Now, a team of astronomers from Boston University
released images revealing details of Mercury’s surface in the May issue
of The Astronomical Journal and at the American Geophysical Union in
Washington, DC.

“More than a quarter-century ago, the Mariner 10 spacecraft flew past
Mercury and for the first and only time transmitted satellite-based
photos of half of the surface of the planet closest to the Sun.,” says
lead author Jeffrey Baumgardner, senior research associate in the Center
for Space Physics at BU. “Capturing similar images from a ground-based
telescope represents a significant milestone in advanced instrumentation,”
he adds.

The BU images, taken on August 29, 1998, at the Mt. Wilson Observatory
in California, reveal surface markings similar to the bright craters
and dark lunar mare found on the Moon. The BU images captured using
a digital camera and stored on CD-ROMs for subsequent processing show
never-before- seen-portions of Mercury.

Photographing Mercury is challenging because of the planet’s proximity
to the Sun. Mercury only has a few viewing times, before sunrise or
shortly after sunset. At rare times when ‘the seeing’ is right, the
air is clear and researchers are looking through less turbulence in
Earth’s atmosphere. Opportunities to photograph Mercury from space are
also limited because light sensitive equipment, such as the Hubble Space
Telescope, are not allowed to look at objects close to the Sun, such
as Mercury or Venus. This restriction has been established to avoid
the possibility of an accidental pointing error causing too much light
to fall upon an instrument.

“The observations were made shortly after sunrise before the Sun’s
heating of the atmosphere distorted the images captured by the telescope,”
says Michael Mendillo, professor of astronomy at BU.

In order to obtain a clear photograph Baumgardner took images with
very short exposures, 1/60th of a second, continuously for
90 minutes. “That comes to 340,000 pictures,” Mendillo added. “The trick
to getting a clear image was then to find the best ones, say 30 to 60,
that could be added together by computer to create a time exposure of
sufficient duration (.5 to 1 second) in order to capture detail on Mercury’s

Baumgardner and Research Associate Jody Wilson assisted by Mead Misic,
a sophomore in the College of Engineering, all took part in the search
for the perfect images. They developed sophisticated computer techniques
to identify the best images with detail taken during rare instances
of ‘perfect seeing.’

“We captured multiple images of Mercury during these rare instances
of ‘perfect seeing,'” says Wilson. “and by combining these images, a
unique photograph with details and clarity resulted. “

The Boston University team plans to make additional observations of
Mercury this fall, even pushing the technique to try to image the planet’s
weak atmosphere. “Mercury has a thin atmosphere created by the ejection
of atoms from its surface, a process that also occurs on our Moon,”
Mendillo explained. One of the chemical elements in Mercury’s atmosphere
is sodium, a gas somewhat easy to detect because it reflects sunlight
very efficiently. “We hope to try our first sodium detection experiments
this fall,” Baumgardner said. “But that will first involve building
a more sensitive detector system.”

For more information, go to www.bu.edu/csp/imaging_science/planetary/mercury/baumgardner.html

To view the photos, go to www.bu.edu/csp/imaging_science/planetary/mercury/pressfig.gif


  1. Figure (with caption)

(Top) This image of a portion of Mercury’s surface not photographed
by Mariner 10 in 1974-75 was obtained by Boston University astronomers
using observations made at the Mt. Wilson Observatory in August 1998.
Hundreds of thousands of pictures taken with short time exposures (1/60th)
were examined to find the 30 images with the clearest surface markings,
taken during instances of “perfect seeing” through the Earth’s atmosphere.

(Bottom) A Mercator-projection map of the high definition image
of Mercury taken by BU astronomers on 29 August 1998 shown in comparison
to the side of the planet photographed from close range by NASA’s Mariner
10 spacecraft in 1974-75.

Photo credit: Baumgardner et al., Boston University