Moon's thin atmosphere extends farther than thought
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Moon's thin atmosphere extends farther than thought
(c) 1995 Copyright Nando.net
(c) 1995 N.Y. Times News Service
WASHINGTON (Nov 28, 1995 - 23:20 EST) -- The wispy atmosphere of the
Moon has been found to extend at least 9,000 miles above its surface,
almost twice the distance previously believed, scientists say.
Many people, with visions of astronauts bounding on the lunar surface
in spacesuits, believe that the moon is an airless body. But
scientists have known for decades that Earth's rocky satellite is
enclosed by an extremely thin atmosphere caused by gaseous particles
dislodged from its surface.
Now researchers at Boston University, who two years ago determined
that the rarefied gas bubble surrounding the Moon extended 5,000 miles
high, say new studies show that the lunar atmosphere reaches out twice
as far.
In a paper in the Oct. 5 issue of the journal Nature, the Boston
astronomers say their new measurements are based on a unique set of
observations made during a total lunar eclipse on Nov. 29, 1993.
The astronomers, Dr. Michael Mendillo and Dr. Jeffrey Baumgardner of
the Center for Space Physics at Boston University, said that during
the eclipse the Moon was totally in Earth's shadow, blocking the
bright moonlight that obscures observations of gases in the lunar
atmosphere. Under these conditions, the astronomers were able to
detect the faint glow of sodium gas, which serves as a marker for
other gases in the lunar atmosphere.
Sodium atoms are easy to detect because they scatter, or reflect,
sunlight very efficiently, even when present in small quantities, the
researchers said.
"We were surprised to find that this glow extended to over nine times
the radius of the Moon, to a height of about 14,000 kilometers, or
9,000 miles above the Moon's surface," Mendillo said.
The observations were made at the McDonald Observatory in Fort Davis,
Texas, using a small telescope with special filters over a four-inch
lens, coupled to a very sensitive light detection system.
The researchers say their observations have enabled them to rule out
some theories on the origin of the lunar atmosphere. They believe that
the most likely explanation is the evaporation of atoms from the lunar
surface when it is struck by light particles called photons coming
from sunlight. Sodium
and other elements escape the surface through erosion caused by the
bombardment of photons, as well as heat from sunlight warming surface
materials, they said.
The astronomers earlier ruled out a suggestion that the lunar
atmosphere was formed by the constant bombardment of the surface by
micrometeorites. If the micrometeorite theory was true, they said, the
atmosphere would be evenly distributed instead of being irregular in
shape, as their measurements indicate.
Another theory holds that solar wind -- charged particles streaming
from the Sun -- kicks up surface atoms as it lashes the lunar surface.
But the researchers said this theory now appeared to be eliminated
because Earth's magnetic field traps solar wind and shields the lunar
surface during
the full-moon phase, when their observations show the tenuous lunar
atmosphere fully extended above the surface.