Lunar Prospecting With NASA’s Chandra X-Ray Observatory
Observations of the bright side of the Moon with NASA’s Chandra X-ray
Observatory have detected oxygen, magnesium, aluminum and silicon over a
large area of the lunar surface. The abundance and distribution of those
elements will help to determine how the Moon was formed.
“We see X-rays from these elements directly, independent of assumptions
about the mineralogy and other complications,” said Jeremy Drake of the
Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass,
at a
press conference at the “Four Years with Chandra” symposium in
Huntsville,
Ala.
“We have Moon samples from the six widely-space Apollo landing sites,
but
remote sensing with Chandra can cover a much wider area,” continued
Drake.
“It’s the next best thing to being there, and it’s very fast and
cost-effective.”
The lunar X-rays are caused by fluorescence, a process similar to the
way
that light is produced in fluorescent lamps. Solar X-rays bombard the
surface of the Moon, knock electrons out of the inner parts of the
atoms,
putting them in a highly unstable state. Almost immediately, other
electrons rush to fill the gaps, and in the process convert their energy
into the fluorescent X-rays seen by Chandra.
According to the currently popular “giant impact” theory for the
formation
of the Moon, a body about the size of Mars collided with the Earth
about 4.5
billion years ago. This impact flung molten debris from the mantle of
both
the Earth and the impactor into orbit around the Earth. Over the
course of
tens of millions of years, the debris stuck together to form the Moon.
By
measuring the amounts of aluminum and other elements over a wide area
of the
Moon and comparing them to the Earth’s mantle, Drake and his colleagues
plan
to help test the giant impact hypothesis.
“One early result,” quipped Drake,” is that there is no evidence for
large
amounts of calcium, so cheese is not a major constituent of the Moon.”
The same Chandra data have also solved a long-running mystery about
X-rays
from the dark side of the Moon, as reported by Brad Wargelin also of
CfA.
Wargelin discussed how data from the German Roentgen satellite (ROSAT)
obtained in 1990 showed a clear X-ray signal from the dark side. These
puzzling “dark-Moon X-rays” were tentatively ascribed to energetic
electrons
streaming away from the Sun and striking the lunar surface.
However, Chandra’s observations of the energies of individual X-rays,
combined with simultaneous measurements of the number of particles
flowing
away from the Sun in the solar wind, indicate that the X-rays only
appear to
come from the Moon. In reality they come from much closer to home.
“Our results strongly indicate that the so-called dark Moon X-rays do
not
come from the dark side of the Moon,” said Wargelin. “The observed X-ray
spectrum, the intensity of the X-rays, and the variation of the X-ray
intensity with time, can all be explained by emission from Earth’s
extended
outer atmosphere, through which Chandra is moving.”
In the model cited by Wargelin and colleagues, collisions of heavy ions
of
carbon, oxygen and neon in the solar wind with atmospheric hydrogen
atoms
located tens of thousands of miles above the surface of Earth give rise
to
these X-rays. In the collisions, the solar ions capture electrons from
hydrogen atoms. The solar ions then kick out X-rays as the captured
electrons drop to lower energy states.
“This idea has been kicking around among a small circle of believers for
several years supported by theory and a few pieces of evidence,” said
Wargelin. “These new results should really clinch it.”
NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra
program for the Office of Space Science, NASA Headquarters, Washington.
Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the
prime
development contractor for the observatory. The Smithsonian
Astrophysical
Observatory controls science and flight operations from the Chandra
X-ray
Center in Cambridge, Mass.
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