The Earth - in Gamma-Rays!
This is a false-color
image of the Earth in three gamma-ray energy bands,
analogous to the colors red (lower energy), green (mid
energy) and blue (higher energy) in the visible
spectrum. The lower right shows a composit image.(Image credit: NASA/CGRO/EGRET/ Dirk Petry)
A NASA-funded scientist has produced a new type of picture of the Earth
from space, which complements the familiar image of our "blue marble".
This new picture is the first detailed image of our planet radiating
The image portrays how the Earth is constantly bombarded by particles
from space. These particles, called cosmic rays, hit our atmosphere and
produce the gamma-ray light high above the Earth. The atmosphere blocks
harmful cosmic rays and other high-energy radiation from reaching us on
the Earth's surface.
"If our eyes could see high-energy gamma rays, this is what the Earth
would look like from space," said Dr. Dirk Petry of NASA Goddard Space
Flight Center in Greenbelt, Md, and assistant research professor at the
Joint Center for Astrophysics of the University of Maryland, Baltimore
Country. "Other planets -- most famously, Jupiter -- have a gamma-ray
glow, but they are too far away from us to image in any detail."
Petry assembled this image from seven years of data from NASA's Compton
Gamma-Ray Observatory (CGRO), which was active from 1991 to 2000. The
Compton Observatory orbited the Earth at an average altitude of about 420
km. From this distance, the Earth appears as a huge disk with an angular
diameter of 140 degrees. The long exposure and close distance enabled
Petry to produce a gamma-ray image of surprisingly high detail. "This is
essentially a seven-year exposure," Petry said.
The gamma rays produced in the Earth's atmosphere were detected by
Compton's Energetic Gamma-Ray Experiment Telescope (EGRET) instrument. In
fact, 60 percent of the gamma rays detected by EGRET were from Earth and
not deep space. Although it makes a pretty image, local gamma-ray
production interfered with CGRO's observations of distant gamma-ray
sources, such as black holes, pulsars, and supernova remnants.
But Petry created this gamma-ray Earth image to better understand the impact
of "local" cosmic-ray and gamma-ray interactions on an upcoming NASA
mission called GLAST*, the Gamma-ray Large Area Space Telescope. GLAST is
planned for launch in 2007. Its main instrument, the Large Area
Telescope, is essentially EGRET's successor.
In 1972 and 1973 the NASA satellite SAS-II captured the first resolved
image of the Earth in gamma rays, but the detectors had less exposure
time (a few months) and worse energy resolution.
*GLAST launched on June 11, 2008.