Gamma Ray Spectrometer - 2001 Mars Odyssey Lunar and Planetary Lab The University of Arizona


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Frequently Asked Questions

WHY IS MARS RED?
The surface of Mars is rusted, or rich in oxidized iron.

WHAT IS A GAMMA RAY?
A gamma ray is a high energy photon of light, or electromagnetic radiation. The different energies of light include radio waves, infrared waves, visible light, ultraviloet light, x-ray light , and gamma ray light. While all travel at the speed of light, gamma rays represent the most energetic of these forms of light. Gamma rays are invisible to the naked eye, but they can be detected with a gamma ray detector.

WHAT IS A SPECTROMETER
A spectrometer is an instrument designed to detect specific energies of light. The Mars Gamma Ray Spectrometer (GRS) detects gamma ray light coming from the surface of Mars. Because different atoms release different energies of gamma rays, we can map the presence of different elements on the surface of Mars using this instrument.

WHAT TYPES OF ICE ARE FOUND ON MARS?
There are actually two types of ice on Mars-carbon dioxide ice and water ice. Carbon dioxide, also known as dry ice, freezes out most of the atmosphere at the poles during the wintertime and retracts back into the atmosphere during the summertime. Water ice has been detected with the Mars Gamma Ray Spectrometer (GRS) buried at the surface of the planet in the polar regions.

HOW DO WE KNOW THERE IS WATER ICE ON MARS?
The Mars Gamma Ray Spectrometer (GRS) has detected an extremely large amount of hydrogen in the polar region of Mars. Based upon what we know about ices and rock, this hydrogen is most likely in the form of water (H20). Additionally, we find the high hydrogen signal in colder regions of the planet where we expected water ice to exist.

DID WATER EVER EXIST ON MARS?
As far as current Mars is concerned, we do not find stable liquid water for any length of time on the surface of the planet. The atmospheric pressure is much lower on Mars then on Earth, therefore it is easier for water vapor to escape(evaporate). However, there is evidence for water on Mars in the past. We have found channels and valley networks consistent with running water that may have been there for a short time.

HOW DEEP CAN XRAY, NEUTRON, AND GAMMA RAY PROBES DETECT?
If you assume a typically dense rocky surface of about 2g/cm^3, x-ray detection is of the order of micrometers for most elements (x-ray detections varies for each element). Both neutron and gamma ray probes can sense tens of centimeters into a surface. Neutrons can sense larger concentrations of hydrogen down to about half a meter. The depth of gamma ray detection depends on gamma ray energy. Lower energy gamma rays are absorbed more than high energy gamma rays, so gamma ray detectors can detect high energy gamma rays from deeper than low energy gamma rays.


Email us your questions at: sanlyn@lpl.arizona.edu

The technical FAQ provides a detailed overview of the Odyssey spacecraft.

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