Imaging the Surface of Mars

Researchers at Johns Hopkins University have developed a spectrometer with a spatial resolution up to 100 times higher than any imaging instrument previously taken into orbit. The spectrometer is currently circling Mars on NASA’s Mars Reconnaissance Orbiter (MRO), mapping the chemical composition of the planet’s surface.

The higher-resolution instrument helps scientists see features on the Martian surface, such as clay rocks, that were completely invisible using other instruments, says Scott Murchie, a scientist at Johns Hopkins University Applied Physics Laboratory (JHUAPL) and the principal investigator on the spectrometer project. Scientists are observing the planet’s features and mapping its mineral makeup to identify a landing site for the Mars Science Laboratory (MSL), which is scheduled to launch in 2009. Last week, scientists met to discuss the 46 possible sites for the lab and bring the total down to 5, using the data gathered by the new spectrometer as a guide.

The Mars rover’s objective is to assess the habitability of the planet and look for a chemical record of life. To do so, the rover needs to land at a site with rock types known to preserve or bury organic material, says Murchie. The new spectrometer is the only instrument with the spectral power to image the chemical composition of these rocks in great detail, because it covers such a wide range of the electromagnetic spectrum.

The instrument measures the different colors of sunlight reflected off Mars’s surface. It has up to 544 individual spectral channels, or colors, ranging from the ultraviolet portion of the electromagnetic spectrum, through the visible spectrum, and all the way to wavelengths (segments of the electromagnetic spectrum) of almost four microns. (The naked human eye can only see wavelengths up to 0.7 microns.) Geological materials, such as rocks and dust, have a “spectral fingerprint” that represents their chemistry, so in essence, every mineral has a color, says Frank Seelos, a scientist at JHUAPL and a member of the spectrometer team.

The instrument records how much energy it is receiving at each one of the 544 wavelengths. An onboard computer uses that data to construct a picture of the surface of the planet. Each picture covers a six-mile-wide area and has a spatial resolution of 20 meters per pixel.

With a flick of a switch, the spectrometer can be turned from a high-resolution camera, capturing data on 544 wavelengths, to a low-resolution camera that captures data on just 72 wavelengths. In the past, two completely different instruments would have been required. While the high-resolution setting is great for zooming in on a particular area, it captures so much data that it’s difficult to create a single map of the entire planet. Switching to low resolution makes this task much easier, says Murchie. The scientists assess the global map to find areas for targeted observation or high-resolution imagery.

“Targeted observations are really what characterize the surface in detail, and global mapping puts it into a broader context,” says Murchie.

The instrument is controlled remotely, from Earth. The new spectrometer has taken approximately 2,300 high-resolution images and 2,700 atmospheric measurements since it first started gathering data on Mars in September 2006. Every two weeks, it records the amount of trace dust and ice in the atmosphere to create an air-quality map that indicates the evolution of water vapor and dust clouds.

The spectrometer will continue to observe landing sites for the Mars Science Laboratory through 2008.