HiRISE Stereo Color Images Detail Mars Terrain that Tantalizes Explorers
The High Resolution Imaging Science Experiment aboard NASA’s Mars Reconnaissance Orbiter has returned more than 8,214 gigapixel-size images of the martian surface since the start of the science phase of the mission in November 2006.
HiRISE scientists recently released 1,005 observations of Mars made between April 26 and July 21 to NASA’s mission data archive, called the Planetary Data System, and also to the public. The new images, a total 3.4 terabytes of data, can be found on the HiRISE Web site.
The HiRISE team has so far released a total 26.9 terabytes of data in more than 7,100 observations with 718,000 different image products derived from those observations, said HiRISE operations manager Eric Eliason of The University of Arizona’s Lunar and Planetary Laboratory.
That amounts to more data than has been released by all previous deep space missions combined. The image products include color images and stereo pairs, as well as monochrome images.
"If I showed each HiRISE image for 10 seconds, it would take me about 4 years to show them all," said UA’s Alfred McEwen, HiRISE principal investigator.
Despite this massive data volume, HiRISE images cover less than four-fifths of one percent of the area of the planet.
Stereo imaging has been a very important part of the HiRISE experiment, McEwen said. The camera has produced 803 stereo pairs. Each pair consists of two images that were taken on two different orbits as the spacecraft pointed off nadir precisely on target.
Acquiring stereo pairs is a lot of work, but worth it, he added.
McEwen and his team published more than 20 scientific papers on HiRISE results in 2007 and 2008. They based much of their analysis on stereo data used to produce digital elevation models that are accurate to within about three feet and to within about eight inches in height.
A new processing pipeline to produce stereo images was recently completed by Eliason and his team. Within a few months all of the stereo image pairs will be processed and posted.
Color is another important HiRISE capability. The camera uses a focal plane system that consists of 14 charge-coupled devices, or CCDs, that include 4 middle CCDs for full color coverage.
Spacecraft motion pushes this electronic array so that it records the view down to Mars’ surface at a ground speed of about 3.2 kilometers per second, or about 7,000 miles per hour.
Skeptics doubted that a technique called "time integration delay," needed to compensate for extremely short exposure times ? about one ten-thousandth of a second per pixel ? could produce sharp, unsmeared images.
But the technique has worked "wonderfully well," thanks to accurate spacecraft pointing and stability and precise exposure time calculations, McEwen said.
Colors in HiRISE images are not natural colors seen with normal human vision. But the colors are important to distinguish ice, frost, dust and rock on the martian surface. Colors enhance differences in morphologies, brightness variations and topographic shadings.
The Mars Reconnaissance Orbiter’s Compact Imaging Spectrometer for Mars, or CRISM, identifies minerals on Mars, and HiRISE colors are useful in extrapolating that information down to the scale of individual rock outcrops. That information is needed for science evaluations in choosing future Mars landing sites. HiRISE data is also used for engineering analysis of candidate landing sites.
McEwen, who is involved in selecting the best possible landing site for NASA’s Mars Science Laboratory planned for 2009 launch, said recent evidence from both U.S. and European Mars orbiters shows clays that contain iron, magnesium and aluminum are widespread throughout ancient Mars bedrock.
"That’s a shocking discovery," McEwen said. "The actual environments and processes that deposited and/or altered these clay minerals is poorly understood. But these places have very quickly become where we have to go with future rovers and sample-return missions."
Scientists theorize that life in our solar system may have formed during or even before the Late Heavy Bombardment of 3.9 billion years ago.
"We know that Mars underwent heavy bombardment and that it was water-ice rich," McEwen said. "Impact-generated hydrothermal systems may have created habitat on Mars similar to Earth’s, and the rock record from this time is much better preserved on Mars than it is on Earth."
HiRISE images show that part of Mars’ hardened, ancient crust is exposed on the planet’s surface as "megabreccia," a conglomeration of large, chaotic rock fragments that formed from energetic events, mostly large impacts on ancient Mars.
Three candidate Mars Science Laboratory sites are very near megabreccia deposits and would be easily accessible to NASA’s future megarover, McEwen noted.