"We have revealed this canyon like no one before us." So proclaims Olivier de Goursac, a French spatial imaging specialist who, along with colleague Adrian Lark, produced the spectacular image (left) of Mars' Valles Marineris, or Mariner Valley. Named for the Mariner 9 satellite that first revealed it some three decades ago, the chasm is often called the Martian Grand Canyon, though at 2,500 miles long, it is actually five times longer (about the span of the continental United States) and as much as four times deeper than Earth's. The image is based largely on satellite data produced by the Mars Orbiter Laser Altimeter (MOLA), an instrument that was launched aboard the Mars Global Surveyor spacecraft in 1996. MOLA measures Martian topography by transmitting a laser beam and tracking the time the beam takes to reach the planet's surface, bounce off, and return (the longer it takes, the lower the altitude of the surface measured). De Goursac and Lark combined that data with evidence about the Martian surface and then used imaging software to generate the landscape. The final result, says de Goursac, astonished even its creators.
How much does this rendering look like the real thing? "The gross features are right," says NASA's David Smith, chief scientist on the MOLA project. "But the colors are artificial and the texture has nothing to do with MOLA data." De Goursac agrees. He says he chose a yellowish-brown color to match color-balanced pictures of Mars taken by the Viking lander in 1976 and the Pathfinder satellite in 1997. The image also reflects evidence that about 30 percent of Martian terrain is covered with stones, and dust on the horizon reflects what's known about Mars' atmosphere. Smith cautions that the height of the mountains may be exaggerated due to distortions that can occur in altimeter readings. But he adds that he has spoken to de Goursac about the methods used in creating the image and "can't see anything wrong with their protocol."
To produce the image above (and the three images directly below this column), de Goursac and Lark began with data generated by MOLA, 1. (Second from top) The satellite's orbits are shown in red, and the Valles Marineris region is boxed in yellow. NASA scientists used the data to generate a detailed topographical map of the Martian surface, (third from top). To produce far more detailed renderings of Valles Marineris, (left, bottom, and main image, top), Lark sometimes had to supplement MOLA data with approximations. For instance, there were gaps in the instrument's readings due to interruptions in the signal. In such cases, Lark made estimations based on data from surrounding areas. Ultimately, he was able to produce a map of the Valles Marineris region based on precise altimeter readings taken every 500 meters. This map has since been checked against more recent, highly sensitive MOLA readings and found to be accurate.
To view more images, follow the link in the right column. (Images courtesy of NASA)