There are tantalizing indications emerging from the thousands of infrared images taken so far by NASA's Mars Odyssey spacecraft that Mars experienced a series of environmental changes during active geological periods in its history.

Infrared imaging from NASA's Mars Odyssey spacecraft shows signs of layering exposed at the surface in a region of Mars called Terra Meridiani. The brightness levels show daytime surface temperatures, which range from about minus 20 degrees to zero degrees Celsius (minus 4 degrees to 32 degrees Fahrenheit). Many of the temperature variations are due to slope effects, with sun-facing slopes warmer than shaded slopes. However, several rock layers can be seen to have distinctly different temperatures, indicating that physical properties vary from layer to layer. These differences suggest that the environment on this part of Mars varied through time as these layers were formed. Credit: NASA/JPL/Arizona State University

"We knew from Mars Global Surveyor that Mars was layered, but these data from Odyssey are the first direct evidence that the physical properties of the layers are different. It's evidence that the environment changed over time as these layers were laid down," said Dr. Philip Christensen, principal investigator for Odyssey's camera system and professor at Arizona State University, Tempe. "The history of Mars is staring us in the face in these different layers, and we're still trying to figure it all out."

"I expect that the primitive geologic maps of Mars that we have constructed so far will all be redrawn based on Odyssey's new information," said Dr. R. Stephen Saunders, Odyssey's project scientist at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

A mosaic of daytime infrared images of the layered Terra Meridiani region shows a complex geology with craters and eroded surfaces, exposing at least four distinct layers of rock. Though the image does not include the infrared "colors" of the landscape (showing surface mineral composition), it does map the temperatures of the features, with surprising results.

Color differences in this daytime infrared image taken by the camera on NASA's Mars Odyssey spacecraft represent differences in the mineral composition of the rocks, sediments and dust on the surface. The image shows a portion of a canyon named Candor Chasma within the great Valles Marineris system of canyons, at approximately 5 degrees south latitude, 285 degrees east (75 degrees west) longitude. The area shown is approximately 30 by 175 kilometers (19 by 110 miles). Credit: NASA/JPL/Arizona State University

"With these temperature data, Odyssey has already lived up to our expectations, but Mars, in fact, has itself exceeded our expectations," said Christensen. "It would have been entirely possible for the rocks of Mars to have been very similar and thus give us all the same temperatures, but Mars has a more interesting story to tell and we have the data to tell it."

Christensen is presenting his findings today at the spring meeting of the American Geophysical Union in Washington, D.C.

"When we look at these distinct layers we see that the temperatures are very different, indicating that there are significant differences in the physical properties of the rock layers," Christensen said.

The differences in surface temperature could be caused by the fundamental differences in either the size of the rock fragments in the layer, the mineral composition or the density of the layers.

Odyssey's imaging team is working on fully processing the infrared images, a complex and difficult task. When finished, the data will help them test some important theories about what causes the layers on Mars by examining the mineral composition of the specific layers. Plausible explanations include a history of volcanic activity depositing layers of lava and volcanic ash; a history of different processes that created the layers through wind and water; or a history of climate change that varied the nature of the materials deposited.

Christensen theorizes that the layers are caused not by surface effects, but by changes in the planet's subsurface water table. The presence or absence of water and the minerals carried in it can significantly affect how sediment particles are cemented together. With no clear evidence for surface water, precipitation or runoff, Christensen believes that changes in levels of underground water percolating through layers of buried sediments could account for differences in rock composition between layers. More complete infrared data will help to confirm or disprove this and many other hypotheses concerning Mars' geology.

"Looking at craters, we're seeing new distributions of rock on the surface that are helping us understand events in martian geology, and we are getting our first glimpses of 'color' infrared images, which will help us precisely determine the composition of the Mars' surface. This is just the beginning," Christensen said.

JPL, a division of the California Institute of Technology in Pasadena, manages the 2001 Mars Odyssey mission for NASA's Office of Space Science in Washington. Investigators at Arizona State University in Tempe, the University of Arizona in Tucson and NASA's Johnson Space Center, Houston, operate the science instruments. Additional science partners are located at the Russian Aviation and Space Agency and at Los Alamos National Laboratories, New Mexico. The thermal emission imaging system was provided by Arizona State University in collaboration with Raytheon Santa Barbara Remote Sensing. Lockheed Martin Astronautics, Denver, is the prime contractor for the project, and developed and built the orbiter. Mission operations are conducted jointly from Lockheed Martin and from JPL.