Martian Deltas

River Runs Through It?

A region near the southeast edge of the fan shows inverted channels at different levels within the sedimentary deposit, indicating a long and complex history of water-related processes. One ridge crosses over another just left of the center of this frame. The top of the lower ridge is the former floor of a channel that was transporting water and sediment toward the lower right. The top of the upper ridge was once the floor of a channel that moved material toward the right or upper right. The lower ridge is the older channel. It was completely filled and buried beneath the surface when the upper channel formed. Image Credit: JPL/MSSS

Newly seen details in a fan-shaped apron of debris on Mars may help settle a decades-long debate about whether the planet had long-lasting rivers instead of just brief, intense floods.

Pictures from NASA’s Mars Global Surveyor orbiter show eroded ancient deposits of transported sediment long since hardened into interweaving, curved ridges of layered rock. Scientists interpret some of the curves as traces of ancient meanders made in a sedimentary fan as flowing water changed its course over time.

"Meanders are key, unequivocal evidence that some valleys on early Mars held persistent flows of water over considerable periods of time," said Dr. Michael Malin of Malin Space Science Systems, San Diego, which supplied and operates the spacecraft’s Mars Orbiter Camera.

"The shape of the fan and the pattern of inverted channels in it suggest it may have been a real delta, a deposit made where a river enters a body of water," he said. "If so, it would be the strongest indicator yet Mars once had lakes."

Malin and Dr. Ken Edgett, also of Malin Space Science Systems, have published pictures and analysis of the landform in today’s online edition of Science Express. The images with captions are available online from the Mars Orbiter Camera team, and from NASA’s Jet Propulsion Laboratory, Pasadena, Calif.

The fan covers an area about 13 kilometers (8 miles) long and 11 kilometers (7 miles) wide in an unnamed southern hemisphere crater downslope from a large network of channels that apparently drained into it billions of years ago.

The fan is a fossil landform. That is, it is an eroded remnant of a somewhat larger and thicker deposit. The originally loose sediment was turned to rock and then eroded over time to present the features seen today. The channels through which sediment was transported are no longer present. Instead, only their floors remain, and these have been elevated by erosion so that former channels now stand as ridges. The floors of former channels became inverted in this way because they were more resistant to the forces of erosion, indicating they either were more strongly cemented than surrounding materials, or they have more coarse grains (which are harder to remove), or both.

"This latest discovery by the intrepid Mars Global Surveyor is our first definitive evidence of persistent surface water," commented Dr. Jim Garvin, NASA’s Lead Scientist for Mars Exploration, NASA Headquarters, Washington, D.C. "It reaffirms we are on the right pathway for searching the record of martian landscapes and eventually rocks for the record of habitats".

"Such localities may serve as key landing sites for future missions, such as the Mars Science Laboratory in 2009," continued Garvin.

White boxes overlaid on the image show the locations of those two areas. A region near the center of the fan has a loop, the inverted floor of a former meandering stream that was cut off as the channel adjusted its course. Meanders and cut-off meanders are the prime evidence for persistent flow of water through this area sometime early in martian history. Credit:MSSS/JPL

"These astounding findings suggest that ‘following the water’ with Mars Global Surveyor, Mars Odyssey, and soon with the Mars Exploration Rovers, is a powerful approach that will ultimately allow us to understand the history of habitats on the red planet."

No liquid water has been detected on Mars, although one of the previous major discoveries from Mars Global Surveyor pictures suggests that some gullies have been cut in geologically recent times by the flow of ephemeral liquid water.

Another NASA orbiter, Mars Odyssey, has discovered extensive deposits of near-surface ice at high latitudes.

Mars’ atmosphere is now so thin that, over most of the planet, any liquid water at the surface would rapidly evaporate or freeze, so evidence of persistent surface water in the past is also evidence for a more clement past climate.

Malin and Edgett estimate that the volume of material in the delta-like fan is about one-fourth the volume of what was removed by the cutting of the upstream channels. Their analysis draws on information from Mars Global Surveyor’s laser altimeter and from cameras on Mars Odyssey and NASA’s Viking Orbiter, as well as images from the Mars Orbiter Camera.

"Because the debris in this fan is now cemented, it shows that some sedimentary rocks on Mars were deposited by water," Edgett said. "This has been suspected, but never so clearly demonstrated before."

The camera on Mars Global Surveyor has returned more than 155,000 pictures since the spacecraft began orbiting Mars on Sept. 12, 1997. Still, its high-resolution images cover only about three percent of the planet’s surface. The picture of persistent river regions are a mosaic of images acquired between August 2000 and September 2003. What is important about it?

First, it provides unequivocal evidence that some valleys on Mars experienced persistent flow over considerable periods of time, as rivers do on Earth.

Second, because the fan is today a deposit of sedimentary rock, it demonstrates that some sedimentary rocks on Mars were deposited in a liquid environment.

Third, the fan’s general shape, the pattern of its channels, and its low slopes provide circumstantial evidence that the feature was an actual delta — that is, a deposit made when a river or stream enters a body of water. If so, this landform is a strong indicator that some craters and basins on Mars once held lakes. Hundreds of other locations on Mars where valleys enter craters and basins have been imaged by the Mars Orbiter Camera, but none has shown landforms like these deltas.

What’s Next

Twin NASA MER rovers and one European Space Agency probe are planned for Mars landing in late 2003 and early 2004. The first Mars Exploration Rover, or MER, will arrive at Mars on Jan. 4, 2004, the second, Jan. 25. The first European Mars mission will arrive on Christmas day, and perform biological analysis on martian soil.

JPL, a division of the California Institute of Technology, Pasadena, manages Mars Global Surveyor for NASA’s Office of Space Science in Washington. JPL’s industrial partner is Lockheed Martin Space Systems, Denver, which developed and operates the spacecraft. Malin Space Science Systems and the California Institute of Technology built the Mars Orbiter Camera. Malin Space Science Systems operates the camera from facilities in San Diego.