The Many-Layered Saga of Terby Crater
Terby Crater is an old, medium-size impact scar that lies on the northern rim of the Hellas Basin, an older and vastly bigger impact basin in Mars' southern hemisphere. What gives it great interest to scientists are the thick sediments filling much of its interior. These sediments should preserve a long record of Martian history, possibly including a time when the planet may have had life.
With a diameter of about 170 kilometers (105 miles), Terby was once one of the candidate landing sites for NASA's next-generation rover, the Mars Science Laboratory (MSL), scheduled for launch in 2011. The four finalist sites are Mawrth Vallis, Holden and Eberswalde craters, and Gale Crater.
While not chosen for MSL, Terby still offers scientific attractions to keep it on lists of potential landing sites for future rover spacecraft.
Having formed on the edge of Hellas, Terby tips southward into the basin, with a northern rim standing higher than the southern. Much of Terby's floor is smooth and flat. Toward the north, the flat floor drops about 600 meters (2,000 feet) to a lower smooth layer that scientists have dubbed "the Moat." Continuing northward across the Moat, several large, finger-like mesas extend south from Terby's rim.
Erosion has etched the mesas, which rise about 2,000 meters (6,500 feet) above the Moat. One knob in particular (see first side image below) shows a layer-cake structure, but high-resolution images show that all the mesa edges are stair-stepped as well.
The main image shows almost all of Terby in false colors, taken by the Thermal Emission Imaging System (THEMIS). THEMIS is an visible and infrared camera on the Mars Odyssey spacecraft. Here researchers have combined daytime and nighttime infrared views, color-coding them so that areas with rocks and harder sediments appear reddish, while bluer tones indicate areas with softer and finer sediments.
Dropping Into the Moat
Spacecraft engineers say rovers need a landing target about 20 km (12 miles) in diameter that's level, flat, and free of big rocks. A circle 20 kilometers across will just fit within the Moat area seen in the small image at left.
As the minimum driving range for a future rover will likely be 20 km, landing a rover here should put some of the mesa sediments within reach. From preliminary studies, it appears that a rover might also be able to drive at least partway up the ramped sediments that flank the mesas. (This assumes the ground isn't too soft or loose for traction, and that no cliffs bar the way.)
Studies of the mesas also show that their layers do not lie perfectly flat, but rather slope gently (about 1°) southward. This has led some scientists to propose a history for Terby that involves long-ago episodes of flooding and erosion.
As they reconstruct it, after forming more than 3 billion years ago, Terby was partly filled with sediments that washed in or blew in by the wind. This may have been the origin of the main floor material.
Then, at some point, Hellas was filled with a giant lake that submerged Terby, which accumulated debris washed in by waters flowing into the Hellas lake from the north. This debris formed layered deposits that later eroded to make the mesas.
If anything like this occurred, the mesa sediments should preserve a valuable record of early Mars - one that may have seen life.
Oozing With Ice
The key to understanding ancient epochs on Mars is the fact that the planet undergoes big swings in climate over periods of hundreds of thousands of years. Today, Mars is a cold, dry desert. But during earlier periods Mars must have seen deposits of snow and ice even down to equatorial latitudes.
This unnamed crater on Terby's rim, just 23 km (14 mi) wide, shows clear signs that material flowed out of the crater through its southern rim. Nor is this crater the only place on Mars showing such flow feature: many others do also.
In earlier times, scientists estimate, large portions of the planet were blanketed with thick layers mixing fine-grain dust and snow. Over time, these ice-rich mixtures settled, compacted, and often flowed. Some of these mantled areas, mainly ones much closer to the poles than Terby, likely contain ice even today.
Fan Site
The debris that washed out of the 23-km crater spread across part of Terby's floor. (Look closely at the top center of the small image at left.) While this area isn't as smooth or flat as the Moat, it might be good enough to serve as a landing strip for a rover. If one did set down here, it would be rolling across what geologists call an outwash fan.
These are accumulations of debris that include samples of all kinds of rocks that lie upstream. Outwash fans are a mixed blessing for field geologists because they present a trade-off: You find many samples drawn from a large area - but you may not know where each sample came from.
On Earth, where mobility is easy, a trade-off like this doesn't matter too much because you can usually search upstream. But on Mars, where a rover will face stringent limits on where it can go, scientists might choose to send it to a site where the origin of each rock it finds is clearer.
The Saga of Terby Crater
