Juventae Chasma's Gypsum Hills

On the north side of Mars' great equatorial rift Valles Marineris lies a large depression called Juventae Chasma. Juventae stretches for 180 kilometers (110 miles) east-west and 250 km (155 mi) north-south. Most of its floor lies 5,000 meters (16,000 feet) or more below the surrounding surface. It's a big hole in the ground.

In ages past, this big hole also held a lot of water. Three to four billion years ago, deep pools of water in Juventae Chasma created large deposits of clays, plus gypsum and other sulfate minerals.

Today, only relic hills and ridges remain of these deposits. But as testimony to an ancient wet period that may have seen life on Mars, the layered hills on Juventae's floor are drawing much scientific interest. 

In the southwest corner of Juventae lies one of the numerous possible landing sites scientists explored for the Mars Science Laboratory (MSL). Due for launch in 2011, MSL will be NASA's next Mars rover.
Although scientists did not choose Juventae for MSL due to several reasons, the site still offers enough scientific attractions to keep it on lists of potential landing sites for future rover spacecraft.
The false-color scene here - Mars is not actually green! - combines day and night infrared images taken by the Thermal Emission Imaging System (THEMIS), a 15-band camera on NASA's Mars Odyssey spacecraft. The false colors show (in redder tones) areas where hard sediments and rocks predominate, while greens and blues show where fine sand and dust cover the surface.
Sulfate Ridge
Scientists think Juventae Chasma formed as faulting and volcanic heat opened cracks in the ground, letting water and melting subsurface ice burst out. This generated floods and caused the ground to collapse.
The water was plentiful enough that some spilled out of Juventae's northern end. From there it flowed northward, carving a channel named Maja Valles, before pouring into the Chryse impact basin.
Eventually, Juventae Chasma ceased growing and ice covered the remaining water ponded in the depression. Inside the canyon, the water altered the rocks and surface materials, turning them into gypsum and other aqueous minerals detected from orbit today.
The elongated ridge seen in the small image is 27 km (17 mi) long and 9 km (6 mi) wide. Its highest point rises more than 2,700 m (9,000 ft) above the foot of the ridge. The potential landing area for a future rover - a circle 20 km (12 mi) wide - lies to the lower left of the ridge's southern tip.
Studies by the OMEGA instrument on the European Space Agency's Mars Express orbiter show the ridge consists of two kinds of sediments. The upper (and younger) one is rich in gypsum (calcium sulfate) and shows more than a dozen distinct layers. Below the gypsum stand layers of kieserite, a magnesium sulfate. Both minerals are what geologists call evaporites - they often form where lakes of salty water dry up.
If NASA decides to send a rover here, scientists will likely want to drive north along the ridge's west side. This would let the rover examine the sequence of layers within the ridge. If life existed on Mars when these layers were deposited, the rover could find remaining traces of it.
Wherever subsurface water and melting ice undermined the surface, the result is sinkholes, depressions, and long troughs and rifts. Juventae is a good example of the process, and Valles Marineris a vastly larger one. Here, on the other hand, is a "small" sinkhole, roughly 16 km (10 mi) across.
This depression has a two-level floor. The upper level lies about 700 m (2,300 ft) below the surface, while the other goes down 1,700 m (5,500 ft). This is a greater depth than the Grand Canyon on Earth.
A neat, round blanket of debris surrounds the impact crater nearby. This was flung out by the explosion that dug the crater, which is now 5.3 km (3.3 mi) across and 500 m (1,600 ft) deep.
"Splat marks" like this are thought by scientists to indicate the ground was saturated with water (or ice) at the time of impact. The meteorite's blast mixed together shattered rock and water, and tossed out a ground-hugging sheet of wet and steamy debris. This flowed and surged, piling up a lip - in this case, 20 to 30 m (65 to 100 ft) high - at its farthest reach.
Spurs and Gullies
Canyons such as Juventae don't grow only by sinkhole-style undermining. Once canyon walls are exposed, rocky materials in them become unstable and often slide and fall.
In earlier epochs, when the climate was wetter and the atmosphere thicker, Mars likely saw rain and snow. These would have eroded the slopes of canyon walls in much the same way as in desert regions on Earth.
Typically, a small notch in the rim channels water down a particular path. This cuts the notch a little deeper, and more water follows. The result is this kind of spur-and-gully landscape. Debris piles up at the bottom of the slope, typically burying the foot of the wall.
In this case, the debris at the bottom has been cut by small channels, suggesting the last stages of erosion involved only a little flowing water.
Not At Fault
When new and fresh, this crater spread 8 km (5 mi) wide and it was wrapped in a circular splash of ejecta just like the smaller crater mentioned before. Here the outer edge of the ejecta blanket rises about 50 m (160 ft).
But since the time of impact, the eroding canyon wall has nibbled away about a third of the ejecta. Had the impact struck right at the edge of the canyon, the crater bowl itself would be less round and more open to the yawning canyon.
Below the crater lies another feature. Toward the foot of the canyon wall, a faint line runs roughly east-west. This is no imaging flaw but a geological fault, a crack in the ground that likely goes the full depth of the canyon. In places along it, reddish colors show where rocks may stand exposed.
It was activity along faults such as this that built Juventae in the first place. These allowed groundwater to escape and the canyon grew by undermining and sapping.
Here, however, the process halted for unknown reasons. Did the forces driving the fault activity stop? Had all the groundwater drained away? Such questions remain unanswered - for now. But if a rover ever visits Juventae, scientists will discover some answers. 


Vital Statistics

4.9°S, 296.9°E
Image Size: 

177.9 x 177.8 km
110.5 x 110.4 mi
1779 x 1778 pixels

100m (330 ft)