Water, water somewhere, so where should we land?
4 Mar 2008
In 2009, NASA plans to launch the Mars Science Laboratory (MSL), a new rover spacecraft with a driving range and instrument payload that greatly improves on the Mars Exploration Rovers Spirit and Opportunity. MSL's task is to assess whether Mars ever had an environment able to support microbial life.
But where should MSL go?
Scientists have a 'short list' of potential landing sites that runs to 36 entries, with locations scattered across much of Mars. As there won't be 36 MSLs heading for Mars anytime soon, scientists are working to choose a single site on which to place all their MSL chips.
One possible place lies in western Melas Chasma, a corner of the vast Valles Marineris 'grand canyon' system that slices across equatorial Mars for thousands of kilometres. (Another MSL site, already profiled, lies in eastern Melas, about 450 km away.)
The image above shows the western Melas landing site, which tucks into a small basin below the south rim of Valles Marineris. While the basin is more than 5,000 metres (16,000ft) lower than the south rim, it still stands 1,200m (4,000ft) above the floor of Valles Marineris. Geologists describe such a feature as a 'perched basin.'
According to research by Catherine Weitz (Planetary Science Institute), Cathy Quantin (National Air & Space Museum), and others, the basin likely contains deposits left when it was once filled with a lake. Even long after the lake waters drained away or dried up, such sediments could preserve evidence of former microbial activity, if it existed.
The image is a false-colour mosaic of photos taken by the Thermal Emission Imaging System (THEMIS), a multi-wavelength camera on NASA's Mars Odyssey orbiter. As Mars Odyssey flies in a near-polar orbit, THEMIS scans almost all of Mars, capturing its surface in five visible colours and 10 infrared ones. This mosaic is made of infrared frames taken by day and by night.
The false colours hint at the nature of the ground surface. Rusty tones show where rocks and hard deposits abound, while greens and blues indicate the areas where gravel, fine sand, and dust particles predominate.
The MSL landing site lies centred in this closed, oval basin that measures roughly 20 x 30 km. The little crater at left centre spans 1.4 km. Engineers figure that MSL will have a minimum driving range of 20 km, so nearly all the basin should lie within the rover's reach, unless the ground proves too rough to drive on.
The swirling lines, reddish-brown in false colour, mark where hardened sediments lie exposed at the surface. The curves suggest a complex pattern of outcrops, heavily sculptured by erosion. Calculations show the basin theoretically could have held about 160 cubic kilometres of water—more than 300 times the volume of Sydney Harbour. The basin has a maximum depth of about 300 metres (1,000 ft), but most of it would have been much shallower.
If the lake behaved like Earth's desert lakes (such as Utah's Great Salt Lake), the waters could have come and gone several times in succession, as changes in the martian climate waxed and waned.
Zooming in on the basin's west end using THEMIS visual-wavelength images—five times sharper than the infrared images—a complex array of deposits (picture at left) comes to light.
The basin floor slopes down from the west (left side), and Mars geologists argue that the carved and sinuous terrain is the remains of a fan of debris washed into the lake. Channels interweave across each other, with softer sediments eroding and leaving tougher deposits standing high. (A smaller fan spills into the closed basin on its eastern end.)
At the foot of the south rim's slope (bottom of the picture) lies a possible high watermark, or beach deposit. This can be traced around the basin's perimeter at an elevation of -1,540 meters (-5,050 ft). Such deposits, as well as both debris fans, would be highly valuable for MSL to examine.
At a watershed
To the west of the little basin lies a feature commonly seen on Earth, but less so on Mars. Running from upper right to lower left across the zoomed-in image at left is a watershed, a divide between two adjacent drainages. While Earth's active hydrosphere gives it uncountable numbers of drainage systems, scientists have found fewer on Mars, which makes this one of considerable interest.
The branching valleys on the east side of the divide slope down into the basin where MSL may be targeted, while those to the west slope in a westerly direction. The height of land between the two stands 4,600 metres (15,000 ft) below the south rim of Valles Marineris.
Water that flowed west from the drainage divide formed what geologists call a mature drainage system—channels, branches and tributaries are all well developed. On Earth, such features typically emerge after about a thousand years of erosion. (The valleys also appear more deeply cut than those that lead into the closed basin on the east.) The west-flowing waters went down onto the main floor of Melas Chasma, carrying sediments onto the big valley's floor.
So where did the water come from? In their branching form, the valleys resemble what develops when rain falls on the ground and runs off unhindered. This remains the most plausible explanation, and it suggests that in Melas at least, ancient climatic conditions allowed for rain.
Outcrop
A low ridge cuts across the floor of Melas Chasma and slices through part of an impact crater 9.6 km wide. The rusty tint indicates (in THEMIS' false colours) that here lies exposed a resistant outcrop of rock or hardened sediments—in fact, the outcrop forms part of the crater's southern wall.
A light coating of sediments covers most of the Melas Chasma floor, but not so thickly as to bury the debris apron that surrounds the crater. This has multiple lobes with upraised outer edges. Planetary geologists have concluded that such features show the ground was saturated with water (or ice) when the impact occurred.
Double whammy
Telescopic and radar surveys of asteroids have found that a sizeable number of them are actually double objects. Some are two separate bodies locked in mutual orbit by gravity, while others are better described as two big rocky lumps in contact.
With Mars orbiting the Sun close to the asteroid belt, it's not surprising to find impact craters that clearly formed when a double-object struck. This double crater (above left) measures about 9 x 18 km and even shows traces of a butterfly pattern in its ejecta apron, the material that 'splashed out' after the impact.
Such patterns are common in two cases: when a meteorite strikes at a very shallow angle and when double-asteroids impact.
Adapted from information issued by NASA / JPL / Arizona State University.
