Tooting Crater's Tangled Tale
If you wish to study a textbook example of fresh Martian impact, it would be hard to beat Tooting Crater. This feature, named for a London suburb, spans 29 kilometers (18 miles) and sits on a pool-table-flat expanse of young lava flows west of Olympus Mons.
Lava floods have covered some craters on Mars, others have been whittled by sharp winds; others yet are emerging from under thick layers of eroding sediments. Not Tooting. The few impacts it has collected show the crater's age may be measured in only hundreds of thousands of years, thus it remains very fresh, at least in Martian terms.
Tooting's smooth surroundings and unaltered aspect have let a group of researchers measure the crater's properties in detail. These have provided a clear picture of how Tooting formed. The group, led by the University of Hawaii's Peter Mouginis-Mark (who grew up in Tooting, England), finds the crater has a two-level interior floor and its flower-like pattern of ejecta is unexpectedly thin.
This mosaic of Tooting blends images taken at visible wavelengths by the Thermal Emission Imaging System (THEMIS), a multi-band camera on NASA's Mars Odyssey orbiter. (Small gaps in coverage have been filled with lower-resolution images taken at infrared wavelengths.)
As it strikes the ground while moving at many kilometers or miles per second, an impacting meteorite creates, in a flash, a large cavity filled with vaporized and molten rock. As the cavity expands to its limit, the floor buckles upward, hoisting deeply buried rocks and raising a central peak.
Surrounding the central peak, molten target rock pooled like butterscotch around a glob of ice cream. Tooting's central peak, about 1 km (3,300 feet) high, looms unusually large for craters its size. The researchers think this is because Tooting is so young that very little sediment has accumulated within the crater. As a result, the central peak lies buried less deeply than most.
Tooting's floor isn't perfectly level, either. The northern half stands some 150 m (500 ft) higher than the southern. The north also displays a knubbled texture lacking in the south.
What caused the difference? The Hawaii team notes that the ejecta pattern around the crater extends twice as far (about 50 km or 30 mi) to the northeast than to the southwest. This suggests the meteorite struck obliquely, traveling from southwest to northeast and spraying most of its debris downrange.
The floor levels thus reflect the impact's asymmetry. As debris rained out of the column of superheated rock vapor that rose over the newborn impact, more of it fell on the northern side of the central peak. In laying down a thicker layer, the cascading debris also produced dimples a few hundred meters in diameter that mottle the northern floor.
Less affected by fallback debris, the southern floor lies lower and is marked with only a handful of flow channels and polygonal fractures. These resemble those seen in pools of impact melt in lunar craters such as Tycho, Aristarchus, and Copernicus.
To the Ramparts
South of Tooting's rim, sheets of broken rock and debris flew across the ground in the aftermath of the impact, reaching a distance of about 30 km (20 mi). Scientists have long wanted to know exactly how much debris ends up being thrown out by an impact - and Tooting offers a perfect test case.
The MOLA laser altimeter on the Mars Global Surveyor made nearly 25,000 individual elevation measurements across Tooting and its ejecta blanket. The figures, combined with the flat terrain, let scientists determine the ejecta thickness. These show that much of the southern ejecta layer is 3 to 5 m (10 to 16 ft) thick, while the rampart that marks its greatest extent rises 65 to 125 m (200 to 400 ft) high, being higher toward the north, west, and southeast.
Taken all together, the Hawaii researchers found the total debris from Tooting's birth adds up to about 425 cubic kilometers (100 cubic miles). This figure is about 11 percent more than the measured volume of the impact bowl itself. The difference may indicate the ejecta is loosely packed compared to the original layers and thus appears to bulk bigger than it is.
Theoretical studies that modeled the ejecta blanket's formation suggest it took roughly 10 minutes to half an hour to fall into place. Debris-laden blast winds would have flown at speeds of several hundred kilometers per hour near the rim, but slowed as they advanced and possibly developed a rolling motion. Eventually the outward-spreading debris came a halt, piling up the rampart.
Tooting differs from a similar-appearing impact feature, Bacolor Crater, which lies in Utopia. Bacolor's debris blanket was built in two stages, powered by the explosive effects of groundwater turning to steam. With Tooting, scientists say, the ground probably contained less water at the time of impact, and the blast had only a single phase.
Undermining the Scene
Yet subsurface water had once played a role in the region. The plains where Tooting formed lie flat to within 20 m (65 ft) over an expanse roughly 150 km (90 mi) square. This surface is older than Tooting, but by how many years scientists do not know.
This depression feature, however, predates Tooting, and its origin has nothing in common with the crater. The irregular depression formed, scientists think, when a small amount of water escaped from the subsurface through a fault. As the water escaped, the ground above collapsed, a process scientists term sapping. It has occurred in many areas of Mars.
The water may have gotten into the ground during an earlier and wetter period in Mars' climate - or it could have come from subsurface volcanic activity. Tooting lies near the giant volcano Olympus Mons, part of the vast Tharsis volcanic province, and the water might have come from the lavas that produced Olympus Mons.
A close look at the channels meandering across the surface shows numerous small craters in the channel beds. These are likely secondaries - satellite craters born when chunks of rocky debris thrown from the Tooting impact crashed down on the surface.