Posted: October 21, 2008
New HiRISE images have revealed two rare sightings of impact craters in the Mars’ northern polar regions.
In the first image, an unusual solitary mound protruding from a depression in a slope in Mars’ north polar layered terrain was brought to the attention of HiRISE scientists. Mars’ layered terrain is made up from stacks of ice and dust several kilometres thick, and is thought to contain much of the planet’s water reservoir. Its formation is believed to be strongly linked to atmospheric processes, and scientists believe the deposits record details of climate changes on the red planet.
This cater surprised scientists because of its location in the north layered terrain where craters are rare, its non-circular shape possibly arising from flowing ice deforming the bowl-shaped cavity, and the mound of bright ice rising out of the crater. The surrounding terrain has lost most of its ice cover, whereas that in the crater could be protected by the crater walls. The image is colour enhanced to show ice as blue/white and the surrounding terrain as yellow. Image: NASA/JPL/University of Arizona.
The new HiRISE image shows an exposed 500 metre thick section of this layering, and part of what could be the remnant of a once buried impact crater. The image reveals a 40 metre high conical mound sticking out of the slope that is made up of polygonal blocks as big as 10 metres across. The blocks are covered with reddish dust, but otherwise resemble ice-rich blocks seen in other images of the north polar layered deposits.
"The mound may be the remnant of a buried impact crater, which is now being exhumed," says planetary scientist Shane Byrne from the Lunar and Planetary Laboratory in Arizona. Impact craters would have been buried by ice as the layered deposits accumulated, with layers wrapping around and infilling the crater. But this is a rare case, since almost no craters exist on the surface of this terrain. "In this case, erosion formed a trough that uncovered one of these structures,” continues Byrne. “For reasons that are poorly understood right now, the ice beneath the site of the crater is more resistant to this erosion, so that as this trough formed, ice beneath the old impact site remained, forming this isolated hill."
This is the crater seen against the vast expanse of the north polar cap. Colours have been enhanced to show dusty regions as red and ice of large grain sizes as blue. A smooth area stretching away from the crater to the upper right of the image may be caused by winds around the crater or by fine-grained ice and frost blowing out of the crater. Image: NASA/JPL/University of Arizona.
In another new HiRISE image a second impact crater 115 metres in diameter is witnessed on the north polar cap itself, where such features are hardly ever observed. The deficit of impact craters in these high northern latitudes suggests that either the north polar cap is only about 100,000 years old or that crater impacts into the ice disappear as the ice relaxes over time.
Since the Mars Reconnaissance Orbiter's HiRISE camera began operations in 2006, it has returned more than 8,200 gigapixel-sized images of the Martian surface that are giving planetary scientists brand new insight into the geology of the red planet.