(Sen) - More evidence that water once flowed on Mars has been revealed by the Mars Express orbiter and NASA's rover, Opportunity.
The Mars Express orbiter captured images of an area that appears to have been sculpted by flowing liquid. The orbiter's high resolution camera took images last June of the area where the western edge of the Acidalia Planitia, a huge basin, meets with Tempe Terra, a higher terrain.
Some of the valleys descending from the Tempe Terra into the basin show evidnce of drainage patterns believed to have been formed by flowing water from rain or melting snow. Scientists studying the data believe that the the deep valleys were probably formed by a process known as "sapping" or "undermining" where the softer layers at the bases of cliffs are eroded. When the soft rock is eroded there is no longer support for the upper harder rock so it falls from the cliff face. This process can be seen in the Colorado Plateau on Earth.
Looking at the main image above, the lower left part of the image appears to be in shadow but the darker 'shadow' is in fact due the surface being covered with dark sand whilst the rest of the surface pictured is covered with lighter dust.
Faults in the Martian crust can also be seen - these are believed to have exposed subsurface reservoirs, releasing water that possibly turned craters into lakes.
Some of the craters also appear to have sediments left by surface water in the past.
Some valleys appear to start at the rim of the craters which scientists believe is evidence that water was released from the craters into the surrounding terrain.
The Mars Express images provide scientists with further evidence that water must have flowed on the Red Planet a long time ago.
Further evidence of a watery past has been gathered by NASA's Opportunity rover. Opportunity reached the Endeavour Crater last summer. The crater, which measures 22 kilometres (14 miles) is believed to have been formed 4 billion years ago. Reviewing the data captured by Opportunity, scientists believe that when the asteroid hit the surface and made the crater, subsurface water was released. In essence the surface sprung a leak.
Scientists also believe water flowed through cracks around the edge of the crater leaving deposits of the mineral gypsum.
This approximate color view of a mineral vein called "Homestake" taken by NASA's Mars rover Opportunity. The vein is about the width of a thumb and about 18 inches (45 centimeters) long. Image Credit: NASA/JPL-Caltech/Cornell/ASU
The discoveries made by Opportunity at the Endeavour Crater are detailed in a new report published in the May 4 edition of the journal Science.
Timothy Parker, a co-author of the report, said "Exploring Endeavour Crater is like having a new landing site. That's not just because of the difference in the geology here compared to what we saw during most of the first eight years, but also because there's a whole vista before us inviting much more exploration."
Opportunity and its twin rover Spirit landed on Mars in 2004 and completed their three month prime missions. The rovers continued for many years and only in 2010 did Spirit stop communicating. Opportunity is still capturing data of the Martian surface and sending the data back to Earth for scientists to interpret.
In August 2010 NASA's newest rover Curiosity is scheduled to land at the Gale Crater.