Mars’ Reull Vallis: A river ran through it

The European Space Agency’s (ESA) Mars Express imaged the striking upper part of the Reull Vallis region of Mars with its high-resolution stereo camera last year.

Reull Vallis, the riverlike structure in these images, is believed to have formed when running water flowed in the distant martian past, cutting a steep-sided channel through the Promethei Terra Highlands before running on toward the floor of the vast Hellas Basin.

This sinuous structure, which stretches for almost 900 miles (1,500 kilometers) across the martian landscape, is flanked by numerous tributaries, one of which clearly can be seen cutting into the main valley toward the upper (north) side.

The new Mars Express images show a region of Reull Vallis at a point where the channel is almost 4 miles (7km) wide and 1,000 feet (300 meters) deep.

The sides of Reull Vallis are particularly sharp and steep in these images, with parallel longitudinal features covering the floor of the channel itself. These structures are believed to be caused by the passage of loose debris and ice during the Amazonian period, which continues to this day, due to glacial flow along the channel.

The structures were formed long after it was originally carved by liquid water during the Hesperian period, which is believed to have ended between 3.5 billion and 1.8 billion years ago.

Similar lineated structures, believed to be rich in ice, also can be found in many of the surrounding craters.

In the wider context image, the tributary intersecting the main channel appears to be part of a forking of the main valley into two distinct branches farther upstream before merging back into a single main valley.

The right (northern) part of the main image is dominated by the Promethei Terra Highlands with their high and soft-rounded mountains shown in these images, rising around 8,200 feet (2,500m) above the surrounding flat plains.

The perspective view shows one of these mountains with nearby sediment-filled impact craters. This region shows a striking resemblance to the morphology found in regions on Earth affected by glaciation. For example, we can see circular steplike structures on the inner walls of the sediment-filled crater in the foreground of the second perspective view. Planetary scientists think they may represent former high water or glacial levels before ice and water sublimated or evaporated away in stages at various times.

The morphology of Reull Vallis suggests it has experienced a diverse and complex history, with analogies seen in glacial activity on Earth. These analogies are giving planetary geologists tantalizing glimpses of a past on the Red Planet not too dissimilar to events on our world today.