The rocks left behind by ancient Martian rivers have be observed in unprecedented detail using NASA's High-Resolution Imaging Science Experiment (HiRISE). Billions of years after water last flowed in abundance on the Martian surface the effects can be clearly seen from orbit.

Once, the question of whether Mars ever had surface water was a matter of hot debate. That's long been settled, but there are still questions as to whether rivers and lakes were long-lasting or the product of brief floods. A new discovery appears to settle that.

“We've never seen an outcrop with this amount of detail on it that we can definitely say is so old,” the Natural History Museum, London's Dr Joel Davis said in an emailed statement.

The outcrop is near the north-western rim of the 4 billion-year-old Hellas Impact Crater. The crater is 2,000 kilometers (1,200 miles) across, far larger than any surviving impact crater on Earth, where plate tectonics and much greater water erosion have eliminated any trace of such reminders of the Solar System's most dangerous days.

The Crater's floor is more than 9.000 meters (5.6 miles) below the rim – a distance greater than from sea level to the top of Mount Everest. Such a low altitude makes the atmosphere much thicker, and therefore better suited to liquid water.

Consequently, Hellas is a prime target for HiRISE's capacity to image the Martian surface with a resolution of 25 centimeters (10 inches) per pixel. In Nature Communications, Davis and colleagues describe a 200-meter-high (700 feet) set of layered rocks left by flowing water. “[T]he striking similarities to sedimentary rocks on Earth leaves very little to the imagination.” said first author Dr Francesco Salese of Utrecht University.

The shifting sandbanks within the rocks prove the deposition was slow. “'The rivers that formed these rocks weren't just a one-off event – they were probably active for tens to hundreds of thousands of years,” Davis said. The sedimentary rocks studied by the Martian rovers are tiny by comparison, and could have been the product of brief floods, rather than lasting rivers.

These rocks could be perfect for checking whether life ever established itself on Mars. If life existed at the time is must surely have flourished in a watercourse like this, and left its mark in the sediments.

Mars almost certainly once had many sedimentary outcrops like this, but most that survived erosion from subsequent waters have been buried beneath sand dunes. Nevertheless, it is likely more survive, but unlike this one are not perfectly angled to be seen by HiRISE.

The formation of the cliffs 3.7 billion years ago coincides with the most recent evidence we have of a Martian magnetic field, reported just yesterday. That may not be a coincidence – magnetic fields protect atmospheres from the solar wind, which in turn sustains liquid water.

We also learned last week Mars had organic material at the time, another requirement for the existence of life. If Mars never supported life we will have to ponder why not.