Pebbles prove streams once flowed across Mars - and raise hopes of finding life on the red planet



First time that evidence of sustained water flows on Mars has been uncovered

Rounded pebbles are only formed when they are carried through water over long distances

Scientists have discovered pebbles on Mars, showing that a stream has flowed on the planet - and giving more weight to the theory that it was once able to support life.



The rounded pebbles found by the team from Nasa's Curiosity rover mission, and published in journal Science today, are only formed when they are carried through water over long distances, according to the researchers.



It is the first time that evidence of sustained water flows on Mars has been uncovered.



The individual stones in the area are rounded like pebbles on Danish shores and were probably transported down through a ravine, Peace Vallis, which is carved in the southern slope of the northern crater wall

HOW THEY DID IT

The images are taken by the spectroscopic stereo camera, Mastcam, at an angle from a vantage point two meter above the Martian ground and therefore gives a slightly distorted image of the size of the stones depending on their location in the image field.



The further away you look, the greater the distortion of the size of the stones in the image. The Danish researchers from the Niels Bohr Institute, together with a German collaborator and the camera team built a system that can rectify the image so that sizes are directly comparable.



Dawn Sumner, a geologist at University of California, Davis, said the discovery of the pebbles - which were laid down more than two billion years ago - was due to choosing the right place for Curiosity to land.



She said: 'The main reason we chose Gale Crater as a landing site was to look at the layered rocks at the base of Mount Sharp, about five miles away.



'We knew there was an alluvial fan in the landing area, a cone-shaped deposit of sediment that requires flowing water to form.

'These sorts of pebbles are likely because of that environment. So while we didn't choose Gale Crater for this purpose, we were hoping to find something like this.'



The report authors noted that conditions on Mars at the time the pebbles were deposited were vastly different to the cold, dry environment of the present day if water was to flow over several kilometres.



They added: 'These ancient fluvial deposits indicate sustained liquid water flows across the landscape - a finding that raises prospects for the former presence of habitable environments on Mars.



More work needs to be carried out into the risks of radiation to astronauts wanting to take a closer look at the geography of the red planet ahead of any future missions to Mars however, according to a separate study published in Science today.



The study area, which has been named 'Hottah', is by all accounts the remains of sediments from the bottom of an ancient stream, which had a relatively strong current

A radiation assessment detector (RAD) on board Curiosity during its 253-day journey to Mars made measurements of the environment inside the spacecraft, and its results could have important implications for manned missions.



Cary Zeitlin of the Southwest Research Institute in Boulder, Colorado, said astronauts would be subject to a high dose of radiation - representing a large fraction of the amount that is acceptable over a lifetime.



He said: 'In terms of accumulated dose, it's like getting a whole-body CT scan once every five or six days. Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep space destinations is critical for planning future crewed missions.



'Based on RAD measurements, unless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself.'



The report looked at the exposure to galactic cosmic rays and solar energetic particles that the spaceship experienced during the journey to Mars.



Multiple outcroppings of rocks like this one (termed a pebble conglomerate) were observed along the first 275 meters traversed by the rover with the high-resolution Mastcam

Dr Zeitlin said a spacecraft carrying astronauts would have protection against solar particles, but that cosmic rays are much more difficult to stop.



The next step will be to investigate radiation while on the surface of the planet itself, said Dr Zeitlin, who added this was critical in light of some of the proposed scenarios for Mars missions which envisaged astronauts spending up to 500 days on Mars.

