MRO revealed rocks containing a mineral similar to opal
A Nasa space probe has discovered a new category of minerals spread across large regions of Mars.
The find suggests liquid water remained on Mars' surface a billion years later than scientists had previously thought.
The US Mars Reconnaissance Orbiter (MRO) spacecraft found evidence of hydrated silica, better known as opal.
The discovery adds to the growing body of evidence that water played a crucial role in shaping the Martian landscape and - possibly - in sustaining life.
Hydrated, or water-containing, minerals are telltale signs of when and where water was present on ancient Mars.
Researchers made the discovery using the Compact Reconnaissance Imaging Spectrometer (CRISM) instrument on MRO.
The $720m (£449m; 565m euro) MRO robotic probe reached the Red Planet in 2006; its objectives are to study Mars' geology, climate and atmosphere from orbit, as well as to search for signs of water.
Details of the latest findings appear in the November issue of the academic journal Geology.
The minerals were recently found at Gusev Crater on Mars by the Spirit rover. This study reveals that they are widespread and occur in relatively young terrains.
Gem of a find
"This is an exciting discovery because it extends the time range for liquid water on Mars, and the places where it might have supported life," said Scott Murchie, from Johns Hopkins University Applied Physics Laboratory in Maryland.
Dr Murchie, who is chief scientist on the CRISM team, added: "The identification of opaline silica tells us that water may have existed as recently as two billion years ago."
MRO has been orbiting the Red Planet since 2006
CRISM works by "reading" over 500 colours in reflected sunlight to detect particular minerals on the Martian surface - including those that formed in the presence of water.
Until now, only two major groups of hydrated minerals, phyllosilicates and hydrated sulfates, had been observed by spacecraft orbiting Mars.
Clay-like phyllosilicates formed more than three-and-a-half billion years ago where igneous rock came into long-term contact with water. During the next several hundred million years, until about three billion years ago, hydrated sulfates formed from the evaporation of salty and sometimes acidic water.
The newly discovered "opaline silicates" are the youngest of the three types of hydrated minerals.
They formed where liquid water altered materials created by volcanic activity or meteorite impacts on Mars' surface. One such location is the large Martian canyon system known as Valles Marineris.
"We see numerous outcrops of opal-like minerals, commonly in thin layers extending for very long distances around the rim of Valles Marineris and sometimes within the canyon system itself," said Ralph Milliken of Nasa's Jet Propulsion Laboratory (JPL) in Pasadena, California.
In some locations, CRISM observed opaline silica with iron sulfate minerals, either in or around dry river channels.
This suggests the acidic water remained on the Martian surface for an extended period of time. Dr Milliken and his colleagues think that in these areas, low-temperature acidic water was involved in forming the opal.
However, in areas where there is no clear evidence that the water was acidic, deposits may have formed under a wide range of conditions.
"What's important is that the longer liquid water existed on Mars, the longer the window during which Mars may have supported life," said Ralph Milliken.
"The opaline silica deposits would be good places to explore to assess the potential for habitability on Mars, especially in these younger terrains."
Nasa is due to send a robotic rover, Mars Science Laboratory (MSL), to the planet in 2009 to look for signs of past or present life.
The European Space Agency (Esa) also plans to send a rover to investigate Mars' habitability. This mission, called ExoMars, is now scheduled to launch in 2016.