WASHINGTON, Oct. 28 (Xinhua) -- U.S. Mars Reconnaissance Orbiter has observed a new category of minerals spread across large regions of Mars, which suggests liquid water remained on the planet's surface 1 billion years later than scientists believed, the U.S. space agency NASA announced Tuesday.

    Researchers examining data from the orbiter's imaging spectrometer for Mars found evidence of hydrated silica, commonly known as opal. The hydrated, or water-containing, mineral deposits are telltale signs of where and when water was present on ancient Mars.

    "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, the spectrometer's principal investigator at Johns Hopkins University. "The identification of opaline silica tells us that water may have existed as recently as2 billion years ago."

    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 3.5 billion years ago where igneous rock came into long-term contact with water. During the next several hundred million years, until approximately3 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 impact on the Martian surface.

    Their findings will appear in the November issue of Geology. The study reveals that the minerals, which also were recently found in Gusev Crater by NASA's Mars rover Spirit, are widespread and occur in relatively young terrains.