WASHINGTON, May 4 (Xinhua) -- NASA's Mars Rover Spirit has discovered evidence of an ancient volcanic explosion at "Home Plate" area on Mars, NASA scientists reported Friday in the journal Science.

    This is the first explosive volcanic deposit identified with a high degree of confidence by Spirit or its twin, Opportunity, scientists said.

    "Home Plate" is a plateau of layered bedrock approximately 2 meters high within the "Inner Basin" of Columbia Hills, at Spirit's landing site in Gusev Crater.

    One of the strongest pieces of evidence for an explosive origin for Home Plate is a "bomb sag" preserved in layered rocks on the lower slopes of the plateau. Bomb sags form in volcanic explosions on Earth when rocks ejected skyward by the explosion fall into soft deposits, deforming them as they land.

    Evidence shows the area near Home Plate is dominated by basaltic rocks. "When basalt erupts, it often does so as very fluid lava, rather than erupting explosively," said Steve Squyres, the principal investigator for the rovers' science instruments. "One way for basaltic lava to cause an explosion is for it to come into contact with water -- it's the pressure from the steam that causes it to go boom."

    Scientists suspect that the explosion that formed Home Plate may have been caused by an interaction of basaltic lava and water." When you look at composition of the rocks in detail, there are hints that water may have been involved," Squyres said. One example is the high chlorine content of the rocks, which might indicate that basalt had come into contact with a brine.

    Spirit arrived at Home Plate in February 2006 and spent several months exploring it in detail before driving to "Low Ridge" to pass the Martian winter. Spirit has now returned to Home Plate to continue exploration there. Spirit's continued exploration of HomePlate will focus largely on testing the idea that water was involved in its formation process.