NASA - National Aeronautics and Space Administration

Blowing off steam can tell observers a lot about a situation. Take for example Home Plate on Mars.

Scientists have found evidence that Home Plate at Gusev crater on Mars is composed of debris deposited from a hydrovolcanic explosion. The finding suggests that water may have been involved in driving an eruption that formed the deposits found on Home Plate.

In a paper published in the May 4, 2007, issue of the journal Science, the team of scientists reports that Home Plate is composed of debris deposited from a hydrovolcanic explosion that occurred when molten volcanic basalt came into contact with subsurface fluid, perhaps brine. Steve Squyres of Cornell University is the lead author of the paper.

Image to right: NASA's Mars Exploration Rover Spirit acquired this high-resolution view of intricately layered exposures of rock while parked on the northwest edge of the bright, semi-circular feature known as "Home Plate." The rover was perched at a 27-degree upward tilt while creating the panorama, resulting in the "U" shape of the mosaic. (A) The northern edge of Home Plate, (B) the coarse-grained lower unit, (C) the fine-grained upper unit. Spirit acquired 246 separate images of this scene using 6 different filters on the panoramic camera (Pancam) during the rover's Martian days, or sols, 748 through 751 (Feb. 9 through Feb. 12, 2006). The field of view covers 160 degrees of terrain around the rover. Credit: NASA/JPL-Caltech/USGS/Cornell

Home Plate may be an area where heat, energy and water -- key ingredients for life -- may have existed.

"If you find areas where energy, heat and water are present, you have more likelihood for the formation or the existence of life," said Doug Ming, participating scientist on the Mars Exploration Rover (MER) mission and one of the co-authors of the paper. "These are the places to go to look for potential past indications of life. And that’s why this is so important.”

Home Plate is a light-toned, quasi-circular plateau 90 meters in diameter within the Inner Basin of the Columbia Hills at Spirit’s landing site in Gusev crater. Images of Home Plate show a thick stack of layered rock, with a lower coarse-grained unit and an upper finer-grained unit.

Scientists believe the lower unit represents accumulation of pyroclastic materials, whereas the upper unit may represent reworking of the same pyroclastic materials by weathering or other alteration processes. Scientists believe the pyroclastic materials were formed by fragmentation as a result of volcanic action.

"Pyroclastic material is volcanic ash coming down in layers," said Dick Morris, co-investigator on the MER mission and one of the co-authors of the paper. "When the volcano switches back and forth between erupting quickly or not so quickly, you’ll get these layers forming. Materials on the lower unit were deposited during the actual volcanic pyroclastic event.”

A particularly notable feature in the lower unit is a clast with deformed layers beneath it, interpreted to be a bomb sag. Bomb sags are found in volcanic deposits on Earth, where large clasts, or fragments of rock, ejected from an explosive event are emplaced ballistically into deformable materials, causing depression of layering. Bomb sags on Earth typically indicate that the deformed materials were wet at the time of emplacement, but the scientists cannot rule out the possibility of sag formation resulting from compaction and gas-supported flow of dry materials.

Image to left: "Home Plate" is viewed from orbit. North is at the top, and the path followed by the rover Spirit is shown. This image was taken by the Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment camera on Nov. 22, 2006. Spirit parked at "Low Ridge" with an 11-degree northerly tilt to maximize sunlight on the solar panels during the southern winter season. Credit: NASA/JPL-Caltech/Univ. of Arizona

The multilayered composition seen in Home Plate, as well as the bomb sag, indicate an explosive process in its origin. Also lending to this theory is the high concentration of chlorine and other volatile elements detected in Home Plate rocks.

“We see a lot of higher concentrations of some of the more volatile elements such as chlorine, bromine and zinc," said Ming. "We also see another element we haven’t seen very often in the past—germanium--and that’s kind of unusual. These elements are often linked to volcanic processes, but the chlorine enrichment suggests an interaction of the volcanic rocks with a salt-rich fluid, or brine."

In addition, loose rocks atop Home Plate include the small, irregularly shaped rock Fuzzy Smith. It has the highest silica content reported to date on the mission. Scientists believe hydrothermal processes "bleached" out the primary elements in the rock, leaving the silica behind.

Ming and Morris are NASA planetary scientists in the Astromaterials Research and Exploration Science Directorate at Johnson Space Center in Houston.

To view the Science paper online, visit:

www.sciencemag.org/

For images and information about the rovers, visit:

http://www.nasa.gov/rovers