[meteorite-list] Rover Spirit Finds Evidence of Early Martian Volcanic Activity - And Further Hints of a Watery Past

[Ron Baalke]

Chronicle Online e-News

Rover Spirit finds evidence of early Martian volcanic activity at 
Home Plate plateau -- and further hints of a watery past

http://www.news.cornell.edu/stories/May07/squyres.pyroclastic.html

May 8, 2007

By Lauren Gold
Cornell University
LG34 at cornell.edu

A plateau on Mars known as Home Plate shows evidence of long-past 
explosive volcanic activity, say scientists on NASA's Mars 
Exploration Rover mission. And data collected during the rover 
Spirit's initial pass across the 90-meter (295 feet) wide plateau 
also supports earlier findings indicating that water once existed at 
or beneath the planet's surface.

The research appeared in the May 4 issue of the journal Science.

Home Plate's finely layered appearance made it one of the most 
tantalizing targets within Spirit's reach in Gusev Crater, said Steve 
Squyres, the mission's principal investigator and the Goldwin Smith 
Professor of Astronomy at Cornell. The rover captured its first 
panoramic image of Home Plate in August 2005 from the summit of 
Husband Hill and reached the plateau in the Columbia Hills' inner 
basin in February 2006.

It quickly sent back an image Squyres called "one of the neatest 
pictures we've taken with the rovers." The image shows a small (4 
centimeter) rock fragment nestled within a downward deflection in 
otherwise ruler-straight lines of layering -- a feature likely to be 
what geologists call a bomb sag. These usually form when a rock 
fragment (the bomb) is thrown upward in an explosion; then lands in 
deformable material, causing the material to sag beneath it.

Chemical analysis shows the rock is made of the same material 
(basalt) as volcanic rocks around it, indicating the explosion was 
not the result of an impact by an exotic source (such as a 
meteorite). The rock also shows tiny spherical particles that look 
like accretionary lapilli -- coagulated bits of ash that typically 
rain down after a volcanic explosion.

Any volcanic activity at Home Plate probably happened billions of 
years ago -- but part of what makes it intriguing, said Squyres, is 
its similarity to regions on other parts of the planet.

"There are lots and lots of places on Mars where, from orbit, you see 
layered deposits locally that kind of look like this," said Squyres, 
"and so it really raises the possibility that a lot of these things 
all over the planet could be explosive volcanic deposits."

That the rocks at Home Plate are basalt -- not a material normally 
associated with explosions -- also hints that water was involved. 
"When basalt erupts, it often does so as very fluid lava, rather than 
erupting explosively," Squyres said. But a notable exception comes 
when hot basalt meets water to cause a steam-driven explosion.

The bomb sag -- now dry, but shaped as if the rock sitting in it 
landed with a splat instead of a thud -- is a second hint that the 
surface was once wet. A third is the material's high chlorine 
content, which may point to past exposure to a briny fluid.

Home Plate may be the site of an early impact crater that was later 
filled in by volcanic debris, according to stereo images that show 
the layered rocks around its edge all sloping in toward the plateau's 
center. Billions of years of erosion could have stripped away the 
surrounding material but left the debris protected by the crater's 
rim -- resulting in the current plateau.

The Science paper is based on data collected during a frenetic few 
months in 2006, as Spirit was chugging down the Columbia Hills toward 
a safe place to ride out the Martian winter.

The route to safety included a path across Home Plate -- leaving 
Spirit's drivers on Earth with a dilemma.

"There was all this fabulous science around us," Squyres said. But 
with winter approaching, the team had the harrying task of getting 
Spirit to its destination in time, while gathering as much data as 
possible along the way. "We got an amazing amount of science done, 
all things considered," he said. "But there's more work to be done 
here." Spirit is now back at Home Plate, continuing exploration there.

Another sol, another discovery ... (and no one's yawning)

A year after Spirit first reached Home Plate plateau, the rover and 
its twin Opportunity are still healthy and plugging away.

Spirit, having made it through the winter, is back for a second pass 
at Home Plate (now driving ably on five wheels after its right front 
wheel died last year). And the baseball theme for names at Home Plate 
continues: Last year, rocks in the area were named to honor players 
from the Negro Leagues of the early 20th century; discoveries this 
year celebrate women from the All-American Girls Professional 
Baseball League of the 1940s and '50s.

Meanwhile, Opportunity, on Meridiani Planum on the opposite side of 
the planet, has been exploring the rim of Victoria Crater and is now 
heading back to an alcove called Duck Bay. From there, it will look 
for a place to start the tricky descent into the crater.

Both rovers are well past sol 1,100 of their 90-sol warranty (a sol 
is a Martian day). Mission members in Ithaca and at NASA's Jet 
Propulsion Lab in Pasadena have settled into a routine Squyres calls 
"a sort of Earth time-Mars time hybrid." And there's always more to 
do.

"We now have an operations concept that is sustainable in the long 
run," said Squyres. "We can keep doing this as long as we need to."
--