The first in-situ chemical measurements ever obtained of a rock on Mars - nicknamed Barnacle Bill for its rough, barnacle-like surface - surprised scientists and raised questions about the duration of volcanic activity occurring on Mars in its early formation.
Dr. Rudolph Rieder, of the Max Planck Institute for Chemistry, Germany, and principal investigator on the Alpha Proton X-Ray Spectrometer (APXS) team, reported that Barnacle Bill, an 8-to-10-inch tall rock near the Mars Pathfinder lander, was unusually rich in silicon, which is more characteristic of Earth rocks than Martian rocks. On Earth, volcanic rocks contain significant amounts of free silica in the form of quartz. The rich silicon content puts Barnacle Bill in one of the most common categories of volcanic rocks on Earth, known as "andesites."
"It turns out this rock has some rather peculiar chemical characteristics, which make it very unlike the other SNC meteorites," said Dr. Hap McSween, University of Tennessee, who is a participating scientist on the APXS team. (The SNC meteorites are those found on Earth that are believed to be of Martian origin.)
"In particular, it has a very high content of silicon or silicon dioxide (quartz)," McSween said. "It appears that Barnacle Bill falls into a category called 'andesites,' which are among the most common volcanic rocks on Earth."
Andesites are mixtures of very fine crystalline and other minerals that are formed through a process known as differentiation. Differentiation is the process by which crustal materials deep within a planet's interior are repeatedly melted and remelted, thereby shaping and reshaping the surface of the planet. Mars today has very few volcanoes and no continental plates like those found on Earth to suggest it was internally active for very long. Barnacle Bill's chemical signature may throw that theory into question.
Today's weather report was similar to yesterday's: at 3 p.m. local Mars time, it was about 5 degrees Fahrenheit, pressure was about 6.74 millibars, with very light winds out of the northwest.
"The weather on Mars is pretty boring," said Dr. Jeffrey Barnes, Oregon State University, who is a member of the atmospheric/meteorological experiment. "Northern summer in the subtropics on Mars is pretty much the same from day to day. Fifty or 60 days from now, we'll start to see dramatic changes with fall."
Atmospheric opacity - or how clear the sky is according to Pathfinder's atmospheric experiment - showed that Mars is moderately dusty up to about 40 kilometers (25 miles) above the surface. The dust appears to be uniformly distributed, and is expected to rise as Mars approaches its dusty season in the fall, Barnes said. The visibility on Mars was estimated to be about 32 kilometers (20 miles) or more, roughly equivalent to a moderately smoggy day in Los Angeles.
The rover's next task later today will be to perform a chemical analysis of the soil around a large rock named "Yogi." Once the soil measurements are taken, Sojourner will then back up to the left side of the rock and begin a chemical analysis using the APXS instrument.
On the fifth day of surface operations since Pathfinder's historic July 4 landing, all spacecraft and rover systems continue to operate extremely well. Pathfinder is returning data at an unprecedented rate of more than 8,500 bits per second and has returned 1,575 images of the Martian surface to date. A 360- degree, color panorama of the Ares Vallis landing site is expected to be released within the next few days.