No Hostage to Earth’s Geology

Pasadena, Sol 16

Spirit’s first rock targets, in field of near driving targets
Credit: NASA/JPL

"I started my career [studying] lunar samples," said Dr. Dave Des Marais, who is from NASA’s Ames Research Center and lead scientist for the Spirit rover’s long-range planning group. "So this is a repeat of that rewarding experience…Beyond the challenge of adjusting in the first week, the big challenges are in dealing with the outside world, being careful when calling people. We all wear two watches on our wrist–one for Mars’ time, the other for Earth. [For science, the challenge] is not to be hostage to having learned geology on Earth."

The alien landforms, and the martian history they may represent, have not yet succombed to a single explanation, but whether the rocks in Gusev crater may have originated predominantly from a volcano, impacts, or wind and water weathering is something Des Marais and the science team want to probe. Des Marais described the current plan as one to "extend the database with suitable rocks."

Having set their sights first on a particular mound-shaped rock called ‘Adirondack’ [see banner image], the planning team wanted to find a "representative rock," said Des Marais, describing what may be a volcanic or "basaltic rock similar to meteorites [from Mars] that have reached Earth." Since there are around 28 or so highly studied examples of martian meteorites, many of which have been analyzed by tens to hundreds of labs around the world, Des Marais said "from an analytical point of view, that was a good place to start. From there, we seek a broad range of rocks that may be reasonably close to where the rover is now, to flex our capabilities."

"This is very significant, our first drive on Mars since we egressed from the lander," said NASA’s Eddie Tunstel, Spirit’s mobility engineer, describing the first ten feet needed to get within a arm’s reach of Adirondack and the team’s first science target. Assessing the rover’s overall mobility at Gusev crater, Tunstel said it is a "pretty benign site. We might avoid rocks combined with sandy soil, so we don’t lose [wheel] traction, and [avoid] some of the slopes in the far distance. It is pretty wide open where we are," a flat feature of what may eventually prove to be an ancient crater lakebed.


The rover’s landing site at Gusev crater was predicted to have less than eight percent rock coverage, a desired smoothness to ease longer drives. After viewing the panoramic images, scientists have assessed the site to be even more favorable with around three percent. Previous landing sites for Viking and Pathfinder have averaged around six to seven times more rocks than Gusev, with up to twenty percent total coverage and significantly larger varieties.

Dr. Dave Des Marais: "My research interests extend to the new and exciting field of Astrobiology, which seeks to identify life in other planets, and to make predictions about what life forms future space missions may encounter. I am thus interested in current missions to Mars, and the wealth of information these probes are collecting regarding the extent of aqueous (caused by water) alteration of the Martian crust, as well as the search for fossil evidence of a Martian biosphere (that is: life on Mars). My field work in geology and biology has taken me to Australia, Canada, South Africa, and Mexico, in addition to many locations in the United States."
Credit: NASA/ARC

The science team may spend several martian days, or Sols, around Adirondack. "We may extend to late Wednesday," said Des Marais, "while using the Rock Abrasion Tool (RAT), Mossbauer and APXS", the primary diagnostics onboard the rover’s robotic arm. "From what we can see [of Adirondack], the rock has enough weight to apply the RAT tool to." Since the abrasive tool called RAT resembles a high-speed, cutting drill, the application of the RAT works best on weighty, stationary rocks to remove any dust covering.

The rover’s tool-carrying arm is called the Instrument Deployment Device (IDD) because it houses four diagnostic instruments used to answer question about Mars’ water history. The arm has a rotating head or ‘turret’, that can place various instruments into action against a rock, soil, or boulder: the Microscopic Imager, Mossbauer, Alpha Proton X-ray Spectrometer (APXS), and the Rock Abrasion Tool (RAT).

"There is interest in the pitting [on Adirondack] for example, and whether there is any larger crystalline structure," said Des Marais. "We want to grind into it and get a good pristine surface interior to the rock."

By placing the microscopic imager on this cleaned surface, the geologists can look for smooth, round grains as broadly supportive of a sedimentary origin or aqueous minerals, while glassy or crystalline grains indicate the rock may have been heated either by martian volcanoes or asteroid impacts.

That distinction between a watery or fiery origin for Mars strikes at the core of what Des Marais described as the challenge not being "hostage" to one’s terrestrial training. Despite many terrestrial analogs to a dry lake bed, nothing like Gusev would persist unweathered by biology and weather on Earth in quite the same wayw that Mars may have preserved these ancient rocks. If Adirondack turns out to be somewhat representative of martian meteorites, mainly having survived heating by volcanoes or impacts, differences will remain.

As Dr. Bill Hartmann of the Mars Global Surveyor team noted in his book ‘Traveler’s Guide to Mars’, such surface rocks may not be the most likely to become eventual meteorites on Earth: "computer simulations show that rocks launched from Mars by impacts would come from near the surface (top 50 meters?) but may favor fragments from intact lava layers instead of loose rocks."

Whether Adirondack is indeed a loose rock or part of a larger ‘iceberg-like’ subterranean boulder, Dr. Des Marais speculated about what may in fact lie beneath it. "The rock may extend beneath the surface, if pieces got knocked off when hit by something, which might be an impact. If [Adirondack] extends below soil depth, so much the better [for planned RAT cleaning]…Among the science team, there is some speculation about the left side of the rock, which has a yellowish caste, and what that is."

What’s Next

After exploring Adirondack for a few days, the science team wants to park the Spirit rover in a good position for a quiet series of diagnostics. There is a planned "three Sol shutdown for Opportunity’s arrival and the immediate aftermath," said Des Marais, referring to the drama of the second rover’s entry, descent and landing near the end of the week, around January 25-26. "We want to make sure we are where we would like to be, with the instrument deployment device [or robotic arm] out, examining fine grain soils. Then the Mossbauer instrument can look for small concentrations of chemical elements, in an extended observation."

Saying that the planning team may not see the need to visit the depression called Sleepy Hollow, Des Marais said they "have already looked at a number of fine grain materials, the darker soil materials were a good start," and share some of the same types of looser soil as the depression in the distance. "We would like to get some of the pinkish, light materials, which is even finer dust…That is a mini-drive to looser, sandy material and closer than Sleepy Hollow," although he noted that there are still members of the team advocating for the longer drive.

"We are asking the team to make a menu of next places to move," said Des Marais. After Wednesday or Thursday, one good place for Spirit might be within one day’s drive to a parking spot. Since long integration times using the Mossbauer instrument can last more than 20 hours, Spirit may be looking to get within a few meters of pinkish soil shortly, as its next place to go. As Dr. Charles Elachi, JPL’s Center Director, pointed out during the mission’s first night, each day for Spirit is like a new landing on Mars, because their laboratory is mobile.