In relatively short order, the Mini Cooper-size rover answered in broad terms the key question the mission's scientists wanted to answer: Was the environment in Gale Crater, the rover's exploration zone, potentially hospitable for life?
Curiosity found evidence that liquid water accumulated or flowed across the crater floor for extended periods, and hints contained in minerals and rocks suggest the answer to the mission's overarching question is "yes."
At this point, Curiosity is now 406 sols into its mission, but researchers are tying a bow around the first phase of the mission by publishing formal reports on the early findings in five papers in Friday's issue of the journal Science, as well as a paper set for publication in the Journal of Geophysical Research: Planets.
Sol 100 represents a geological turning point in the mission, where Curiosity's focus shifted from looking at soil and loose material on the surface to exposed bedrock, says John Grotzinger, a planetary scientist at the California Institute of Technology in Pasadena, Calif., and the mission's project scientist. The papers "wrap this up very nicely."
One of the most surprising discoveries involved a pyramid-shaped rock some 20 inches tall the team named Jake Matijevic, after an engineer who played a key role in designing Curiosity but who passed away shortly after the craft landed on Mars on Aug. 6, 2012.
Analysis of the rock, detailed in one of the five Science papers, showed that while it had a volcanic origin, its composition had never been seen before on Mars.
If the rock had been found on Earth, "we would have been hard-pressed to tell from its whole-rock chemical composition that it is Martian," wrote Edward Stolper, a CalTech geologist who led the team analyzing the rock. Instead, its composition was comparable to the the basaltic rock found along mid-ocean ridges or on volcanic islands in the ocean, where magma wells up from Earth's mantle and forms fresh crust.
Based on its chemistry, the rock would have been right at home among the volcanic rocks found on Tenerife, the largest of the Canary Islands, the team found.
The remaining papers in Science and Journal of Geophysical Research focused on the Martian soil – one of the science team's high-priority targets. Getting a picture of Martian geology is crucial to eventually piecing together Martian history.
Missions ranging from Viking to the Mars Phoenix Lander have analyzed Martian soil, but not with instruments as capable as those on Curiosity. Understanding the composition of the fine-grained dust at Gale Crater opens a window on the composition of the dust planetwide, since it has been blowing across Mars for hundreds to perhaps a billions years, Dr. Grotzinger says. It's thought to be a good surrogate for the composition of the Martian crust as whole.
Among the discoveries about the Martian soil:
- Teams analyzing the soils found that the coarser material matched the composition of some of the loose rock and rubble Curiosity encountered during its early travels – material that was dominated by lighter elements – while the finer dust was rich in iron and magnesium.
- From 30 to 45 percent of the particles in the soil samples lacked any structure, compared with the rest, which had high concentrations of minerals in crystalline form.
- When some of the finer silts were heated in the Sample Analysis at Mars (SAM) laboratory inside Curiosity's chassis, they released water vapor, sulfur dioxide, carbon dioxide, and molecular oxygen. Another instrument, ChemCam, also detected a spike in hydrogen in the soils, leading researchers to estimate that the soil contains between 1.5 percent and 3 percent water by weight.
As if to add a cautionary note to Curiosity's explorations, the rover's instruments uncovered perchlorate salts, which can destroy the very organic compounds that scientists are looking for as soil samples are being heated and tested.
“The presence of perchlorates isn’t good news for some of the techniques we’re currently using with Curiosity,” said Daniel Galvin, a researcher at the Goddard Space flight Center in Greenbelt, Md., in a prepared statement. “This may change the way we search for organics in the future on Mars.”
It's possible Curiosity's experiments can be tweaked to reduce the impact of perchlorates on the results. Researchers will be exploring such tweaks in the near future, he suggested.
The team's results also point to Earth as a likely source for some of the chlorine-based hydrocarbons that appeared during efforts to analyze the soil samples. The chlorine was Martian, but there's no definitive evidence that the carbon-based components of those hydrocarbons are native to Mars, Dr. Glavin's team notes. One in particular, benzene, could have hitched a ride from Earth, the team notes.