How an obscure mineral provided a vital clue to Martian water

These images, taken by cameras on the Mars rover Opportunity, show a close-up of the rock outcrop dubbed "El Capitan," located in the rover's landing site, a crater at Meridiani Planum. Inset, a detail of the rock showing one of the tiny spherules, nicknamed "blueberries."

PASADENA, Calif. -- On the southeastern coast of Spain, the Sierra Almagrera range provides a bounty for geologists. One area, in particular, the Jaroso ravine, has yielded a huge catalog of unusual minerals. Among them is one that will be forever tied to Martian history.

In 1852 a German mineralogist discovered an unusual amber-yellow-brown mineral made of potassium iron sulfate hydroxide in Jaroso. He named the mineral jarosite.

Since then the world has had little use for jarosite. Until now.

On Tuesday, March 2, Cornell's Steven Squyres, principal investigator on the twin-rover Mars mission, told a press briefing at NASA headquarters in Washington, D.C., that his team had found jarosite on Mars. Since the mineral only forms in dilute sulfuric acid in ground water, the discovery was a clear indication that water once abounded in the area around the rover Opportunity's landing site in a crater on a vast plain called Meridiani Planum.

This modern voyage of discovery started in NASA's Jet Propulsion Laboratory (JPL) on Jan. 25, the day following the rover Opportunity's landing, when Jim Bell, Cornell associate professor of astronomy and the scientist in charge of the two rovers' panoramic cameras, received the rover's first color image of the crater in which it had landed.

When the image appeared on television monitors in JPL's von Karman auditorium at 2 a.m., Squyres reacted by saying, "This is the first outcrop ever found on Mars." Bedrock outcrops, he pointed out, usually provide strong clues to geologic history.

Squyres was prophetic. Beneath the dusty veneer and the rocky crust, jarosite awaited.

For the next few weeks, Opportunity cruised around the crater while JPL scientists and engineers tested the rover's platoon of Cornell-built and Cornell-partnered geologic tools.

By Feb. 20, or Martian day (sol) 27, the rover examined the outcrop, now dubbed El Capitan, with its panoramic cameras, miniature thermal emission spectrometer (Mini-TES) and microscopic imager.

The following day Opportunity placed its Mössbauer spectrometer and its alpha particle X-ray spectrometer (APXS) on the rock surface to assess mineral presence.

Opportunity performed its first rock abrasion tool (RAT) operation on Feb. 24 on a rock target known as McKittrick Middle Rat at El Capitan. The tool shaved the rock over a period of two hours, grinding into a total depth of about 4 millimeters (.16 inches).

After the abrasion tool retracted, the scientists took microscopic images of the hole, and the APXS was later pointed inside the rock.

"Finding evidence of water hasn't been an 'Aha!' moment," said Bell. "It's been a series of data sets building in our minds. The measurements trickle in and we wait for data. Then we interpret the data, throw ideas around, reach a consensus and we get a snapshot of the consensus."

During the Martian morning hours of Opportunity's sol 31 on Feb. 25, the APXS and the Mini-TES took measurements from inside the hole for five hours. Later in the day, the rover swapped the APXS for the Mössbauer spectrometer, and it continued to collect data during a "leisurely 24-hour observation," according to JPL.

On sol 32 on Feb. 26, the Mossbauer continued to examine the hole for spectral signatures of iron-bearing minerals.

This led the science team to discover gray spheres, dubbed "blueberries," which had likely been solidified from a water source.

When all the data was in, the APXS had detected large amounts of sulfur and the Mössbauer had detected jarosite, a finding that the late Roger Burns, a geologist at the Massachusetts Institute of Technology, had predicted several years ago.

The last piece in this early stage Martian water puzzle fit, Squyres realized, when the last Mössbauer data returned Friday, Feb. 27. Immediately NASA officials began working with him to organize the March 2 press conference.

"Most of the scientists went into this mission armed with hopes and prejudices," said Squyres. "It's been fun over the past few weeks to watch the puzzle come together right before my eyes."

The jarosite evidence for the existence of water will not be the last finding the science team makes. The rovers have many meters to go and many sols remaining. Squyres, smiling before the media in Washington, D.C., said: "So far the mission has been very gratifying, we've enjoyed working one centimeter at a time. But we're just getting started."