ASU Research E-Magazine: In Search of Martian Oceans

Surf’s up…on Mars? Perhaps millions of years ago.

Thanks to NASA’s unmanned planetary exploration program, evidence of the existence of past oceans on Mars has been accumulating for years. But what was the chemical composition of those oceans? No one has been able to say—until now.

ASU scientists and colleagues provided the answer based on their recent analysis of the interior of a 1.2 billion-year-old Martian meteorite known as the Nakhla Meteorite. Their findings indicate that ancient Martian oceans had a chemical composition similar in variety and concentration to Earth oceans. The analysis detected the presence of water-soluble ions. The researchers think that the ions may have been deposited in cracks by evaporating brine.

Carleton Moore, ASU Regents Professor of Chemistry and Geology, led a team that included Douglas Sawyer of Scottsdale Community College, ASU graduate student Michael McGehee, and Julie Canepa of Los Alamos National Laboratory. The researchers published their findings in the July 2000 issue of the journal Meteoritics and Planetary Science.

“We have concluded that we have extracted salts that were originally present in Martian water,” Moore says. “The salts we found mimic the salts in Earth’s ocean fairly closely.”

Moore is the director of ASU’s Center for Meteorite Studies. He decided to examine the ion content of Martian meteorites in ASU’s sizable meteorite collection, when he noticed an oddity in chemical analyses done by Canepa, then a graduate student at ASU, 15 years ago.

“She studied chlorine and sulfur in basalts from all over the solar system, including the moon, the Earth, and the meteorites. At the time, we didn’t realize that some of the meteorite basalts came from Mars,” Moore explains. “Then one day I realized that some of the meteorites were high in chlorine and some were low in chlorine. When I checked on it, it turned out that all the high chlorine meteorites were Martian meteorites and the low chlorine meteorites were all asteroidal.”

The now-famous study of Martian meteorite ALH84001 helped Moore make a second connection: “When the study of this meteorite revealed not just possible evidence of life but also the presence of salts, I said to myself ‘Aha! Perhaps our meteorites’ chlorine is the remains of salt that had gotten into the meteorites.’ If this was so, it would most likely be from salt water that had leaked in through cracks in the Martian rock the meteorites came from.”

Moore chose the Nakhla meteorite to test, since it had the highest chlorine content of all those in the survey. Nakhla is named for El-Nakhla in northern Egypt, where it was found following a meteorite shower in 1911.

“We had a very nice piece of the Nakhla meteorite, about the size of a golf ball. The piece provided a clean, uncontaminated interior for us to study,” Moore says. Sawyer and McGehee prepared the meteorite and carefully drilled into its interior.

Moore tested for chlorine using an ion chromatograph first on the sample and then on water to which the sample was later exposed. The results showed that a high percentage of the element present was water-soluble. That told the scientists the element had probably originated from a water solution—from salt water.

“We tested for other elements and found the highest concentrations of negative ions were chloride, sulfate, fluoride, and a little dissolved silica. We found positive ions of sodium, magnesium, and calcium,” Moore says.

“The elements in highest abundance were sodium and chloride—like the salt water on Earth. In ocean water, these are the predominant ionic elements. We are interpreting the elements that we have extracted as having come from an early Martian ocean.”

The only significant difference between the ionic elements Moore found in the Martian rock and those found in Earth ocean water was the abundance of calcium, which was significantly higher in the Nakhla meteorite than in seawater.

Moore points out, however, that the lower calcium concentration in seawater may be due to the mineral being removed biologically by plants, corals. and shellfish. When the Nakhla meteorite was ejected from Mars 1.2 billion years ago, life on Earth had not yet evolved to these higher forms (shells only appear in the fossil record about 600 million years ago).

To Moore, the finding is interesting because it implies a chemical similarity between the planets that may improve the likelihood of finding life on Mars. It also provides a window of sorts into the Earth’s own past.—James Hathaway