26 June 2000

Meteorite Indicates Mars Had Oceans

by Kate Melville

Thanks to NASA's unmanned planetary exploration program, evidence of the existence of past oceans on Mars has been accumulating for years, but no one had ever been able to say what the overall chemical composition of those oceans might actually have been like -- until now.

A recent analysis of the interior of a 1.2 billion-year-old Martian meteorite known as the Nakhla Meteorite has shown the presence of water-soluble ions that are thought to have been deposited in cracks by evaporating brine, according to a study by Arizona State University Regents Professor of Chemistry and Geology Carleton Moore, Douglas Sawyer of Scottsdale Community College, ASU graduate student Michael McGehee and Julie Canepa of Los Alamos National Laboratory. The finding, announced in the July issue of the journal Meteoritics and Planetary Science, indicates that ancient Martian oceans had a chemical composition similar in variety and concentration to Earth oceans.

"We have concluded that we have extracted salts that were originally present in Martian water," said Moore. "The salts we found mimic the salts in Earth's ocean fairly closely."

Moore, who is the director of the ASU Meteorite Center, 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. 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."

Then the now-famous study of Martian meteorite ALH48001 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 so that there was a clean, uncontaminated interior for us to study," Moore said. Sawyer and McGehee prepared the meteorite and carefully drilled into its interior so to get a convincingly uncontaminated sample.

Using an ion chromatograph first on the sample and then on water to which the sample was exposed later, Moore tested for chlorine in both. The results showed that a high percentage of the element present was water soluble and therefore had probably originated from a water solution -- from salt water.

"Then we tested for the other elements and we found the highest concentrations of negative ions were chloride, sulfate, fluoride, and a little dissolved silica , and, in positive ions, sodium, magnesium and calcium," Moore said.

"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 Moore found between the ionic elements 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 sea water. 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 left 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 not just a chemical similarity between the planets that may improve the likelihood of finding life on Mars, but also because it provides a window of sorts into the Earth's own past.

"There was apparently a uniformity between the planets. The inference that the early Martian ocean was very similar to our current ocean also implies that the early Earth's ocean may have been very similar to what it is today. This is a clue to what it might have been."