A highly detailed study of 11 meteorites that have come from Mars shows that although 10 of them contain the organic element carbon, the source of the carbon is not biological in origin. Instead, the carbon lies as molecules inside oxide minerals that formed in molten magma. When the minerals crystallized, the heat and pressure were such as to rule out biology as a source for the carbon.
TOO HOT FOR LIFE. A proposed crystallization sequence, given in schematic form, shows various minerals including carbon (MMC), spinel (SP), and pyroxene (Px) crystalizing from igneous mantle rocks that erupt at the surface. (Image taken from Figure 3 in the paper.)
The study, published May 24 in Science Express, was carried out by a group of scientists led by geochemist Andrew Steele (Carnegie Institution of Washington).
“Ten of the meteorites,” the team writes, “contain abiotic macromolecular carbon (MMC) phases detected in association with small oxide grains included within high-temperature minerals.” Scientists have detected carbon in Mars meteorites before, but disagreed over how the carbon formed.
The meteorites in the study are known to be pieces of Mars because they contain trapped gases whose composition fits the Martian atmosphere. They have ages ranging from 4.4 billion years old (ALH84001) to 190 million years (Zagami). These ages record when the rock was last melted or severely shocked. (Three of the meteorites have undetermined ages.) Regarding how long ago each landed on Earth, the meteorites span a range of 60,000 years (Dar al Gani 476) to 6 months (Tissint); four of the 11 have no known terrestrial arrival date.
The question of terrestrial contamination looms large in any study of extraterrestrial samples. Steele’s team reports that the carbon detected was located deep inside the mineral crystals, making it unlikely to have gotten there after the meteorites landed on Earth.
“With a combination of transmitted and reflected light,” Steele and colleagues write, “we determined the distance from the oxides to the surface and confirmed their isolation from any visible cracks.”
As these molecules were found in Martian meteorites of such an large span of ages, their presence means that Mars has been making its organic compounds throughout its history and apparently continues to do so today.
The scientists note, “The youngest MMC-bearing meteorite (about 190 million years old), demonstrates that reduced carbon phases have been generated recently in Mars’ history, and therefore, the Martian reduced carbon budget was in flux during the late Amazonian, hinting that a true Martian carbon cycle may still be active.”
The researchers write, “Our results imply that primary organic carbon is nearly ubiquitous in Martian basaltic rocks. It formed through igneous, not biological, processes and was delivered over most of Martian geologic history to the surface as recently as the late Amazonian.” The Amazonian is the current Martian geologic era.
While Curiosity, NASA’s next Mars rover that’s due to land August 6, carries instruments to detect organic elements, the meteorite study suggests a caveat. As the team explains, because basaltic rocks are widespread on Mars, “a positive detection of organics by the Mars Science Laboratory…may be detecting this abiotic reservoir.”