Curiosity finds clues to lost atmosphere on Mars
Sen—NASA's Curiosity rover has provided evidence about the loss of much of Mars' original atmosphere.
Curiosity's Sample Analysis at Mars (SAM) suite of laboratory instruments inside the rover has measured the abundances of different gases and different isotopes in several samples of Martian atmosphere. Isotopes are variants of the same chemical element with different atomic weights due to having different numbers of neutrons.
SAM checked ratios of heavier to lighter isotopes of carbon and oxygen in the carbon dioxide that makes up most of the planet's atmosphere. Heavy isotopes of carbon and oxygen are both enriched in today's thin Martian atmosphere compared with the proportions in the raw material that formed Mars. This provides supportive evidence for the loss of much of the planet's original atmosphere and a clue to how the loss occurred.
"As atmosphere was lost, the signature of the process was embedded in the isotopic ratio," said Paul Mahaffy of NASA Goddard Space Flight Center. He is the principal investigator for SAM and lead author of one of the two papers about Curiosity's findings.
The enrichment of heavier isotopes measured in the dominant carbon-dioxide gas points to a process of loss from the top of the atmosphere, favoring loss of lighter isotopes, rather than a process of the lower atmosphere interacting with the ground.
Curiosity measured the same pattern in isotopes of hydrogen, as well as carbon and oxygen, consistent with a loss of a substantial fraction of Mars' original atmosphere. Enrichment in heavier isotopes in the Martian atmosphere has previously been measured on Mars and in gas bubbles inside meteorites from Mars.
Meteorite measurements indicate much of the atmospheric loss may have occurred during the first billion years of the planet's 4.6-billion-year history. Perhaps volcanic eruptions, or a massive collision caused the atmospheric loss. The Curiosity measurements reported this week provide more precise measurements to compare with meteorite studies and with models of atmospheric loss.
SAM's mass spectrometer and tunable laser spectrometer independently measured virtually identical ratios of carbon-13 to carbon-12.
Lab demonstration of the measurement chamber inside the Tunable Laser Spectrometer, part of the Sample Analysis at Mars investigation on NASA's Curiosity rover. Image credit: NASA/JPL-Caltech.
"Getting the same result with two very different techniques increased our confidence that there's no unknown systematic error underlying the measurements," said Chris Webster of NASA's Jet Propulsion Laboratory. He is the lead scientist for the tunable laser spectrometer and the lead author for one of the two papers. "The accuracy in these new measurements improves the basis for understanding the atmosphere's history."
Artist concept of MAVEN spacecraft. Image credit NASA's Goddard Space Flight Center.
The Curiosity measurements do not directly measure the current rate of atmospheric escape, but NASA's next mission to Mars, the Mars Atmosphere and Volatile Evolution Mission (MAVEN), will do so. "The current pace of the loss is exactly what the MAVEN mission now scheduled to launch in November of this year is designed to determine," Mahaffy said.
The Neutral Gas and Ion Mass Spectrometer, a network of electrically charged rods aboard MAVEN, will measure the composition of the current atmosphere and how variables like time of day change the gas particles over time. This critical information can then be used to build simulations of both the current Martian atmosphere and the atmosphere billions of years ago.