August 20, 2013
It has been over a year since NASA's Curiosity rover landed on Mars. One of its objectives was to assess Martian atmospheric processes and to tell us why and how did Mars lose its atmosphere. Thanks to its two instruments based on different techniques, the Curiosity has managed to find out what happened to the Red Planet.
View of the Martian surface from the Curiosity rover.
Mars' atmosphere today is hundred times thinner than Earth's, although it must have been more dense before, for the first billion of Mars' 4.5 billion years. The water is believed to have been present on Mars at that time and this indicates that the air must have been around for the same time period. However, with Mars having only 11% of Earth's mass and 38% of its gravity, it was just a matter of time before Mars began losing its original atmosphere. There are couple of way this could happen to a planet.
One mechanism is when elements of the atmosphere bond with the soil. Meteorites that landed made their way from Mars' surface to Earth have often included gas bubbles, giving evidence that some of Mars' atmosphere retreated into the soil. Another possibility is that the planet lost its atmosphere by giving it away into the vacuum of space, without the gravity strong enough to keep it. As the atmosphere became thinner, meteors reached the surface with ever greater ease, bombarding the surface with greater force, thus blasting out more and more atmosphere into the space.
Curiosity's Sample Analysis at Mars (SAM) instruments were used to settle the matter. SAM is a collection of sensors that collect the air and analyse its chemical makeup, particularly the mix of isotopes. This is important because elements don't appear in just one form. Carbon 12 and carbon 13, for example, have a different number of neutrons in the nucleus and differ in weight because of that. Elements weight is crucial in studying atmosphere, since heavier gases remain close to the surface, while lighter ones take their place above them. Previous measurements of Mars' current atmosphere had always shown a high concentration of the heavy isotopes of carbon and oxygen. This is something unique to Mars as it is today, because the isotope mix of the sun and the early solar system as a whole had more lighter isotopes. This means that Mars would have started with the same isotope mix as other planets and since the heavy isotopes dominate its atmosphere today, it means that lighter, high-altitude gases had since then been lost to the vacuum of space.
"As atmosphere was lost, the signature of the process was embedded in the isotopic ratio," said Paul Mahaffy, principal investigator for the SAM team. This was the theory before Curiosity finally confirmed it with SAM. Its results are considered reliable because Curiosity used two instruments that are based on two different systems. One is the tunable laser spectrometer that analyses how two different frequencies of infrared laser are reflected by the air previously pumped into a chamber. The other is the mass spectrometer that measures the entire spectrum of elements according to their mass. “Getting the same results with two very different techniques increased our confidence that there's no known systematic error,” said NASA’s Chris Weber, lead author of one of the new papers published in Science.
It is thought that this atmosphere losing process is still ongoing, but in order to find out how fast this is happening, NASA will launch another probe in November, as a part of the Mars Atmosphere and Volatile Evolution (MAVEN) mission.