Examination of the Martian atmosphere by NASA's Curiosity Mars rover confirms that some meteorites that have dropped to Earth really are from the Red Planet. A key new measurement of the inert gas argon in Mars' atmosphere by Curiosity's laboratory provides the most definitive evidence yet of the origin of Mars meteorites while at the same time providing a way to rule out Martian origin of other meteorites. The new measurement is a high-precision count of two forms of argon accomplished by the Sample Analysis at Mars (SAM) instrument inside the rover. These lighter and heavier forms, or isotopes, of argon exist naturally throughout the solar system. On Mars the ratio of light to heavy argon is skewed because much of that planet's original atmosphere was lost to space. The lighter form of argon was taken away more readily because it rises to the top of the atmosphere more easily and requires less energy to escape. That left the Martian atmosphere relatively enriched in the heavier isotope, argon-38.
One reason scientists have been so interested in the argon ratio in Martian meteorites is that it was -- before Curiosity -- the best measure of how much atmosphere Mars has lost since the planet's wetter, warmer days billions of years ago. Figuring out the planet's atmospheric loss would enable scientists to better understand how Mars transformed from a once water-rich planet, more like our own, into today's drier, colder and less-hospitable world.
While argon makes up only a tiny fraction of the gas lost to space from Mars, it is special because it's a noble gas. That means the gas is inert, not reacting with other elements or compounds, and therefore a more straightforward tracer of the history of the Martian atmosphere.