NASA's Mars Curiosity rover may have found evidence as to how Mars lost its atmosphere, and why it keeps thinning.

Curiosity's Sample Analysis at Mars (SAM) measured the concentration of different isotopes and gases in Mars' atmosphere, a University of Michigan press release reported.

"Isotopes are variations of the same chemical element that contain different numbers of neutrons, such as the most common carbon isotope, carbon-12, and a heavier stable isotope, carbon-13, which contains an additional neutron," the press release stated.

The rover measured ratios of lighter and heavier isotopes in the Martian atmosphere, along with oxygen levels.

Curiosity's measurements showed the "heavy isotopes of carbon and oxygen were more abundant in today's thin atmosphere compared with the proportions in the raw material that formed the planet." This suggests Mars has lost a lot of its atmosphere over the past billions of years.

The findings support the theory that Mars lost its atmosphere through the top of its environment.

"The isotope data are unambiguous and robust, having been independently confirmed by the quadrupole mass spectrometer and the tunable laser spectrometer, two of the SAM suite instruments," Sushil Atreya, professor of atmospheric, oceanic and space sciences at the University of Michigan said. "These data are clear evidence of a substantially more massive atmosphere, hence a warmer, wetter Mars in the past than the cold, arid planet we find today."

NASA is not planning on measuring the current rate of atmosphere loss on the red planet during the current mission, but the Mars Atmosphere and Volatile Evolution Mission (MAVEN) which will launch in November of this year will do just that, a NASA news release reported.

"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," Paul Mahaffy of NASA Goddard Space Flight Center, said.