Mars Atmosphere Gradually Lost to Space, Curiosity Rover Data Shows

Evidence delivered by the Mars Rover Curiosity instruments support the theory that Mars lost much of its original atmosphere by a process of gas escaping from the top of the atmosphere. Rover team members reported diverse findings on Monday at the European Geosciences Union 2013 General Assembly, in Vienna.

The Sample Analysis at Mars (SAM) instrument analyzed an atmosphere sample last week using a process that concentrates selected gases. The results provided the most precise measurements ever made of isotopes of argon in the Martian atmosphere. Isotopes are variants of the same element with different atomic weights. "We found arguably the clearest and most robust signature of atmospheric loss on Mars," said Sushil Atreya, a SAM co-investigator at the University of Michigan, Ann Arbor.

The evidence comes in the form of argon, one of the most Universe's most unreactive elements, so it doesn't disappear and hide by forming molecules with other elements and concentrating somewhere. The ratio, coming from the fundamental element mixture that formed all objects of our solar system, should be 5.5 argon-36 atoms for every one argon-38 atoms -- this ratio is found on the gas planets and the Sun.

But Mars was now confirmed to only have a ratio of 4.2, leading Atreya and his colleagues to conclude that the theory of a gradual atmosphere loss is very likely because it indicates that the lighter atoms in Mars' atmosphere are slowly filtering out and abandoning the heavier ones as they head out into space.

"We've been seeing the same kind of behavior in the carbon dioxide isotopes and the water isotopes - they're all telling us the same story; that gases have been escaping from Mars over time, and the argon isotope just really nails it," Atreya said.

Quite simply, there is nowhere else the essentially unreactive element could have gone, Atreya explained, since it doesn't respond to anything and Mars had every reason to start off with the same ratios as all other planets.

Curiosity also measures several variables in today's Martian atmosphere with the Rover Environmental Monitoring Station (REMS), provided by Spain. While daily air temperature has climbed steadily since the measurements began eight months ago and is not strongly tied to the rover's location, humidity has differed significantly at different places along the rover's route. These are the first systematic measurements of humidity on Mars.

Possible interchange of water molecules between the atmosphere and the ground is studied by a combination of instruments on the rover, including the Dynamic Albedo of Neutrons (DAN), provided by Russia under the leadership of DAN Principal Investigator Igor Mitrofanov.