Meteorites blasted off Mars have provided indications of the early Martian atmosphere, including signs of how its differences with that of the early Earth emerged.
When large asteroids strike a planet they can blast rocks off the surface hard enough to achieve escape velocity. The smaller the planet the more likely this is to occur, so quite a few objects have lifted off Mars over its lifetime, and some eventually make it to Earth and survive entry into our solar system. So far 69 meteorites have been found that are believed to have come from Mars
Past analysis of some of these has triggered controversial claims about water and even life on Mars. While these remain in dispute Curiosity confirmed last year which asteroids are actually of Martian origin.
Now the largest study ever conducted on Martian meteorites, taking in 40 of them, has been published in Nature. The authors concluded that sulfur was incorporated into Martian magma in very different ways to on the early Earth, suggesting the atmospheric chemistry of the two planets was quite distinct, even before abundant life transformed the gas shroud around the third rock from the sun.
The use of so many asteroids allows us a longitudinal insight into Mars' evolution. The senior example is 4.1 billion years old, while the youngest are a mere 200-500 million years old.
Mars has a lot of sulfur; one theory holds that sulfur dioxide, probably of volcanic origin, warmed Mars in its early years enough to allow liquid water to exist at the surface. This would explain how Mars could once have been warm enough to allow water, despite the sun at the time having put out less heat than it does today, and there being no sign of the limestone deposits we would expect if there was a runaway greenhouse effect based on carbon dioxide. On Earth sulfur dioxide has a net cooling effect, as it condenses to aerosols that reflect more light than is trapped as heat, but the different circumstances on Mars at the time would have created the opposite effect.
By comparing the different sulfur isotopes incorporated into the rock, Dr Heather Franz of the University of Maryland was able to confirm that some of the sulfur in the meteorites had been in the Martian atmosphere before being deposited on rocks and incorporated into magma when nearby volcanoes erupted. However, the chemical reactions the sulfur had been through while in the atmosphere were different from those occurring on Earth at the same time, suggesting Mars had already lost most of its initial gasses.
"Climate models show that a moderate abundance of sulfur dioxide in the atmosphere after volcanic episodes, which have occurred throughout Mars' history, could have produced a warming effect which may have allowed liquid water to exist at the surface for extended periods," Franz said. "Our measurements of sulfur in Martian meteorites narrow the range of possible atmospheric compositions, since the pattern of isotopes that we observe points to a distinctive type of photochemical activity on Mars, different from that on early Earth."
The study provides mixed news on the burning question of life on Mars. Microorganisms on Earth process sulfur isotopes in ways that leave a distinct signature, and none of the Martian asteroids show similar signs. One the plus side, however, Franz's findings help explain how water could have existed on Mars, perhaps for substantial periods of time. In the light of other evidence to the contrary, the idea that Mars was wet enough for long enough for life to evolve helps keep hope alive.