Meteorites yield clues to Martian early atmosphere - Sulphur

A microscope reveals colorful augite crystals in this 1.3 billion-year-old meteorite from Mars, which researchers studied to understand the red planet's atmospheric history. 

Credit: James Day

Geologists who analyzed 40 meteorites that fell to Earth from Mars unlocked secrets of the Martian atmosphere hidden in the chemical signatures of these ancient rocks.

Their study, published April 17 in the journal Nature, shows that the atmospheres of Mars and Earth diverged in important ways very early in the 4.6 billion year evolution of our solar system.

The results will help guide researchers' next steps in understanding whether life exists, or has ever existed, on Mars and how water—now absent from the Martian surface—flowed there in the past.

Heather Franz

Heather Franz, a former University of Maryland (UMD) research associate who now works on the Curiosity rover science team at the NASA Goddard Space Flight Center, led the study with James Farquhar, co-author and UMD geology professor.

The researchers measured the sulfur composition of 40 Mars meteorites—a much larger number than in previous analyses. Of more than 60,000 meteorites found on Earth, only 69 are believed to be pieces of rocks blasted off the Martian surface.

The meteorites are igneous rocks that formed on Mars, were ejected into space when an asteroid or comet slammed into the red planet, and landed on Earth.

James Farquhar

The oldest meteorite in the study is about 4.1 billion years old, formed when our solar system was in its infancy. The youngest are between 200 million and 500 million years old.

Studying Martian meteorites of different ages can help scientists investigate the chemical composition of the Martian atmosphere throughout history, and learn whether the planet has ever been hospitable to life.

Mars and Earth share the basic elements for life, but conditions on Mars are much less favourable, marked by an arid surface, cold temperatures, radioactive cosmic rays, and ultraviolet radiation from the Sun.

Still, some Martian geological features were evidently formed by water – a sign of milder conditions in the past.

Scientists are not sure what conditions made it possible for liquid water to exist on the surface, but greenhouse gases released by volcanoes likely played a role.

Sulphur, which is plentiful on Mars, may have been among the greenhouse gases that warmed the surface, and could have provided a food source for microbes.

Because meteorites are a rich source of information about Martian sulphur, the researchers analyzed sulfur atoms that were incorporated into the rocks.

In the Martian meteorites, some sulphur came from molten rock, or magma, which came to the surface during volcanic eruptions.

Volcanoes also vented sulphur dioxide into the atmosphere, where it interacted with light, reacted with other molecules, and settled on the surface.

The team's work has yielded the most comprehensive record of the distribution of sulphur isotopes on Mars.

In effect, they have compiled a database of atomic fingerprints that provide a standard of comparison for sulphur-containing samples collected by NASA's Curiosity rover and future Mars missions.

This information will make it much easier for researchers to zero in on any signs of biologically produced sulphur, Farquhar said.

More information: Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars, Nature, DOI: 10.1038/nature13175