Scientists Who Study the Evolution

Scientists who study the evolution of our planet sometimes look to other planets for data that is not available on Earth. That is because the Earth is a dynamic environment where climactic changes, erosion, and the shifting of huge tectonic plates deep within the planet's crust have eliminated any evidence of certain aspects of planetary processes. Mars is no longer climactically active and therefore may offer clues as to its early evolution and development, as well as that of the Earth.

By analyzing soil and rock samples collected by the Mars rovers launched from Earth to explore that planet, scientists compare those analyses with elements of certain geological and chemical processes known to have occurred on Earth. However, sometimes, initial assumptions about those processes lead to confusing situations that make more difficult to learn about the geology of one planet by studying another planet.

For one example, on Earth, volcanic eruptions releases vast amounts of carbon dioxide, which is subsequently dissolved into the waters of the oceans, where it forms limestone (or calcium carbonate) buried in underwater sedimentary rock. For years, scientists hoping to understand how Mars could have supported an environment that included water have focused on the same carbon cycle that characterizes oceanic environments on Earth. Independent evidence has suggested that Mars did indeed contain water at one time in the past, but scientists have not been able to explain the absence of sedimentary calcium carbonate on the Martian surface because that is a universal feature of water environments on this planet.

That paradox seems to have been solved by the finding of a mineral known as jarosite on Mars, contained in samples collected by the rover Opportunity. Since jarosite is known to form only in strongly acidic water, planetary scientists at Harvard University and MIT realized that the presence of jarosite on Mars may finally have provided a plausible explanation for the unexpected absence of limestone on a planet thought to have once had a significant portion of its surface covered by water.

On Earth, volcanic eruptions trigger the carbon cycle that accounts for the eventual concentration of calcium carbonate wherever water once existed. On Mars, the absence of calcium carbonate and, even more importantly, the presence of jarosite suggests that the element sulfur may have played a comparable role to that of carbon on Earth. Specifically, Martian volcanoes apparently ejected vast quantities of sulfur dioxide (instead of carbon dioxide released in volcanoes on this planet) where it was eventually dissolved in Martian oceans to form silicates and sulfites including calcium sulfite. At the same time, the presence of calcium sulfite would also have inhibited the formation of carbonates like limestone.