Like life, rocks on Earth evolve and “speciate.” Heat, water and oxygen drive the creation of new minerals—early collisions, melting and volcanic processes provide the necessary heating forces. Plate tectonics helps by pushing crustal elements and minerals down to new pressure conditions. Remarkably, on top of all that much of our planet’s mineral diversity today owes its existence to Earth’s life-forms and biological processes. Some 2.3 billion years ago newly formed bacteria transformed the carbon dioxide atmosphere into a more oxygen dominated one. The increased oxygen in turn interacted with minerals at the planet's surface, which in turn interacted with evolving life. Such transformations and regroupings continue today.
Robert Hazen of the Carnegie Institution for Science in Washington, D.C., laid out this paradigm shift in mineralogy in a 2010 feature “The Evolution of Minerals” in Scientific American. He and his colleagues established a time line for the evolution of minerals, thereby determining that Earth's mineral population grew more diverse as the planet changed. Ultimately, interactions with life added more than 2,500 new minerals to the catalogue of the roughly 1,500 minerals that had been forged during Earth’s first two billion years.
Now, Hazen and his colleagues have applied their time-line to minerals on other planets and moons where the mineral populations could provide insights into past and present habitability. Just as minerals on Earth coevolved with the planet and its life, their presence or absence on other bodies inside and outside the solar system could suggest where planets stand in terms of biological evolution, and if they ever were capable of hosting life. Hazen detailed his latest work on this topic in April at the Space Telescope Science Institute (STScI) symposium, Habitable Worlds across Time and Space.
The explosion of new planet discoveries in the past four years by the Kepler spacecraft and other telescopes makes such investigations all the more intriguing. "It completely changes how we think about a planet's evolution, because it means we can study their near-surface mineralogy and learn about the biology," Hazen said.
Within in a generation or two improved resolution in next-gen space telescopes may enable the study of the light waves emitted by minerals and other features on exoplanets, Hazen says. Orbiters and rovers at Mars already perform this task to locate minerals there. Such instruments could find a mineral composition similar to those on Earth or dramatically different ones as distinctive, local processes interact to create previously unknown minerals species. For instance, Saturn's moon Titan, a planet-size body harboring a methane atmosphere, is thought to be one of the most likely habitable places in the solar system. It hosts methane/ethane pools that, as they evaporate, leave behind saltlike crystal deposits completely different from any found on Earth.
If minerals that on Earth require biochemical processes to form could be detected on other planets, in the solar system or beyond, their presence has the potential to indicate whether a planet hosted life at some point in its past. Any remote detection of minerals on exoplanets would depend on the existence of large formations, but the presence of massive outcrops on Earth provides hope. In Morocco, for instance, carbonate rocks can cover hundreds of square kilometers. If the pixel resolution on a space telescope is fine enough to detect an area of around 130 square kilometers, scientists might be able to detect such signals from Earth, Hazen said.
Peter McCullough of STScI, who was present at Hazen's lecture, pointed out that finding minerals associated with life processes on Earth would kick off discussions about whether they evolved in association with life or independently in a new setting.
Whereas processes dominated by heat and water should be fairly similar on other bodies, a third force driving the creation of new minerals on Earth is its oxygen atmosphere, which creates what Hazen said were some of the most colorful and beautiful minerals. For such minerals to exist on another planet harboring a different atmosphere, another chemical method of stripping electrons from the near-surface rocks would be required, said Christopher Burke, a Kepler scientist at SETI Institute and at NASA Ames Research Center who was present at Hazen’s lecture. "If we don't find life,” he added, “maybe we could find minerals that suggest that the planet got to some stage in its [mineral] evolution but didn’t quite reach where we did on Earth."