Why Complex Mineral Surfaces Could Be Indications Of Life
Minerals are highly abundant on Earth and other planets. A “mineral” is a solid substance that has a well defined atomic crystal structure and chemical composition. Ice, for example, is considered a mineral because it has a crystal structure (think of six-sided snowflakes) and the composition H2O. Rocks are naturally occurring solids that are composed of one or more minerals.
At an April presentation for the Space Telescope Science Institute in Washington, D.C., Robert Hazen explained that as life evolved and grew more complex on Earth, so did the number of minerals. By extension, this could mean that a planet with more minerals would have life on it.
“The basic idea is if you look at Earth and other terrestrial planets, the near surface environment has changed dramatically through time — 4.5 billion years of history — and those planets manifested very dramatic changes in mineralogy,” said Hazen, an earth science professor at George Mason University in Washington, D.C.
In simpler terms, that means that when a planet is formed, it would only have a few minerals and that over time, the presence of life would add to the mineral complexity. At first glance, this seems counter intuitive since minerals are lifeless. Hazen said mineral abundance increases because life alters the near-surface environment, forming complex chemical and physical micro-environments that change over time.
What’s more exciting is that scientists are just beginning to figure out the composition of planets outside of our solar system, which means that it could one day be possible to estimate if a planet has life based on the elements visible in telescopes.
Hazen’s work is detailed in several scientific journals, including a November 2013 paper in the American Journal of Science titled “Paleomineralogy of the Hadeon Eon: A Preliminary Species List,” which talks about the Earth’s first 500 million years.
Another paper in American Mineralogist from 2013 titled “Clay mineral evolution,” points out that clays appeared on Earth due to our planet’s evolution over time, particularly during major events such as when plate tectonics formed and when oxygen became prevalent in the atmosphere.
Part of Hazen’s research focuses on creating a timeline for how and when minerals emerged on our own planet, with an eye to making predictions for other planets. The timelines are under debate, he said, but as a rough estimate it is a starting point for mineralogists to make predictions.
As Earth formed a surface more than 4.5 billion years ago, about 420 minerals were there in its earliest stage. The first mineral formed in the Universe was likely diamond, which arose out of supernovas. Other elements such as nitrogen and silicon were also important to Earth as it formed.
Incidentally, this 420 mineral figure is close to what scientists believe are on two other planets in our solar system. The planet Mercury has about 300 minerals, and likely stayed in that state because it is a dry planet. Mars is frozen at about 450 minerals, including clays and hydroxides that arose from processes such as glaciation and vulcanism.
Things changed quickly on the early Earth, however. Plate tectonics and granite formation occurred somewhere in the Earth’s first billion years. (Hazen said a few experts claim plate tectonics could have begun as long as 4.4 billion years ago, but most geologists peg it as closer to 3.5 billion years ago)
“The earliest life didn’t really create new minerals,” he said. “Those microbes essentially lived off the chemical energy of the rock itself.”
As an example, when basalt breaks through to the surface of a continent or the ocean through geologic processes, its weathering creates energy that microbes can feed from and speed up chemical reactions on the surface.
A bigger chemical imbalance occurred when microbes learned to use the Sun’s energy for photosynthesis, which created a “huge chemical imbalance” in the atmosphere. Oxygen interacting with these minerals on a large scale about 2.3 billion years ago created oxidization that you can see in the rock record, in elements such as uranium, copper, nickel, iron and manganese.
“Those biological changes created thousands of new minerals,” Hazen said.
Life-spotting on other planets or moons?
While many other milestones in Earth’s history led to the 4,900 or so minerals we have today, Hazen identified a few more key ones that increased the relative abundance. In the neoprotozeroic period, about 750 million years ago, global glaciations altered the surface. Following the retreat of the ice, a “big pulse” of oxygen brought levels of oxygen in the atmosphere from only a few percent of modern levels, to close to what we have today.
Then there was the rise of life on land, particularly multicellular life and the first plants and animals. The greenery covering Earth’s surface produced more oxygen, which created the ozone layer and protected more species on the ground from ultraviolet radiation — a feedback between the atmosphere and life itself.
“It seems as if the diversity of minerals you see at or near a planet’s surface may be an indicator of biological processes,” Hazen said.
While Earth’s environment could have direct applicability to Mars, which has organic materials on its surface, Hazen said this understanding could also be applied to an environment that is less an analog, such as Titan, Saturn’s moon.
Titan is a strange environment when compared to Earth, with ethane and methane forming its lakes, and hydrocarbons covering its surface. On Earth, if the oceans were to evaporate they would leave behind sodium chloride or other organic salts for microbes to colonize.
Environments close to Earth would show organic molecule evaporates such as crystals of amino acids, which are substances that you wouldn’t expect to see unless life was there. Oxidized minerals could also be an indicator of life, although Mars’ life (if it existed or exists) would be unlikely to display it since it would be in the subsurface.
What can a telescope show us?
The launch of NASA’s James Webb Space Telescope in 2018 is expected to herald a new era in exoplanet characterization by helping to reveal the chemical composition of planets.
“We can get a rough idea of whether a planet has salt on its surface, or sandy beaches,” Hazen said. “We can also detect atmospheric signals as a planet transits its sun. We can get some light passing through its atmosphere, and read the absorption signals [of its elements].”
These acts together would reveal the basic mineralogy of a planet’s surface, which could indicate to geologists what is present. Finding the presence of life, however, would require a more multidisciplinary approach. Hazen said that one of the strengths of entities such as NASA’s Astrobiology Institute is the willingness of different disciplines to work together — be they biologists, chemists, astronomers, geochemists or other types of scientists.
More detailed investigations would likely require an orbiter or a lander to see what is happening on the ground, making it unlikely for the time being that detailed information would be available from a planet’s surface. An Earth-sized planet relatively close to our own, however, could reveal a lot of information for us to extrapolate to other planets, Hazen added.