Plate tectonics is the process of continents on the Earth drifting and colliding, rock grinding and scraping, mountain ranges being formed, and earthquakes tearing land apart. It makes our world dynamic and ever-changing. But should it factor into our search for life elsewhere in the universe?
Tilman Spohn believes so. As director of the German Space Research Centre Institute of Planetary Research, and chairman of ESA's scientific advisory committee, he studies worlds beyond our Earth. When looking into the relationship between habitability and plate tectonics, some fascinating possibilities emerged.
It is thought that the best places to search for life in the universe are on planets situated in "habitable zones" around other stars. These are orbital paths where the temperature is suitable for liquid water; not so close to the star that it boils away, and not so far that it freezes. Spohn believes that this view may be outdated. He elaborates, "you could have habitats outside those, for instance in the oceans beneath ice covers on the Galilean satellites, like Europa. But not every icy satellite would be habitable. Take Ganymede, where the ocean is trapped between two layers of ice. You are missing a fresh supply of nutrition and energy."
So planets and moons that lie beyond habitable zones could host life, so long as the habitat, such as an ocean, is not isolated. It needs access to the key ingredients of life, including hydrogen, oxygen, nitrogen, phosphorous and sulphur. These elements support the basic chemistry of life as we know it, and the material, Spohn argues, must be regularly replenished. Nature's method of achieving this on the Earth appears to be plate tectonics.
Plate tectonics — essential for life?
It is an idea growing in popularity among planetary scientists. Says Spohn, "plate tectonics replenishes the nutrition that primitive life could live on. Imagine a top surface that is depleted of the nutrition needed for bacterial life. It needs to be replenished, and plate tectonics is a method of achieving this."
Spohn found that the further he delved into the issue, the more important plate tectonics seemed to be for life. For example, it is believed that life developed by moving from the ocean to the kind of strong and stable rock formations that are the result of tectonic action. Plate tectonics is also involved in the generation of a magnetic field by convection of Earth's partially molten core. This magnetic field protects life on Earth by deflecting the solar wind. Not only would an unimpeded solar wind erode our planet's atmosphere, but it also carries highly energetic particles that could damage DNA.
Another factor is the recycling of carbon, which is needed to stabilize the temperature here on Earth. Spohn explains, "plate tectonics is known to recycle carbon that is washed out of the atmosphere and digested by bacteria in the soil into the interior of the planet from where it can be outcast through volcanic activity. Now, if you have a planet without plate tectonics, you may have parts of this cycle, but it is broken because you do not have the recycling link."
It has also been speculated that the lack of tectonic action on Venus contributed to its runaway greenhouse effect, which resulted in the immense temperatures it has today.
All this evidence adds up to paint a convincing picture of many lifeforms only surviving on worlds where plate tectonics are active. For astrobiologists, there is another interesting element to this story. Many within the planetary science community believe that to have plate tectonics, the near-surface rock must be weakened. The molecule most effective at doing this is H2O — water. So worlds with plate tectonics are likely to have water as well, which means they feature two ingredients theoretically necessary for life.
This presents an exciting option: searching for plate tectonics on distant worlds as a sign of life. Spohn agrees that this is a possibility, but remains level-headed. "It's an interesting idea, but is just speculation at the moment," he explains. "As a biosignature it would be very difficult to detect, especially with current technology."
The problem is how challenging it is to spot plate tectonics from orbit even on our own Earth. The jig-saw puzzle shape of continents along with the presence of mountain belts provides indirect evidence. Mid-oceanic ridges are more convincing, but these are covered with water and not visible from space. To see features on an extrasolar planet would require a probe in orbit, and this is far beyond our technological ability. Even if we were able to achieve this, the evidence would still be indirect. Currently there is no conclusive way of remotely determining tectonic action on a planet.
So perhaps using these markers as an indication of life on other worlds is a step too far, but as our technology becomes ever more complex it could become a possibility in the future. Imagine detecting an Earth-sized planet with an atmosphere, water, organic materials, and plate tectonics. It would unquestionably raise hopes for finding life in the universe.