It Looks Like Plate Tectonics Aren’t Required to Support Life

When looking for potentially-habitable extra-solar planets, scientists are somewhat restricted by the fact that we know of only one planet where life exists (i.e. Earth). For this reason, scientists look for planets that are terrestrial (i.e. rocky), orbit within their star’s habitable zones, and show signs of biosignatures such as atmospheric carbon dioxide – which is essential to life as we know it.

This gas, which is the largely result of volcanic activity here on Earth, increases surface heat through the greenhouse effect and cycles between the subsurface and the atmosphere through natural processes. For this reason, scientists have long believed that plate tectonics are essential to habitability. However, according to a new study by a team from Pennsylvania State University, this may not be the case.

The study, titled “Carbon Cycling and Habitability of Earth-Sized Stagnant Lid Planets“, was recently published in the scientific journal Astrobiology. The study was conducted by Bradford J. Foley and Andrew J. Smye, two assistant professors from the department of geosciences at Pennsylvania State University.

The Earth’s Tectonic Plates. Credit: msnucleus.org

On Earth, volcanism is the result of plate tectonics and occurs where two plates collide. This causes subduction, where one plate is pushed beneath the other and deeper into the subsurface. This subduction changes the dense mantle into buoyant magma, which rises through the crust to the Earth’s surface and creates volcanoes. This process can also aid in carbon cycling by pushing carbon into the mantle.

Plate tectonics and volcanism are believe to have been central to the emergence of life here on Earth, as it ensured that our planet had sufficient heat to maintain liquid water on its surface. To test this theory, Professors Foley and Smye created models to determine how habitable an Earth-like planet would be without the presence of plate tectonics.

These models took into account the thermal evolution, crustal production and CO2 cycling to constrain the habitability of rocky, Earth-sized stagnant lid planets. These are planets where the crust consists of a single, giant spherical plate floating on mantle, rather than in separate pieces. Such planets are thought to be far more common than planets that experience plate tectonics, as no planets beyond Earth have been confirmed to have tectonic plates yet. As Prof. Foley explained in a Penn State News press release:

“Volcanism releases gases into the atmosphere, and then through weathering, carbon dioxide is pulled from the atmosphere and sequestered into surface rocks and sediment. Balancing those two processes keeps carbon dioxide at a certain level in the atmosphere, which is really important for whether the climate stays temperate and suitable for life.”

Map of the Earth showing fault lines (blue) and zones of volcanic activity (red). Credit: zmescience.com

Essentially, their models took into account how much heat a stagnant lid planet’s climate could retain based on the amount of heat and heat-producing elements present when the planet formed (aka. its initial heat budget). On Earth, these elements include uranium which produces thorium and heat when it decays, which then decays to produce potassium and heat.

After running hundreds of simulations, which varied the planet’s size and chemical composition, they found that stagnant lid planets would be able to maintain warm enough temperatures that liquid water could exist on their surfaces for billions of years. In extreme cases, they could sustain life-supporting temperatures for up to 4 billion years, which is almost the age of the Earth.

As Smye indicated, this is due in part to the fact that plate tectonics are not always necessary for volcanic activity:

“You still have volcanism on stagnant lid planets, but it’s much shorter lived than on planets with plate tectonics because there isn’t as much cycling. Volcanoes result in a succession of lava flows, which are buried like layers of a cake over time. Rocks and sediment heat up more the deeper they are buried.”

Image of the Sarychev volcano (in Russia’s Kuril Islands) caught during an early stage of eruption on June 12, 2009. Taken by astronauts aboard the International Space Station. Credit: NASA

The researchers also found that without plate tectonics, stagnant lid planets could still have enough heat and pressure to experience degassing, where carbon dioxide gas can escape from rocks and make its way to the surface. On Earth, Smye said, the same process occurs with water in subduction fault zones. This process increases based on the quantity of heat-producing elements present in the planet. As Foley explained:

“There’s a sweet spot range where a planet is releasing enough carbon dioxide to keep the planet from freezing over, but not so much that the weathering can’t pull carbon dioxide out of the atmosphere and keep the climate temperate.”

According to the researchers’ model, the presence and amount of heat-producing elements were far better indicators for a planet’s potential to sustain life. Based on their simulations, they found that the initial composition or size of a planet is very important for determining whether or not it will become habitable. Or as they put it, the potential habitability of a planet is determined at birth.

By demonstrating that stagnant lid planets could still support life, this study has the potential for greatly extending the range of what scientists consider to be potentially-habitable. When the James Webb Space Telescope (JWST) is deployed in 2021, examining the atmospheres of stagnant lid planets to determine the presence of biosignatures (like CO2) will be a major scientific objective.

Knowing that more of these worlds could sustain life is certainly good news for those who are hoping that we find evidence of extra-terrestrial life in our lifetimes.

Further Reading: PennState, Astrobiology

What is a Subduction Zone?

Transform Plate Boundary

IF you don’t know anything about plate tectonics you might be wondering about what is a subduction zone. A subduction zone is a region of the Earth’s crust where tectonic plates meet. Tectonic plates are massive pieces of the Earth’s crust that interact with each other. The places where these plates meet are called plate boundaries. Plate boundaries occur where plates separate, slide alongside each other or collide into each other. Subduction zones happen where plates collide.

When two tectonic plates meet it is like the immovable object meeting the unstoppable force. However tectonic plates decide it by mass. The more massive plate, normally a continental will force the other plate, an oceanic plate down beneath it. This is the subduction zone. When the other plate is forced down the process is called subduction. The plate enters into the magma and eventually it is completely melted. That is how the surface of the earth makes way for the crust created over time at other plate boundaries.

Subduction zones have key characteristics that help geologist and seismologist identify them. The first is mountain formation. Subduction zones always have mountain ranges caused by plate subduction. The next is volcanic activity as a plate is subducted the pressure and heat turns it into magma. These pockets of magma find paths to the surface and create volcanoes. A good example is the subduction zone near Chile. The final sign is deep marine trenches. These are the best evidence of a subduction zone as they are visible evidence of the crease formed by subduction of a plate. The most famous is the Mariana Trench.

There are some interesting theories about why Subduction occurs in the Earth’s crust. One common theory is that subduction was initiated by major impacts by asteroids or comets early in Earth’s history. This makes a lot of sense due to the geologic evidence of large impacts scattered around the world.

Understanding how subduction zones work is important because it helps scientist to identify areas of high volcanic and seismic activity. Monitoring these areas can help them warn people who live near them of imminent events and also people who could be affected by the side effects of such events such as ash clouds or tsunamis.

Subduction continues to be one of the most powerful and dynamic processes on planet Earth and as technology improves we can come to understand more about this amazing process.

We have written many articles about the subduction zone for Universe Today. For example, here is one on the Ring of Fire and plate boundaries.

You should also check out plate tectonics and subduction.

If you’d like more info on the subduction zone, check out the U.S. Geological Survey Website. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded related episodes of Astronomy Cast about Plate Tectonics. Listen here, Episode 142: Plate Tectonics.

Sources:
http://en.wikipedia.org/wiki/Subduction
http://myweb.cwpost.liu.edu/vdivener/notes/subd_zone.htm

Subduction is a process in geology where one tectonic plates slides underneath another one and merges into the Earth’s mantle. The denser plate is the one that slips under the less dense plate; the younger plate is the less dense one. The process is not a smooth one. The tectonic plates grate against each other, which often causes earthquakes. The plate that slips under does not stay that way. Due to the heat caused by it rubbing against the other plate as well as the natural heat of the mantle, the plate melts and turns into magma. The area where subduction occurs is known as the subduction zone.

When one plate begins to slip underneath another one a trench is formed. The earthquakes that result due to the plates grinding against each other often cause magma to spill out through the trench in submarine volcanoes. Various formations such as mountain ranges, islands, and trenches are caused by subduction and the volcanoes and earthquakes it triggers. In addition to causing earthquakes, subduction can also trigger tsunamis.

When the older plate is holding a continent however, it does not sink, which is reassuring. Instead, the less dense material slips into a trench behind the denser oceanic crust where it gets stuck. The pressure continues to build until the trench flips over and the less dense plate slips underneath the one with the continent.

It is possible for a whole tectonic plate to disappear. This happens when the plate goes through subduction faster than new material can be added to the plate through seafloor spreading. The spreading pushes the plate slowly toward the subduction zone until the whole thing disappears. When this happens, the other tectonic plates rearrange to cover the area.

Subduction zones are mainly located in the Pacific Ocean. This is because seafloor spreading – the process by which new oceanic crust is created – occurs mostly in the Pacific. Thus the new material pushes the older plates outward and then they need to undergo subduction. This also explains why so many earthquakes originate in the Pacific Ocean near the Ring of Fire. That is where the subduction zones are concentrated.

Continental plates also converge, but this is not considered subduction because these plates do not have different densities and thicknesses to subduct. Landforms such as the Himalayas are formed from these convergences though.