Does a planet need life to create continents?

January 6, 2014 by Charles Q. Choi
Does a Planet Need Life to Create Continents?
Earth may not have possessed the continents it does now if not for life, instead becoming a planet covered nearly entirely in ocean.

If not for life, Earth may not have possessed the continents it does now, instead becoming a planet covered nearly entirely in ocean, researchers say.

These new findings suggest that any continents astronomers may one day see on alien worlds may potentially be signs of , scientists added.

Earth is currently the only known planet in the universe that has liquid water on its surface. There is life virtually wherever there is liquid water on Earth, so one main focus of the search for extraterrestrial life as we know it is the region around a star where it is neither too hot nor too cold for to exist on a planet, an area known as the star's habitable zone.

Although water covers most of Earth's surface, nearly 30 percent of the planet is covered by land, sustaining a dazzling variety of life. Scientists might one day be capable of telling if distant planets are similarly covered by land, oceans and clouds by looking for reddish, bluish or grayish tints in the color of those worlds. Researchers have already developed maps of clouds on a giant planet orbiting a distant star.

Now researchers suggest Earth would have been a water world with very few continents, if any at all, without the presence of life.

A great deal of research has shown that life has had a major impact on the evolution of Earth's atmosphere and oceans. Plants and other photosynthetic life generate oxygen, giving Earth the only known atmosphere in the universe with significant levels of oxygen. Life also greatly influences how much carbon is in the atmosphere and oceans in the form of carbon dioxide and methane. These greenhouse gases trap heat and can dramatically affect Earth's climate, which in turn has an effect on how much of Earth's water is frozen as ice. Oxygen can also indirectly cool Earth's climate by removing methane from the atmosphere—in fact, the dramatic rise of oxygen in Earth's atmosphere about 2.4 billion years ago, known as the Great Oxidation Event, may have cooled the planet enough to for it become a frozen "Snowball Earth."

"However, much less is known about whether life has had any effects on the deeper interior of Earth," said study author Tilman Spohn, a planetary scientist at the German Aerospace Center's Institute of Planetary Research in Berlin.

Past research noted the oldest signs of life found so far are roughly 3.5 billion years old, about the same time continents began growing, and suggested a potential link between these events. The scientists then explored whether or not the evolution of life on Earth could have influenced the evolution of the planet.

The investigators focused on biological weathering, by which life breaks down rock. This crumbled rock gets blown and washed away by wind and water, sediment that eventually makes its way to subduction zones, the areas where one tectonic plate of the Earth's crust dives or subducts under another.

Does a planet need life to create continents?
Astronomers have discovered many planets orbiting the star Gliese 581. This artist’s representation shows Gliese 581 e (foreground), which is only about twice the mass of our Earth. Other confirmed planets in the system are 16 (planet b, nearest to the star), 5 (planet c, center), and 7 Earth-masses (planet d, with the bluish color). Credit: ESO

"Lichens that cover rocks provide constant moisture that can weaken rock, while acid from bacteria can dissolve rock," Spohn said.

These sediments are up to 40 percent water by weight. This means sediment that gets subducted can ferry huge amounts of water into the mantle layer that lies between the Earth's crust and core. Once this sediment reaches the pressure and heat found at depths of about 60 miles (100 kilometers), it releases its water, reducing the melting temperature of the surrounding rock. This leads more rock to melt and rise, eventually bursting from volcanoes as lava that helps add to continental mass. In other words, although life helps wear continents down, it also helps build continents up.

"When the Earth's surface is recycled in subduction zones, it affects processes in the deep interior," said study lead author Dennis Höning, a planetary scientist at the German Aerospace Center's Institute of Planetary Research in Berlin.

The magnitude of the effects biological weathering have on continental erosion globally are currently under debate, with estimates varying quite widely. To see what these effects might be, Spohn and his colleagues developed models of continent production and erosion that assumed that a world with no life had some fraction of the continental erosion rates currently estimated for Earth.

When the scientists ran their model of Earth with present-day continental erosion rates, it simulated a planet with a wet mantle that, after roughly 4 billion years, had a surface that was covered about 40 percent by continents, roughly similar to the real Earth. However, when the investigators ran their model with continental erosion rates 60 percent or less of present-day values, which one might see on a lifeless Earth, it simulated a planet with a dry mantle that, after some 4 billion years, "had continents covering only about 5 percent of its surface," Spohn said.

"The biggest surprise for me was the pronounced difference we saw," Spohn said. "We hoped to see a difference, but we didn't expect to see such a big difference." Höning, Spohn and their colleagues Hendrik Hansen-Goos and Alessandro Airo detailed their findings online Oct. 25 in the journal Planetary and Space Science.

The evolution of photosynthesis starting at least 3.4 billion years ago may have had an especially large impact on Earth's .

"The invention of photosynthesis allowed life to be much more productive, allowed the biosphere to rely on a much larger energy source then before and grow very rapidly in mass, which would have increased the effect of biological weathering," Spohn said.

Does a planet need life to create continents?
A Sun glint on Earth is captured (center of the black circle) in this image taken by NASA's Deep Impact spacecraft as it looked at the north pole. The reddish area is North America, and the glint is coming from a body of water in California. Credit: Don Lindler, Sigma Space Corporation/GSFC

Spohn added that if the mantle was dry, one might not see plate tectonics, which drives the motions of the tectonic plates that make up the surface of the planet and underlies tectonic activity such as earthquakes and volcanoes. "Plate tectonics needs water to operate," Spohn said.

Geophysicist Norman Sleep at Stanford University in California, who did not take part in this research, noted that biology could have other effects on Earth's interior. For instance, past laboratory experiments have shown that microbial activity can help transform the soft clay mineral smectite to much stronger and denser illite. Since illite is less permeable to water than smectite, microbial activity could make more available at greater depths. Preliminary experiments that Spohn and his team conducted suggest this effect could enhance the difference between a life-filled and a lifeless Earth even more.

These findings suggest "if we find a planet somewhere in the universe with a continental coverage similar to the Earth, it may be a good place to search for life," Höning said.

Still, Höning cautioned that larger planets, such as super-Earth worlds up to 10 times Earth's mass, evolve in a strongly different way than Earth has, so these findings might not apply. Moreover, an exoplanet with a hot surface, a thick atmosphere or strong winds might also experience large erosion rates even without .

"All these factors have to be taken into account when we try to model continental growth on other ," Höning said.

Future research can examine the tendency of cracks to form in sediment in subduction zones and the effects this may have on activity in Earth's interior, Sleep said. It can also investigate if volcanic activity at varies with levels of sediment flow, Sleep added—with lava from volcanoes eventually building new land.

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Jan 06, 2014
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Vera Paunova
3.8 / 5 (13) Jan 06, 2014
I would opine that the article's logic is backwards. Continents were made to support life (c.f. Gen 1:9), not the other way around.

That was a joke, right? Quoting a book of (bad) mythology on a SCIENCE site?
Jan 06, 2014
This comment has been removed by a moderator.
3.7 / 5 (6) Jan 06, 2014
Changing erosion rates by .6 is a bit over the top. I sincerely doubt that biological life contributed THAT greatly to the continental rate. In either case, life on this planet would have persisted as the article mentions the Oxidation event, which was caused by anaerobic organisms to begin with.
4.5 / 5 (17) Jan 06, 2014
"Many famous scientists in history had an utmost respect for the Bible"

True, but one must keep in mind that in the past, western societies resembled islamic countries today, belief was beyond expected, it was mandatory, a non-christian couldn't even be a scientist or hold any kind of position in society. In modern times we recognize the Bible for what it is, a work of mythic fiction combined with fractured cultural history of a small group of people in the middle east a long time ago. And discussion of that book has no place here.
5 / 5 (3) Jan 06, 2014
I would suggest that the effects of an impact between proto-Earth and Selene to form the Moon and Earth would have had a more significant effect on the nature and distribution of the earth's crust than any microscopic processes.
1.8 / 5 (5) Jan 06, 2014
I think you should first ask the question of whether life is possible on this planet without continents. I am not the only person to suggest that it may not be so.

Strangely enough, all macroscopic life on this planet is related, including the organisms know to exist near hydrothermal vents, for example crabs and worms living on the Bacillus Infernos' sugar products, are closely related to the other organisms which survive on sunlight based food coming from the surface. Ultimately, there is not much special about the crabs, shrimp, and fish that live in these environments, except that they happen to eat something else that lives there, yet they somehow manage to move between such vents to populate them elsewhere, as do the worms and the bacteria.

To me evidence suggests life based on the Sun moved to the vents, not the other way around, because the fish have eyes, and the crabs and shrimp have eyes and/or eye sockets.
1.8 / 5 (5) Jan 06, 2014
So if you were on a planet without tectonics, or even without continents, where materials could be eroded and chemical nutrients become suspended in water, how would micro organisms as we know them exist? A deep ocean with no continents would have very little over-turning of nutrients, close to none in fact, towards the surface for solar life to exist.

Combine all of these facts, and especially the last sentence in the above post, and you'll see that typical explanations of where life came from appear to be the reverse of what happened.

Them: Life started in cess pools and hydrothermal vents.

Contradictory Observation1: Organisms with eyes (from elsewhere) migrated to hydrothermal vents from wherever life really originated.

Contradictory Observation2: Extremophiles actually migrated TO the extreme environments, not FROM them. After all, hot springs and vents are often unrelated to one another and thousands of miles apart with no obvious connection to one another.
1.7 / 5 (6) Jan 06, 2014
Contradictory Observation3: There is no mechanism to move extremophiles from a hydrothermal vent or other volcano (such as Toba) to a volcanic system in the center of a continent such as Yellowstone, where we find them now. Since Yellowstone does not pre-date North America (obviously) then there is no sensical connection between the extremophiles per se, in terms of geography. Not to mention that one type of extremophile is an algae while the other type is a bacterium, thus unrelated.

This means that microscopic life provably moved TO a hydrothermal vent at least one time, since there are two unrelated types. Once we know it can move TO that environment from elsewhere at least once, there is no longer need for the conjecture that life started in a hydrothermal vent, because we know life moved to the hydrothermal vent from another location, which means life started somewhere else at least once.

If life started somewhere else at least once, then that is all that is needed.
3 / 5 (3) Jan 06, 2014
5 / 5 (8) Jan 06, 2014
The Bible is a book written by men at a time in human history that made it impossible to know any truths at all. So it's not surprising that they got everything wrong; from "creation", to morals, to history. Also, its hilarious to suggest that the Bible teaches science. The definition of science as a practice was invented in the late 15th century. But it doesn't matter what I say; if religious people were to think using reason and logic, they wouldn't be religious.
not rated yet Jan 07, 2014
Venus thanks to a lack of plate tectonics has to turn its crust over every few million years via vast volcanic eruptions.

As a result of that lack, continental sized areas make up far less of Venus surface than they do Earth's.

I wonder if they simply excluded life's effects from their formulations without factoring in the process that happens in its place based on evidence from Venus.

Would a worldwide ocean dramatically affect what a planet turning its crust over like Venus does or would it make no difference?

5 / 5 (3) Jan 07, 2014
Many famous scientists in history had an utmost respect for the Bible, but apparently you are not one of them.

Many (well, all) scientists today find the Bible irrelevant to their pursuit of science. And yet, oddly enough, the pace of scientific advancement is fast. Faster overall than any other period of history.
not rated yet Jan 08, 2014
- The earliest putative fossils are now Isua BIFs and carbons, which despite being metamorphosed may be acceptable in recent analyses. That puts life > 3.8 Ga bp.

- Also, Sd ratios indicates that Archean Earth was in a stagnant-lid regime (but with at least 10 % of time having subduction) up to 3 Ga bp. That puts plate tectonics in a 3 Ga, not 4 Ga, regime.

It is useless to feed the myth-texts-on-magic-ideas spouting antiscience trolls. They are encouraged whatever the response is.

@tadchem: That collision was before later life appeared, setting the initial conditions. Technically the planetoids were named Tellus (proposed by geoscientists) and Theia (proposed by astronomers).
not rated yet Jan 08, 2014
Oops, the edit lost my initial lines from the earlier comment:

Generically, superEarths have the opposite problem of too much water. But it seems in recent work that allowing for the larger pressure, they will still have continents.

Specifically, the timing is off.
not rated yet Jan 08, 2014
@Jonseer: I think they considered habitable zone planets. Venus is today considered to be too close to the Sun.

@Returners: "Them: Life started in cess pools [sic] and hydrothermal vents." "Strangely enough, all macroscopic life on this planet is related,"

"Them"? That is a major abiogenesis theory. "Strangely"? Evolution is _the_ basis for biology, implying a few (and in our case one - the best observation of all of science) ancestral populations. Are you a creationist?

If not trolling: There was no "cess" to pool - no life.

Alkaline hydrothermal vent (AHV) abiogenesis is a specific theory. Modern life has nothing to do with abiogenesis, and whatever environment you suggest for earliest populations and their protocells they have been replaced with modern cells long since.

In fact, if you read the papers you find that modern life conceals some of the abiotic chemistry of AHVs vital for abiogenesis research.

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