Massive Earth-like planets that have both continents and oceans may be better at harboring extraterrestrial life
than those that are water-only worlds.
A new study gives hope for the
possibility that many super-Earth planets orbiting distant stars have
exposed continents rather than just water-covered surfaces.
Super-Earths likely have more stable climates as compared to
water-worlds, and therefore larger habitable zones where alien life
could thrive. In the new study, researchers used the Earth as a starting
point for modeling how super-Earths
might store their water on the surface and deep underground within the
mantle. The work is detailed in a preprint paper titled “Water Cycling
Between Ocean and Mantle: Super-Earths Need Not Be Waterworlds” that was
published in the January issue of The Astrophysical Journal.
Researchers typically expect super-Earths to exist as water-worlds
because their strong surface gravity creates relatively flattened
surface geography and deep oceans. But the new study found that
super-Earths with active tectonics can have exposed continents if their
water is less than 0.2 percent of the total planetary mass.
"A planet could be ten times wetter than Earth and still have exposed
continents," said Nicolas Cowan, a planetary scientist at Northwestern
University and co-author on the new paper. "That's important for what
the planet looks like and how it ages."
Cowan and Dorian Abbot, a climate scientist at the University of
Chicago, built the model in the study. The model uses Earth as a
starting point in defining how a planet's water distribution could end
up balanced in a steady state between the surface oceans and the mantle,
which allows the researchers to calculate whether a super-Earth is
likely to be a water-world or not.
The movement of tectonic plates on Earth transfers water continuously
between the surface oceans and the mantle. Ocean water enters the
mantle as part of deep-sea rocks when one tectonic plate slides under
another and goes down into the mantle.
"Earth is the only known planet with plate tectonics,
a deep water cycle, etc., so it's a good place to start," Cowan said.
"On the other hand, if it turns out that Earth's deep water cycle in
nowhere near a steady-state, then our conclusions are way off the mark. "
Water in the mantle can re-enter the ocean when volcanic activity
splits the planet's crust at mid-ocean ridges. The loss of the crust
causes a drop in pressure that leads the underlying mantle rock to melt
and lose volatiles such as water. (An additional twist is that
super-Earths with their stronger gravity could have greater seafloor
pressure that suppresses the mantle's loss of water, so that more of the
planet's overall water remains in the mantle.)
There are other uncertainties that could make a big difference in the
model's accuracy in predicting a super-Earth's likelihood of having dry
continents. One unknown is the amount of water hidden deep within
Earth's own mantle; Cowan and Abbot cite estimates of one to two oceans
worth of water. Another factor is whether or not super-Earths have
tectonic processes. If the researchers' assumptions about either factor
are wrong, that would change their model's calculation of the
"water-world boundary," which represents the mathematical model's
dividing point between water-worlds and worlds with dry continents.
"If some of our input parameters are wildly off, then the actual
water-world boundary might differ by an order of magnitude," Cowan said.
"No matter how you cut it, though, the water-world boundary is unlikely
to be as damning as previously thought."
The debate over super-Earths will continue until space missions begin
collecting hard data on how much water exists on such planets. A space
telescope with an interior coronagraph or exterior starshade could block
the blinding light of distant stars to get a peek at orbiting planets.
But no active space telescopes can currently do the necessary work of
mapping the surface of super-Earths.
""At the very least, you'd need a space telescope with a mirror a few
meters wide, coupled to a starshade tens of thousands of kilometers
away," Cowan explained. "NASA is mooting this idea, but it is not the
next priority." One space telescope that could fit the bill would be
NASA's Wide-Field Infrared Survey Telescope
(WFIRST) — a planned 2.4-meter telescope with an instrument for imaging
exoplanets. The $1.6 billion mission remains up in the air until NASA
can squeeze it into the budget, but Cowan expects that WFIRST could get
off the ground by the mid-2020s or 2030s. If so, that would bring
researchers one step closer to understanding whether super-Earths truly
work like our own world.
Research by astronomers at the Harvard-Smithsonian Center for Astrophysics
in 2012 showed that if Earth had been slightly smaller and less
massive, it would not have plate tectonics - the forces that move
continents and build mountains. And without plate tectonics, life might
never have gained a foothold.
"Plate tectonics are essential to life as we know it," said Diana
Valencia of Harvard University. "Our calculations show that bigger is
better when it comes to the habitability of rocky planets."
Plate tectonics -the movement of huge chunks, or plates, of a
planet's surface- are crucial to a planet's habitability because they
enable complex chemistry and recycle substances like carbon dioxide,
which acts as a thermostat and keeps Earth balmy. Carbon dioxide that
was locked into rocks is released when those rocks melt, returning to
the atmosphere from volcanoes and oceanic ridges.
"Recycling is important even on a planetary scale," Valencia explained.
Valencia and her colleagues, Richard O'Connell and Dimitar Sasselov
(Harvard University), have examined the extremes to determine whether
plate tectonics would be more or less likely on different-sized rocky
worlds. In particular, focusing on "super-Earths"-planets more than
twice the size of Earth and up to 10 times as massive.
"It might not be a coincidence that Earth is the largest rocky planet
in our solar system, and also the only one with life," said Valencia.
"There are not only more potentially habitable planets, but MANY
more," stated Sasselov, who is director of the Harvard Origins of Life
Initiative.
In fact, a super-Earth could prove to be have volcanic "rings of
fire" that could span the globe while the equivalent of Yellowstone Park
would bubble with hot springs and burst with hundreds of geysers. An
Earth-like atmosphere would be possible, while the surface gravity would
be up to three times that of Earth on the biggest super-Earths.
Sasselov observed that although a super-Earth would be twice the size
of our home planet, it would have similar geography. Rapid plate
tectonics would provide less time for mountains and ocean trenches to
form before the surface was recycled, yielding mountains no taller and
trenches no deeper than those on Earth. Even the weather might be
comparable for a world in an Earth-like orbit.
"The landscape would be familiar. A super-Earth would feel very much like home," said Sasselov
The Daily Galaxy via Astrobio.net and Harvard-Smithsonian Center for Astrophysics
Image credit: David A. Aguilar (Harvard-Smithsonian CfA)
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