According to scientitsts at the Harvard Smithsonian Center for Astrophysics, habitable worlds are most likely found on large, rocky planets that are up to ten times the size of Earth and contain plate tectonics. Plate tectonics play a critical role in determining the rate of cooling of a potentially habitable planet by creating the optimum temperature ranges for the development of intelligent animal life -as continents grow, planets cool.
Our planet is changing before our eyes, and as a result, many species are living on the edge. Research by astronomers at the Harvard-Smithsonian Center for Astrophysics has shown 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.
Exoplanet searches have turned up five super-Earths already, although none have life-friendly temperatures. If super-Earths are as common as new Kepler data suggests, then it is inevitable that some will enjoy Earth-like orbits, making them excellent havens for life.
"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.
Within our own Solar System, Mars has been found to be at a primitive stage of plate tectonics. "It gives us a glimpse of how the early Earth may have looked and may help us understand how plate tectonics began on Earth," said An Yin, a UCLA professor of Earth and space sciences. Yin made the discovery during his analysis of satellite images from a NASA spacecraft known as THEMIS (Time History of Events and Macroscale Interactions during Substorms) and from the HIRISE (High Resolution Imaging Science Experiment) camera on NASA's Mars Reconnaissance Orbiter. He analyzed about 100 satellite images — approximately a dozen were revealing of plate tectonics.
Yin has conducted geologic research in the Himalayas and Tibet, where two of the Earth's seven major plates divide.* "When I studied the satellite images from Mars, many of the features looked very much like fault systems I have seen in the Himalayas and Tibet, and in California as well, including the geomorphology," said Yin, a planetary geologist.
"You don't see these features anywhere else on other planets in our solar system, other than Earth and Mars," said Yin.
The surface of Mars contains the longest and deepest system of canyons in our solar system, known as Valles Marineris (image below)-Latin for Mariner Valleys and named for the Mariner 9 Mars orbiter of 1971, which discovered it. It is nearly 2,500 miles long — about nine times longer than the Earth's Grand Canyon. Scientists have wondered for four decades how it formed. Was it a big crack in Mars' shell that opened up?
"In the beginning, I did not expect plate tectonics, but the more I studied it, the more I realized Mars is so different from what other scientists anticipated," Yin said. "I saw that the idea that it is just a big crack that opened up is incorrect. It is really a plate boundary, with horizontal motion. That is kind of shocking, but the evidence is quite clear.* "The shell is broken and is moving horizontally over a long distance. It is very similar to the Earth's Dead Sea fault system, which has also opened up and is moving horizontally."
The two plates divided by Mars' Valles Marineris have moved approximately 93 miles horizontally relative to each other, Yin said. California's San Andreas Fault, which is over the intersection of two plates, has moved about twice as much — but the Earth is about twice the size of Mars, so Yin said they are comparable.
"Earth has a very broken 'egg shell,' so its surface has many plates; Mars' is slightly broken and may be on the way to becoming very broken, except its pace is very slow due to its small size and, thus, less thermal energy to drive it," Yin said. "This may be the reason Mars has fewer plates than on Earth."
Earth endured a tectonic upheaval 1.1 billion years ago that saw the world's continents collide and form a single supercontinent, accoring to Martin Van Kranendonk of the University of New South Wales and Christopher Kirkland of the Geological Survey of Western Australia, who set out to reconstruct the history of plate tectonics to determine if Earth has been getting more or less active over time.
It was the most active period of tectonic activity in Earth's history but the globe has been getting calmer since, they said. Their analysis of 3,200 rock samples from around the world suggests tectonic activity increased from 3 billion years ago, when the Earth was very young, to a peak around 1.1 billion years ago and then declined.
During the peak, all the continents collided and merged into a vast supercontinent called Rodinia, the Austrailian researchers said, spanned by a mountain range that dwarfed today's Himalayas.
The Daily Galaxy via UCLA and www.cfa.harvard.edu/
Image credit of Mars : With thanks to space artist, Walter Myers
Image Credit of Mars' Valles Marineris: from Google Mars, created by MOLA Science Team)