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In Iceland, at one end of the 10,000-mile-long Mid-Atlantic Ridge, a visitor could see new crust being born as magma oozes up from the interior. (© 2013 Robert Postma/ FirstLight.com)

Planet Earth: A Guide for Alien Scientists

If astronomers from another world sent a probe to study ours, where would you tell it to land?

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A fundamental problem for scientists studying the universe is that it’s big. They can’t point telescopes at every star in the sky, or send planetary rovers traversing millions of square miles, so they have to choose the best targets.

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John Grant knows about choosing. A geologist with the National Air and Space Museum, he co-chaired the research committee that selected Gale Crater as the landing site for NASA’s Curiosity rover. (He did the same for the Mars Exploration Rovers Spirit and Opportunity 10 years earlier.) “There are two things you want to do when you go to a particular place on Mars,” Grant says. First, naturally, you want to make sure the site meets the mission’s scientific requirements. For Curiosity, that meant a place with many geologic layers, so the rover could study what conditions were like in the Martian past, when at least some parts of the planet were habitable. “The second thing is, you want to be able to put those results into a global framework.” Observations from Mars orbit have shown other landscapes on the planet similar to those at Gale Crater, so the rover’s limited view on the ground can be extrapolated to tell us about Mars as a whole.

That led us to wonder: If intelligent beings wanted to land a probe on Earth to learn as much as possible about our geology, what locations would reveal the most? We asked a few experts to be the aliens’ John Grant and tell us: Where is Earth’s Gale Crater?

Fire and Birth: Iceland
Brent Grocholski, a postdoctoral fellow with the Division of Meteorites at the Smithsonian’s National Museum of Natural History, suggests a one-stop-shopping site for the extraterrestrial geologists. “Our planet is defined by plate tectonics, but also has volcanoes fueled by heat coming from the deep mantle,” he says. “Wouldn’t it be great if you could land someplace where you could find both an active rift zone and a volcanic hotspot? Welcome, aliens…to Iceland.”

The active rift zone here is the Mid-Atlantic Ridge, which reaches from below the southern tip of Africa along the floor of the Atlantic Ocean to southwest Iceland, where it emerges onto land. At the ridge, two tectonic plates are slowly drifting apart, creating a fissure where magma rises from the mantle and cools to form new crust. These ridges exist all over the planet, mostly underwater. In Iceland, a rover could watch this fundamental geological process from dry land.

Iceland also is one of the most volcanically active places on Earth. A rover wouldn’t have to travel far to see wide swaths of land covered in lava rock. It would find geothermal activity like hot springs and fumaroles—holes in Earth’s crust emitting super­heated water that turns to steam and often reeks of sulfurous gas belched up from below. Navigating the rough ground might be tricky, but the rover would be able to explore fissures, craters, and calderas formed from the constant movement of magma under the ground. It might even catch an eruption in progress: Iceland has one every few years.

If the probe landed near Breiðafjörður, a large, shallow bay on Iceland’s western coast, the aliens could even learn something about astrogeology. Watching the tides go in and out over the course of a day would reveal the physical relationship between Earth and its moon.

The Depths: Mariana Trench
The first thing alien astronomers are sure to notice when observing Earth from afar is that our pale dot is distinctly blue, says Deborah Hutchinson, a marine geologist with the U.S. Geological Survey. So a list of potential landing sites almost certainly should include one that involves a splashdown. But, she asks, “Do you want to go to where the extremes are, or is there more value in going to something that’s more representative?” Hutchinson says that extreme places can actually offer broader insights: The features are more obvious, the results are more dramatic, and they give you one endpoint to use for calibration.

Therefore, we wouldn’t want aliens to target just any place in the ocean. Perhaps the best location would be over the Mariana Trench. The deepest point, the Challenger Deep, is about 200 miles southwest of Guam, in the western Pacific Ocean. Though the trench isn’t visible from the surface, aliens could find the underwater canyon by using gravity mapping, Hutchinson says, the way NASA’s GRAIL spacecraft recently mapped the moon. Space-faring aliens would likely have the technology to deal with the pressure their probe would have to withstand at a depth of 36,000 feet: eight tons per square inch.

“My experience is that nature doesn’t usually operate in long, linear features,” Hutchinson says. In other words, in gravity maps, the ocean trenches should stand out. Studying the trench, the visitors would learn what causes those long features: plate tectonics. The massive trench is the result of two plates converging in the subduction zone where the large, heavy Pacific plate is sliding slowly under the smaller, lighter Mariana plate. Learning how the plates move and how the movement creates trenches, volcanoes, mountains, and earthquakes would teach the aliens a lot about Earth’s geophysics.

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