“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.
The Power of Water: The Grand Canyon
A rover sent to study Earth’s geological history would do well to visit Arizona’s Grand Canyon, where the Colorado River has slowly, over millions of years, carved a slice through the planet’s crust. “If aliens were to land here, they would see a wonderful layer cake of Earth history,” says Richard Murray, professor of Earth sciences at Boston University.
Karl Karlstrom, a geologist at the University of New Mexico who has spent his career studying the canyon, calls it “one of the most spectacular and informative geological laboratories on Earth.” The first thing a visitor will see is how powerfully water changes the landscape. The Colorado River drains a huge area of the western United States, so “a visit here would provide information about water-driven erosion at the scale of half a continent,” he says. It would also show that different types of rock have different degrees of resistance to erosion. Not even a volcano can stop the water for long. “Lava dams formed several times over the last half-million years when volcanoes erupted on the rims,” says Karlstrom. “But each dam was quickly [over thousands of years] eroded by the not-to-be-harnessed river.”
A trip down into the mile-deep canyon would be a journey back through nearly half of Earth’s 4.5-billion-year history. The canyon floor has veins of pink granite made from cooled and hardened magma that rose to the surface when the North American plate collided with an island chain about two billion years ago. Moving up the canyon walls from the floor, sedimentary rock shows that the area was covered by a shallow ocean about 500 million years ago. Each layer tells more of the story, from the iron-rich sandstone formed from a muddy coastal plain about 280 million years ago (the iron gives the canyon much of its reddish color) to the limestone in its topmost layer, which is more than 250 million years old.
History on Ice: Antarctica
Studying an ocean trench or volcano would teach extraterrestrials that Earth’s crust is continually moving, and that this movement forms many of our planet’s most conspicuous features. But if they land their probe on an ice sheet, they’ll get a slice of history.
Cin-Ty Lee, a Rice University geochemist and petrologist who calls himself a “natural historian of the Earth,” would send a rover to an ice shelf, like Greenland or Antarctica’s Ross Ice Shelf, not far from where the U.S. McMurdo research station is located. There, a rover outfitted with proper equipment could drill down to the base of the ice sheet. An ice core sample, Lee says, is “like a tape recorder of the planet.” The thick sheets are created over millions of years, as snow and sleet falls, carrying samples of the atmosphere with it. The snowfall is gradually compacted, forming discrete layers as the seasons change. Analysis of contaminants in the layers can reveal when a volcano erupted. Trapped bubbles can show the amounts of oxygen, carbon dioxide, and other gases in the atmosphere over time, and water isotopes are evidence of temperature changes.
Given enough time to look around, the rover would discover that the ice shelf is holding back enormous glaciers, which otherwise would slide rapidly toward the sea. The aliens “would recognize how glaciers dramatically transform landscapes over thousands of years,” says the Smithsonian’s Brent Grocholski. A clever alien scientist might notice that glaciers come in two main types: Alpine, which forms in mountains, and continental—large unbroken sheets. Both can be studied in Antarctica.
The Heights: Everest
Having pointed our extraterrestrials to Earth’s deepest spot, should we also send them to the tallest peak? In fact, 29,029-foot-high Mount Everest, part of the Himalayan range in Asia, would reward any spacefaring geologist.
“As the point of highest elevation on the planet, it is natural that any alien may want to land there,” says Richard Murray. But our visitors don’t even need to land on the summit. Anywhere in the vicinity of Everest, aliens would see dramatic evidence of Earth’s tectonic plates plowing into each other. Unlike the Mariana Trench, where a heavier plate is burrowing under a lighter one, the Himalayas were created 40 million years ago when two light plates, the Indian and Eurasian, crunched together, pushing the land upward. If the alien probe stayed around to take occasional altitude measurements, it would detect that these already tall mountains are getting even taller, by almost an inch a year.
What if the rover did land at the summit? “The aliens may be a bit confused,” says Murray, “because Mount Everest includes fossils from shallow seas.” Before the land rose, the Tethys Sea sat between the two colliding landmasses, and because its sea floor is made up of light rock, the collision produced mountains instead of a trench. About 20 million years after the range began to form, the water dried up, but the remains of long-dead sea creatures continued to rise. “The visitors may wonder why fossils would be found so far above sea level,” Murray says, “but if they knew what was going on, it would certainly tell them how geologically active our planet is.”