Still, there are ways to explore strange new worlds without leaving Earth. “Terrestrial analogues”—places where the geology or the climate mimics other planets in our solar system—are scattered across the globe. Astronauts and scientists use these sites to prepare for space missions and search for clues to extraterrestrial life. Spacesuits not required.
1. THE MOON OF ARIZONA
When Neil Armstrong first set foot in a crater, he wasn’t on the moon. He was outside Flagstaff, Arizona. In 1963, he and eight other Apollo astronauts visited Meteor Crater, one of the best-preserved impact sites on the planet, to see what they could expect on the lunar surface.
The scenery was quite different 50,000 years ago, when a 150-foot-wide iron-nickel meteorite interrupted the tranquil lives of the giant sloths, mammoth, and bison that roamed northeastern Arizona’s grassy hills and woodlands. When it struck the ground, it released the kinetic energy equivalent to a 15-megaton explosion, excavating 175 million metric tons of rock. The Earth’s crust melted at the impact site, and a fireball scorched some three miles of surrounding land. The crater it left is still three-quarters of a mile wide and about 600 feet deep.
This geological carnage offers a hands-on opportunity for scientists to understand cratering and the physical history of the moon. (Telescopes and orbital imagery alone don’t cut it, says David Kring, a senior scientist at the Houston-based Lunar and Planetary Institute, which organizes field studies at the site.) Visiting the crater was essential for astronauts during the Apollo missions. “One of the points that I often make, both with post- doctoral researchers and the astronauts, is that this is but a single crater here on Earth,” Kring says. “If you were standing on the rim of a similar-sized crater at the Apollo 16 landing site, within your field of view would be two other craters of approximately the same size.”
Today, Meteor Crater remains essential for researchers who are analyzing lunar meteorites or rocks collected by the Apollo astronauts. “They’re studying those completely without context,” Kring says. “If they can see the types of rocks that are produced in a real crater, it will enhance their ability to extract meaningful information from those samples.”
2. TRINIDAD’S TITAN
Saturn’s largest moon, Titan, is the indisputable badass of our solar system. The surface is so cold that ice is as hard as granite. Its bleak dune landscape is drenched by methane monsoons and pockmarked with hydrocarbon seas named for mythical monsters and mountain formations named after the works of J.R.R. Tolkien. Future explorers will say they sailed the Kraken Mare and climbed Mount Doom.
It’s hard to imagine, but Titan’s earthly doppelgänger is in the Caribbean. The black, gooey cousin of Titan’s hydrocarbon seas is the largest asphalt lake on Earth: Trinidad’s Pitch Lake. Legend says the lake once transformed into a viscous maw to swallow a tribe of Chaima Amerindians as punishment for eating hummingbirds that contained their ancestors’ souls. Sir Walter Raleigh made a pit stop there in 1595 to scoop up tar to caulk his ships, and by the 19th century, tons of asphalt was excavated and used to pave city roads worldwide. Today, the 114-acre lake teems with microbial life.
Each gram of hot toxic sludge contains a diverse community of up to 10 million microbes, which make their home in minuscule droplets of water and survive by feeding on hydrocarbons. Chemical analysis of the droplets suggests the water originated underground, perhaps from ancient seawater. That’s important because it means Titan may have a subsurface ocean, says astrobiologist Dirk Schulze-Makuch. Titan’s ocean may be a mixture of water and ammonia, a combination that has a lower freezing point than pure water. Titan may also be geologically active, meaning a hot interior keeps some of that water from freezing.
Put those facts together and you reach an intriguing conclusion: The heavier hydrocarbons at the bottom of Titan’s methane-ethane seas may be home to tiny droplets of water-ammonia. Kept in a liquid state, these could be home to microbes similar to those in Pitch Lake. Someday, scientists may learn that Titan is home to millions of tiny creatures from the Black Lagoon.
3. MARS IN CHILE
Mars had a promising life before it became the rusty, freeze-dried rock quarry we know today. Around four billion years ago, a cozy atmosphere kept the planet warm. Rivers of water emptied into lakes and seas. But after about 100 million years, the Martian atmosphere began leaking into space. As Mars slowly choked to death, its water froze. Much of it is still buried beneath its surface.
Things turned out better for Earth—except in Chile’s Atacama Desert. Spanning 40,000 square miles, Atacama is the driest place outside of Antarctica. While the average annual rainfall in most deserts hovers below 400 millimeters, Atacama is lucky to hit 2mm. Some areas have gone three to four centuries without a single drop! Wind and occasional tremors are the only natural forces that leave a mark. Some of the boulders scattered on the ground haven’t moved in one million to two million years.
Atacama is bone dry because it’s wedged between two mountain chains—the Andes and the Chilean Coast Range—that block moist air from entering. The Peru Current, which carries cold water from Antarctica along the coast, also keeps rain clouds at bay. Plus, the desert is on a plateau that’s 13,000 feet above sea level. The thin, dry atmosphere at that altitude, coupled with high levels of UV radiation, makes Atacama the closest thing earthlings have to Mars.
For engineers, the landscape is perfect for testing prototype Mars-roving equipment. More exciting, however, is that life is still eking out an existence in Atacama’s nearly sterile soil. Photosynthetic bacteria have been found inside local halite, or rock salt. The translucent crystals absorb sunlight but block lethal doses of UV radiation. The salt also swallows some water from the air, making life possible.
For scientists, this suggests that Martian saline deposits may be a viable habitat for alien life. The salt would lower the ice’s freezing point—so it could temporarily melt during the Martian spring and summer—and then absorb that water to sustain a community of tiny organisms.
4. CANADA’S ODE TO EUROPA
In 1990, a helicopter pilot flying over Ellesmere Island in the Canadian arctic ran into rough weather and took a detour through a valley called the Borup Fiord Pass. Geologist Benoît Beauchamp was on board, and he looked down to see a strange yellow patch on the glacier below.
A few weeks later, he returned with a group of students. “The aircraft had not yet touched the ground when the unmistakable smell of rotten eggs inundated the cabin,” he wrote in the journal Arctic. “While the students at the back of the ma- chine blamed each other for what they thought was an afterthought on a rather spicy meal the night before, it was clear to me that the smell came from the glacier itself and that it was the scent of hydrogen sulfide; as for the yellow stuff staining the ice: No doubt it had to be native sulfur.”
It was a surprising discovery. Sulfur is typically found at hot springs, volcanoes, or salt domes—not glaciers near the North Pole. Later, scientists learned that hydrogen sulfide was bubbling to the surface from underground saltwater springs. Microbes that had adapted to the cold environment then fed on the hydrogen sulfide, producing sulfur as a chemical byproduct.
That’s interesting because Jupiter’s icy, sulfur-rich moon, Europa, contains a salty body of water bigger than all of Earth’s oceans combined. If it’s anything like Ellesmere, the sulfur on Europa’s frozen exterior may be evidence of alien bacteria. To determine whether that’s the case, scientists tested Ellesmere. They’ve found telltale biosignatures in Ellesmere’s sulfur, including traces of protein and fatty acids and a rare mineral, rosickyite. NASA can use that chemical roadmap to look for life on Europa. All they need to do is scoop up some samples 390 million miles from home.