More than half a century ago, a young geologist named Eugene Shoemaker took one of his first road trips through the Southwest. He and his wife, Carolyn, were down to pennies and short on time, but they wanted to see the famed Meteor Crater, 35 miles east of Flagstaff. The crater's origin was the subject of some debate: Despite its name, many scientists suspected the 4,000-foot-wide hole in the ground was actually the result of a volcano.

The Shoemakers pulled off the entrance road to the privately owned attraction and climbed to the rim, stealing a quick peek. The volcano explanation just didn't sit right with the fledgling geologist.

A few years later, new research suggested that Meteor Crater had been formed by a collapsed salt deposit. But Shoemaker brought rocks from the crater to his laboratory and found quartz that had been fused by temperatures too hot for either a volcano or a salt deposit. Meteor Crater's real nature was clear: Only an extraterrestrial impact could have made it. Shoemaker then turned his microscope on samples from the 1.5-mile-wide Upheaval Dome at Canyonlands National Park, where he found evidence of a long-ago pummeling and a shock of heat -- again indicating an ancient, other-worldly intruder.

The deserts of the Southwest, Shoemaker realized, not only revealed astronomical history; they could also serve as models for extraterrestrial landscapes. Shoemaker set up the U.S. Geological Survey's Astrogeology Branch in Flagstaff in the 1960s, and later became a key leader of the Apollo program. Thanks in part to his influence, the desert Southwest became a proving ground for lunar landings, including the July 20, 1969, landing of Apollo 11 -- the first time human beings walked on the moon.

During the Apollo era, Flagstaff-based geologists and their photographer partners made astronaut-training videos at New Mexico's Zuni Salt Lake and tested lunar rovers at Bonito Crater northeast of Flagstaff, where the rough, rocky surface mimicked what some geologists expected to find on the moon.

Astronauts in training flew over Sunset Crater in Cessna 182s, taking aerial photos of landforms in preparation for similar research on the moon. The comparisons continue today. Just last fall, NASA tested a pressurized rover at its research compound at the Black Point lava flow, on Babbitt family ranchlands north of Flagstaff. This September, NASA and the Houston-based Lunar and Planetary Institute will conduct a 14-day lunar mission simulation at the same site.

Meanwhile, space missions have visited our nearest siblings in the solar system -- and discovered that some of them look a lot like home. Recent missions to Mars have revealed copious salt deposits, evidence of a warm, wet early history -- not unlike more recent history in the Southwest, nearly 600 million years ago, when much of the region was submerged under shallow seas, and marine life left its mark as the fossils we find today. Like the rivers, rims, steppes and ridges of the West, many of Mars' gullies, canyons and branched channels were carved by water, ice or both.

On Mars, the NASA rovers Spirit and Opportunity have found silica deposits indicating historic Yellowstone-like geysers. And last year, NASA's Phoenix lander spotted falling snow, which evaporated before reaching the ground -- much like virga in the Southwest.

Perhaps these extraterrestrial echoes are part of the reason so many of us love the Southwest. Here, Earth is naked. Its crust has been shaped by magma, water and ice. Like Mars and the moon, its surface has been gouged by meteorites. When the Earth's rotation pushes the desert toward the sun, it is lit and heated mercilessly. Turned away, it is cold. In the Southwest, it's clear that the Earth is made of the same building blocks, and shaped by the same processes, as any other rocky planet near our Sun.

But although life may exist elsewhere in the universe, human beings are probably unique. Earth went through a long and improbable series of steps -- first geological, then biological -- to support life and, eventually, the phenomenon of human consciousness.

So the Southwest is a place where we can contemplate the barest bones -- the iron and magnesium, silica and dust -- that connect us to other planets, all the while marveling at the consciousness that allows us to entertain such notions in the first place.

And when we hike our sky islands, descend deep into canyons, and wander across the vast, open deserts, we now can imagine other, more distant explorations. The latest maps of Mars, for example, are just as inked up with names as the local U. S. Forest Service map. They are standing invitations to daydream of hiking along the Valles Marineris, a Martian canyon 3,000 miles long and 300 miles wide. Or of climbing Olympus Mons, a 15-mile-high volcano -- perhaps on an evening when the Earth eclipses the Sun.

Anne Minard, a freelance science writer, is currently a Ted Scripps Fellow in Environmental Journalism at the University of Colorado in Boulder.