Researchers interested in what makes the red planet tick can't study the planet in person-at least not yet. To help them interpret what they see in Mars images and other remote sensing data--and to test their instruments and procedures--they turn to Earth.

Though the two planets are very different, Earth offers many similarities, or analogs, to Mars. Some of these, such as Antarctica, are definitely off the beaten track. Others, however, such as Mt. Etna, are places where ordinary travelers might find themselves-- although perhaps unaware that what they're seeing is anything like our neighboring planet.

"A site can be like Mars in a variety of ways," says JPL geologist Dr. Tom Farr, one of the participants in the "Exploring Mars' Surface and its Earth Analogues" workshop at Mt. Etna. "Since Mars is really cold, the first places you think of are Antarctica and the Arctic. These places provide a way to see some of the processes that probably take place on Mars- glaciers and permafrost. But a place can also be like Mars by having similar geological features, such as volcanoes, or processes like erosion and weathering."

Volcanoes in Common
The prominent volcanoes on Mars are large, old and apparently no longer active. "Though small by Mars' standards, Etna, like the majority of volcanoes on Earth, is basaltic," says Mars Odyssey Project Scientist Dr. Jeffrey Plaut, who was also in the Etna workshop. "We believe that Mars' volcanoes have the same composition."

Etna also has an example of a volcanic process that scientists think may occur on Venus, the Moon, and possibly Mars, but until recently hadn't been seen before on Earth. "We see some long narrow channels on those planets that don't look like they were eroded by water," says Farr. "We inferred that they were produced by lava, but until their discovery on Etna, we had never actually seen that happen."

Not all of Earth's volcanoes match those on Mars. "Mt. St. Helens is not a good analog," says Plaut, "it's silica-rich and is a result of plate tectonics that do not seem to occur on Mars". For good examples of large shield volcanoes, the most common type on Mars, Plaut picks Mauna Loa and Kilauea. "The big island of Hawaii, which is the largest volcano on Earth, has been a tremendous Mars analog."

On Mars, super-sized volcanoes sculpted the landscape by releasing huge amounts of lava. It's possible to see what that sort of event did on Earth along the Columbia River in Washington. "Some of the largest lava flows on Earth took place there," says Plaut. The area was repeatedly flooded by lava, which formed the great basaltic cliffs called the Columbia River Basalts. And in Idaho's Snake River Plain, "rift lava seeped over a large flat surface creating a volcanic plain that serves as a good terrestrial analog for extensive sheet lavas on Mars as well as Venus and the Moon," says Farr.

Mars in the Desert
Earth's deserts have many examples of geological processes at play on Mars.

"Processes in arid environments tend to create dunes and landforms eroded and etched by winds like those we see on Mars," says Plaut. "We also like the desert because there's not much vegetation and the geology is exposed at the surface as it is on Mars."

Drier and cooler than most deserts, the Atacama in Chile is often considered a Mars analog. In the warm deserts of Tunisia at the edge of the Sahara and California's Mojave, wind-blown sand creates Mars-like dunes and landforms.

Deserts in North Africa, China, Asia, and North America are home to wind-sculpted ridges known as yardangs, also common in the Martian landscape.

On both planets, some of today's deserts were probably yesterday's lakes.

The same process that created Utah's Bonneville Salt Flats may have shaped the dry lakebeds that dot the Martian landscape. "Mars seems to have had catastrophic floods," says Plaut. "not unlike those that took place in the Bonneville area in the ice age. As the glaciers retreated, the rapid draining of a large lake carved up the landscape creating distinctive landforms. There was a lot of water and a lot of energy."

One of the most famous planetary analogies and laboratories is Death Valley.

"It's like Mars in its tectonic, erosional and sedimentation processes," says Plaut. "It looks like Mars, too. There's a spot called Mars Hill that reminds people very much of the Viking 2 Lander site."

Impact Zone
Many a meteor made its last stop on Mars and on Earth. Mars' surface is pockmarked with impact craters. Here on Earth, most are buried or have eroded away. The Haughton Crater on Devon Island in the Canadian Arctic is a well-known site for Mars-related studies. More accessible is the meteor crater in Winslow, Ariz. "It's fairly recent," says Plaut, "and well-preserved, like many we see on Mars."

Geology may not be all that Mars and Earth have in common. In the search for life in extreme environments, like those which may exist on Mars, researchers are looking in places like Yellowstone and Hot Springs, Ark.

"Because the Mars environment is so cold and dry, getting liquid water to the surface today may require hot spring activity," says Plaut. "But people are studying all kind of ground water environments, hot or not, and caves as possible Mars analogs."

As Mars exploration continues, new common ground between that planet and this one is likely to emerge. "Even as we learn more about Mars from the new missions," says Farr, "we'll go out and try to find places on Earth that are similar, continuing our search for better Mars analogs."

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