Venus has its volcanoes. Mars has a canyon grander than any on Earth. Eros the asteroid is pockmarked by impact craters and littered with boulders.
Many Sun-orbiting objects have geologic features that are analogous to those here on Earth. New research reveals that even comets, the dirty balls of ice from the edge of the solar system, can remind us of home.
Last September, while on its last leg, the Deep Space 1 (DS1) spacecraft zipped by comet Borrelly, taking some of the most detailed images ever of a comet's core. Examining these images, scientists noticed mesas, ridges and hills, all resembling terrestrial surface features.
On Earth, analogous structures are carved out largely through the erosive forces of wind and rain. On a ball of dust and ice (with perhaps some rock) hurtling through space, however, geology is formed when a material turns directly from a solid into a gaseous state, a process called sublimation.
"It's basically all physics," says Dan Britt, a geologist from the University of Tennessee and a member of the DS1 science team.
The mesas on Borrelly are more than 300 feet (100 meters) tall and can be 20 times as wide. Britt says they resemble the mesas in the American Southwest, which are formed when a cap of hard rock overlies softer material that erodes faster. The cap acts as a type of shield.
On Borrelly, the caps are made of the dust and rock left behind when volatiles, such as water and methane ice, sublimate. Sublimation continues from the sides of the mesa, and a resistant cap finally drops down when it is undermined, Britt said. During the course of Borrelly's seven-year orbit around the Sun, Britt adds, the mesas erode as much as thirty feet (10 meters).
There are also regions on Borrelly that experience slower sublimation-related erosion, a fact Britt figures is responsible for making the hills and linear features on the comet. Overall, he says, sublimation removes about three feet (1 meter) of Borrelly every cycle.
"That's actually pretty active erosion, even in geologic terms," Britt said in a telephone interview. "If your yard eroded one meter every seven years, you'd be upset."
Britt and his colleagues also observed ridges on Borrelly. These were formed, they believe, when one part of the comet broke off and was pushed back at an angle.
"When you have two moveable objects pushing against each other, you make ridges," he said. "That's how you make mountains on Earth."
As simple as it may sound to draw a correlation between Earth and comets, these findings are somewhat surprising, Britt says.
"Comets, up until now, have been really astronomical objects, sort of dots on a photographic plate, or blobs," he said. "I've never really thought that a ball of ice and dust would make interesting surfaces and have interesting processes and produce interesting pictures."
Britt and his colleagues compiled DS1's images of Borrelly to create 3-D composites. They then carefully examined and measured the features on the comet. They presented their findings earlier this month at a meeting of the Division of Planetary Sciences in Birmingham, Alabama.
Astronomers have been taking pictures of comets for more than a decade, but none have been as detailed as those of Borrelly, made when DS1 passed within 1,250 miles (2,000 kilometers) of the comet.
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