Overview - page 4

Mars/Earth Analogs

Since scientists can't yet travel to Mars to study its geologic features in person, they're doing the next best thing: finding similar landforms and environments on Earth that might provide clues to how things work on Mars. One research team is studying craters on Earth to develop a set of criteria that distinguish two different types of craters: impact craters, which are created when a comet or meteor strikes a planet, and maar craters, which form as a result of volcanic processes. Through a series of field studies at Meteor Crater in Arizona (an impact crater) and El Elegante Crater in Sonora, Mexico (a maar crater), the researchers can compare on-the-ground observations with satellite imagery from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument. This process will enable them to learn which characteristics are associated with each of the two crater types. Next, they can begin to apply these criteria to craters they see on Mars via imagery from the Thermal Emission Imaging System (THEMIS), an instrument that currently flies onboard the Mars Odyssey Orbiter and produces high-resolution images that have nearly the same wavelength and pixel size as ASTER imagery. Since maar craters are associated with underground liquid water, the ability to discern these craters from impact craters will help scientists learn about the existence of water-driven features on Mars, which could be an indicator of the planet's climate history.

Impact Craters on Mars

The surface of Mars is covered with impact craters, bowl-shaped depressions that are created when an asteroid or comet collides with a planetary surface. Scientists estimate that on Mars, there are more than 43,000 impact craters with diameters greater than 5 kilometers (3 miles). Yet, only about 120 impact craters have been identified on the surface of the Earth. Both Mars and Earth experienced a period of intense bombardment by interplanetary debris early in their geologic history, which occurred about 4 billion years ago. So why don't we see a similar number of impact craters on both Earth and Mars? For one thing, the Earth's surface is still geologically active. Because of ongoing volcanic eruptions and plate tectonics, many land surfaces are still fairly young. In addition, erosional processes happen much faster on Earth than on Mars, largely because Earth has flowing water on its surface and rain. So, if you look at the Earth's surface, your first impression would be that there aren't many impact craters. In reality, lots of older impact craters exist on Earth, but they've been "erased" from view by erosional processes and geologic activity.

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