October 14, 2008
The crater-counting system that scientists have used since the 1970s to determine the age of large geologic features on Mars also will allow them to date small features, such as riverbeds and lava flows, according to William K. Hartmann, a senior scientist at the Tucson-based Planetary Science Institute (PSI), and a member of Astronomy's Editorial Advisory Board.
Hartmann presented the results of his study at the Division of Planetary Sciences meeting, which began October 10 and is running through October 15 at Cornell University in Ithaca, New York.
Crater counting relies on the density, or crowding, of craters to determine the age of planetary surfaces. It works on the assumption that older landforms have been exposed for longer periods and more meteorites have hit them than have hit the younger surfaces.
While most scientists recognize this method as valid for large, miles-wide craters, some scientists think the rate at which small craters form is not understood well enough and is not constant enough to predict the age of a landform accurately.
The issue didn't arise until 1997, when the small craters first became visible in Mars Global Surveyor high-resolution camera images. In recent years, the HiRISE camera aboard the Mars Reconnaissance Orbiter has produced a large selection of high resolution images.
The crater-counting system, which Hartmann first proposed in the 1960s, was originally developed for counting large craters that are several miles wide.
"Using small craters to measure the age of landforms is complicated," Hartmann observed. While a single event forms large craters, many small craters can form simultaneously when a large meteorite slams into the planet and throws debris into the air. This smaller debris then falls as secondary meteorites, he explained. Meteor showers also can produce many small craters in a short time.
These phenomena caused some scientists to question the lower limits of crater-counting accuracy.
To test whether small craters can determine the age of planetary features, Alfred McEwen, principal investigator for the HiRISE camera, proposed that researchers add up the number of small craters that form inside some of Mars' youngest large craters. The idea was that, if the system works, these small craters should give roughly the age expected for the youngest crater.
Young craters were needed because there's general agreement on the rate at which miles-wide craters form on the red planet. If a crater is "new," scientists can assume it formed sometime between now and a single interval of crater formation. For instance, if a certain size crater forms every 500,000 years, they can assume that a new one formed between zero and 500,000 years ago. This result is based on the fact that all the small craters would have formed after the large crater was created, Hartmann explained.
McEwen, of The University of Arizona Lunar and Planetary Laboratory, and his colleagues identified some of the youngest large craters in HiRISE images, Hartmann said. These were craters in the range of about 2 to 10 miles (3 to 16 kilometers) in diameter with sharp rims that show no signs of erosion, which suggests they formed in recent geologic time.
"We've tested this theory on about eight of those large craters and in every case the count of small craters has given the expected approximate age," Hartmann noted. This gives him confidence that counting the number of small craters around other martian formations, such as a dry river channel or lava flow, will yield an accurate age for that feature.
"Of course, we never claim that this method has the precision of radiometric dating that can be done with an actual rock sample," he said. "But it is valuable to know if the features we're looking at were formed in the first 10 percent or the last 10 percent of the planet's history."
Some planetary scientists had previously suspected that the age estimates produced by counting small craters could be off by as much as a factor of 1,000. "We have found it's within a factor of two, which sounds pretty good to planetary scientists," Hartmann said. "It really allows us to sketch out the overall history of the martian surface."
A factor of two means that if crater counting shows a feature was formed 20 million years ago, it's likely to be at least between 10 million and 40 million years old.
"Whether it's 10 million or 40 million, that's still incredibly young on Mars," Hartmann said. "It's within the last 1 percent of the planet's history, and that's what's important. You don't want to go around saying there are features formed by water within the past 10 million years and then discover they are billions of years old."
Hartmann and his colleagues have used crater counting to determine the age of surface features on Mars since the 1970s.
The system has worked well, particularly after astronauts returned rock samples from the Moon that radiometric methods could date. Lunar crater densities could then be tied directly to rocks of known age to accurately calibrate the system.
Over the years Hartmann and others have further refined the crater-counting system by taking into account factors such as the effect of the martian atmosphere on slowing and burning up small meteors. They've also factored in the closer proximity of Mars to the asteroid belt. This location causes the planet to be hit by about twice as many meteorites as the Moon.
The high-resolution cameras now circling Mars also are making further refinements possible, Hartmann said. The Mars Global Surveyor Camera detected about 20 new craters forming during a 7-year period of observations. "This was a tremendous advance," Hartmann said. "Now we can actually begin to measure how fast small craters are forming, how long it takes for a 10-meter-wide crater to form in a square mile, for instance."
"Once you know those rates, then you can begin to get dates for small features on Mars without even having to go there to pick up rock samples," he said. "Given that ability, we'll soon understand the modern-day geological processes on Mars."