The crater-counting system that scientists have used since the 1970s to determine the age of large geologic features on Mars will also 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.
Hartmann, who works out of PSI’s Tucson office, presented the results of his study at the Division of Planetary Sciences meeting, which began yesterday and is running through Wednesday at Cornell University in Ithaca, N.Y.
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 a longer periods and have been hit by more meteorites than younger surfaces.
While the method is widely recognized as valid for large, miles-wide craters, some scientists had questioned whether the rate at which small craters form is well enough understood and constant enough to be trusted in predicting the age of a landform.
The issue didn’t arise until 1997, when the small craters first became visible in images returned by the Mars Global Surveyor high-resolution cameras. In recent years, many more high-resolution images have come from the HiRISE camera aboard Mars Reconnaissance Orbiter.
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 the large craters are formed by a single event, many small craters can be formed simultaneously when a large meteorite slams into the planet and throws debris into the air, which 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 reliably 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 the youngest large craters on Mars. 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 the crater is “new” they 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, scientists 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 two to 10 miles in diameter that have perfectly sharp rims that show no signs of erosion, indicating that 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 ten percent or the last ten 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 very 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 one 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 wonderfully well, particularly after astronauts returned rock samples from the moon that could be dated by radiometric methods. 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, which causes it 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 seven-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.”
An electronic version of this release and an accompanying image is available at: http://www.psi.edu/press/archive/20081009cratercounting/
CONTACT:
William K. Hartmann
Senior Research Scientist
520-322-8925
hartmann@psi.edu
PSI INFORMATION:
Mark V. Sykes, Director
1-520-622-6300 sykes@psi.edu
PSI Homepage http://www.psi.edu
THE PLANETARY SCIENCE INSTITUTE:
The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to solar system exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972.PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork in North America, Australia and Africa. They also are actively involved in science education and public outreach through school programs, children’s books, popular science books and art.
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