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Re-writing the cratering history of the Moon

Posted: October 14, 2008

In two separate reports planetary scientists have presented new insights into the cratering history of the Moon, and used small craters to help date the ages of geological features on Mars.

Counting the number of craters per unit area on a planetary surface has been the fundamental way of determining the relative ages of different terrains since the beginning of the Apollo era, when the exact ages of different lunar rock samples returned to Earth could be determined and matched to certain parts of the Moon to calibrate the system. The age of any surface can be determined by looking at the number of craters present, and this is plotted as a graph known as a chronology curve. In general, the more craters there are in a given area, the older that surface is, since it has been exposed to impact for a longer period of time than a young surface that may only have a few craters.

By counting craters, planetary scientists can determine the relative ages of different terrains. In this image, the large crater, measuring about 30 kilometres across, is relatively young since it contains very few craters in its interior compared with the heavily cratered surrounding terrain. Image: NASA.

“Sample analysis provides the real ages of the rocks, and hence of the lunar terrains,” describes Simone Marchi of Padova University in Italy. “These ages are fundamental in order to calibrate the chronology curve: without them it would have been impossible. But we only have ages for a dozen or so lunar regions, which cover 4.3 billion years of lunar history. Therefore we do not have accurate time sampling of the history of the Moon. For instance, there is no data points from one billion years to three billion years ago.”

In the new study, Marchi and colleagues from the German Space Agency in Berlin determined the cratering history of the Moon by modelling the incoming flux of impactors striking the lunar surface. “In our approach, we use the most updated dynamical models to obtain the flux of impactors, and hence craters, on the Moon,” he tells Astronomy Now. “This information has been used to derive a new chronology curve. This new approach has been possible only in recent times thanks to big improvements in the understanding of the inner Solar System dynamics, and also thanks to telescopic survey that discovered more and more near-Earth asteroids, which represent the incoming flux.”

Marchi’s model and the traditional crater counting model agree to within a factor of two, which, says Marchi, is surprising since they are derived in very different ways. “It may not seem a big difference comparing with old methods but indeed it is. There are a few discrepancies that need to be analysed and better understood that may be important.”

Indeed, the new results suggest an irregular flux in recent times, which has implications for the ages of young regions. “If you double the flux you reduce the age [of that surface] by two,” says Marchi.

For the moment Marchi has focused just on the Moon-Earth system, since the Moon acts as the clock to which all other planetary surfaces are set. But in another new and separate study, William Hartmann of the Planetary Science Institute in Arizona, and who first proposed the crater-counting system in the 1970s, applies the traditional crater counting method to small craters, to help work out the ages of different terrain on Mars.

This image, from NASA's Mars Global Surveyor, shows a young lava flow (darker formation in lower middle part of picture) lying atop an older surface on Mars (lighter regions, upper right and lower left). The lava flow has visibly fewer impact craters than the background terrain, illustrating the general principle that crater numbers can reveal ages of surfaces. Image: NASA, JPL and Malin Space Science Systems.

"Using small craters to measure the age of landforms is complicated," says. While the crater counting method is widely recognised as valid for large kilometre-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. This is because large craters are generally formed by a single event, but collections of small craters can form simultaneously when material from a large event is thrown into the air, falling as secondary debris that also form craters. Meteor showers also have the same effect.

To test whether small craters can reliably determine the age of
planetary features, Alfred McEwen, principal investigator for the HiRISE camera onboard the Mars Reconnaissance Orbiter, 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 these small craters should give roughly the age expected for the youngest crater.

Craters between about three and 16 kilometres in diameter that showed no sign of erosion such as complete crater rims, were used in the study and classified as ‘new’ craters. Young craters were needed because there is general agreement on the rate at which kilometres-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.

Some small craters are known as secondary craters since they result from debris thrown out of a single large impact crater. The white arrows in this image point to chains of secondary craters produced in the event that made the large crater on the left. It is important that these secondary craters are not biasing the results of crater counts. Image: NASA.

"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," says Hartmann. This gives the team 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 says. "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," says Hartmann. "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," says Hartmann. "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 colleagues have also 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 have also considered the closer proximity of Mars to the asteroid belt, which causes it to be hit by about twice as many meteorites as the Moon. And thanks to the high resolution cameras on Mars-orbiting spacecraft, the formation of craters can also be studied as it happens. Indeed, the Mars Global Surveyor Camera detected about 20 new craters forming during a seven-year period of observations, which allows planetary scientists like Hartmann to begin to measure how fast small craters are forming.

"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 says. "Given that ability, we'll soon understand the modern-day geological processes on Mars."

Both Dr Simone Marchi and Dr William Hartmann presented their research at the American Astronomical Society’s Division of Planetary Sciences meeting held at Cornell University this week.