Fresh Impacts Viewed on Mars, Moon
New impacts observed on the moon and Mars allow space scientists to learn about crater formation in near real time. What conclusions can be drawn?
Flash shot: On March 17, a flash was reported on the moon by NASA cameras (see video clip on Space.com). The object was the size of a small boulder going 56,000 miles per hour. The crater is estimated to be 65 feet wide. It was the brightest of 300 such impacts seen, by a factor of ten, since NASA monitoring lunar impacts in 2005. The video clip states, “Lunar meteor showers have turned out to be more common than anyone expected.” About 55% come from known meteor swarms; the others are random stragglers.
The Earth’s atmosphere protects us from many such objects, but the fireball over Russia last Feb. 15 (Space.com), made by an object estimated to be 50 feet across (50 times larger than the lunar impactor), made big news as the biggest meteoroid to hit Earth in more than a century.
Orbiting spacecraft like the Mars Reconnaissance Orbiter (MRO) are allowing planetary scientists to gather “ground truth” data about meteoroid impact rates on another planet. A press release from the University of Arizona discussed the 250 fresh craters detected by the high-resolution camera on MRO, based on before-and-after images. Even though this implies hundreds of hits per year, the rate is lower than expected:
Taking before and after pictures of Martian terrain, researchers of the UA-led HiRISE imaging experiment have identified almost 250 fresh impact craters on the Red Planet. The results suggest Mars gets pummeled by space rocks less frequently than previously thought, as scientists relied on cratering rates of the moon for their estimates.
Can the new data provide any information on the age of Mars? “Estimates of the rate at which new craters appear serve as scientists’ best yardstick for estimating the ages of exposed landscape surfaces on Mars and other worlds,” the article says. There are, however, many variables to consider. Meteoroids span a whole range of sizes, from dust particles to asteroid-size, each with its own probability of impact. Objects above a certain threshold can spawn secondary impacts (9/25/07), making date calculations essentially unreliable (see 5/22/12)
Today’s rates, furthermore, cannot be extrapolated into the distant past without making unverifiable assumptions. So when Alfred McEwen says, “Mars now has the best-known current rate of cratering in the solar system,” he can only speak authoritatively of knowledge in the observational period of the space program.
In addition to the major problem of secondaries, consider other complications when making calculations of crater rates and dates: speed of the impactor, angle of impact, gravity of the target, composition of the impactor and the target surface (e.g., porosity), atmospheric drag, magnetic field, sunlight pressure, focusing effects of other orbiting bodies. Above all else it is impossible to know whether impact rates have been episodic. One major swarm can throw all the dates off. Planetary scientists try to infer a “Late Heavy Bombardment” from lunar data, but as we have seen, that inference is not without critics (4/26/12, 1/09/12, 9/17/10, 2/16/10).
By analogy, there are rivers in large canyons (including the Grand Canyon and, at Mt. St. Helens, Loowit Canyon) that did not carve the canyons—the rivers are relicts of catastrophic events. Modern viewers might look at these canyons and conclude that the canyons formed by slow, gradual erosion over long periods of time, but we know from Mt. St. Helens that is not true. The Yellowstone fossil forests were similarly thought to have taken eons of slow processes, but now are thought to represent catastrophic deposits.
So when you look at a crater-filled moon or Mars, or any of the other bodies in the solar system, you cannot know just from the number of craters how old the surface is, no matter what the textbooks and TV documentaries say. If you won’t take our word for it, look at what Xiao and Strom said in Icarus a year ago (5/22/12).