NEAR Science Update

Science Update August 07, 2000

Perhaps the most prominent features on the surface of Eros are the craters, which are scars left by impacts on Eros. The craters are important because they tell us something of the age and the history of Eros, but also because in some cases they provide hints about what lies beneath the surface.
What we know about cratering is based mainly upon studies of craters on the Earth and the Moon, as well as laboratory experiments and theoretical modeling. Craters are found on nearly all the objects in the solar system, except that the gas giants do not have solid surfaces and therefore do not have craters (though we remember that the impact of Shoemaker-Levy 9 on Jupiter left darkened spots in the atmosphere that persisted for months) and the planet Pluto has never been imaged clearly enough to distinguish any craters. Likewise, no comet nucleus has been imaged clearly enough for us to be convinced that we have seen impact craters, and because of cometary activity, we are not even sure if we should expect to see them. On Earth, however, we know of more than 100 impact structures. The first of these to be identified was the Barringer Crater (which is also called Meteor Crater) in Arizona. This 1 km diameter crater is named after Daniel M. Barringer, who concluded in 1906 from geologic arguments that the crater was formed by the impact of an iron meteorite and not by any type of volcanic activity. Barringer was a well-to-do fellow who graduated from Princeton University in 1879 and who went hunting with Theodore Roosevelt. He also owned a silver mine, and he hoped to make a fortune by mining meteoritic iron beneath the crater. Unfortunately for him, the amount of iron beneath the crater was far less than he believed, because he thought that the meteorite must be almost as large as the crater and must still be beneath the crater. Barringer was wrong about the physics of hypervelocity cratering, and his Meteor Crater Exploration and Mining Company ran out of money and shut down in 1929. He died shortly thereafter, having lost nearly all his fortune (who says physics doesn't matter?). In any case Barringer is remembered for championing the impact origin of Barringer Crater, as is our namesake Eugene Shoemaker who proved the impact origin of the crater conclusively in the early 1960s (see update of March 22, 2000).
Barringer Crater is only 50 000 years old - barely an instant in geologic terms. Much older craters are found on the Moon, which underwent a period of especially intense bombardment from about 4.5 to about 3.8 billion years ago. The Earth underwent a similar bombardment at that time, but the surface features created then on Earth have been long since destroyed by weathering and plate tectonics. The cratering history of the Moon was reconstructed from lunar samples returned by the Apollo missions. While the relative chronology of lunar craters had already been pieced together, the absolute chronology was fixed by radioactive dating of Apollo samples. The total number of impacts experienced by the lunar surface was determined as a function of the surface age and the size of the impact. The approximate ages of areas on the Moon could then be estimated from the numbers of craters of a given size per unit area - the more heavily cratered an area is, the older it must be.
The lunar cratering chronology is a triumph of planetary science, but we cannot be sure how well it applies to other planets and other bodies in the solar system. This is because we do not know cratering rates may have varied with time and with location in the solar system. We have not been able to construct an absolute cratering chronology of any object other than the Moon, because the Moon is still the only object in the solar system from which samples have been returned to Earth.
Where does this leave us for Eros? We are not able to give absolute ages for regions on Eros. Although Eros is a near-Earth asteroid, it probably spent most of its lifetime somewhere in the main belt of asteroids between Mars and Jupiter - we can't say where - and will spend only a brief geologic interlude, on the order of a hundred million years, close to its present orbit (a topic for another time). Most of the craters on Eros presumably formed while it was in the main belt, in the midst of a vast swarm of potential impactors. We do not know what the cratering rates would have been at that time. We are confident, however, that areas on Eros on which very few craters are found, such as the saddle (see images of the day for May 31, June 2, June 26 or July 6) must have relatively young surfaces. But Eros is a primitive asteroid and must be very old (at least 4.5 billion years old), so these relatively young surfaces must have been wiped clean of craters. Our challenges are to figure out how this 'resurfacing' may have happened, and at least to piece together a possible sequence of events, even if we can't say just when these events may have occurred.

Andrew Cheng
NEAR Project Scientist Science Update Archive