Square Craters

In the pantheon of cosmic geometry, curves rule. Astronomy texts are filled with spiral galaxies, elliptical orbits, and ring nebulae. There are no chapters on triangles or rectangles -- after all, who ever heard of a square planet? Some of the simplest shapes, common in the handiwork of humans, are just plain rare in space. Rare, but not impossible...

Last month, astronomers were studying pictures of asteroid 433 Eros when they noticed some unusual craters. Most impact craters are circular, but these were square!

Right: NASA's NEAR-Shoemaker spacecraft spotted these square-shaped craters on asteroid 433 Eros. [more information]

An overzealous fan of Star Trek might mistake the impact scars for places where cube-shaped Borg vessels touched down and lifted off again, but scientists say they are natural -- albeit unusual -- features.

"These square craters are not just novelties, they tell us something very interesting," says Andy Cheng of the Johns Hopkins University Applied Physics Laboratory. Cheng is the project scientist for NASA's Near Earth Asteroid Rendezvous spacecraft, which is orbiting Eros. "It's an indication that Eros is permeated with an extensive system of fractures and faults.

Typically on Earth when we find this type of fractured area, the fractures form intersecting systems. Craters in such a terrain look square; we call them jointed craters. The best example is the Barringer Meteor Crater in Arizona."

Square craters add to accumulating evidence that Eros is riddled with cracks and ridges that extend the entire 33 km length of the peanut-shaped space rock. "We first saw long grooves in global pictures of the asteroid when NEAR was entering orbit around Eros in February 2000," continued Cheng. "Now, if we look carefully, most of the closeup pictures seem to show signs of grooves and ridges."

"We have to ask ourselves how these cracks could have formed. Presumably they are the result of large impacts. The question is: did these impacts take place after Eros was its present size and shape or while Eros was part of a larger parent body?"

It's a question that goes to the heart of the asteroid's origin.

Scientists believe that billions of years ago, when the solar system was young and planets were newly-forming, Eros circled the Sun in an orbit between Mars and Jupiter. It was a denizen of the asteroid belt. Since then, collisions with other asteroids and gravitational perturbations by Mars and Jupiter have altered Eros's orbit, so that now it comes close enough to Earth to study with spacecraft like NEAR.

We know a great deal about Eros today, but what was it like at the dawn of the solar system, before it became a "Near-Earth" asteroid? Was Eros once part of a moon-sized planet between Mars and Jupiter, or has it always been an isolated space rock?

Above: The Barringer "Meteor Crater" on Earth has a square-shaped rim, indicative of fractured terrain around the impact site.

"If continued mapping confirms that faults and ridges extend from one end of Eros to the other, I would consider it to be strong evidence that Eros is a piece of something that was once much larger," says Cheng. If all of the rocks in the modern-day asteroid belt were assembled, they would form a small planet about 1500 km in diameter -- roughly half the size of Earth's moon. Such a body might have existed in an orbit between Mars and Jupiter billions of years ago, before it shattered as a result of collisions with other planetoids.

But if Eros is a "chip off the old block," there's a new mystery to consider. When rocky planets like the Earth and its moon (and maybe the parent body of Eros) are formed, heavier elements sink to the core while lighter ones remain near the crust. This leads to a core-mantle structure with distinctive chemical signatures in each layer.

The looming conundrum is that Eros does not exhibit the chemical signatures of differentiation. NEAR X-ray spectrometer data show that aluminum, magnesium, and silicon on Eros have the same relative abundances that they do in the Sun and in the early solar nebula. Evidently, Eros was not part of a body that experienced the Earth-like process of heating and segregation of metals from silicates to form an iron core and rocky mantle.

"Eros is an example of a very primitive body ... nothing much has happened to it other than formation and cratering. If you want the most pristine material in the solar system [where very little has happened] Eros is a good example," says Joe Veverka, professor of astronomy at Cornell University, and the principal investigator for two of NEAR's cameras.

Can Eros be both -- a primitive, undifferentiated body and a fragment from a long-ago planetoid? It's a possible contradiction that puzzles researchers.

"Even before we visited Eros we knew that asteroids were a mixed group -- some appear to be differentiated and some not," says Cheng. "The largest asteroid of all, 933 km-wide Ceres, is not differentiated. Yet, we believe it's possible for objects even smaller than Ceres to melt and chemically segregate. We simply don't know why some asteroids appear to be more primitive than others. We have to reserve a little skepticism here and pursue this mystery."

Above right: This global mosaic of asteroid Eros shows some of the grooves that hint at fractures and faults that may permeate the rocky asteroid.

Cheng says that a global map of Eros's grooves and ridges -- and possibly more square craters -- will likely shed new light on the asteroid's history. For now researchers and asteroid enthusiasts wait with anticipation as NEAR Shoemaker continues its first-ever and often surprising survey 433 Eros, knowing that the best answers and most perplexing mysteries may be yet to come.