Atmosphere blocks many small stony asteroids. (Protective Blanket).

Although Earth and the moon inhabit the same cosmic neighborhood, our planet has far fewer scars from extraterrestrial impacts because incoming objects burn up in its atmosphere. A new computer model suggests that Earth's thin layer of air is an even better shield than previously thought.

Scientists have identified fewer than 200 impact craters on Earth (SN: 6/15/02, p. 378), and only for a few do they suspect the type of object that gouged the hole. Of those, most were blasted out by asteroids, which come in rocky and iron-rich varieties, says Philip Bland, a planetary scientist at Imperial College in London. From observations of asteroids in space, and analysis of the composition of their meteorite remnants on Earth, scientists believe that about 5 percent of the asteroids that enter the upper atmosphere are of the iron-rich type.

That proportion roughly matches the data from Earth's craters larger than 10 kilometers in diameter and of known origin, says Bland. However, 16 of the 17 craters less than 1.5 km across and with known impactor types apparently were blasted by iron-rich bodies. According to Bland and Natalia A. Artemieva of the Russian Academy of Sciences in Moscow, the disparity stems from the filtering effect of Earth's atmosphere.

The researchers' computer simulations, described in the July 17 Nature, calculate the aerodynamic forces on an object passing through Earth's atmosphere. Besides affecting the body's motion, these forces pummel it, break it apart, and burn away its surface, says Bland. Because a mass of rock doesn't withstand such pounding as well as a lump of iron does, most small rocky asteroids don't survive their trip through the atmosphere. The team's model suggests that even large rocky bodies partly disintegrate but that remaining chunks can still smash a big crater.

On average, the model suggests, asteroid fragments at least 3 meters across and capable of blasting a crater 100 m wide will strike Earth once every 300 years or so. A piece of asteroid about 220 m across--one large enough to cause a dangerous tsunami if it were to strike the ocean--might smack Earth only once every 170,000 years. That's a rate about one-fiftieth of that estimated by other scientists using other models. However, Bland notes that major impacts might occur even less frequently because stony asteroids may not be as strong or as dense as he and Artemieva assumed.

The type of aerodynamic simulation the researchers used is far better than simulations used in previous analyses, says Douglas O. ReVelle, an atmospheric scientist at Los Alamos (N.M.) National Laboratory. However, he notes, observations of meteors disintegrating high in the atmosphere suggest that the objects' strength and density vary widely, so Bland and Artemieva's results may still be no more than ballpark estimates of how frequently objects of various sizes punch through the atmosphere.

Nevertheless, the team's report is "important work" that suggests that Earth isn't as vulnerable to extraterrestrial impacts as some scientists had thought, says William K. Hartmann of the Planetary Science Institute in Tucson. Also, he notes, similar simulations should shed light on the cratering rates on other celestial objects with atmospheres, such as Venus, Mars, and Saturn's moon Titan.