Crash! 



A close encounter of the cometary kind 



By Charles Liu 



The term "astrogeology" (from 

 the Greek for "star" and 

 "earth") is either an overgen- 

 eralization or an oxymoron. If you're 

 an astrogeologist, do you study Earth, 

 or the universe beyond Earth, or both? 

 And if you study both, what else is left? 

 So, let's avoid the paradox by defining 

 astrogeology as the study of cosmic ob- 

 jects with the methods and tools de- 

 veloped for studying our own planet. 



It's easy to see the attraction of this 

 rapidly growing subfield ot astronomy. 

 What rock hound wouldn't love to 

 chisel into a Martian mountain or crack 

 open a geode on Ganymede? And tru- 

 ly dedicated astrogeologists aren't about 

 to be deterred by something so mun- 

 dane as, say, a multimillion-mile com- 

 mute. With scientific ingenuity, no 

 stone is too far out in the solar system 

 to stay unturned. 



Maybe the most explosive example 

 of astrogeological research so far is the 

 Deep Impact mission, led by Michael 

 E A'Hearn, an astronomer at the Uni- 

 versity of Maryland in College Park. 

 On July 3, 2005, the Deep Impact space 

 probe released an 800-pound, battery- 

 powered "smart impactor" the size of 

 a washing machine into the path of 

 Comet 9P/Tempel 1 , then some 80 

 million miles from Earth. Twenty-four 

 hours later, cameras on probe and pro- 

 jectile, as well as elsewhere in space and 

 on Earth, snapped away as comet and 

 impactor barreled toward each other at 

 more than 20,000 miles an hour. Then, 

 caught on images that will be analyzed 

 for years to come, came the ultimate 

 Independence Day fireworks. 



The energy of the crash, which pul- 

 verized the impactor into a fine cop- 

 pery mist, was like detonating nearly 



five tons of TNT or — 

 tor those of us who 

 enjoy more visually en- 

 gaging comparisons — 

 like dropping 800,000 

 baby grand pianos out 

 ot a very large third- 

 story picture window. 



But aside from the 

 visceral joy of watching, 

 what's the point of causing such a crash? 

 First of all, the collision gouged a hole 

 in the side of a comet the size of a foot- 

 ball field and an estimated hundred feet 

 deep. That gave astronomers the first- 

 ever view beneath the surface of one of 

 those mysterious wanderers. Maybe just 

 as important, knowing a bit more about 

 what's inside a comet might help hu- 

 manity avoid extinction if we ever dis- 

 cover a comet streaking toward Earth. 



c 



omet Tempel 1 is, by all mea- 

 sures, an ordinary comet, orbit- 

 ing the Sun in the region between 

 Mars and Jupiter. That was just fine 

 with A'Hearn and his collaborators: 

 studying a typical example from a 

 group of objects gives more insight 

 about the group than the extreme 

 cases do. But examine anything close- 

 ly enough, and it becomes interesting. 

 Intensive studies of Tempel 1, done 

 well before the impact, showed that 

 even such an undistinguished comet is 

 an intriguing object. Shaped like a fist, 

 and less than five miles across at its 

 widest, Temple 1 is covered with pits 

 and pockmarks left by more than 4 bil- 

 lion years of cosmic collisions. 



Comets are thought to have formed 

 early in the solar system's history and to 

 have undergone little geologic change 

 since that time. Hence their internal 



Comet 9P/Tempel 1 is pictured sixty-seven seconds aher Deep 

 Impact's impactor was intentionally crashed into it at 20,000 

 miles per hour. The collision created a bright burst of light at 

 the point of impact. The photograph was made by the main 

 Deep Impact spacecraft. 



composition should hold fossilized clues 

 about the chemical origins of the plan- 

 ets. From thousands of spectroscopic ob- 

 servations made before and after the col- 

 lision, the Deep Impact team was able 

 to determine the relative proportions of 

 the elements and compounds that orig- 

 inally made up the comet and the ma- 

 terial ejected by the impact. 



The data showed that, in the two- 

 tenths of a second following the im- 

 pact, the temperature of the impact site 

 flashed above 2,000 degrees Fahren- 

 heit. More than a thousand tons of ma- 

 terial were thrown into space: comet- 

 stuff containing water vapor, carbon 

 dioxide, cyanide gas, and an unex- 

 pectedly large amount of organic mat- 

 ter rich in carbon and hydrogen atoms. 



But the water vapor, surprisingly, 

 made up only a small fraction of the 

 ejected material — not what you'd ex- 

 pect if, according to conventional wis- 

 dom, comets are made mostly of ice. 

 Deep Impact's preliminary results thus 

 seem to confirm a more recent propos- 

 al: that comets are made mostly of rock, 

 not ice. In short, they may be "snowy 

 dirtballs" instead of "dirty snowballs." 



I 



n one sense, Deep Impact did its job 

 too well. The collision kicked up so 

 much cometary material, which in 

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NATURAL HISTORY April 2006 



