The solid surfaces of almost all planets and satellites, from Mercury and the Moon to Jupiterís satellite Ganymede, are marked with impact craters; the one exception is Jupiterís moon Io whose volcanic outpourings of lava have erased all the craters from its surface. The impact craters are the scars of past collisions with cosmic objects speeding through space. The terrestrial planets originated by the coalescence of these objects, and one of them tore enough material out of our planet to forge the Moon. Large comets or asteroids might even have brought water to the young Earth. And the cosmic barrage continues today. A hail of cosmic objects is now pelting the Earth as it sweeps through space. Some of them are tiny, and burn up in the atmosphere. Every month at least one house-sized object is blowing up when it enters our air, producing a blast as forceful as a nuclear bomb. Now and then a bigger one gets through, gouging a crater out of the ground and even threatening the inhabitants of Earth.
Explosions in the atmosphere
The largest object to strike the Earth in the 20th century wasnít quite big enough to reach the ground. It disintegrated between 5 thousand and 10 thousand meters up, over the Podkamennaya Tunguska River in central Siberia. The shock wave generated by the ensuing explosion leveled trees over 2 trillion (2 x 1012) square meters of the underlying land, an area larger than New York City and surrounding suburbs (Fig. 14.8). The energy produced was equivalent to the aerial explosion of the nuclear bomb that leveled Hiroshima. So much devastation, yet it failed to produce a crater.
The terrestrial impact record
Even the largest craters, produced by the biggest comets or asteroids, will gradually disappear from sight with the passage of time. The same forces that erode mountains, deposit sediments, eject lava and shift continents are erasing the craters and removing them from sight. If not for these dynamic forces, the craters accumulated over the ages would be as densely distributed and prominent as the overlapping craters on the Moon.
Only about 160 terrestrial impact craters have managed to survive the ravages of time. They have been identified on images taken from space, using airplanes, the Space Shuttle, or satellites such as Landsat. These craters can be first identified from aerial photographs, by their circular shapes and uplifted and overturned rims (Figs. 14.9, 14.10, and 14.11). But since other processes, such as volcanism and erosion, can also leave circular holes, confirming evidence of an impact origin must be gathered from rocks in and around the crater.
How do geologists know that some terrestrial craters are due to the explosions of projectiles coming from space? They look for rocks that have been transformed under the conditions of extreme temperature, pressure, and shock associated with a high-velocity, external impact. The most apparent shock effect is the formation of conical structures called shatter cones, which point toward the center of the impact (Fig. 14.12). Other evidence includes glassy, previously molten material formed at high temperature, and minerals with a deformed crystal structure produced by a shattering, high-pressure impact. Roughly ten percent of the craters also contain meteorites that had to come from space.
Catastrophe from the sky
Collisions by objects from outer space have always been a menace to life on Earth. During the planetís first billion years, the barrage was probably so intense that living things could not exist on the Earthís surface. After those early times, the rate of bombardment slowed down, so impacts of exceptionally large cosmic projectiles became less frequent. But these giant impacts continued every once in a while, with devastating consequences. The most recent death rock arrived 65 million years ago, resulting in the wholesale removal of life on Earth. Such an abrupt destruction of an entire species of living things by a force of nature is known as a mass extinction.
A thin, worldwide layer of clay, just 0.01 meters thick, provided the initial evidence that an asteroid or comet collision wiped out the dinosaurs. The clay layer was deposited at the right time, and it contained unusual amounts of the rare element iridium that had to come from outer space.
Walter and Luis Alvarez, and their colleagues concluded in 1980 that the iridium deluge came from outside the Earth, delivered by a large asteroid or huge comet that struck the Earth and vaporized about 65 million years ago. According to their hypothesis, the iridium was lofted into the atmosphere along with other debris by the fireball of hot gas created during the collision, and then carried by the winds over much of the globe. The worldwide cloud of iridium-rich dust then slowly filtered back down to the ground where it produced a thin global layer that contained relatively large amounts of iridium. They estimated that a layer 0.01 meters thick covering the entire Earth would be deposited by an asteroid about 10 thousand meters in diameter.
After years of searching, the telltale crater was found straddling the northern coastline of the YucatŠn Peninsula (Fig. 14.13). It is located below the Mayan village of Chicxulub (pronounced Cheek-shoe-lube, a Maya phase for ""horns of the devil""), and is hence known as the Chicxulub impact basin. The discovery of this crater and the subsequent confirmation of its age at 65 million years led most scientists to accept the impact hypothesis for the demise of the dinosaurs.
High-resolution gravity maps in the 1990s revealed the size and structure of the buried Chicxulub crater (Fig. 14.14). Regions of high density and greater gravitational pull are distributed in several concentric rings, with an outermost diameter of at least 180 thousand meters and perhaps as much as 250 thousand to 280 thousand meters. Its size and bulls-eye pattern is similar to the largest impact basins on the Moon and Mercury, created by cosmic collisions during the early days of the solar system about four billion years ago.
The Earth does not occupy a secure niche in space. Our planet is instead immersed in a cosmic shooting gallery, subject to a steady bombardment by lethal, Earth-approaching asteroids and comets. Somewhere in space, one of them is hurtling toward a future collision with Earth (Fig. 14.15). And if it is large enough, the impact will severely disrupt terrestrial life upon impact. Itís only a matter of time.
Thus, to estimate the risk of being hit in a way that matters, the potential impacting projectiles first have to be sorted according to size (Fig. 14.16). Fragments smaller than a few tens of meters across burn up in the atmosphere and rarely reach the ground Ė except for the exceedingly tiny particles of cosmic dust that drift down into your hair. Asteroids or comets a hundred meters in diameter are expected to strike Earth every thousand years on average. They could take out a city and cause severe local damage, but pose no threat to the Earth as a whole.
Finding the hidden threat
While we know that asteroids and comets have collided with the Earth in the past, and that they will inevitably hit our planet in the future, we do not yet know if any them are now headed for a deathly collision with our solitary outpost of life. Astronomers are therefore taking a census of everything out there that is big enough and close enough to threaten us (Fig. 14.17). Once all of these near-Earth objects are located, and their current trajectories known, astronomers can use computers and refined observations to determine their precise future paths and establish whether and when any of them will strike the Earth.
Doing something about the threat
Sooner or later we will discover an asteroid or comet headed toward collision with the Earth. But unlike most natural disasters, the impact can be forestalled once the killer object has been fingered and we can see it coming. Evasive action will depend on the lead-time available and the expected physical nature of the object, including whether the object is a binary system.
And what do we do if we find a large asteroid or comet headed our way? The Earth cannot be moved out of the way, but we could launch an intercept mission to redirect the objectís course. If the impact is many years away and the threatening object relatively far away from us, all you have to do is give it a little nudge. By the time the asteroid or comet reaches the Earthís vicinity, that small change in trajectory will make a big difference, enabling it to bypass the planet.
If the warning time is only a matter of months or less, the sole recourse might be to send a high-powered rocket armed with a bomb powerful enough to redirect the object or blow it up. Such a possibility has sparked the interest of bomb designers and some members of the military. A conventional nuclear weapon might be used to deflect or destroy a small, solid, rocky asteroid, but a much larger explosion could be needed to divert or pulverize a loosely bound object, like some large asteroids and most comets.
(Fig. 14.18. Summary Diagram)