{ IMPACT SCIENCE } Earth under fire Our planet’s surface is riddled with craters formed by high-speed cosmic impacts. ⁄⁄⁄ BY MIKE REYNOLDS ost of us are familiar with the major features of the world — oceans, continents, mountain Mranges, valleys, deserts, rivers, lakes — the list goes on and on. One feature not likely to make this list is an impact crater. Craters appear few and far between on Earth’s surface, and most of the obvious ones formed THE GOSSES BLUFF CRATER in Australia (above) is the from volcanic activity, not impacts. Yet, that doesn’t result of an impact 142 million years ago and has suffered mean impacts haven’t occurred. The evidence lies all the ravages of erosion since. It measures 15 miles (24 km) around us — on Earth and on its neighbors. across and 3 miles (5 km) deep and covers most of this Take a look at the Moon through a small telescope, image. The circular ring of hills at center spans 3 miles and and one thing stands out even to casual observers: Cra- rep resents the remains of the crater’s central uplift. NASA/USGS ters cover its surface. Time and time again, rocks have AOROUNGA CRATER (upper left) spans 10.5 miles (17 km) pummeled the Moon. Our satellite’s pockmarked face is in northern Chad. An asteroid or comet impacted this spot evidence that our solar system was and still is an active several hundred million years ago. Sediments buried the place. Spacecraft views of our planetary neighbors pro- original crater, but later erosion has partially revealed it. It vide the same image of impacts and cratering. shows in full glory in this space-shuttle radar image. NASA/JPL © 2011 Kalmbach Publishing Co. This material may not be reproduced in any form without permission from the publisher. www.Astronomy.com astronomy ⁄⁄⁄ august (LPL/UA) (LPL/UA) Copernicus CONSOLIDATED LUNAR ATLAS LUNAR CONSOLIDATED ATLAS LUNAR CONSOLIDATED FROM FROM FROM NASA/JPL THE CRATER Copernicus marks the spot THE MOON’S southern highlands are MERCURY looks a lot like the Moon, where an asteroid smashed into the Moon’s chock-a-block with impact craters dating with craters covering most of the visible surface 800 million years ago. Debris from more than 4 billion years ago. surface. The high crater density implies thrown out from the initial impact created Younger craters dot the floors and rims Mercury’s surface is among the oldest in many of the small craters surrounding it. of older, degraded craters. the solar system. Planetary scientists now realize that rocks of various sizes have Because they don’t have solid surfaces, the gas-giant planets been colliding with solar system members since the whole system don’t show any signs of impacts. Still, we know they happen. formed some 4.65 billion years ago. Some of the impacting bodies We had front-row seats in July 1994, when fragments of Comet were quite small and hardly made a dent. Others were miles across Shoemaker-Levy 9 smashed into mighty Jupiter. The dark splotches and caused major changes on the impacted body’s surface. Our that resulted from these impacts lasted for months, dramatic proof Moon owes its very existence to a colossal impact; it formed from of what can happen when you release an energy equivalent to all debris created when a Mars-size object struck the early Earth. the nuclear arsenals on Earth. Even some of the small objects in our solar system — asteroids Even with our Moon providing evidence to anyone with a tele- and comets — show numerous impact scars. It would come as a scope, astronomers and geologists had a hard time believing Earth NEAR-EARTH asteroid Eros huge surprise if Earth were unique in the solar system as the only had suffered from major impacts. Most scientists thought terres- shows lots of craters, which rocky object without impact craters. No need to worry — telltale trial craters were volcanic in nature. After all, they have attributes proves even small objects signs of cratering in both the distant and recent past exist. The similar to known volcanic features. Some researchers even argued have been battered since evidence, however, is often subtle. lunar craters could be volcanic. the solar system’s beginning. The large, saddle-shape fea- The solar system’s impact history can tell us The debate wasn’t resolved until the middle of the ture partially in shadow is about Earth’s past as well as give us a peek into 20th century. This was when geologists Eugene LAKE MANICOUAGAN in northern Quebec is the remnant of one of the largest impact features left on Himeros, a region with fewer the future. We know the early solar system was Shoemaker and Edward Chao discovered a Earth. The ring-shape lake surrounds the impact’s central uplift. The crater measures about 45 miles craters (suggesting relatively a far more dangerous place than it is today. MOST form of quartz known as coesite at several (70 km) across and resulted from an impact some 212 million years ago. NASA/USGS recent resurfacing). NASA/JHU/APL Objects with ancient surfaces, like the Moon crater sites. Coesite has a structure that can and Mercury, show impacts used to happen ASTEROIDS WERE be formed only quickly and under high How to make a crater impact scars have been eroded — in some cases entirely, in many much more frequently than they do now. pressure. This turned out to be the first Scientists understand the dynamics of crater formation better others, enough to make identification difficult. And impacts that Astronomers estimate as little as 5 percent blown to smithereens unambiguous signature of cosmic impacts. today. This is attributable in large part to sophisticated computer occurred in bodies of water — better than a 50-50 proposition — of the solar system’s original number of after losing a game Until a couple of decades ago, impacts simulations. Several variables play important roles in what kind of leave little traditional evidence. asteroids still remain. The rest were casual- by extraterrestrial bodies were considered crater an impact will create. The key factors include the size and So far, scientists have identified about 150 impact craters on ties, blown to smithereens after losing a of celestial interesting from a geological perspective, composition of the impacting body, the speed at which it is travel- Earth, with several new ones joining the list each year. For example, game of celestial chicken. but not of any major consequence. As astron- ing, and how its orbit intersects the object being struck. Another the Tswaing Crater in South Africa was thought to be volcanic in chicken. omers and geologists continued to examine the critical factor is whether the object being hit has an atmosphere origin just 20 years ago. After several meticulous expeditions, geol- Chicken Little evidence for impacts and bombardment through- and, if so, how dense it is. ogists confirmed Tswaing as an impact crater that formed about Perhaps the most surprising discovery to come from out the solar system, however, it became obvious that It all comes down to kinetic energy — the impactor’s mass and 220,000 years ago. Tswaing appears similar in size to its more- spacecraft reconnaissance of our solar system has been the collisions were a major force shaping the surfaces of these velocity at the moment of impact. (A planet’s atmosphere comes recent — and more-famous — cousin: Meteor Crater in Arizona. degree of cratering found on its bodies. Although, in hindsight, worlds. Eventually, scientists realized evidence of early impacts into play because it will slow the impactor and may break it up.) In this shouldn’t have been a shock if we had accepted what the Moon should be visible on Earth as well. comparing two similar-size impacting bodies, one composed of Craters on Earth was trying to tell us. Most terrestrial planets, moons, asteroids, and Impact research gained a lot of momentum in 1980, when phys- iron and the other richer in silicate rock or ice, the iron impactor Meteor Crater has to be at the top of the list for any American who comets show evidence of a severe battering. Saturn’s moon Mimas icist Luis Alvarez and his geologist son, Walter, suggested a giant will prove much more devastating. wants to see what happens when a big rock hits Earth. This well- has a crater so big, it indicates the impact probably came close to impact caused the extinction of about 50 percent of all living spe- On worlds like the Moon and Mercury, the evidence of impacts preserved crater not only has a classic structure, but it’s also on a splitting the moon in half. The near-Earth asteroid Eros not only cies, including the dinosaurs, approximately 65 million years ago. has remained mostly undisturbed with the exception of additional scale you have to see in person to believe. The crater measures shows multiple craters but also one large impact site. Apparently, a comet or asteroid some 6 miles (10 kilometers) in impacts. This is obvious to anyone who examines the Moon closely: about 4,100 feet (1,250 meters) across and 570 feet (175m) deep. diameter struck near the tip of the Yucatán Peninsula and formed Craters appear within and on top of other craters. If you converted the crater into a football stadium, it would seat a Mike Reynolds is an astronomy professor and the Dean of Mathematics a crater called Chicxulub roughly 100 miles (160 km) across. The On Earth, signs of the earliest cratering have been erased.