March 24, 2015

After at least 300 million years, one of the largest impact events in Earth’s history is finally coming to light. Scientists from the Australian National University (ANU) in Canberra have discovered the remains of giant craters—evidence of huge strikes by a meteorite that split in two—measuring some 250 miles (402 kilometers) across. Their findings were reported this month in the journal Tectonophysics. The meteorites that struck in the Australian outback were at least 6 miles (9.6 kilometers) wide, the size of the asteroid that killed the dinosaurs.

World Book illustrations by Paul Turnbaugh

A crater is a bowl-shaped depression on the surface of a planet, moon, or other solid body. Most are impact craters, which form when an asteroid, comet, or other solid body strikes the surface of a larger body, such as a planet or moon, at high speed (above, Step 1). Such an impact releases a tremendous amount of energy in the form of shock waves (Step 2). Shock waves are waves of energy that travel away from the impact. The shock waves travel through the surface until their energy is used up. As they travel, they push material away from the impact site, forming the crater (Step 3). The shock waves force some of this material upward and outward to form the wall of the crater (Step 4). The impact also tosses some of the material into the air. This material, called ejecta, settles around the crater in a layer called the ejecta blanket.

While drilling in the Warburton Basin in central Australia, the ANU team found shocked quartz in a layer of rock dating from 300 to 600 million years ago. Shocked quartz is a type of rock that can only be formed by the extreme pressures of a meteorite impact. The scientists also found other evidence of the impact in Earth’s crust, such as variations in the magnetism of the rocks there.

If this impact was so large, where are the craters? On Earth, erosion (the breaking down and moving of rock and soil from one place to another) and plate tectonics (the slow movement of pieces of the earth’s crust) work to erase impact craters. On planets and moons where these forces aren’t at work, such as Earth’s moon, impact craters are more easily preserved. Hundreds of millions of years of erosion and plate tectonics have made the Australian craters invisible. Only features below the surface hint at the cataclysmic events of the past.

December 10, 2012

Almost the entire surface of the moon was fractured by impacts billions of years ago, according to the most detailed map of the lunar gravity field ever created. The map is based on high-resolution data collected by NASA’s twin GRAIL orbiters. (GRAIL stands for Gravity Recovery and Interior Laboratory.) The map shows many of the moon’s structures in great detail, including features in the crust, volcanic formations, and numerous other landforms. The map has also revealed that the lunar crust is marked with many holes. Scientists believe this finding indicates that the surface was battered by a heavy bombardment of meteorites and other objects early in its history. The gravity map also indicates that the crust of the moon is only about 21 to 27 miles (34 to 43 kilometers) thick, about 6 to 12 miles (10 to 20 kilometers) thinner than scientists had thought.

A gravity map of the moon reveals mountains and other higher areas (red) and craters and other lower areas (blue). (NASA/ARC/MIT)

GRAIL was designed to measure changes in the moon’s gravitation as a way to reveal structures beneath the surface. The orbiters, which were launched in September 2011, travel around the moon on opposite sides at an altitude of about 34 miles (55 kilometers). They use radio signals to precisely measure the distance between them, which changes with the density of the rock and soil below. The system is so accurate that it can measure distances less than the width of a human hair. Scientists can link the satellites’ measurements to structural differences in the moon’s surface and underlying crust. The probes were named “Ebb” and “Flow” in January 2012 by elementary students in Bozeman, Montana, in a nationwide contest. The GRAIL mission is scheduled to end later this month.

Another interesting discovery by the orbiters is collections of long, linear dikes (thin,vertical bodies of solidified magma) beneath the surface. The dikes, which extend for hundreds of miles (kilometers), crisscross the surface. Scientists think that the dikes, which are among the oldest features on the moon, likely formed as the moon’s crust expanded. Such geologic events could have occurred if the moon formed as a result of a collision known as the Giant Impact or the “Big Whack.” According to this idea, Earth collided with a planet-sized object 4.6 billion years ago. During the impact, a cloud of vaporized rock shot off Earth’s surface and went into orbit around Earth. The cloud cooled and condensed into a ring of small, solid bodies, which then gathered together, forming the moon. The rapid joining together of the small bodies released much energy as heat. Consequently, the moon melted, creating an “ocean” of magma. Over time, the magma ocean slowly cooled and solidified into rocks called basalts.

Additional World Book articles:

• Space exploration (Lunar probes)
• How the Moon Was Born (a special report)

April 27, 2012

A giant fireball, a meteor, lit up the sky above the Sierra Nevada Mountains near Sutter’s Mill, California, early on April 22. Seen from as far away as Sacramento, the state capital, and Reno, Nevada, the fireball was caused by a meteoroid, a piece of primordial matter from space.

A meteoroid becomes visible when it enters Earth’s atmosphere from space at very high speed. As the meteoroid collides with the atmosphere, friction causes it to heat up so that it glows, creating a shining trail of hot gases. Most meteoroids are smaller than a pebble and break apart in a second or less.

A meteor appears in the sky when an object called a meteoroid enters the atmosphere from space. (World Book illustration by Greg Maxson)

Scientists believe that the meteoroid that hit the atmosphere with a loud explosion over California probably weighed about 154,300 pounds (69,990 kilograms) and was as large as a minivan. They calculated that it hit the atmosphere with about one-third the explosive force of an atomic bomb. The loud boom heard as the meteoroid streaked across the sky was caused by the speed with which the rock entered the atmosphere. Scientists estimate the meteoroid was traveling up to 44,000 miles (80,000 kilometers) per hour–many times faster than the speed of sound–when it hit the atmosphere. This caused the gigantic sonic boom heard across two states.

Thousands of meteorites rained down over hundreds of square miles as the fireball broke apart in the sky. Meteorites are meteoroid fragments that reach the Earth’s surface intact. By midweek, collectors had found several meteorites weighing about 1/3 ounce (10 grams) each. After examining the fragments, scientists determined that the large meteoroid was a rare, carbon-rich type known as a carbonaceous chondrite. This type of meteorite is made up of the same material from which the planets formed and dates to the very beginnings of the solar system more than 4 billion years ago. Scientists hope to find even more fragments of this rare meteor event.

Additional articles in World BooK:

• Meteor Crater
• Shoemaker, Eugene Merle
• Stones from Space (a Special Report)

Jan. 9, 2012

The mystery of how a highly unusual material found in a rock from a Russian mountain could have been created naturally on Earth may have been answered: It didn’t. A new study by scientists from Princeton University suggests that the material, called a quasicrystal, came from outer space in a meteorite. Moreover, the meteorite may be older than Earth itself. The finding indicates that materials that could never form naturally on Earth can form in space–which has scientists wondering what other materials are out there.

In a crystal, atoms are arranged in an orderly manner, with a regularly repeating pattern. The atoms in a quasicrystal also have an orderly arrangement, but they do not have a regularly repeating pattern. Scientists once thought that quasicrystals were an impossibility. In fact, when Daniel Shechtman, an Israeli engineer, reported his discovery of quasicrystals in 1982, the scientific community dismissed his findings completely. (In 2011, Shechtman won the Nobel Prize in chemistry for his discovery.) Several years after his discovery, other scientists made more quasicrystals in various arrangements. Then in 2009, scientists reported finding a naturally occurring quasicrystal in a rock from Russia’s Koryak Mountains.

Now Princeton scientists have reported that the Koryak quasicrystal appears not to have formed naturally on Earth after all. The rock apparently is the remains of a meteorite that formed 4.5 billion years ago, even before Earth had taken shape in the early solar system. In the rock sample, the scientists found a tiny grain of a mineral called stishovite, which occurs only at the kind of high pressure achieved in meteorite impacts and collisions. The quasicrystal was encased in the stishovite.

Both quasicrystals and crystals, such as these salt crystals, contain atoms that are arranged in an orderly pattern. But the atoms in quasicrystals do not follow the kind of repeating pattern found in crystals. (c) Charles Falco, Science Source from Photo Researchers

Many quasicrystals have practical uses. Certain quasicrystals can be particularly strong and hard. They are mixed with such metals as aluminum and steel and other alloys (mixture of metals) to increase a metal’s strength and hardness. Quasicrystals may also exhibit nonstick abilities, such as those found in the nonstick coatings on cookware.

Additional World Book articles:

• Mineral
• Symmetry