Why study impact craters?
Part of Hall of Meteorites.
©AMNH/C. Chesek Why Study Impact Craters?
Craters are windows into the past, present and future.
Lessons from impact craters have been essential to developing our understanding of the past and present of the Solar System. Scientists study craters on planets, asteroids and moons to learn about the geological history of those bodies. The Apollo lunar missions opened our eyes to the changing rate of impacts throughout the history of the Solar System.
Knowledge of the ages of lunar craters opened a path for investigating other planetary surfaces. Today, scientists can make inferences about the geologic histories of planetary bodies without ever landing on their surfaces. Impact craters give scientists a unique look into the past, present and future of collisions throughout our dynamic Solar System.
Our Dynamic Solar System
Impacts on the Moon, Mars and other rocky bodies tell us that the Solar System was once a much more violent place. About 4.4 billion years ago, the inner Solar System—including Earth—was pummeled by impacts much larger and more frequent than anything seen today. This period lasted millions of years or longer, but the craters no longer survive on Earth. Instead, the Moon provided the first evidence of the bombardment, which peaked about 3.8 billion years ago. The surfaces of Mars, Mercury and some moons of outer planets also show evidence of this tempestuous time.
So why the cosmic tumult? In the young Solar System, unfinished planetesimals, asteroids and comets were in unstable orbits and prone to collision. Moreover, the giant outer planets were closer to the Sun. As they migrated to their present positions, smaller bodies were thrown into disarray and became impactors. Many details of this period are active areas of research—and impact craters are key to unraveling this complex history.
Ever–Changing Surfaces
One way scientists assess the history of planetary surfaces is by counting impact craters. Older surfaces tend to be densely cratered, while surfaces with few or no craters formed more recently. Using these benchmarks, scientists can evaluate the relative ages of these surfaces without ever landing a spacecraft.
Sometimes, the best way to study impact craters is to make one. In 2015, NASA’s MESSENGER probe crashed into Mercury’s surface at 14,000 kilometers (8,750 miles) per hour—but this was no accident. The freshly exposed soil of the crater is now changing through irradiation and more impacts, processes known as space weathering. The next probe to survey Mercury will gather information about the new crater, giving scientists insight into how these processes change Mercury’s surface.
The Planetary Deep Past
Impact craters allow scientists to study a planet’s geological history—even when the records are buried beneath the surface. During an impact, buried material is ejected while outward pressure pushes the rock at the crater’s edge upward, forming a rim. These processes expose old, long-buried materials, making them accessible to probes and rovers sent to investigate these craters.
On Mars, impact craters are a key focus in the hunt for the planet’s warmer, wetter past. Rocks ejected by impacts contain minerals that formed in the presence of liquid water; some craters also show signs of ancient lakes. Layered sediments in the rims of craters bolster the evidence that Mars not only had flowing water, but a complex water cycle.
Impact craters on Mercury helped scientists solve a long-standing mystery. Mercury’s surface is unusually dark—and until flyovers by the MESSENGER probe, no one knew why. Infrared spectrometry showed dark, carbon-rich rock in and around impact craters. The impacts had blown away the younger surface rocks, revealing Mercury’s original crust: a layer of graphite that floated on the planet’s early magma ocean.
Earth’s future
Don't panic!
In the very unlikely event that astronomers discovered a civilization-ending asteroid or comet destined to impact Earth in a month, we wouldn't have enough time to stop it. But with more warning, we should be able to deflect a doomsday rock. How would we go about protecting ourselves from a catastrophic impact?
Knock Off–Course
Our best option would be to deflect an incoming body—blowing it up would just mean more debris. If the object is solid, we may be able to hit it before it hits us by using a projectile with enough mass to knock the object away from Earth. However, this method might not work with the “rubble piles”—asteroids made of loosely bound rocks—that are more common among near-Earth asteroids.
Our best option would be to deflect an incoming body—blowing it up would just mean more debris. If the object is solid, we may be able to hit it before it hits us by using a projectile with enough mass to knock the object away from Earth. However, this method might not work with the “rubble piles”—asteroids made of loosely bound rocks—that are more common among near-Earth asteroids.
Gravity Tractor
Another option is the Gravity Tractor, which would fly alongside a hazardous asteroid or comet, so that gravity eventually drags the object out of the path of collision. The craft would have to fly near the body for years or even decades to be effective, but it would also give engineers more control over the offending object.
Paint It White
Other options include painting half of the object white, relying on the change in sunlight reflecting off the lightened surface to shift it off-course, or using a laser to blast dust or rock off the asteroid, changing its mass and, therefore, its orbit. At the moment, however, humanity continues watching, finding and studying the massive objects streaking through space.