On Sunday, the United States Geological Survey (USGS) turned an impressive 134 years old. Founded by President Rutherford Hayes on March 3, 1879, this scientific agency has been charged with many tasks over the years from mineral exploration to earthquake monitoring to biodiversity assessment. But one of its most fascinating endeavors is surely the planetary mapping of its astrogeology program, the results of which bear a striking relevance to our recent encounters with asteroids last month.
The USGS turned its gaze skyward in the 1960s when scientists at the agency began training the astronauts on the Apollo Lunar missions in basic geology. To compliment their work, the USGS also began mapping the Moons pockmarked surface using images beamed back from the Lunar Orbiter and samples brought home by the astronauts themselves. This work produced beautiful geologic maps like the image above of the north side of the Moon. These maps are so detailed its hard to remember their contents lie 240,000 miles away in the cold vacuum of outer space.
Nearly all of the donut-shaped objects on this lunar map are impact craters from asteroids like the one that exploded over Siberia a few weeks ago. The different colors indicate different kinds of rocks or craters of different ages. In fact, most of what we know about the impact history of the Earth comes from studying the surface of the Moon and other rocky planets. Because the Moon has no atmosphere, every object headed its way hits the ground (small meteorites do not vaporize in the air as they do on Earth). And because the Moon has no water, tectonic activity, or other agents of erosion, a crater remains visible for billions of years after an impact.
Using maps like this, scientists have studied the overlapping patterns of lunar craters to understand the frequency and size of planetary impact events in our corner of the solar system. They discovered that objects a few meters in diameter, like the recent Russian bolide, intercept our planet about once a year. These often go unnoticed because they usually appear over the oceans, which cover three-quarters of the Earths surface. Very large asteroids, on the other hand, like the one that wiped out the dinosaurs 65 million years agoa whopping 10 kilometers of rocky catastrophe hurtling through spaceonly threaten the Earth once every 100 million years or so. And thats a number that makes even the USGS feel young again.
Check out the biggest impact craters on Earth at National Geographic.
Discover more fascinating science happening at the USGS Astrogeology Center.
Learn how to read a geologic map from the National Park Service.
Written by Julia Rosen
Julia Rosen is a freelance science writer and PhD student at Oregon State University. She received a Bachelors degree in Geological and Environmental Sciences from Stanford University before beginning her doctoral research on polar ice cores and climate change. In between, she did her Master's in backpacking around the world and skiing. Julia is a periodic contributor to Oregon States research magazine, Terra, and helps write blog content and develop learning modules for Visionlearning.