The Earth is full of craters that display a long history of large objects hitting the planet. For centuries, scientists have studied craters that shape the topography and influence the past, present, and future human uses of the lands and waters.
Wylie Poag, U.S. Geological Survey (USGS), identified the Chesapeake Bay Crater, which was formed when an object hit the Atlantic Ocean over 35 million years ago in the late Eocene and is largely responsible for creating the Chesapeake Bay and surrounding area. Poag assembled an international team to investigate its characteristics and consequences.
Thirteen years ago, the U.S Geological Survey (USGS) drilled a 2,700-foot-deep (823 meters) hole at NASA’s Langley Research Center, which sits near the edge of the of the impact crater. Underground sediment and rock were brought up for analysis.
According to the USGS, the cores sampled a sandy rubble bed, which contained hand-size to person-size chunks of clay, limestone, and sand. Small pieces of the deeply buried granitic basement rocks were scattered throughout the rubble and contained shocked quartz and melted grains, which confirm an impact.
The structure and geometry of the crater were determined by seismic profiling from ships in the bay, according to Gerald Johnson, Professor Emeritus of Geology at the College of William and Mary, who spoke during an August presentation at the Hampton History Museum about the Chesapeake Bay Crater impact and its effect on the geology of Hampton Roads.
According to Johnson, the space debris created a crater about 56 miles across and 1.2 miles deep (90 kilometers across, 1.9 kilometers deep) when it slammed into the ocean, through several hundred feet of ocean water and a couple thousand feet of sediments, near the present-day mouth of the Chesapeake Bay. The excavation is twice the size of Rhode Island and as deep as the Grand Canyon, according to the USGS.
According to Johnson, the Chesapeake Bay Impact Crater is the largest known in the United States and sixth largest in the world.
“I’m just sad we can’t have a piece of it,” he said.
As Johnson explained, the object vaporized, only leaving behind changes to the atmosphere, ocean, land and biosphere.
An enormous tsunami modeled at 1,500 feet (457 meters) rushed westward, Johnson says, and engulfed the land to the foot of the Blue Ridge Mountains.
At the time of the impact, sea level along the East Coast was much higher and most of eastern Virginia was submerged. According to Johnson, the ancient shoreline was somewhere in the vicinity of Richmond before the impact.
Johnson stated that the Chesapeake Bay itself did not form until after the Wisconsin glaciation ice sheet melted 18,000 years ago.
Discovery of the giant crater revised our understanding of Atlantic Coastal Plain evolution. Studies revealed several consequences of the impact that still affect citizens around the bay today: land subsidence, river diversion, disruption of coastal aquifers, ground instability, and location of Chesapeake Bay.
Though we have learned much from the geology of the Chesapeake Bay crater, Dan Mazanek, a near-Earth object (NEO) expert at NASA Langley, explained that there is still much left to learn. There are also several schools of thought about the specific type of impactor that created the crater -- an asteroid, a comet, or a fragment of a larger NEO that may have also been responsible for the Toms Canyon impact in New Jersey and possibly the Popigai crater in Siberia, Russia.
While attempting to keep up with research from down under, Mazanek keeps his focus on remaining threats.
“There are more than 10,107 discovered asteroids,” Mazanek said. “There are hundreds of thousands or even millions of asteroids of a threatening size, larger than 30 to 50 meters (98 to 164 feet).”
Mazanek referenced the recent Russian meteor event in February, which injured thousands and caused millions of dollars in structural damage. That meteor was about 17 meters (56 feet) in diameter when it exploded about 23 kilometers (14.3 miles) above the Earth.
“But that asteroid still had a major effect on the ground, and there are potentially millions of them,” Mazanek said. “Another meteor of similar size to that would be the next likely event.”
Mazanek says that there are threats all around us in the Earth-Moon system. Very small objects pass by the Earth on a daily basis or burn up in the Earth’s atmosphere. Larger objects, about 30 meters or larger, come within several lunar distances on a monthly or yearly basis, but pass harmlessly by the Earth.
"The frequency is always a question," he said. "We know that the larger objects are less frequent, but they have more devastating affects."
For objects one kilometer and larger, there are only 6 percent that are yet to be discovered, Mazanek says. For objects larger than approximately 100 meters (328 feet), there are still about 90 percent left to be discovered.
"The percentages are based on dynamical modeling of the asteroid population and observations to date to help verify those models," Mazanek said. "There is uncertainty in these models so the percentages should not be taken as exact numbers, but rough guides."
As he explained, the focus of current observation efforts is to find the NEOs 140 meters (roughly 460 feet) or larger.
“Some objects are black as coal and some are white as chalk,” Mazanek said. “The reflectiveness makes them easy or difficult to locate.”
That darkness of some adds to a particular population uncertainty for smaller Near-Earth Asteroids that are observed by ground-based telescopes, Mazanek says.
Radar is a powerful tool for characterizing the orbits and physical properties of NEOs. The trick, Mazanek says, is knowing where to look, much like a police officer knows where to point a radar gun to determine the speed of vehicle.
Earth impacts are a very infrequent event, but having a 100-year impact event, doesn’t mean it will be another hundred years until it happens again.
“It’s not like a bus or a train schedule, it just happens that frequently on average," Mazanek said of objects entering Earth’s atmosphere. “It’s like a coin toss. Even though it’s a 50-50 heads or tails on average, it could be heads ten times in a row or tails ten times in a row.”
According to Mazanek, NASA’s NEO Observation Program has been responsible for a vast majority, about 99 percent, of all NEO discoveries since 1998.
Just last month, NASA announced the Asteroid Grand Challenge, which aims to locate all asteroid threats to human populations and to know what to do about them.
The challenge is a large-scale effort that will use multi-disciplinary collaborations and a variety of partnerships with other government agencies, international partners, industry, academia, and citizen scientists. It complements NASA's recently announced mission to redirect an asteroid and send humans to study it.
As research of the past collides with efforts to understand potential events of the future, the overall knowledge gained will benefit all humankind.
NASA Langley Research Center