A shattered asteroid may have bombarded Earth 800 million years ago

A violent collision in the main asteroid belt may have triggered a prolonged wave of impacts across the inner solar system about 800 million years ago, according to a study led by the Southwest Research Institute. The researchers propose that the breakup of the parent object that formed the Eulalia asteroid family sent large amounts of debris toward Earth, the Moon, and Mars. If the connection is correct, the event may have produced geological changes across several worlds and could even have influenced Earth's climate and biosphere. Ancient Impacts and the History of Life "The role impacts have played in shaping the origin and evolution of life in our solar system is poorly understood," said Dr. William Bottke, an executive director in SwRI's Solar System Science and Exploration Division in Boulder, Colorado. He also directs the Center for Lunar Origin and Evolution (CLOE), SwRI's team in NASA's Solar System Exploration Research Virtual Institute, and is lead author of a paper describing this research. "The heavily cratered surface of the Moon serves as a reminder of the large impacts in Earth's past, but so far, only the Chicxulub impact event 66 million years ago has been strongly linked to a specific effect on life, namely the mass extinction of the dinosaurs." Chicxulub is the enormous impact crater buried beneath Mexico's Yucatán Peninsula. The asteroid strike that created it is widely associated with the extinction event that wiped out all nonavian dinosaurs and many other species. Far older collisions are much harder to reconstruct. Geological evidence from impacts more than 650 million years old is scarce because Earth's surface is constantly being altered and recycled. Volcanoes create new rock, plate tectonics reshape the continents and ocean floors, and weathering gradually breaks down exposed landscapes. Together, these processes erase or bury many ancient impact craters. To investigate these missing chapters of Earth's history, scientists can study asteroid showers, periods when fragments from a major collision repeatedly strike planets and moons across the inner solar system. "These rare events, triggered by large, well-positioned collisions in the main asteroid belt, bombard all inner solar system worlds," Bottke said. "So, evidence preserved on the Moon's static surface can be used to infer what happened on Earth and Mars in ancient times." The Moon Preserves an Ancient Impact Record Unlike Earth, the Moon has no active plate tectonics, flowing water, or substantial atmosphere to rapidly erase old craters. Its surface therefore serves as a much more complete archive of ancient impacts. Previous research has suggested that the Moon experienced a significant increase in large impacts around 800 million years ago. That conclusion was based on the estimated ages of major lunar craters and the ages of impact glass collected during the Apollo missions. Impact glass forms when a collision generates enough heat to melt rock. The molten material cools into glass, preserving chemical and chronological clues that scientists can use to estimate when the impact occurred. Although the lunar evidence pointed to an impact surge, researchers still needed to identify a realistic event in the asteroid belt that could have produced it. "Our cosmic forensics team used collisional and dynamical models to link these to the formation of the Eulalia asteroid family, when a primitive carbonaceous chondrite-like object collided with another object," Bottke said. "The location of the parent asteroid was key -- it broke up on the brink of the gravitational 3:1 mean motion resonance with Jupiter." Carbonaceous chondrites are primitive, carbon-rich meteorites that contain some of the oldest material formed in the solar system. They can also contain water-bearing minerals and organic compounds. Jupiter's Gravitational Escape Route The orbital region described by Bottke is called the J3:1 resonance. In this configuration, an asteroid travels around the Sun three times during every single orbit completed by Jupiter. Repeated gravitational nudges from Jupiter can gradually destabilize asteroids in this region. As a result, the resonance acts like an escape route from the main asteroid belt, pushing objects into elongated orbits that cross the paths of planets. Many asteroids currently found near Earth are believed to have escaped from the asteroid belt through the J3:1 region. According to the simulations, the position of the Eulalia parent body made the breakup especially consequential. About half of the fragments entered the J3:1 resonance almost immediately. The resonance then scattered this planetary debris through the inner solar system, increasing the number of impacts on the Moon, Earth, Mars, and possibly other rocky worlds. The bombardment did not end quickly. During the following 100-150 million years, another 25% of the fragments gradually moved into the resonance through the Yarkovsky effect. The Yarkovsky effect is a subtle force caused by heat. An asteroid absorbs sunlight and later releases that energy as infrared radiation. Because the heat is emitted unevenly, it produces a tiny push that can slowly change the asteroid's orbit over millions of years. A Barrage Across the Inner Solar System The modeling shows that the Eulalia breakup could plausibly explain the increase in lunar craters dated to roughly 800 million years ago. It also suggests that the collision may have produced much broader effects across the inner solar system. Earth would have received far more impacts than the Moon because it is much larger and has stronger gravity. Researchers estimate that for every large object that struck the Moon, roughly twenty objects of a similar or greater size hit Earth. Most physical evidence of those impacts has since disappeared from Earth's surface. However, the timing of the bombardment overlaps with a period of widespread cooling and major biological changes, raising the possibility that the impacts affected the planet's environment. The study does not establish that the asteroid barrage caused those changes, but the coincidence offers a compelling target for future research. "Given that the peak of this barrage coincides with a period of widespread cooling and major shifts in our biosphere, it is tempting to suggest that the former produced the latter," Bottke said. "On Mars, these impacts would have triggered substantial episodes of seismic shaking and can be linked in time with a surge in volcanic activity. Together, this showcases how certain catastrophic collisions in the main belt could have had far-reaching consequences for the history of the terrestrial planets."