A massive asteroid that broke apart around 800 million years ago may have sent a huge number of rocky fragments towards Earth, the Moon and Mars, according to a new study led by researchers at the Southwest Research Institute.Scientists believe the collision happened in the main asteroid belt, the region between Mars and Jupiter where millions of asteroids orbit the Sun. The shattered pieces may have travelled through the inner solar system over millions of years, causing a long period of impacts on several rocky planets.The researchers say the event could help explain why the Moon has many large craters dating back to around 800 million years ago.

They also suggest that the bombardment may have coincided with important changes in Earth's climate and the evolution of life.

Looking for clues

Scientists have long known that asteroid impacts have shaped planets throughout the solar system. However, understanding how those impacts affected life on Earth has been difficult because much of the evidence has disappeared."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," said Dr William Bottke, executive director in the Southwest Research Institute's Solar System Science and Exploration Division and lead author of the study, Science Daily reported.

Finding evidence of much older impacts on Earth is far more difficult. Over hundreds of millions of years, Earth's surface keeps changing because of volcanoes, plate tectonics and weathering. These natural processes bury or erase ancient impact craters. Because of this, scientists often look beyond Earth for answers.

Moon has missing record

Unlike Earth, the Moon has no plate tectonics, flowing water or thick atmosphere to wear away its surface.

As a result, impact craters remain visible for billions of years.

Earlier studies suggested that the Moon experienced a rise in large impacts about 800 million years ago. Scientists reached this conclusion after studying the ages of major lunar craters and impact glass collected during the Apollo missions.Impact glass forms when an asteroid strikes the surface with enough force to melt rocks. As the melted material cools into glass, it preserves chemical information and allows researchers to estimate when the impact happened.

Asteroid family may explain

The new study also points to the breakup of the Eulalia asteroid family as the likely source. According to the researchers, a large carbon-rich asteroid, known as the Eulalia parent body, collided with another object in the main asteroid belt. The collision shattered the asteroid into countless fragments."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.Carbonaceous chondrites are among the oldest materials in the solar system. They are rich in carbon and can also contain water-bearing minerals and organic compounds, making them important for scientists studying the early solar system.

Jupiter's role

The asteroid broke apart near a region called the J3:1 resonance, also known as the 3:1 mean motion resonance with Jupiter. This is a gravitational region where an asteroid completes three orbits around the Sun for every one orbit completed by Jupiter.Although the name sounds technical, scientists describe it as a kind of gravitational gateway. Jupiter's powerful gravity repeatedly pulls on objects passing through this region. Over long periods, these repeated tugs can push asteroids out of the main asteroid belt and onto paths that cross the orbits of planets. Many near-Earth asteroids are believed to have reached our neighbourhood through this same route.

The researchers' computer simulations suggest that about half of the fragments from the Eulalia collision entered this gravitational pathway almost immediately. Those fragments were then scattered across the inner solar system, increasing the chances of impacts on Earth, the Moon and Mars.Over the next 100 to 150 million years, another quarter of the asteroid fragments slowly drifted into the same region because of the Yarkovsky effect.The Yarkovsky effect is a very small force created when an asteroid absorbs heat from sunlight and later releases that heat back into space as infrared radiation. The uneven release of heat acts like a tiny push, slowly changing the asteroid's orbit over millions of years.

Impact on Earth

The team's models suggest the Eulalia breakup can explain the increase in large lunar craters seen around 800 million years ago.Because Earth is much larger than the Moon and has stronger gravity, it would have attracted even more incoming objects.

Researchers estimate that for every large asteroid that struck the Moon, around 20 similar or larger objects may have hit Earth. Most traces of those impacts have since disappeared because Earth's surface has been constantly reshaped.The period of increased, however, impacts overlaps with evidence of widespread global cooling and major biological changes on Earth. The researchers say this does not prove that asteroid impacts caused those events, but they believe the overlap deserves further investigation."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.He added that the impacts would also have affected Mars.