A study published this month shows that around 466 million years ago, during the Ordovician period, Earth may have had a ring of asteroid debris that surrounded our planet for millions of years.
Earth would have had a ring of asteroids a few hundred million years ago
Scientists have long speculated about the origin of the Ordovician meteor event, when Earth was bombarded by space rocks at an unusually high rate, producing dozens of meteorite-filled craters and sediments. Previous research has implicated a large asteroid that broke up in the main belt between Mars and Jupiter and sent rocky shrapnel into the inner solar system, where it hit our planet.
Researchers led by Andy Tomkins, professor of Earth and planetary sciences at Monash University in Australia, are looking at an asteroid that passed thousands of miles from Earth, close enough to be torn apart by the planet’s gravity. The breakup debris then coalesced into a ring around the Equator, a scenario that could be linked to dramatic changes in climate and biodiversity at the time.
“The idea came to me when I read a popular science article about Phobos and Deimos, the moons around Mars, which formed from bits of rings left around Mars”said Dr. Tomkins, whose study was published in Earth and Planetary Science Letters. “That got me thinking about how I would see it in the geological record if the Earth had a ring in the past.”
Dr. Tomkins and his team mapped where on Earth 21 Ordovician craters would have been located at that time. If the debris came from the asteroid belt, the craters should have been distributed all over the planet. But the craters were clustered around the Equator, suggesting they were created by meteorites falling from an equatorial ring.
“That’s what gave him,” said Dr. Tomkins. “They’re all pretty close to the Equator, and it’s pretty hard to do that through normal impact cratering processes.”
The team says there is no evidence to date that Mars and the Moon experienced similar spikes in impact craters at this time, suggesting the debris was located on Earth. In addition, the researchers pointed to Ordovician meteorites in Sweden that show low exposure to space radiation, meaning the rocks fell to Earth within tens of thousands of years after their parent body disintegrated. Asteroid belt debris, on the other hand, is typically exposed to space radiation for millions of years.
The team also wondered whether the shadow cast on Earth by the ring cooled the planet, triggering a major glaciation called the Hirnantian Ice Age, which shook the trajectory of life. Dr. Tomkins said this link was more speculative but merited further study.
These “more evidence” combine in what “we think it’s a plausible hypothesis”said Dr. Tomkins.
Birger Schmitz, professor of geology at Lund University in Sweden, praised the team’s new and creative approach, but said more data was needed.
“The work has a completely new perspective, and this will certainly bring us a step forward in understanding what happened in the Ordovician,” said Dr. Schmitz, who is also affiliated with the Purple Mountain Observatory in China.
Gretchen Benedix, professor of Earth and planetary sciences at Curtin University in Australia, called the study “appetizing”, but he remained unconvinced. She noted that meteorites could have impacted far from the Equator without leaving any surviving geological traces. She was also skeptical of the proposed link between the ring and the Hirnantian Ice Age.
“There’s a lot of assumptions here, and that’s not a bad thing”Dr. Bendix said of the study. “But I think there’s physics and chemistry that needs to be worked out.”
To that end, Dr. Tomkins and his colleagues outlined methods to test their hypothesis, including studies of Ordovician meteorites at different latitudes and more complex models of how a disintegrating asteroid might form an Earth ring.
“What I especially like is that the authors present a testable idea,” Dr. Schmitz said. “By looking for meteorite minerals in sediments at different latitudes, we will get an answer as to whether the Earth really had a ring.”
For now, it’s exciting to imagine a bygone Earth inhabited almost entirely by marine life, surrounded by the remains of a broken space rock, according to space.com.
