Objects that strike the Earth can either originate in the asteroid belt between Mars and Jupiter, or in the outer reaches of the Solar System. So was the dinosaurs' bane a space rock (asteroid) or “dirty snowball” (comet)? Two controversial astronomers have made a case for a comet being more likely, which might undermine efforts to prevent such events in the future.
Asteroids hit the Earth's atmosphere frequently, but few are large enough to cause even local damage. Comets that cross our orbit are rarer still. Even over hundreds of millions of years, a crater as large as the one left at Chicxulub in modern-day Mexico – generally agreed to have been created by the object that kick-started the mass extinction of the dinosaurs when it hit 66 million years ago – looks like an oddity in the modern Solar System. This has left astronomers wondering if the dinosaurs were just very, very unlucky, or if the chance of major impacts is sometimes raised. One hypothesis is that disturbances in the outer reaches of the Solar System sometimes increase the number of incoming comets, raising the odds of an impact.
Harvard graduate student Amir Siraj and Professor Avi Loeb have proposed an alternative explanation. They argue large comets may frequently break up when passing close to the Sun, increasing the chance of a large impact on Earth on the way out by a factor of 10.
Since comets are held together by ice, getting too close to the Sun's warmth can melt them enough they fall apart, as ATLAS did last year, leading to a deeply disappointing show. Comets so large even their fragments could plunge the planet into darkness are much less affected, however. This would appear to undermine the cometary breakup hypothesis, but Siraj and Loeb have pointed to an alternative mechanism in a paper published in Scientific Reports.
"In a sungrazing event, the portion of the comet closer to the Sun feels a stronger gravitational pull than the part that is further, resulting in a tidal force across the object," Siraj said in a statement. "You can get what's called a tidal disruption event, in which a large comet breaks up into many smaller pieces.”
The paper includes modeling showing these tidal forces will frequently be large enough to break comets up. Of course, the comet still needs to get close enough to the Sun for this to occur, but Siraj says this is quite common. "Jupiter, the most massive planet, kicks incoming long-period comets into orbits that bring them very close to the Sun," he said. The paper claims about 20 percent of comets from beyond Neptune become Sun-grazers as a result.
Although the idea is hard to confirm so long after the event, the authors note evidence both Chicxulub and Vredefort craters both were made by carbonaceous chondrites. These account for just 5 percent of meteorites, indicating their rarity in the inner Solar System, but maybe more common among long-period comets.
The orbits of potential inner Solar System threats, such as Apophis can be plotted, giving us plenty of time to take action of one form or another. Long-period comets give much less warning, possibly a matter of months. If we want to be ready to deal with such threats, we need to be prepared well beforehand.
Loeb and Siraj are prolific authors of intriguing scientific papers, but Loeb has recently become associated with the conviction our first interstellar visitor 'Oumuamua was an alien spacecraft, rather than an ordinary asteroid or lump of molecular hydrogen from another star. The idea makes for great headlines and has been run uncritically by many media sites, but has not convinced many of his peers. Nevertheless, some of the pair's work is highly regarded, and it remains to be seen which category this one will fall into.