2024-07-18 13:44:27
These small, rapidly rotating objects roam close to Earth, and most likely came from the far reaches of the Solar System. They can be a source of water and other volatiles – and also a source of serious danger.
In general, comets are very different from asteroids. Comets come from the farthest reaches of the solar system, where temperatures are low enough to freeze molecules like water. Although the orbits of comets are generally stable, they can sometimes be disrupted by gravitational interactions with the giant planets, sending some of the icy rocks hurtling toward the inner Solar System. When this happens, the Sun’s heat causes them to disintegrate, a process that also gives comets their characteristic tails.
And asteroids usually fly in the inner part of the solar system – usually between Mars and Jupiter. They are much rockier than their comet cousins, so they can stay in the Sun’s light longer. However, they too sometimes fall into unstable orbits, which cause them to come dangerously close to Earth.
But there’s a third strange type of space rock that astronomers have only recently begun to identify: dark comets, which behave like both asteroids and comets. Now in an article accepted for publication in the journal Icarus a team of astronomers is trying to determine the mysterious origin of dark comets.
Dark comets are small – just a few tens of kilometers in diameter. They show no visible signs of gas evolution or evaporation of volatile elements such as water. But they don’t move in perfect orbits either. Instead, they show “non-gravitational” acceleration – meaning there are other forces that can gently nudge their orbits.
All small objects in the Solar System, including asteroids, exhibit some degree of non-gravitational acceleration, but astronomers can usually determine its cause. For example, asteroids are heated unevenly by the Sun, causing their orbits to shift slightly – but measurably.
The researchers found that non-gravitational acceleration of dark comets is inconsistent with uneven heating, so there must be another source of acceleration. The team believes that dark comets are indeed ejecting gas, which can cause a peculiar non-gravitational acceleration – just at a imperceptible level.
Dark comets also spin very quickly – which means they must have enough internal force to keep from tearing apart. From this, scientists concluded that dark comets are similar in composition to asteroids, and are probably the result of the breakup of a larger object.
Based on this evidence, scientists suspect that dark comets likely originate in the main asteroid belt between Mars and Jupiter, and their orbits are distorted by the gravitational interaction with Saturn. So, dark comets are likely asteroids, but of a strange kind, asteroids that contain unusually large amounts of light molecules (such as water) that can evaporate when the objects enter the inner solar system. Therefore, scientists say that dark comets may be a possible candidate for explaining how the early Earth got its water.
Meanwhile, the unstable orbits of dark comets and their incredible combination of properties make them particularly dangerous near-Earth objects. They are small, fast and hard to detect. Most importantly, they don’t behave like the more familiar asteroids and comets — and are therefore unpredictable, the researchers found.
To protect Earth from potential threats, we will need to study nebulae — such as dark comets — in much greater detail to better detect them and predict their future movements, Live Science writes.
2024-07-18 13:44:27