Is it an asteroid? Is it a comet? Well not as we know them! Astronomers are investigating the properties of mysterious members of the Solar System called Main Belt Comets by trying to determine the source of their dust tails. 

With only four discovered to date, Main Belt Comets (MBCs) are comet-like bodies which reside in the main asteroid belt between Mars and Jupiter. Most comets originate in the Kuiper Belt and the Oort Cloud, which are at much greater distances than the main belt.

Icy bodies prone to cometary tails were not expected to exist among the rocky asteroids, as it was thought that ice wouldn’t survive. However studies have shown that ice can survive deep beneath the surface.

Main Belt Comets are a relatively recent discovery in our Solar System. The first object in this class, 133P/Elst-Pizarro, was discovered in 1992 but it wasn’t until successive perihelion passages in 2002 and 2007 showed a cometary tail that it became clear that this was no ordinary asteroid. The paucity of the discoveries so far is mostly because the activity they exhibit is very faint. “But I think they should be quite common. I suspect that many of them will be discovered when more powerful telescopes will be dedicated to this task” Simone Marchi from the University of Nice Sophia Antipolis tells Skymania News.

It is thought that the comet-like activity of these bodies is caused by the impact of a projectile. Maria Teresa Capria, lead author of a paper on the activity of MBCs, along with Simone Marchi and their colleagues have investigated the impact theory further. “You have to make the assumptions that some ice is buried under a thick crust. An impact is disrupting the mantle, and exposing ice to Sun. Then, the body behaves like a comet, with activity going up when it is close to the Sun and down when it is far,” explains Capria.

There are two possible impact scenarios according to their calculations. The impact of the projectile can cause a crater large enough to rip open the mantle and expose the underlying ice to the Sun. The ice will then change directly from a frozen state to a gaseous one, known as sublimation, and cause a tail as it drags dust away from the MBC. The second scenario is a less violent impact which doesn’t completely expose the subsurface ice. However, even if the protective mantle is partially stripped away, the ice beneath can still sublimate if the layer above it is quite thin.

A small impact which doesn’t reveal the icy interior couldn’t be the cause the MBCs activity. For example the dust expelled from the collision of NASA’s Deep Impact probe with a comet in 2005 was only visible from Earth for a few days, where as MBCs show seasonal activity when the crater points towards the Sun.

“We believe the impact scenario makes most sense according to all the information we have to date. Of course, the situation could change as more data on Main Belt Comets will be gathered. This is a new class of objects and it may hide lots of surprises” adds Marchi.