A new study has found out why some asteroids have strange watermelon-shaped moons trapped in their orbits, contrary to the predictions of typical asteroid formation theories. The unusual shapes of the small asteroids Dimorphos and Ceram have puzzled astronomers for years, but a new study finally explains why they became so strange. It also suggests that these oddly shaped “little moons” may be more common than scientists thought.
Binary asteroids (pairs of asteroids that are essentially miniature versions of the Earth-Moon system) are fairly common in our cosmic neighborhood. This includes the duo Didymos and Dimorphos, which are the stars of NASA’s Double Asteroid Redirect Test (DART) mission in 2022. Previous studies have suggested that such binary asteroids form when a remnant “parent” asteroid, consisting of loosely held rock fragments, spins so fast that it loses some of its mass and grows into a second, smaller moon, or “moon” asteroid that connects.
Most small moons orbit a parent moon, usually in the shape of a vertex, so they look like a rounded football with an upright tip. Such small moons are called “prolate moons”. However, some have strange shapes. Take Dimorphos for example. That is, before DART hit it. It was an “oblate spheroid”, a sphere squashed at the poles and elongated in the middle, like a watermelon. And Little Ceram, a moon of the recently discovered asteroid Dinkinesh (also known as “Dinky”), is even stranger. It consists of two connected spheres of rock.
The odd shape of the little moon has puzzled astronomers, including John Wimarson, a doctoral student at the University of Bern in Switzerland and lead author of the new study. “We’ve never seen an asteroid moon like this before, and it can’t be directly explained by traditional models of binary asteroid formation.” To understand the asteroid’s odd shape, Wimarson and his colleagues from European and American universities developed two sets of detailed computer models. In the first set, they simulated how the parent asteroid changes its shape as it rapidly rotates and ejects debris.
In the second set, they assumed that the debris forms a ring-shaped zone around the parent asteroid, a so-called debris disk. Then, an algorithm tracked the movement of all the fragments as they collide under gravitational forces from each other and the parent asteroid to form the aggregate. The researchers also looked at two parent asteroids similar in size and density to the “rubber duck” Ryugu and Didymos.
The results, published online July 20 in the journal Icarus, showed that there are two main factors that determine the final shape of the Moon’s small asteroids: the gravitational force exerted by the parent asteroid and the type of impact the small asteroid causes with the Moon. Other rocky objects in the debris disk will also be affected. However, other parameters determine which of these factors play a bigger role. One of the parameters is the density of the parent asteroid. A denser asteroid like Didymos rotates faster than a lighter asteroid like Ryugu, creating a wider debris disk, which results in the formation of small moons further away from the parent asteroid.
The researchers found that moons that form at a certain distance from the parent planet usually adopt an elongated shape. At this distance, called the Roche limit, the parent planet’s gravity balances the internal forces of the tiny moon, allowing it to maintain its shape as it slowly grows by colliding and merging with other debris.
source: https://www.sciencedirect.com/science/article/pii/S0019103524002835?via%3Dihub