Everybody knows that Saturn has rings. It’s pretty much the image that most of us conjure up when we think of other worlds and they are, truly, beautiful. It is little known, however, that Jupiter, Uranus and Neptune all have rings too. Whilst they are much less dense and not at all as easy to see they do nonetheless exist.
The inner, rocky planets; Mercury, Venus, Earth and Mars; do not have ring systems; Mercury and Venus don’t even have a moon. Understanding why this is the case has been keeping planetary scientists busy for some time but one theory is that the huge planets of the outer solar system can use their enormous gravity to, literally, tear one of their many moons apart and spread its trail of destruction around itself like a cloak. The puny inner worlds simply don’t have the heft to pull this off. Or do they?
A new paper in Nature Geoscience proposes that humble Mars may be about to shatter its own largest moon, Phobos, even though it has little more than a tenth of the mass of the earth. So how can lil old Mars pull off such a feat of strength? The key lies in the composition of Phobos. The new analysis shows that it is very weak, quite porous and already heavily damaged. Whilst it is technically in one piece you only need to give it a stern talking to for it to begin coming apart. Think back to your childhood when you left your plasticine out in the air for too long and it began to dry out. You pick up the loose bits and pieces and try squishing them back together but it’s not really happening. That’s Phobos.
Mars may be small, then, but it has more than enough grunt to make Phobos crumble. The researchers predict that the weaker parts of Phobos will begin breaking off within 20-40 million years, and this will form the basis of a new ring system that will have a similar density to that of Saturn. It won’t last forever, though, after no more than 100 million years the bulk of the debris field will thin out, either falling to the surface of Mars or scattering to form more of a cloud than a ring. Ultimately Phobos will be eradicated and the only trace of it for future astronomers to find will be the increasingly pock marked surface of Mars itself, baring the tell tale scars of it’s turbulent adolescence.