Methone is small moon of Saturn which was discovered by Cassini spacecraft on 1 June, 2004. The Cassini has made two visits to Methone and its closest approach was made on May 20, 2012 with a minimum distance of 1900 km form it. Methone orbits the planet Saturn between Mimas and Enceladus at the distance of 194,000 km.
Methone is not like all the other moons of the solar system, the diameter of Methone is just 3 km but the surface of Methone is very smooth without any craters of asteroid or meteorite hit.
Objects in the solar system are frequently bombarded by asteroid and comets but planets and moons have different mechanisms for erasing impact of hit and smoothing their surface. For example planets use their strong gravitational pull to reshape them in spherical form. Some objects have enough internal energy that melted lava flows on the surface and fills the craters resulting in smooth surface.
This is not the case with small moon because they do not have enough gravity and internal heat. “When we look at objects less than 200 km in radius, they are like potatoes. They have lumps, grooves, craters” says Cassini team member Peter Thomas, at Cornell University in Ithaca, New York.
Therefore it is really shocking how can Methone be round and smooth?
There are many small bodies in the rings of Saturn which have smooth appearance due to collection of dust on surface but they still have craters. It is hard to understand that how could dust settle on this small body?
One hypothesis says that Methone is composed of fluffy ice which is unable to hold any craters. This fluffy ice should be loose like liquid which have no definite shape. The density of Methone is estimated to be 0.31 g/cm3 which are nearly one third of density of water. In this way, Methone is body in the solar system which has smallest density.
The roundness is explained by the Hydrostatic equilibrium Hypothesis in which it is believed that the tidal force applied by the planet Saturn and gravitational force of Methone is such that the resultant shape is round.
Although the roundness of Methone is explained by the hydrostatic equilibrium explanations but this may challenge the definition of “planets” where one of the requirements was the roundness of body due to gravitational full.
Our solar system is full of wonders. Exploration of solar system solves some problems but raises lots of new mysteries. This is the way in which science develops.
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