Four Moons Larger than Luna

Our Solar System is so much more than just the eight planets that inhabit it.  One of the things I learned a bit later in my career as an astronomer (my teenage years; been doing this for awhile) is how unique and diverse the natural satellites are.  Our own moon seems somewhat tame, and it’s easy to think the same of all moons.  Many of them are boring quiescent rocks with little more scientific value than asteroids, but the largest hide deep and profound secrets that we are just starting to unlock.  Four moons in our solar system are larger than Earth’s Moon, and they all hold incredible secrets of their own.

Largest Natural satellites of the Solar System – Credit:

Ganymede as the largest moon is the only natural satellite with a magnetic field, and it harbors a vast liquid water ocean deep below the surface, unknown until recently.  Titan comes in second with its own amazing surprises, being the most Earth-like of all solar system objects, containing a liquid hydrocarbon ocean and an active liquid cycle.  Callisto is tidally locked with Jupiter, meaning an observer on Callisto would see Jupiter in the sky all the time with little to no movement.  It also has the oldest and most heavily cratered surface of any in the solar system, meaning there has been little to no volcanism over its history.

And then we reach Io, the densest moon and innermost of the Galilean four, with incredible tidal heating that changes its diameter by up to 100 meters. This tidal heating makes Io the most volcanically active body in the solar system, with hundreds of volcanoes dotting the surface made predominantly of Sulphur and Sulphur Dioxide.  The largest of the volcanic features, named Loki, is a part of a huge depression named pantera, where the cooling lava episodically sinks, revealing the lava lake and creating a rise in the emissions from the region.  A new photo from NASA’s Large Binocular Telescope (LBT) shows the most detailed image of the depression ever taken, with an early Voyager image overlaid.

The LBT image of Loki Patera (orange) laid over a Voyager image of the volcanic depression. The emission (in orange color) appears spread out in the north-south direction due to the telescope point-spread function; it is mainly localized to the southern corners of the lake. Credit: LBTO- NASA

This is one of the first uses of the LBT as a large interferometer, useful for looking at tiny features of distant objects. “Interferometry is the way to combine the light coming from each of the two main mirrors of the telescope in a way which allows us to make it look like coming from a single mirror as big as the most distant points of the LBT’s two mirrors: 22.8 meters instead of the 8.4 meters of each main mirror,” explained Christian Veillet, director of the LBTO. “If we want to look in the vicinity of a star to find dust or planets, we use the information coming from both mirrors in a way that will ‘erase’ the bright star and allow us to look at faint objects around the star such as planets or a disk of dust.”

The deeper study of the volcanoes and plumes of Io gives insights into the inner structure of the Moon, and the intricate dance it performs between Jupiter and the other Galilean satellites.



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