Beauty, mystery leave earthlings moonstruck
By Amanda Jermyn

Reach for the StarsÖ Amanda Jermyn
March, 2011
Our moon, with its mysterious beauty, has captured the imaginations of both romantics and scientists throughout the ages. The first time any object was observed orbiting a celestial body other than the earth or sun was in 1610, when Galileo discovered Jupiterís moons Ganymede, Callisto, Io and Europa. This helped lead to the realization that we earthlings are not at the center of the universe.

But what exactly is a moon, and how many are there out there? A moon is a natural satellite that orbits a planet or smaller body such as a dwarf planet or minor planet. About 175 moons have been discovered orbiting the major planets, but over 300 orbit smaller bodies in our solar system. Moons come in many shapes and sizes and have a variety of origins. Some were formed from the same disk of dust and debris orbiting the sun that formed the planets. Others were asteroids or Kuyper belt objects captured by their planetís gravity. Neptuneís moon Triton is thought to be a captured dwarf planet. Our own moon most likely originated over 4 billion years ago when a Mars-sized object collided with earth, spewing debris into space which coalesced under gravity to form our one and only natural satellite.

Mercury and Venus have no moons. Mars has two. The large gas giants have many. Jupiter has 63, and Saturnís include the shepherd moons whose gravity keeps the planetís famous rings in orbit, preventing their constituent ice and rock from spiraling in towards Saturn. Some moons such as our own are roughly spherical; other smaller ones are potato shaped. Moons are tidally locked to their planet, meaning that the same side of the moon always faces the planet. The only known exception to this is Saturnís moon, Hyperion which rotates chaotically under the gravitational influence of Titan.

Our solar systemís moons contain bizarre environments such as Titanís thick nitrogen atmosphere where the rain, rivers and lakes are liquid methane instead of water. Jupiterís moon Io is the most volcanically active body in the solar system, averaging 9 active volcanoes, compared with about 6 on earth. Jupiterís Europa, Saturnís Titan and Enceladus, and Neptuneís Triton all show evidence of tectonic activity and ice volcanoes. Enceladus and Triton have geysers. Most tantalizing of all, Europa, Ganymede, Callisto and Titan are thought to have oceans of liquid water beneath their frozen surfaces. Future space probes will likely explore these for signs of life. While other stars may have exotic moons orbiting their planets, many mysteries remain to be explored among the moons of our own solar system.

This month join the Springfield STARS Club on Tuesday, March 22nd at 7:30pm at the Springfield Science Museum for a talk by amateur astronomer Jack Megas on Wacky Moons of the Solar System. A retired medical technologist, Jack is a past president of both the STARS Club and the Naturalist Club, a planetarium educator at the Springfield Science Museum, planetarium show developer, and co-founder of The Conjunction, the oldest annual astronomy convention in Massachusetts. Refreshments will be served, and the public is welcome free of charge.