June 7, 2020 – Pick a Moon

Technically there is only one Moon, Earth’s Moon, also called Luna.  Most planets have their own “Moons”, or satellites.  Only Mercury and Venus are without a known satellite.  Why is that?  Let’s look at how planets get their satellites, their satellite’s varied forms, and how many there are in our solar system.

The currently accepted explanation for Mercury and Venus is they were/are so close to the Sun with its tremendous mass/gravity that satellites could not attain a stable orbit.  Anything in Mercury’s or Venus’s orbit would either become planet, be eaten by the Sun, or tossed out by the Sun.

How satellites form is subject of endless debate (it’s in the details) but the consensus is there are three general ways to make a satellite.

1 – The gas giants Jupiter, Saturn, Uranus, and Neptune each had enough mass/gravity to create their own disk of orbiting material, much like the Sun. As their disks cooled, things got chunky and coalesced into satellites.

2 – Some satellites exist due to capture by a planet.  Mars’ dinky Phobos and Deimos are examples.  Small objects (relatively speaking) floating around the forming solar system can get caught up in the gravity of a larger object and become its satellite.  The gas giants did it with impunity.  Come be my satellite, pipsqueak!

3 – Our Moon is an example of another way to form satellites. Collision! The current theory is that a planet around the size of Mars slammed into the forming Earth, carving out a big glop of Earthly material.  That would have been a sight! Our Moon is made up of mostly the same materials as our own Earth’s crust.  Mostly is the key word as there are some slight differences.  It makes sense that the Moon would have retained some of the impactor’s material

These are the generally accepted ways to make a planetary satellite.  Why are they so varied in composition and appearance?  That’s for a future article.  Here are some of the more interesting satellites.

Jupiter has Europa, an ice-covered body with a slushy or liquid interior.  It has Io, a volcanic, sulfurous satellite with a surface that covers up meteor strikes.  No visible craters!

Saturn has Enceladus, much like Europa, with super cold water cryovolcanic geysers to boot!  It has Titan, a hydrocarbon world with methane rain and lakes, and thick brown atmospheric haze.  Worse than LA’s 1970s smog.

Uranus has Titania, a mix of rock and ice, maybe some liquid water below the icy layers?

Neptune has Triton, a retrograde (backward compared with most other satellites) orbiting satellite with a nitrogen/methane/CO2 atmosphere.

In all our solar system has 205 “moons” so far:  Earth 1, Mars 2, Jupiter 79, Saturn 82, Uranus 27, Neptune 14

What’s in the Sky?

Mercury is low in the west 30 minutes after sunset.  Use binoculars.

June 8 and 9; Jupiter and Saturn are with the Moon south-southwest in the pre-dawn hours