What is a Moon?

We now have the most impressive picture ever taken of Phobos, which circles Mars.

It’s a rock.  Nothing but a rock.  Granted it’s a big rock.  But, it’s still just a rock.  Is that a moon?  Is anything that orbits a planet truly a moon?  Is there some more definitive definition of a moon?  By the existing definition, Earth has probably 8,000 moons by now, with 7,999 being man-made objects.

That’s an asteroid called “Annefrank”.  It looks a lot like Phobos.  If “Annefrank” suddenly started circling a planet, it would no longer be an asteroid, it would be a moon.  So, you could come to the conclusion that moons are objects that serve no other purpose than to just circle a planet.  However:

That’s Titan, which circles Saturn.  It’s got color, it’s got an atmosphere, and taking a leap of faith, it could support life.

That’s the surface of Europa, which circles Jupiter.  That’s ice you see there, evidence of the basic element needed to support life.  Speculation is life might exist under the frozen shell.

And Io, which looks a lot like a planet.  Less likely to support life, but still has a lot of features a planet would.

What I’m leading to is physically the only difference in definition between a moon and a planet is what it orbits.  That worked fine 100 years ago, but in lieu of being able to actually visit these places, and becoming very familiar with the differences moons have, I would like to see a better definition of what constitutes moons and planets.  If there’s life out there, who cares what it orbits?  To me a moon is something not likely to support life, a planet should have some physical characteristics to define it could support life, such as an atmosphere.  Io and Europa are more of a planet than they are a moon.  Annefrank is as much a moon as Phobos is.  When someone says something is a moon, I’d like to have a clear visualization and expectation of what it is.  If they say something’s a planet, I’d like to know I could get excited about it.

Just another useless rant from Moon, who has a lot of atmosphere.