Moon and Mars 
The roster of known planets includes more than a thousand worlds. A few of them are somewhat similar to Earth — balls of solid rock that are close to the right temperature for liquid water. Yet of all the planets we know about, the one where conditions are most like Earth is right here in our own solar system: the Red Planet Mars.
Mars is a good bit smaller than Earth, so its surface gravity is only about a third as strong. Such weak gravity couldn’t hold on to what was once a fairly thick, warm atmosphere.
But some air remains today. It’s only about one percent as dense as Earth’s, but it does provide some advantages. For one thing, it’s made almost entirely of carbon dioxide — a greenhouse gas that traps heat. That keeps the surface quite a bit warmer than if the planet had no atmosphere at all.
Another advantage is that it allows Mars to have ice caps and clouds made largely of frozen water — a precious resource for life. Some liquid water may occasionally seep to the surface as well. The atmosphere is so thin, though, that the water quickly evaporates.
That combination makes Mars the most Earth-like world yet seen — and it’s right next door.
And early tomorrow, Mars is right next door to the crescent Moon. The planet looks like a moderately bright orange star to the Moon’s left or lower left. The true star Regulus, which is a bit brighter than Mars, stands above them. We’ll have more about Mars on Wednesday.
Script by Damond Benningfield, Copyright 2013