If you stepped off a spacecraft onto the surface of Titan, you might experience a little d j vu. Saturn’s largest moon has many of the same features as Earth. That includes rivers and seas, clouds, and even rainfall – it’s the only world in the solar system other than Earth with bodies of liquid on its surface.

What wouldn’t seem familiar is the temperature – almost 300 degrees below zero Fahrenheit. In that icebox, water is frozen as hard as granite. So Titan’s rivers and seas and clouds are made of liquid methane and ethane.

Titan is a large world – about half-again the diameter of our moon. And it has the densest atmosphere of any moon in the solar system; the surface pressure is equivalent to a depth of 50 feet in Earth’s oceans.

The methane and ethane are quickly broken apart by sunlight, so the supply in the air has to be renewed. The most likely source is cryo-volcanoes – volcanoes that belch frozen water. Methane mixed with the water would waft into the atmosphere.

The volcanoes could be fed by an ocean of liquid water below the surface – perhaps much more water than in all of Earth’s oceans combined. Both the ocean and the liquid bodies on the surface are possible homes for microscopic life – one more similarity to our own world.

Saturn looks like a bright star near the Moon this evening. Through good binoculars or a small telescope, Titan looks like a tiny star quite near the planet.

Script by Damond Benningfield

The planet Mercury is putting in a decent appearance in the evening sky now. It looks like a bright star low in the west during twilight. And tonight it has a prominent companion: the crescent Moon. In fact, they’ll look like they’re almost touching each other.

Mercury is tough to see because it’s the closest planet to the Sun. Because of that, it never moves far from the Sun in our sky. At best, it’s visible for an hour or two after sunset or before sunrise.

Right now, it’s farthest from the Sun in the evening sky. For a few nights, it won’t set until about an hour and 20 minutes after sunset. As twilight begins to fade, though, it’s so low in the sky that you’ll need a clear horizon to spot it.

The Moon is just a day and a half past “new,” when it crossed between Earth and the Sun. So the Sun illuminates only a tiny fraction of the lunar hemisphere that faces our way. The rest of the disk will be faintly highlighted by earthshine – sunlight reflected from Earth. That will enhance the beauty of this duo in the fading twilight.

Two other planets are close by. Saturn is to the upper right of Mercury and the Moon, and looks like a fairly bright star. Venus is heaving into view below them. It’s much brighter than Mercury, but much lower, making it tougher to pick out. But Venus will climb higher over the coming weeks – blazing as the “evening star.”

More about the Moon and Saturn tomorrow.

Script by Damond Benningfield

Car wrecks aren’t all alike, so there’s a wide range in the results.

The same principle may apply to the bodies of the early solar system, when the planets were taking shape. In fact, a recent study says the modern appearance of the planet Mercury could be explained by a glancing blow between two bodies of similar size.

Mercury is an oddball among the rocky planets of the inner solar system. Its metallic core accounts for about 70 percent of its mass – a far higher ratio than for Earth or the other planets. And the core is surrounded by a fairly thin mantle – a layer of lighter-weight rocks.

Some simulations have suggested that was the result of a giant impact – a massive collision between bodies of much different sizes. Such impacts were common in the early solar system; one of them might have led to the creation of the Moon.

But the recent study suggested something else. It found that a glancing blow between Mercury and a similar-sized planet could have stripped away much of Mercury’s mantle. But the research doesn’t tell us what happened to the other planet, or the debris from the impact. So scientists will ponder the possible collision a little longer to understand the planet Mercury.

Mercury is peeking into view in the early evening. It looks like a bright star, but it’s quite low in the west during twilight, so it can be hard to spot. The Moon will join it tomorrow night; more about that tomorrow.

Script by Damond Benningfield