Wednesday, September 11, 2013

An Alien Sea on a Moon of Saturn's

I want to introduce you today to Ligeia Mare (the Ligeian Sea), the second largest known body of liquid outside the Earth. It's on the surface of Saturn's moon Titan, the only moon in the solar system to have a thick atmosphere and, therefore, to have air pressure. It's air pressure that keeps liquids from evaporating and makes rivers, lakes, and seas possible.

And to our delight, Titan, despite the freezing cold temperatures in the Saturn System, has rivers, lakes, and seas. At Titan temperatures, water is frozen until it's harder than rock on Earth. So the bodies of liquid on Titan are not water, but methane and ethane, which can be liquid under Titan conditions. Methane is natural gas (and also what comes out of both ends of the cow after it has been digesting its food for a while.) Ethane is a chemical that is often found in natural gas and is also a product of refining oil.

Take a look at the picture. This is not a photograph, but an image that comes from bouncing radar beams off Titan, something we equipped our clever little Cassini spacecraft to do. On radar pictures, rough things show up light, smooth things (like bodies of liquid) show up dark. The colors are added artificially to make the picture more interesting.

The name Ligeia comes from one of the sirens of Greek mythology, beautiful creatures who lured sailors toward rocky shores and their certain death. Look at Ligeia on the image -- ain't she beautiful? Some people see the shape of a sleeping dog, with its head toward the bottom. But its size is what astounds. The sea is about the size of the Earth's Caspian sea and if you walked completely around it on the shore, you'd walk a total of 1,240 miles.

Where the radar picture is black, the sea is deep; where you see grey, the sea is more shallow, so the radar beam can hit the sea bottom and some of it bounces back. Already scientists have proposed a mission (not approved yet) to splash down a capsule in Ligeia Mare and either drift or purposely navigate around the sea, sending back images to Earth.

It's been winter in the North polar region of Titan since Cassini arrived in the neighborhood. Some scientists think that once the weather starts to warm up on Titan (it will still be outrageously cold compared to Earth,) the Titan winds might blow hard enough to make waves on this alien sea, and our radar might just pick up those waves. (Somebody should tell the Beach Boys that surf might soon be up another world!)

For more details about weather on Titan, see:

Saturday, September 7, 2013

Crescent Moon and Venus Get Close Sunday Night Sept. 8

A minor, but cute, pairing is visible in the sky tomorrow. It happens soon after sunset and you can only see it if you have an unobstructed view of the west-southwest horizon. (For many of us that means we will have to get high -- only in the geographical sense, mind you!! Typically, our view is blocked by buildings, trees, hills, etc. and we can't see the western horizon.) But if you get to a place where you can see low in the west Sunday evening, you will see a very thin crescent Moon and the bright planet Venus just to the right of it.

You can start looking about a half hour after the Sun sets on Sunday and you'll see it best about 45 minutes after sunset. But don't wait too long to catch it; soon both objects will set in the West, and disappear from your view.

When Venus is visible just after sunset, people call it the evening star. But it's not a star at all, but a planet -- our neighbor planet just sunward of the Earth. It reflects quite a bit of sunlight both because it's close to the Sun and because it's covered with clouds that are quite reflective. (In fact Venus is so cloudy, there hasn't been a clear day there in 3 billion years. I like to cheer up my friends in Seattle by telling them that.)

As you can see in the nice diagram, which I borrowed from the good people at Sky & Telescope magazine, if you look further up and to the left, you will also see the planet Saturn in the same part of the sky. The distance in the sky between the Moon and Saturn should be about the width of your clenched fist, if you hold out your hand at arm's length. If you have binoculars or a telescope, you can enhance your view, but if you don't have an instrument, it's still fun to look. If you have kids, bring them outside and show them the sight too. It couldn't hurt to impress them with your knowledge of the universe while they are young.

For more information, you can see the full story at the Sky & Telescope web site:

Wednesday, September 4, 2013

Hints of Water on a Super-Earth

A Japanese team of astronomers, using the giant Subaru telescope, atop an extinct volcano in Hawaii, have found further hints of the presence of water on a planet orbiting the star Gliese 1214, about 40 lightyears away.

The planet orbits its star in only 38 HOURS (not days, folks, but hours!) You might think that a planet this close to a star will soon be french fried, but, in this case, the star itself is much dimmer and cooler than the Sun -- it is what astronomers call a "red dwarf."

The planet circling the red dwarf is one of the few planets outside our own solar system which we have been able to find in two independent ways. We know it's there because its gravity makes the star wiggle a bit, and we also know it's there because we can see the planet move across the face of the star and cause a mini-eclipse (or transit).

This double identification is very helpful, since it lets us measure both the size of the planet and how much stuff (mass) it contains. That's how we know it's a "super-Earth" -- a kind of planet we don't have in the Sun's family. This super-Earth is about three times the size of Earth, and more than 6 times its mass. These characteristics make this alien world denser than Jupiter but less dense than Earth. It could be a little rocky planet with a giant atmosphere, or a planet with some rock and a lot of liquid water surrounding it.

The new Japanese study examined the planet's atmosphere and concluded that the way light of different colors scattered from it was consistent with either the presence of water vapor or with some kind of extensive cloud cover. Combining this work with other studies makes astronomers a bit more sure that this is a water-rich environment. That still doesn't help us pin down exactly what this super-Earth looks like, and it's probably too warm overall for life as we know it. Nevertheless, isn't it amazing that we can now discover not only planets out there in other parts of the Galaxy, but even something about the kind of air they have surrounding them?

(NOTE: The image above is an artist's conception of what the star might look like through a blue filter.  The star itself would look red to your eye.  The planet is the smaller black sphere on the left side of the star.)