All posts for the month August, 2009

On Titan, That Is

Artists concept of Titan surface beneath its foggy atmosphere. Courtesy NASA. Click to embiggenate.

Artist's concept of Titan surface beneath its foggy atmosphere. Courtesy NASA. Click to embiggenate.

Astronomer Mike Brown of CalTech (who tweets under the name PlutoKiller) has a fascinating discussion on his blog about fog banks hovering over Titan’s south pole. Titan, if you haven’t been following outer solar system news, is the largest moon of Saturn. It has this thick atmosphere hanging over a frigid surface which itself boasts pools of hydrocarbons in the form of liquid and ice. The hydrocarbons are in the form of ethane (on the surface) and now it appears that the methane forms fog banks in the atmosphere. Methane breaks down in the presence of sunlight to make ethane, so this whole thing seems to point to some sort of cycle between atmosphere and surface on Titan.

I say “seems” because, as Mike discusses, there’s a lot of atmospheric science work to be done to completely understand what’s happening on this shrouded world to make methane clouds form.  Want to know more and see a cool pic? Run over to Mike’s blog and read what he has to say. He also has a link to his science paper outlining the fogbank on Titan and a nice, insightful discussion on peer review of his paper — and he invites folks knowledgeable in the Titan atmosphere to review his paper before it goes to publication.  How cool is that!


Earth and Moon from LCROSS orbit.  August 17, 2009. Click to embiggen.

Earth and Moon from LCROSS orbit. August 17, 2009. Click to embiggen. Courtesy NASA/Ames.

Imagine you’re on this spacecraft coming to Earth — this is the scene you’d see from a vantage point of 520,294 kilometers from Earth and 880,850 kilometers from the Moon. You’d know there’s a planet there, and its moon would be tantalizingly far away… but it would be exciting to see.  If I were the alien piloting the ship, I’d be excited to see another world and, given the instruments on my ship showing the components of that planet’s atmosphere, I’d know there was life there.

Well, this isn’t a view from an alien ship — it’s the view that the Lunar Crater Observation and Sensing Satellite (LCROSS) mission, which is on a journey to study the Moon and, on October 9, crash a Centaur rocket stage into the south polar region.  The impact should kick up a plume of dust and other materials — and hopefully some of that stuff will be hydrogen or even water vapor. If there is water there, then we’ll know that there’s a supply of ice at the lunar south pole. How much ice is yet to be determined — but if there’s a lot, it could be a useful supply for future moon explorers.

So far, the LCROSS mission is on schedule for its delivery date, despite a sensor anomaly that caused one of the spacecraft’s thruster to fire excessively. That action consumed quite a lot of fuel, but the team estimates that the spacecraft still has enough to complete its full mission. They’re still assessing the situation and trying to figure out the complete chain of events that led to the over-firing of the thruster.  For a nice background on LCROSS from the Astronomical Society of the Pacific, head to their Astronomy Behind the Headlines web page for a podcast interview I did with LCROSS team member Brian Day. This is going to be an exciting mission come early October, so stay tuned!

Star Formation Factory

The Trifid Nebula as seen by the European Southern Observatory Wide-field Imager. Click to embiggen.

The Trifid Nebula as seen by the European Southern Observatory Wide-field Imager. Click to embiggen.

One of the sky sights in the Milky Way that delights summertime stargazers in the Northern Hemisphere (and late winter-early spring gazers in the Southern Hemisphere) is the Trifid Nebula. It lies in the constellation Sagittarius and is a massive stellar factory.  Professional astronomers study it to understand the whys and wherefores of star formation; amateurs just like to look at its gorgeousness.  The European Southern Observatory has just released a new wide-field image of the Trifid that really lets you explore the details of this region, as seen in visible light.

Let’s take a little tour of the image. First, if you can, right-click on the image and open in a separate window.

Now, look at the bluish patch to the upper left. This is what’s known as a reflection nebula. It does what it sounds like it does — the gas in the nebula scatters light from nearby stars that were born in the nebula.  The larger ones shine hotter and brighter, especially in the blue portion of the visible spectrum. Dust grains and molecules scatter blue light more efficiently than red light and that makes this part of the nebula look so very pretty and blue.

The pink-reddish area is a typical emission nebula. That differs from reflection nebula in a very important way — instead of reflecting light, the gases are heated by the hot, young nearby stars and that ultra-hot bath of radiation causes the gases to glow.  They emit the red signature light of hydrogen, which is the major component of the gas.

That’s two kinds of nebula in the scene, but there’s a third type. The gases and dust that crisscross the clouds make up the third kind of nebula. They form what’s called a dark nebula, and they block out the light from the parts of the nebula that lie behind them — similar to the way a dust cloud on Earth blocks out sunlight.  These aren’t dead clouds, however. The remnants of previous rounds of star birth are clumping together and coalescing under the pull of gravity from within.  Eventually, the cloud gets dense and hot enough and the pressure from the coalescence triggers nuclear fusion where the clouds are the thickest — this is the formative event of a newborn star.

Finally, if you look at the lower part of the emission nebula, you can see a finger of gas poking out, pointing directly at the central star powering the Trifid. This is an example of an evaporating gaseous globule, or “EGG”.   At the tip of the finger, which was photographed by Hubble, a knot of dense gas is holding out against the onslaught of radiation from the massive star.

There are star formation sites in many places in our galaxy — and of course, in other other galaxies. Astronomers study them to see how the process of star birth progresses — which, in turn, gives them insight into how our own star formed more than 4.5 billion years ago.