Pluto's spic-and-span moon
A new study indicates that Pluto's moon, Charon, is getting some clean-up work: the dynamic cleaning combo of ammonia and water has been discovered on its surface.
This was the first identification of frozen ammonia in the solar system, if you don't count comets, so the finding indicates that on this tiny moon, at least, cleanliness is being taken seriously.
Seriously, the discovery shows that bitter-cold Charon (the surface is 50 degrees Kelvin, or minus 360 degrees Fahrenheit) was at one point warm enough inside to erupt with a solution of cleaning fluid. Imagine a water-and-ammonia version of a volcano.
Since Pluto and Charon are closer than 1/3,600 of a degree in our sky, Charon was not discovered until 1978, and it's still extremely tough to distinguish its spectrum from Pluto's. But Michael Brown, a professor of planetary astronomy at the California Institute of Technology, and colleague Wendy Calvin lucked out during an exceptionally clear night at Hawaii's 10-meter Keck Telescope. With exceptionally low atmospheric turbulence, they distinguished the two bodies and read their infrared emissions (see "Evidence for Crystalline Water... " in the bibliography).
Why would the presence of that ammonia-water brew signify volcanoes? It's a matter of quantity -- the only realistic source of ammonia is some sort of volcanism, Brown says. Indeed, he speculates that ammonia-water volcanism could explain geologic changes seen on other frozen satellites deep in the solar system.
Brown says the cleaning-fluid eruptions -- which could have happened billions of years ago -- demonstrate that at some time Charon's interior was at least 150 degrees Kelvin. But that heat didn't come from a hot water tap. Whence the heat on a satellite that's roughly 40 times as far from the sun as our planet?
From heat left over from the formation of the solar system 4.6 billion years ago, when all the whirling bodies coalesced from gas and dust. Brown says the internal warmth may also reflect an impact that apparently cleaved Charon from Pluto.
The infrared data also show that the water ice is in the familiar crystalline form. At Charon's frigid conditions, solar radiation would eventually degrade the crystalline ice into amorphous ice, where solid water molecules are packed together rather than structured into crystals. But apparently the impact of micrometeorites is causing the ice to vaporize and recrystallize -- recoating the surface with the more familiar type of ice.
That's interesting, Brown and Galvin wrote, because it proves a general rule, and again points to a general condition in the fringes of the solar system. "The presence of crystalline water ice on all of the well-studied icy satellites in the outer solar system confirms that a ubiquitous mechanism such as impacts must be responsible."
Translated: Meteorite hazard! Bring an umbrella next time you visit Charon!
Cold, but active
The finding of those volcanoes indicates that Charon is -- or was -- churning below decks. "It drives home the fact that Charon's not been sitting out there doing nothing," says Brown. "It has geological activity just like anywhere else in the solar system."
The observation also indicates that Charon is too small to have an atmosphere. Pluto, in contrast, is coated with frozen methane and carbon dioxide, which apparently crystallized from its atmosphere, much as water freezes out of a cold night on Earth. Since the chemicals are absent from Charon, it seems the satellite is too small to retain gas in an atmosphere.
So while you'd want to park your Chevy II in a garage to prevent frosty windows on Pluto, that might not be necessary on Charon.
Beneath the surfaces, both Charon and Pluto probably share a similar composition, says Brown: a combination of water ice with rocks that resemble those found on Earth.
Back to basics
If infrared is helpful in assessing the cleanliness of tiny, distant moons, it's also handy for myriad other astronomical chores. For one thing, as infrared astronomer Marcia Rieke of the University of Arizona points out, dust passes through lots of gooky stuff that blocks visible light, hiding interesting tidbits like stars aborning and black holes.
The center of our galaxy, the Milky Way, is obscured in the visible light by a Dust Busterful of dust, yet it's much clearer in the infrared.
Many of the benefits of infrared astronomy derive from the origin of infrared light in the first place. Cool stuff -- okay, low-temperature bodies -- makes virtually no visible or ultraviolet radiation, but lots of infrared.
We'll get to the list in a second, but first let's note that because anything above absolute zero makes infrared, it's best to cool the detectors, so stray emissions from the telescope structure itself don't bogus the data. In fact, to reduce noise, the upcoming Space Infrared Telescope Facility will be cooled to 4 degrees Kelvin, barely above absolute zero.
Rieke, who is collaborating on that 'scope, notes that it will orbit distant from Earth, to avoid infrared noise from our planet, and will use a shade to ward off sunlight. The last bit of cooling will be accomplished by evaporating liquid helium, which occurs at 4 degrees Kelvin.
Cool explains the power and beauty of infrared -- there's a bunch of low-temperature stuff in space:
Planets! Infrared equipment may eventually make direct images of planets beyond the solar system. In fact, the goal of the new Terrestrial Planet Finder's is to find ET's home place. We covered the search for planets, and a possible first photo of a planet beyond the solar system.
Dust and gas that are heated by ultraviolet and visible light, as commonly occurs during star formation. (X-rays can also make images of the much hotter gas that surrounds invisible objects like black holes.)
Browd dwarfs, the pathetic misfits that exist in a never-never land between planets and stars. Brown dwarfs and similar dross seem to make up a fair portion of the mass of the universe. If we could count this stuff, we'd have a better notion how much mass we can't see but must be there to explain the rotation of galaxies. The search for mass that seems to exist in strange, invisible forms has energized and frustrated physicists for years.
Much of the distant universe, even bodies that emit in visible or ultraviolet regions, are also visible in the infrared. Why? Because of the Doppler shift, which reduces the frequency, and increase the wavelength, when a source of sound or light moves away from the observer. "If you want to study distant galaxies, you're best off in infrared," says Rieke.
If you're not impressed by these "quality" arguments, let's talk quantity for a second. Bodies colder than about 1,700 C emit almost all their radiation in the infrared. Ditto for galaxies. Here's the long and short of it: You want to see the universe? You gotta get with the infrared program!
Still in an infrared fog? Here's a dandy description.
Root around our infrared bibliography.