Star date: 10:10:98
Many amateur astronomers spend evenings
gazing at the four largest moons of Jupiter. The ease of viewing of these
moons is so great that they were among the first astronomical discoveries
made by Galileo in the opening years of the seventeenth century. With his
primitive telescope, not even as good as an inexpensive pair of binoculars
are today, he could see four moons around the king of the planets.
Less well known, but still within the range of small backyard telescopes are the moons of Saturn. The largest of Saturns moons is Titan. Larger than the planets Mercury or Pluto, It is the second largest moon in the Solar system, smaller than only Jupiters moon Ganymede. With an atmosphere 60% thicker than that of Earth, Titan is a world of mystery. The clouds which envelop Titan forever hide its surface from view. Like Venus, our only hope of seeing the surface is to either look at electromagnetic waves (like radar waves) which can penetrate the bodys atmosphere, and be bounced to the surface and back, or to send a spacecraft through its hazy cloudtops to land on its enigmatic surface.
The Hubble space telescope has an infrared camera which can see through Titans murky cloud layers. In 1944, the discovery of methane in the atmosphere of Titan led to the first understanding of the composition of Titans clouds. In the 1970's, hydrocarbons were also discovered in the clouds of Titan. Suprisingly, niether of these gases constitutes the majority of Titans atmosphere. Measurements by the Voyager spacecraft of the early 1980's reported finding mainly nitrogen, similar to the atmosphere on Earth, and only a few percent methane. If Titan were much closer to the Sun than it is, even nitrogen would escape to space, leaving Titan a barren, airless world.
However, at the distance that Saturn and Titan revolve around the Sun, the cloud tops of Titan have a temperature of -94 degrees celsius (-290 F). Since what we call heat is really a measure of how fast the average particle in a substance is traveling, we can see that at these cold temperatures the particles of nitrogen are moving too slow to overcome even Titan's modest gravity. The large amount of nitrogen present was most likely formed from ammonia.
Ammonia is common in the outer solar system, and the frozen
surface of Titan may contain large amounts of frozen ammonia and water ice.
Since ammonia is composed of nothing but nitrogen and hydrogen, when this
ammonia is present in the air, sunlight breaks it apart into nitrogen and
hydrogen. Hydrogen is the lightest element
of them all, and the mass of Titan is far too low to hold on to the hydrogen
(even at these low temperatures), so it escaped to space, leaving behind
nitrogen.
The hydrocarbons in the cloudtops of Titan could be caused by the presence of oceans of methane and ethane, which are the components of natural gas here on Earth. Observations from the Hubble Space Telescope seem to suggest that there are areas of solid land surface at various points around Titan, so a moon wide ocean of natural gas seems unlikely. A more possible explanation would be seas of methane and ethane around the world, punctuated from region to region by continents made from exposed ices.
Like our own moon (and many other moons in the solar system) Titan is locked in a gravitational embrace with it's parent world. Therefore, one side of Titan always faces toward Saturn, and this side seems to be a region which has a continent. The opposite side of Titan may contain islands, or perhaps small island continents in the frozen sea of natural gas.
Look for Titan throughout the fall as you are observing Saturn. How long does it take to revolve around its parent planet? How long does this mammoth moon "disappear" behind Saturns disk before reappearing on the other side?
Clear skies, and good viewing.
James Maynard is head of the astronomy department at Earth Treasures in Keene, N.H.
