100-C 25-C 50-C 75-C C+M 50-C+M C+Y 50-C+Y M+Y 50-M+Y 100-M 25-M 50-M 75-M 100-Y 25-Y 50-Y 75-Y 100-K 25-K 25-19-19 50-K50-40-40 75-K 75-64-64CHAPTER 25
Titan
Athena Coustenis
LESIA, Observatoire de Meudon
Paris, France- Introduction 4. Looking Ahead
- The Atmosphere of Titan Bibliography
- The Surface of Titan
1. Introduction
1.1 Titan’s Discovery, First Observations, and Models
Titan, Saturn’s biggest satellite (second in size among
the satellites in our solar system), has attracted the eye
of astronomers preferentially ever since its discovery by
Dutch astronomer Christiaan Huygens on March 25, 1655.
Titan orbits around Saturn at a distance of 1,222,000 km
(759,478 mi) in a synchronous rotation, taking 15.9 days to
complete. As Titan follows Saturn on its trek around the
Sun, one Titanian year equals about 30 Earth years. The
sunlight that reaches such distances is only 1/100th of that
received by the Earth. Titan is therefore a cold and dark
place, but a fascinating one.
It has been known for a long time that Titan possesses a
substantial atmosphere: Catalan astronomer Jose Comas i
Sol `a claimed in 1908 to have observedlimb-darkening
on Titan. Due to its thick atmosphere, Titan subtends
0.8arcsecin the sky, and it was thought to be the largest
of the satellites in the solar system. This explains the name
it was given (following a proposition by Herschel, who sug-
gested names of gods associated with Saturn for naming
its satellites), until the advent of theVoyagermissions that
showed Ganymede to be a few kilometers larger. Today, we
know that this massive atmosphere is the one most similar
to the Earth’s among the other objects of our solar system
as N 2 is its major constituent and it is host to a complex
organic chemistry.
In 1925, Sir James Jeans showed that Titan could have
kept an atmosphere, in spite of its small size and weak grav-
ity, because some of the constituents which could have been
present in the proto-solar nebula (ammonia, argon, neon,
molecular nitrogen and methane) would not escape. It was
realized later that although ammonia (NH 3 ) is in solid phase
at the current Titan temperatures and could not in principle
contribute to its present atmosphere, it could have evapo-
rated in the early atmosphere and been converted into N 2
at the end of the accretion period when the environment
was warmer.
On the other hand, methane (CH 4 ), the second most
abundant constituent on Titan, is gaseous at present
Titan’s atmospheric temperature range and, unlike molec-
ular nitrogen, exhibits strong absorption bands in the
infrared. These bands were first detected in 1944 by
Gerard Kuiper of Chicago University. Ethane (C 2 H 6 ), mon-
odeuterated methane (CH 3 D), ethylene (C 2 H 4 ) and acety-
lene (C 2 H 2 ) were also discovered later.
Prior to spacecraft observations, two models were pop-
ular: a “thin methane” atmosphere model, which favored
methane as the main component (about 90%) and predicted
surface conditions ofT=86 K for 20 mbar as well as a tem-
perature inversion in the higher atmospheric levels, illus-
trated by the presence of emission features of hydrocarbon