Three views of Titan from observations by the Cassini
spacecraft. Left: a natural color composite made from images taken
through three filters sensitive to red, green, and violet light, showing
roughly how Titan would appear to the human eye. Center: a near-infrared
composite revealing the surface. Right: a false-color composite of
one visible-light and two infrared images. Green areas are where Cassini
can see the surface; red represents regions high in Titan's stratosphere.
Taken on Apr. 16, 2005, at distances ranging from 168,200 to 173,000
km. Credit: NASA/JPL
Voyager 2 photo of Titan, taken Aug. 23, 1981, from
a range of 2.3 million km. The southern hemisphere appears lighter,
a well-defined band is seen near the equator, and a dark collar is
evident at the north pole. All these bands are associated with cloud
circulation in Titan's atmosphere. Credit: NASA/JPL
The largest moon of Saturn and the second
largest moon in the Solar System, after Ganymede.
Titan is bigger than the planet Mercury,
although only half as massive. It is the only moon in the Solar System to
have a dense atmosphere (10 times deeper than Earth's and with a surface
pressure 60% greater). Until the arrival in orbit around Saturn of the Cassini-Huygens
spacecraft, in 2004, little was known about the surface of Titan because
of the obscuring effect of an orange haze in the atmosphere.
Discovery and naming
Titan was discovered by the Dutch scientist Christiaan Huygens
on March 25, 1655 and was the first moon to be found with the aid of a telescope
after the four Galilean satellites of
Jupiter. Huygens called it simply Luna Saturni (moon of Saturn).
However, in keeping with a custom of the time, he didn't announce his discovery
openly. Instead, he disguised the news in the form of an anagram, partly
using a verse by the Roman poet Ovid, "Admovere Oculis Distantia Sidera
Nostris" (They brought the distant stars closer to our eyes), which is etched
around the edge of the lens of the telescope that Huygens used. When decoded
and translated, the anagram reads "A moon revolves around Saturn in 16 days
and 4 hours," a value close to the modern value of Titan's orbital period.
It was John Herschel who proposed the
name "Titan" in his 1847 publication "Results of Astronomical Observations
made at the Cape of Good Hope." In Greek mythology, the titans were brothers
and sisters of Cronos, the Greek equivalent of the Roman god Saturn. Also
in the same publication, Herschel named the six other then-known moons of
Saturn after individual titans.
Comparison of the atmosphere of Earth
with a model of that of Titan.
The possibility of an atmosphere around Titan first arose scientifically
in 1903 when the Spanish astronomer Jose Comas
Solá observed that Titan appears brighter at its center than it
does at its limb. He suggested that this is because sunlight reflected toward
Earth by Titan's limb must pass through more of the moon's atmosphere than
sunlight reflected by the center. The existence of an atmosphere was confirmed
in 1944 by Gerard Kuiper at the University
of Chicago, who identified methane in the
spectrum of Titan.
Further observations, notably by the Voyager probes as they flew by in 1980
and 1981, and, more recently, by Cassini-Huygens, have shown the composition
of Titan's atmosphere to be 98.4% nitrogen
and 1.6% methane, with trace amounts of other gases, including various hydrocarbons
(such as ethane, diacetylene, methylacetylene, cyanoacetylene, acetylene,
and propane), argon, carbon dioxide, carbon monoxide, cyanogen,
hydrogen cyanide, and helium. Apart from Earth, Titan has the only dense
nitrogen-rich atmosphere in the Solar System.
Natural color image of Titan's upper atmosphere obtained
by Cassini at a distance of about 9,500 km (5,900 miles) on Mar. 31,
2005. The haze preferentially scatters blue and UV wavelengths, making
its complex layered structure more easily visible at the shorter wavelengths
used in this image. Credit: NASA
The hydrocarbons are thought to form in Titan's upper atmosphere due to
reactions involving the breakup of methane by the Sun's ultraviolet light
and cosmic rays. This organic photochemistry
produces the orange haze, most dense at an altitude of about 300 km (200
miles), which obscures the surface at visible wavelengths and also reflects
a substantial amount of infrared back into space giving rise to an "anti-greenhouse
effect". Titan is one of only two known bodies – the other is Pluto
– whose surface temperature is lower (by about 10K) than it would
be if it had no atmosphere. The production of a variety of organic materials
in the atmosphere, which presumably drizzle down onto the surface, is one
of the reasons Titan is of such interest to astrobiologists (see Titan,
prebiotic evolution and Titan, life
A person standing on Titan's surface during the day would experience only
about one thousandth the daylight brightness on Earth's surface. This comparison
takes into account not only the thickness of the atmosphere, but also Titan's
greater distance from the Sun. Even so, light levels on Titan's surface
are 350 times brighter than moonlight on Earth under a full moon.
Because the methane in Titan's atmosphere is being continually depleted,
there must be some mechanism on the surface that replenishes it. One possibility
is that Titan has active volcanoes, which
Titan seen by Cassini in infrared (Jul. 3, 2004).
The nature of the white spot toward the south pole remains uncertain.
Prior to the arrival of Cassini-Huygens in June 2004, infrared observations
by the Hubble Space Telescope had provided a crude map of bright and dark
regions on Titan, though the nature of these features remained uncertain.
It had been speculated that oceans or lakes of liquid ethane
might cover a large fraction of the moon's surface and that liquid methane
might fall as rain. According to another model, the bright areas glimpsed
by Hubble might be water ice plateaus thrust above lowlands darkened by
solid and liquid organic molecules.
The mysterious 450-km-wide white feature seen by
Cassini in July 2004. Credit: NASA/JPL/SSI
A more detailed and accurate picture of Titan has begun to build up thanks
to images and other data sent back by Cassini-Huygens. During its first
of more than 40 planned flybys of the big moon, Cassini showed what appear
to be methane clouds and a giant impact crater. The most prominent feature
seen was a bright region of cumulus-like cloud near the south pole, some
450 km across and about 15 km above the surface. The spacecraft's measurements
suggested that the clouds were probably made from hydrocarbons, and might
be associated with a ground feature. Cassini revealed some variations in
the surface brightness were circular and others are linear. There also appear
to be some concentric features near the south pole. This variation in surface
detail suggests that Titan is not heavily cratered and that much geologic
activity has taken place. 1
A mosaic of nine pictures, taken as Cassini flew past Titan on Oct. 26,
2004, has given astronomers their most detailed view yet of the moon's full
disk. Titan's surface features are clearest at the center of the disk, where
the probe had the least atmosphere to peer through. There are no visible
craters, which implies the moon probably has a young surface that is continually
refreshed. Astronomers are still unsure whether the flowing patterns on
Titan's surface are caused by volcanic eruptions, shifting plates of rock,
wind-blown dust or even rivers of liquid hydrocarbons.
Drainage channels and what appears to be a shoreline
taken by the Huygens probe during descent (Jan. 14, 2005). Credit:
First image from the surface of Titan (Jan. 14, 2005).
On Jan. 14, 2005, the Huygens probe successfully
parachuted down to and landed on the surface of Titan, sending back remarkable
pictures both during the descent and on the ground. One stunning black and
white image reveals what seem to be drainage channels on a land surface
leading out into a dark body of liquid. Another shows a flat surface that
is apparently strewn with boulders. Huygens captured more than 300 images
as it dived through the moon's atmosphere.
Speaking about the picture of an apparent shoreline, John Zarnecki, principal
investigator for the surface science package (SSP) on Huygens said: "If
it's not a sea, it could be a lake of tar. And did one see waves?"
Owen, T., Gautier, D., Rautlin, F., and Scattergood, T. "Titan." In
G. Carle, D. Schwartz, and J. Huntington, eds., Exobiology in Solar
System Exploration, NASA Special Publication 512, p. 127. Moffett
Field, alif.: NASA, Ames Research Center (1988).