Pale Blue Dot ( PDFDrive.com ) (1)

that obscure the surface: which colors of light they like to absorb, which colors they pretty much let
pass through them, how much they bend, the light that does pass through them, and how big they are.
(They're mostly the size of the particles in cigarette smoke.) The "optical properties" will depend, of
course, on the composition of the haze particles.
In collaboration with Edward Arakawa of Oak Ridge National Laboratory in Tennessee, Khare
and I have measured the optical properties of Titan tholin. It turns out to be a dead ringer for the real
Titan haze. No other candidate material, mineral or organic, matches the optical constants of Titan. So
we can fairly claim to have bottled the haze of Titan—formed high in its atmosphere, slowly falling
out, and accumulating in copious amounts on its surface. What is this stuff made of?
It's very hard to know the exact composition of a complex organic solid. For example, the
chemistry of coal is still not fully understood, despite a long-standing economic incentive. But we've
found out some things about Titan tholin. It contains many of the essential building blocks of life on
Earth. Indeed, if you drop Titan tholin into water you make a large number of amino acids, the
fundamental constituents of proteins, and nucleotide bases also, the building blocks of DNA and
RNA. Some of the amino acids so formed are widespread in living things on Earth. Others are of a
completely different sort. A rich array of other organic molecules is present also, some relevant to
life, some not. During the past four billion years, immense quantities of organic molecules sedimented
out of the atmosphere onto the surface of Titan. If it's all deep-frozen and unchanged in the intervening
aeons, the amount accumulated should be at least tens of meters (a hundred feet) thick; outside
estimates put it at a kilometer deep.
But at 180°C below the freezing point of water, you might very well think that amino acids will
never be made. Dropping tholins into water may be relevant to the early Earth, but not, it would seem,
to Titan. However, comets and asteroids must on occasion come crashing into the surface of Titan.
(The other nearby moons of Saturn show abundant impact craters, and the atmosphere of Titan isn't
thick enough to prevent large, high-speed objects from reaching the surface.) Although we've never
seen the surface of Titan, planetary scientists nevertheless know something about its composition. The
average density of Titan lies between the density of ice and the density of rock. Plausibly it contains
both. Ice and rock are abundant on nearby worlds, some of which are made of nearly pure rice. If the
surface of Titan is icy, a high-speed cometary impact will temporarily melt the ice. Thompson and I
estimate that any given spot on Titan's surface has a better than 50-50 chance of having once been
melted, with an average lifetime of the impact melt and slurry of almost a thousand years.
This makes for a very different story. The origin of life on Earth seems to have occurred in oceans
and shallow tidepools. Life on Earth is made mainly of water, which plays an essential physical and
chemical role. Indeed, it's hard, for us water-besotted creatures to imagine life without water. If on
our planet the origin of life took less than a hundred million years, is there any chance that on Titan it
took a thousand? With tholins mixed into liquid water-even for only a thousand years the surface of
Titan may be much further along toward the origin of life than we thought.
DESPITE ALL THIS we understand pitifully little about Titan. This was brought home forcefully to me at a
scientific symposium on Titan held in Toulouse, France, and sponsored by the European Space
Agency (ESA). While oceans of liquid water are impossible on Titan, oceans of liquid