This
set of images from NASA's Cassini spacecraft shows Saturn's moon Titan glowing
in the dark. Titan was behind Saturn at the time, in eclipse from the sun. The
image on the left is a calibrated, but unprocessed image from Cassini's imaging
camera. The image on the right was processed to exclude reflected light off
Saturn and it is clear that even where Titan did not receive any Saturnshine,
it is still emitting light. Some light appears to be emanating from high in the
atmosphere (noted by the outer dashed line at about 625 miles or 1,000
kilometers in altitude). But more surprisingly, most of it is diffusing up from
lower down in the moon's haze, from about 190 miles (300 kilometers) above the
surface. Image credit: NASA/JPL-Caltech/SSI
SANDY ANDERSON reports how one of NASA/JPL's veteran spacecraft has detected a faint glow emanating from Saturn's enigmatic moon, Titan.
A
literal shot in the dark by imaging cameras on NASA's Cassini spacecraft has
yielded an image of a visible glow from Titan, Saturn's largest moon. The effect emanates not just from the top
of Titan's atmosphere, but also - surprisingly - from deep in the atmosphere
through the moon's haze. A person in a balloon in Titan's haze layer wouldn't
see the glow because it's too faint - something like a millionth of a watt.
Scientists were able to detect it with Cassini because the spacecraft's cameras
are able to take long-exposure images.
It
turns out that Titan glows in the dark - though very dimly, and it's a little like a neon sign, where
electrons generated by electrical power bang into neon atoms and cause them to
glow. This recently detected glow is caused by light emitted when charged particles collide with nitrogen molecules in Titan's atmosphere.
Scientists
are interested in studying the input of energy from the sun and charged
particles into Titan's atmosphere because it is at the heart of the natural
organic chemistry factory that exists in Titan's atmosphere. JPL's scientists want to know what galvanizes the chemical reactions forming the heavy molecules
that develop into Titan's thick haze of organic chemicals and this kind of work will help them understand what kind of organic chemistry could have existed on an early
Earth.
The
light, known as airglow, is produced when atoms and molecules are excited by
ultraviolet sunlight or electrically charged particles. Cassini scientists have
already seen an airglow from Titan's nitrogen molecules caused by X-rays and
ultraviolet radiation from the sun when Titan was illuminated by the sun.
During 2009, Titan passed through Saturn's shadow, offering a unique
opportunity for Cassini's instruments to observe any luminescence from Titan
while in darkness. Cassini's imaging cameras could see in very dim light by
using exposure times of 560 seconds.
Scientists
expected to see a glow in the high atmosphere (above 400 miles, or 700
kilometers in altitude) where charged particles from the magnetic bubble around
Saturn strip electrons off of atmospheric molecules at Titan. Although an
extremely weak emission was seen in that region, they were surprised to see
Titan's dark face glow in visible wavelengths of light from deeper in the
atmosphere (at about 190 miles or 300 kilometers above the surface), as though
illuminated by moonshine from nearby satellites.
The
scientists took into account sunlight reflected off Saturn. There was still a
glow from the part of Titan that was dark. The luminescence was diffusing up
from too deep for charged particles from the sun to be exciting atmospheric
particles. The area was also not affected by the shooting of charged particles
into the magnetic fields, which is what causes auroras.
Scientists'
best guess is that the glow is being caused by deeper-penetrating cosmic rays
or by light emitted due to some kind of chemical reaction deep in the
atmosphere. They have previously reported that the night side Venus atmosphere also produces a
glow, called the Ashen light. Some have suggested that lightning on Venus is
responsible, although that explanation is not universally accepted. While
Cassini's radio wave instrument has detected lightning at Saturn, it has not
detected lightning at Titan. Scientists plan to keep looking for clues as
Cassini continues to make its way around the Saturn system for another season.
Source: NASA/JPL
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