The optical measurements of solar radiation inside Titan's atmosphere can reveal a great deal regarding the physical processes occurring there. For example, the absorption of sunlight determines the amount of energy available at different levels in the atmosphere available to drive atmospheric dynamic, e.g., winds.
Now, for an explanation of the instruments..
Visible Spectrometer
This instrument measures the light spectrum of upward and downward sunlight
as a function of altitude during the descent. The purpose of this data
includes measuring the profile of energy absorption, that is, how
much sunlight is
able to diffuse through Titan's atmosphere and accordingly, how much
sunlight reaches the planet's surface. This instrument will also be
vital in defining
the composition and nature of Titan's surface by measuring the absorption
of reflected sunlight. Different surface composition will have diagnostically
different spectra. In addition, the visible spectrometer will provide
a vertical profile of methane distribution in the atmosphere and near
the
surface. This
instrument will also measure the optical properties, size, and vertical
distribution of aerosols (particles in the atmosphere) and clouds.
IR Spectrometer
The IR spectrometer contains two parts: an upward-looking infrared
spectrometer (ULIS) and a downward-looking infrared spectrometer
(DLIS), which are useful
in measuring the upward and downward fluxes. This instrument has
much in common with the Visible Spectrometer, but it measures at longer
wavelengths
than the human eye can see. An internal shutter opens and closes
alternately
to account for dark current during the descent measurements.
Solar Aureole Camera
This instrument will measure the intensity of scattered sunlight
near the sun. The size of this “aureole” of light around the sun is sensitive
to particle size. The solar aureole camera will also measure the vertical
and horizontal polarization in the sunward and anti-sunward directions as
the probe spins. The polarization senses the size of components making up
the aerosols. These measurements will be collected at two wavelengths to
provide sensitivity over a range of particle sizes and altitudes. In effect,
the solar aureole camera will retrieve particle size distribution data for
different levels of Titan's atmosphere.
Violet Photometers
The purpose of the violet photometers is to allow measurements
similar to those of the other spectrometers at shorter
wavelengths. This
extension of
measurements enables DISR to cover nearly all the solar
energy within Titan’s
atmosphere. This instrument is also able to determine the absorption properties
of photochemical aerosols in Titan's stratosphere. Like the other spectrometers,
the violet photometer contains two parts: and upward-looking and downward-looking
portion.
Sun Sensor
The timing of all the upward-looking DISR measurements
are all based on the position of the sun in the sky.
The location
of
the sun provides
a
standard
orientation for the other instruments' data. As the
sun crosses this instrument's field of view, it passes through
three
slits. Each slit
produces a “pulse” signal,
which appears in the detector. The slits are arranged such that the elevation
of the sun in Titan’s sky can be measured. The DISR sun sensor will
continue to look for the sun if it disappears behind a passing cloud, so
that the other instruments will always have a "baseline" of instrument
rotation in Titan’s sky.
Surface Science Lamp
This instrument will illuminate Titan's surface, since
very little sunlight is able to reach the surface
at many wavelengths.
The
SSL is a 20-watt
lamp designed like a flashlight to produce a beam
of light. It is activated at
a height of 700 meters above Titan's surface, as
determined by the radar altimeter. This instrument will allow
continuous measurements
of the
spectral reflectivity of the surface throughout
the spectral range using the downward-looking
infrared spectrometer during the last few minutes
of the descent.
