The spectrum of the zodiacal light is a key for understanding the origin of
Solar System dust disk and its connection to dust disks around other
planet-forming stellar systems.  The 5-16 micron range accessible to the
ISOCAM CVF contains both the Wien tail of the spectrum of dust near the
Earth and the 10 micron silicate feature, which is now well-observed in
many comets and in the dust disk around beta Pictoris.  As part of the
performance verification phase, the spectrum of the zodiacal light was
already measured toward one line of sight (elongation 100 in the ecliptic
plane, Reach et al.  1996).  We propose here to measure the zodiacal light
spectrum in three directions, representing the extremes of its expected
variation within the sky visible to ISO.  The spectrum toward the lowest
and highest solar elongations in the ecliptic plane will be measured in
order to determine the spectral variability as a function of distance from
the Sun.  As the spectrum in the 5-16 micron range is extremely sensitive
to the temperature, we expect to detect a large change in spectral shape
between these lines of sight and thereby measure the temperature gradient.
The spectrum of an ecliptic pole will also be measured, in order to
determine whether it is significantly different from the spectra in the
midplane of the disk.  As asteroidal dust is relatively more confined to
low inclinations than cometary dust, we might expect a difference between
the minerology of the dust between the plane and pole.  Cometary dust is
known to produce a strong silicate feature when it is in the cometary coma,
and may also exhibit a strong feature in the interplanetary medium.