Abstract
Pele has been the most intense high-temperature hotspot on Io to be
continuously active during the Galileo monitoring from 1996-2001.
A suite of characteristics suggests that Pele is an active lava lake
inside a volcanic depression. In 2000-2001, Pele was observed by
two spacecraft, Cassini and Galileo. The Cassini observations revealed
that Pele is variable in activity over timescales of minutes, typical
of active lava lakes in Hawaii and Ethiopia. These observations also
revealed that the short-wavelength thermal emission from Pele decreases
with rotation of Io by a factor significantly greater than the cosine
of the emission angle, and that the color temperature becomes more
variable and hotter at high emission angles. This behavior suggests
that a significant portion of the visible thermal emission from Pele
comes from lava fountains within a topographically confined lava
body. High spatial resolution, nightside images from a Galileo flyby
in October 2001 revealed a large, relatively cool (< 800 K) region,
ringed by bright hotspots, and a central region of high thermal emission,
which is hypothesized to be due to fountaining and convection in
the lava lake. Images taken through different filters revealed color
temperatures of 1500 +/- 80 K from Cassini ISS data and 1605 +/-
220 and 14120 +/- 100 K from small portions of Galileo SSI data.
Such temperatures are near the upper limit for basaltic compositions.
Given the limitations of deriving lava eruption temperature in the
absence of in situ measurement, it is possible that Pele has lavas
with ultramafic compositions. The long-lived, vigorous activity of
what is most likely an actively overturning lava lake in Pele Patera
indicates that there is a strong connection to a large, stable magma
source region. (C) 2003 Elsevier Inc. All rights reserved.
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