Sky & Telescope - November 2023 - 52

NOVEMBER 2023 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
" Powder Burns " on Jupiter
J
upiter is a big target. With a diameter
over 11 times greater than
Earth's and two-and-a-half times the
mass of all the other planets combined,
it's a veritable cosmic vacuum cleaner,
with the highest frequency of impacts
of any body in the solar system other
than the Sun. Without Jupiter's protecting
influence, the inner solar system
that we inhabit would be a far more
dangerous place.
For eight days in July of 1994, a
stream of fragments from Comet
Shoemaker-Levy 9 smashed into Jupiter,
producing huge clouds of sooty particulates
that encircled the planet and
lingered for months. These features
were not merely dark but jet black, like
the shadows cast by Galilean satellites.
At the time, it was believed that no one
had ever observed a planetary impact
before, so their appearance caught
astronomers by surprise.
In July 2009 - almost 15 years to the
day after the Shoemaker-Levy 9 events
- Australian amateur Anthony Wesley
captured images of a fresh impact
" powder burn " when it rotated into
view over Jupiter's morning limb. The
52 NOVEMBER 2023 * SKY & TELESCOPE
impact itself was unobserved because
it had occurred only hours earlier on
Jupiter's night hemisphere. At first Wesley
mistook the feature for a satellite
shadow but quickly realized it wasn't.
Observers of the Shoemaker-Levy 9
and Wesley impact scars were struck
by a unique hallmark characteristic of
these features. Normally, markings in
Jupiter's cloud canopy exhibit the highest
contrast when they are located near
the center of the planet's disc. As they
rotate toward the limb, the increased
thickness of overlying hazes in our line
of sight markedly reduces their contrast,
so they tend to disappear just before
they rotate around the limb. This did
not occur with the Shoemaker-Levy 9
and Wesley spots. They were located
high above the haze layers, so as they
neared the limb the contrast of these
sooty remnants wasn't diminished. Satellite
shadows behave this way, too.
During the 14 years since Wesley's
discovery, amateurs armed with video
cameras have recorded the flashes of
9 bright bolides against the backdrop
of Jupiter's sunlit clouds, but none of
these events left a detectable blemish on
Are we overdue for the appearance of visible impact scars?
pAnthony Wesley's discovery image on July
19, 2009 (left) captured a compact, black spot
that he initially mistook for a satellite shadow.
Subsequent images taken on July 24th (center)
and August 3rd (right) record the effects of
zonal winds shearing the impact scar into an
elongated dark streak.
the planet's face. According to Spanish
astronomer Agustín Sánchez-Lavega,
an expert on planetary atmospheres,
impactors larger than 300 meters
(1,000 feet) in diameter are required
to produce dark debris fields visible
through large amateur telescopes that
persist for weeks. He estimates that
these features ought to be seen once
every 7 years or so (S&T: Jan. 2022, p.
52). No doubt many are missed because
there's only an 8-month window each
year when Jupiter is well placed for
observation, and the majority of these
observations cluster around the date of
opposition. In addition, the visibility of
impacts at high latitudes on Jupiter's
oblate globe is poor.
An exhaustive review of three
centuries of Jupiter observations conducted
by University of Northern Iowa
astronomer Thomas Hockey turned up
Sky & Telescope - November 2023

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