Sky & Telescope - September 2022 - 52

SEPTEMBER 2022 OBSERVING
Exploring the Solar System by Thomas A. Dobbins
A Lingering Jovian Mystery
What caused a bright flash on Io in 1983?
I
o is the innermost of Jupiter's four
Galilean satellites and the most
geologically active body in the solar
system. A world of fire and brimstone,
the 3,643-km-wide (2,264-mile-wide)
moon is dotted with hundreds of active
volcanoes powered by tidal heating
generated as the moon is rhythmically
tugged by the gravitational pull of Jupiter
and the other Galilean satellites. The
high and ebb tides raise and lower Io's
crust by tens of meters, supplying the
energy for eruptions that hurl plumes of
vaporized sulfur and sulfur-dioxide gas
to heights of hundreds of kilometers.
Io's brightness can increase by as
much as 15% when it emerges from
Jupiter's frigid shadow, gradually fading
back to normal in about 20 minutes.
It's theorized that during an eclipse,
sulfur dioxide emitted by Io's volcanoes
condenses as brilliant frost that quickly
evaporates when sunlight returns.
The amount of sulfur dioxide present
depends on the level of volcanic activity,
which can vary dramatically on a time
scale of days or even hours (S&T: Jan.
2015, p. 54).
I recently came across an observation
reported in 1993 by planetary scientists
Heidi Hammel and Robert Nelson in
the British scientific journal Nature. On
July 26, 1983, an observing team from
52 SEPTEMBER 2022 * SKY & TELESCOPE
the Jet Propulsion Laboratory led by
Nelson was monitoring Io's brightness
as it emerged from an eclipse. The team
employed a violet filter to maximize the
contrast between Io's white frost and its
yellowish surface materials.
Using a Vidicon camera as a photometer
at the Cassegrain focus of Palomar
Observatory's 1.5-meter (60-inch)
Ritchey-Chrétien reflector, Nelson's
team recorded fourteen 10-second
exposures at 3-minute intervals after
1.5
2.0
2.5
2.5
3.5
3.5
4.5
5.0
pThis simulation shows the positions of
Europa, Io, and Jupiter as the innermost moon
emerged from the shadow of the planet on
the night of July 26, 1983. Europa was on the
sunward side of the planet at the time.
Io emerged from Jupiter's shadow. The
moon's brightness increased by a whopping
50% in the third frame, taken 6
minutes after the eclipse ended. None
of the other frames recorded any appreciable
brightness enhancement.
At the time, Hammel was a member
Palomar Observatory
Mauna Kea
of a University of Hawai'i team that
was also monitoring Io's brightness at
Mauna Kea Observatory that night.
Their series of twenty-one 3-second
frames at 2-minute intervals failed
to record the flash but did bracket it,
constraining its duration to less than
2 minutes. In all other respects the
Mauna Kea data were in excellent agreement
with the Mount Palomar results.
But were the results real? A mal10
20
Minutes
after eclipse
pPhotometric measurements of Io in violet
light with a 1.5-meter telescope at Palomar
Observatory (orange) and with a 0.61-meter
at Mauna Kea (blue). The Palomar data set
contains the anomalous brightening at 6:22:50
UT on July 26, 1983.
30
function of the Nelson team's detector
seems unlikely. The brightness of both
the background sky and the nearby
satellite Europa remained constant in
every frame. All 12 pixels comprising
Io's image brightened during the flash,
so the event wasn't a spurious artifact.
A colossal impact on Io can also
be ruled out. A comparison of highresolution
1979 Voyager images with
Photon counts / 104
SIMULATION OF JUPITER IN 1983: WINJUPOS;
GRAPH: S&T ILLUSTRATION, SOURCE: H. HAMMEL,
R. NELSON, ET. AL. / NATURE 1993
Sky & Telescope - September 2022

Table of Contents for the Digital Edition of Sky & Telescope - September 2022
