ity generally occur on longer timescales, un-
correlated with the orbital period. This con-
trasts with the case for other tidally heated
moons such as Saturn’s Enceladus, for which
the degree of activity varies predictably with
its distance from the planet. Although Io and
Enceladus have very similar orbital eccen-
tricities and periods, a key difference is the
viscosity of the erupting fluid, which is water
on Enceladus and magma for Io. in the near-infrared varies by more than an
order of magnitude. This large data set en-
abled the team to uncover surprising pat-
terns in Io’s volcanic activity. For instance, of
the 18 sites with the brightest eruptions, 16
are on the trailing hemisphere with respect
to Io’s orbital motion. This tendency remains
unexplained; the likelihood of it occurring
from a random spatial distribution is much
less than 1%.
To understand what drives the variations in
the volcanism on Io, a team of astronomers
led by Katherine de Kleer of the California
Institute of Technology has analyzed the
most detailed data set on the moon’s volca-
nic activity to date. The observations were
collected on 271 nights between August
2013 and July 2018 using the Near InfraRed
Imager and spectrometer (NIRI) on Gemini
North with the ALTAIR adaptive optics sys-
tem in natural guide star (NGS) mode and
the Near InfraRed Camera 2 (NIRC2) on the
Keck II telescope, also using NGS adaptive
optics. The Gemini/NIRI data comprise 80%
of the total visits; example NIRI images are
shown in Figure 7. The study has been pub-
lished in The Astronomical Journal and fea-
tured in The New York Times. In a companion paper published in Geo-
physical Research Letters, de Kleer and
colleagues show that the roughly 500-day
variations in the intensity of Loki Patera’s
activity may be related to periodic changes
in the shape of the moon’s orbit. Regular
gravitational perturbations from Europa
and Ganymede, which respectively have 2:1
and 4:1 orbital resonances with Io, prevent
the inner moon’s orbit from circularizing.
Instead, Io’s eccentricity and semimajor axis
vary cyclically with periods of 480 and 460
days, respectively. This evolution in Io’s orbit
is consistent with the timescale of the quasi-
periodic behavior of Loki Patera.
In total, the team has detected at least 75
unique hot spots of volcanic activity. The
most active volcano, known as Loki Patera,
was detected 113 times during the five-year
campaign, essentially every time it was visi-
ble. Three other hot spots were each detect-
ed at least 80 times. Loki Patera appears to
be erupting continuously, but its brightness
January 2020 / 2019 Year in Review
Figure 7.
AO-assisted near-infra-
red images taken with
NIRI on Gemini North
of Jupiter’s moon Io,
showing the eruption
of Isum Patera in May-
June 2018. Isum Patera
is the only bright spot
visible in these Kc (2.27
μm) images; it is seen at
the corresponding loca-
tions in the L’ (3.78 μm)
images. The bright spot
south of Isum Patera in
the L’ images is Marduk
Fluctus.
Figure reproduced from
de Kleer et al., The Astro-
nomical Journal, 158:
29, 2019.
At first, this link between orbital evolution
and volcanic activity may seem surprising,
since the range in the tidal stresses over a
single orbit is larger than the variation in the
mean tides resulting from the change in or-
bital shape. However, the researchers note
that while magma is likely too viscous to
change its flow significantly on the timescale
of one orbit, it can adjust its flow over the
longer period associated with the change
in Io’s orbital shape. If there is a connection,
GeminiFocus
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