Sky & Telescope - July 2020 - 21

May 31

June 16

June 18

June 23

June 25

June 30

3.78 microns

2.27 microns

May 27

IO CH A NG ES: K . DE K LEER E T A L. / ASTR ONOMICAL
JOUR NAL 158:29 (2019); JUNO VIE W OF IO: N ASA / JPLCA LTECH / SWRI / ASI / IN A F / JIR A M

p VISIBLE FROM EARTH The Gemini North telescope captured these infrared images of ionian eruptions in spring 2018. Isum Patera is the only
hotspot in the top row; it appears at the same location in the bottom row. The bright hotspot south of Isum Patera is Marduk Fluctus. The one west of
Isum Patera in some images is Loki Patera.

only by distance from the Sun, but also by orbital dynamics.
This opened the possibility of environments capable of
supporting life inside Europa and Ganymede, into which
energy is also being pumped via the same tidal resonance
as in Io (although to a lesser degree), as well as in Saturn's
Enceladus, Tethys, and Dione. Could volcanism be taking
place at the bottom of the deep Europan ocean, generating an
environment that could support life? And what about inside
plume-rich Enceladus, where Cassini chemical data indicate
that hydrothermal vents probably exist (S&T: Aug. 2017,
p. 10)? The discovery of exoplanets in orbital resonances,
such as Trappist-1's seven planets, also emphasizes the importance of understanding just how tidal heating works.
But we have now reached the limit of what existing Io
data can tell us. While ground-based telescopes of increasing power and capability continue to obtain astonishing
data on the location and magnitude of individual eruptions,
it is impossible to obtain the polar observations needed to
accurately map the moon's heat ﬂow. NASA's Juno spacecraft
is making polar observations of Io at ultraviolet, visible, and
infrared wavelengths, but, while a handful of new hotspots
have been discovered, the instruments onboard cannot map
the heat ﬂow from Io's non-volcanic areas.
What is needed is a new mission dedicated to Io. One
potential mission, the Io Volcano Observer (IVO) proposed
by Alfred McEwen (University of Arizona) and others, would
essentially "follow the heat." In a highly inclined orbit around
Jupiter, IVO would regularly pass Io as the spacecraft crossed
the jovian equatorial plane, determining the nature of Io's
crust and upper mantle and the evolution of Io's orbit. It
would also measure the magnetic ﬁeld induced inside Io as
it moves through Jupiter's own ﬁeld, to determine if Io has a
global magma ocean. Building on decades of data analysis,
specially designed imagers would constrain the lava's eruption
temperature, measure lava composition, identify minerology,
and map surface temperatures across Io's entire surface.

In summary, then, Io is key to understanding the process
of tidal heating - how heat is deposited within a planetary
body, and how it is transported to the surface. Although
existing and future telescopes will continue to obtain data
to better understand some of the volcanic processes taking
place, the big questions - regarding internal structure, heat
ﬂow magnitude and distribution, and lava composition - can
only be comprehensively answered by a new Io-dedicated mission. Only then can we pull aside the curtain to fully appreciate Io's elegant planetary ballet as it is tugged and twisted by
the other bodies in the orbital resonance, all choreographed
by the immutable laws of physics.

¢ ASHLEY GERARD DAVIES is a research scientist at NASA's
Jet Propulsion Laboratory in Pasadena, California, and can't
quite believe he's actually paid to tramp about on erupting
volcanoes.
q JUNO'S VIEW This infrared view of Io's southern hemisphere from the
Juno spacecraft reveals a new hotspot (circled) that could be a previously
undiscovered volcano. Brighter colors indicate higher temperatures.

s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

http://www.skyandtelescope.org

Sky & Telescope - July 2020

Table of Contents for the Digital Edition of Sky & Telescope - July 2020