upper bounds are highly uncertain. For now, the question
of Io’s dominant magma composition remains an intriguing
mystery for future observations to settle.
Fountains of Lava
Figure 3.
The decline in the 3.8micron intensity of the
August 29th outburst,
derived from Gemini
observations. Figure
adapted from de Kleer
et al., 2014.
Geological features on Earth tell us that this
latter type of volcanism was widespread 1-2
billion years ago, during the era when life
was evolving. While we can only infer what
this activity looked like on Earth by surface
features created in the distant past, we see
such eruptions continuing on Io today, allowing us, in a sense, to look back in time.
Eruption temperatures can be derived from
near-infrared (2-5 micron) spectra, where we
see the peak in thermal emission from objects
with temperatures in the 600-1450 K range.
Combining data from the IRTF and Gemini
N we extracted the eruption’s spectrum and
modeled the event as a multi-component
system, including small high-temperature
eruption zones and larger, cooler regions of
spreading lava. We fit the models to the spectrum to determine the temperatures and
emitting areas of the various components.
Figure 4 shows the outburst spectrum with
model spectra for lava temperatures of 1475
K and 1900 K, corresponding to basaltic and
ultramafic lava compositions, respectively.
Our modeling placed a lower bound on the
eruption temperature of 1200-1300 K with
best-fit temperatures above 1500 K. These
upper values indicate ultramafic magma
composition, but the difficulty of observing
Io at the short wavelengths required to constrain these temperatures means that the
6
GeminiFocus
The high eruption temperatures we measured suggest
freshly-exposed lava continuously gushing from an area
of tens of square kilometers.
Ashley Davies, a member of our team and a
volcanologist at the Jet Propulsion Laboratory who specializes in Io, says that the eruption most likely occurred in the form of fire
fountains erupting from long fissures along
Io’s surface.
Volcanic events on Io range from bright
bursts that last only a few hours to hot spots
that persist for months or years. The neardaily observations at Gemini North in the
two weeks following the August 29th detection allowed us to watch the eruption’s
rapid decay in brightness as it transitioned
from vigorous lava fountaining to the resultant fluid flows that spread rapidly over
thousands of square kilometers of Io’s surface while slowly cooling. Figure 3 plots the
change in the eruption’s 3.8-micron brightness in the days following detection.
We measured a peak power of 15-25 terawatts
(TW), making this one of the most powerful
eruptions observed in the Solar System to
date. The highest-power eruption ever observed on Io was at the Surt volcano in 2001; it
emitted around 78 TW, a factor of a few above
this event (Marchis et al., 2002). Both of these
numbers completely overwhelm lava fountains we see on Earth today; for comparison,
the lava fountains associated with the 2010
eruption of Eyjafjallajökull emitted a peak of
only 1 gigawatt (Davies et al., 2013).
October 2014