Taking
advantage of a rare orbital alignment between two of Jupiter’s moons, Io
and Europa, researchers have obtained an exceptionally detailed map of
the largest lava lake on Io, the most volcanically active body in the
solar system.
On March 8, 2015, Europa passed in front of Io,
gradually blocking out light from the volcanic moon. Because Europa’s
surface is coated in water ice, it reflects very little sunlight at
infrared wavelengths, allowing researchers to accurately isolate the
heat emanating from volcanoes on Io’s surface.
The infrared data
showed that the surface temperature of Io’s massive molten lake steadily
increased from one end to the other, suggesting that the lava had
overturned in two waves that each swept from west to east at about a
kilometer (3,300 feet) per day.
Overturning lava is a popular
explanation for the periodic brightening and dimming of the hot spot,
called Loki Patera after the Norse god. (A patera is a bowl-shaped
volcanic crater.) The most active volcanic site on Io, which itself is
the most volcanically active body in the solar system, Loki Patera is
about 200 kilometers (127 miles) across. The hot region of the patera
has a surface area of 21,500 square kilometers, larger than Lake
Ontario.
Earthbound astronomers first noticed Io’s changing
brightness in the 1970s, but only when the Voyager 1 and 2 spacecraft
flew by in 1979 did it become clear that this was because of volcanic
eruptions on the surface. Despite highly detailed images from NASA’s
Galileo mission in the late 1990s and early 2000s, astronomers continue
to debate whether the brightenings at Loki Patera -- which occur every
400 to 600 days -- are due to overturning lava in a massive lava lake,
or periodic eruptions that spread lava flows over a large area.
“If
Loki Patera is a sea of lava, it encompasses an area more than a
million times that of a typical lava lake on Earth,” said Katherine de
Kleer, a UC Berkeley graduate student and the study’s lead author. “In
this scenario, portions of cool crust sink, exposing the incandescent
magma underneath and causing a brightening in the infrared.”
“This
is the first useful map of the entire patera,” said co-author Ashley
Davies, of the Jet Propulsion Laboratory in Pasadena, who has studied
Io’s volcanoes for many years. “It shows not one but two resurfacing
waves sweeping around the patera. This is much more complex than what
was previously thought.”
“This is a step forward in trying to
understand volcanism on Io, which we have been observing for more than
15 years, and in particular the volcanic activity at Loki Patera,” said
Imke de Pater, a UC Berkeley professor of astronomy.
De Kleer is lead author of a paper reporting the new findings that will be published May 11 in the journal Nature.
The
images were obtained by the twin 8.4-meter (27.6-foot) mirrors of the
Large Binocular Telescope Observatory in the mountains of southeast
Arizona, linked together as an interferometer using advanced adaptive
optics to remove atmospheric blurring. The facility is operated by an
international consortium headquartered at the University of Arizona in
Tucson.
“Two years earlier, the LBTO had provided the first ground-based images of two separate hot spots within Loki Patera, thanks
to the unique resolution offered by the interferometric use of LBT,
which is equivalent to what a 23-meter (75-foot) telescope would
provide,” noted co-author and LBTO director Christian Veillet. “This
time, however, the exquisite resolution was achieved thanks to the
observation of Loki Patera at the time of an occultation by Europa.”
Loki Patera is the bright hot spot in the upper part of the disk. Europa appears dark because water ice on its surface absorbs incident sunlight, while the sulfur dioxide ice on Io’s surface is less absorbing at this wavelength. Credit: LBTO
Europa
took about 10 seconds to completely cover Loki Patera. “There was so
much infrared light available that we could slice the observations into
one-eighth-second intervals during which the edge of Europa advanced
only a few kilometers across Io’s surface,” said co-author Michael
Skrutskie, of the University of Virginia, who led the development of the
infrared camera used for this study. “Loki was covered from one
direction but revealed from another, just the arrangement needed to make
a real map of the distribution of warm surface within the patera.”
These
observations gave the astronomers a two-dimensional thermal map of Loki
Patera with a resolution better than 10 kilometers (6.25 miles), 10
times better than normally possible with the LBT Interferometer at this
wavelength (4.5 microns). The temperature map revealed a smooth
temperature variation across the surface of the lake, from about 270
Kelvin at the western end, where the overturning appeared to have
started, to 330 Kelvin at the southeastern end, where the overturned
lava was freshest and hottest.
Using information on the
temperature and cooling rate of magma derived from studies of volcanoes
on Earth, de Kleer was able to calculate how recently new magma had been
exposed at the surface. The results -- between 180 and 230 days before
the observations at the western end and 75 days before at the eastern --
agree with earlier data on the speed and timing of the overturn.
Maps of the temperature and lava crust age within Loki Patera, derived from the LBTO observations. The higher temperatures in the southeast (location 3) indicate that new magma was exposed most recently in this locatiaon. Credit: LBTO
Interestingly, the overturning started at different times on two sides of a cool island in the center of the lake that has been there ever since Voyager photographed it in 1979.
“The velocity of overturn is also
different on the two sides of the island, which may have something to do
with the composition of the magma or the amount of dissolved gas in
bubbles in the magma,” de Kleer said. “There must be differences in the
magma supply to the two halves of the patera, and whatever is triggering
the start of overturn manages to trigger both halves at nearly the same
time but not exactly. These results give us a glimpse into the complex
plumbing system under Loki Patera.”
Lava lakes like Loki Patera
overturn because the cooling surface crust slowly thickens until it
becomes denser than the underlying magma and sinks, pulling nearby crust
with it in a wave that propagates across the surface. According to de
Pater, as the crust breaks apart, magma may spurt up as fire fountains,
akin to what has been seen in lava lakes on Earth, but on a smaller
scale.
De Kleer and de Pater are eager to observe other Io
occultations to verify their findings, but they’ll have to wait until
the next alignment in 2021. For now, de Kleer is happy that the
interferometer linking the two telescopes, the adaptive optics on each
and the unique occultation came together as planned that night two years
ago.
“We weren’t sure that such a complex observation was even
going to work,” she said, “but we were all surprised and pleased that it
did.”
PIO Contacts:
UC Berkeley
Robert Sanders
rlsanders@berkeley.edu
+1 510-643-6998
University of Virginia
Fariss Samarrai
fls4f@virginia.edu
+1 434-924-3778
University of Liège
press@ulg.ac.be
+32 4 366 52 17
Science Contacts:
Katherine de Kleer
kdekleer@berkeley.edu
Imke de Pater
imke@berkeley.edu
Michael Skrutskie
skrutskie@virginia.edu
+1 434-924-7494
Christian Veillet
cveillet@lbto.org
Denis Defrère
ddefrere@ulg.ac.be
+32 4 366 97 13
Reference:
“Multi-Phase Volcanic Resurfacing at Loki Patera on Io,” K. De Kleer et al., 2017 May 11, Nature [https://www.nature.com]. In addition to de Kleer, Skrutskie, Davies, Veillet and de Pater, co-authors of the paper are J. Leisenring, P. Hinz, E. Spalding and A. Vaz of the University of Arizona’s Steward Observatory, and Al Conrad of the Large Binocular Telescope Observatory, A. Resnick of Amherst College, V. Bailey of Stanford University, D. Defrère of the University of Liège, A. Skemer of UC Santa Cruz and C.E. Woodward of the University of Minnesota. The research was supported by the National Science Foundation.
Video 1 (reconstruction of the maps from the light curve):
https://www.youtube.com/watch?v=OAUzzuOd3xg
Lower panels show the intensity of Loki Patera as a function of time as it is covered (ingress) and uncovered (egress) by Europa. The red curve is the model light curve corresponding to the intensity map shown above, the best-fit map to the observations. The animation shows Europa sweeping across the patera and obscuring different portions of its floor. Credit: Katherine de Kleer/UC Berkeley
Video 2 (simulation of the two-phase resurfacing):
https://www.youtube.com/watch?v=GiUp91Ybciw
The animation shows a schematic simulation of two resurfacing waves sweeping around the patera at different rates and converging in the southeast corner. Credit: Katherine de Kleer/UC Berkeley
Imke de Pater’s website:
http://w.astro.berkeley.edu/~imke/
Katherine de Kleer’s website:
http://w.astro.berkeley.edu/~kdekleer/
The Large Binocular Telescope Observatory website:
http://www.lbto.org/
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