Volcanic Eruptions On A Fiery Little Moon

Io is an “oddball” moon circling the colorfully banded, beautiful, behemoth gas-giant planet Jupiter–it is the innermost of the quartet of Galilean moons that were first detected in January 1610 by Galileo Galilei when he aimed his primitive little “spyglass”–one of the first telescopes to be used for astronomical purposes–up into the clear, starlit, winter sky above his home in Padua. Named after a character in ancient Greek and Roman mythology, the fictitious Io was a priestess of Hera (Juno) who became one of Zeus’s (Jupiter’s) many, many lovers. Pockmarked by more than 400 active volcanoes, Io is considered to be the most geologically active world in our Solar System–and its incredible volcanism is responsible for creating many of its unique surface features. Io’s volcanic plumes and eruptions of bizarre, and very alien lava flows, create significant surface alterations on this weird moon-world, painting its strange surface various shades of red, white, black, green, and yellow, primarily due to allotropes and compounds of sulfur. In October 2016, planetary scientists from the University of California, Berkeley, announced that their recent observations reveal that little Io continues to be the most volcanically active body in our Sun’s family.

Allotrope is a term in chemistry, used to refer to the ability of some chemical elements to exist in two or more different forms, in the same physical state. For example, graphite, charcoal, and diamond are all allotropes of carbon.

The Berkeley astronomers’ findings are documented by the longest series of frequent, high-resolution observations of Io’s thermal emission ever obtained. The scientists used near-infrared adaptive optics on two of the world’s largest telescopes–the 10-meter Keck II and the 8-meter Gemini North, both located near the summit of the dormant Mauna Kea volcano in Hawaii. The Berkeley astronomers tracked 48 volcanic hot spots on the surface of the tormented, and very colorful, little moon over a time-span of 29 months–from August 2013 through the end of 2015.

Without the benefit of adaptive optics, Io appears as merely a fuzzy sphere. Adaptive optics is a technique that eliminates the atmospheric blur to sharpen the image. This is because it can separate features just a few hundred kilometers apart on Io’s 3,600 kilometer surface.

“On a given night, we may see half a dozen or more different hot spots. Of Io’s hundreds of active volcanoes, we have been able to track the 50 that were the most powerful over the past few years,” explained Katherine de Kleer in an October 20, 2016 University of California, Berkeley, Press Release. Ms. de Kleer is a UC Berkeley graduate student who led the observations. She and Dr. Imke de Pater, a professor of Astronomy and Earth and Planetary Science at Berkeley, observed the heat emanating from active eruptions, as well as cooling lava flows, and they were able to determine the temperature and total power output of individual volcanic eruptions. The two planetary scientists tracked the evolution of these distant eruptions during the passage of days, weeks, and sometimes even years.

The observation that some of the eruptions appeared to progress across the surface over time is of particular interest. This is because it seemed as if one eruption triggered another 500 kilometers away.

“While it stretches the imagination to devise a mechanism that could operate over distances of 500 kilometers, Io’s volcanism is far more extreme than anything we have on Earth and continues to amaze and baffle us,” Ms. de Kleer noted in the October 20, 2016 Berkeley Press Release.

De Kleer and de Pater discussed their new observations at a media briefing on October 20, 2016 during a joint meeting of the American Astronomical Society’s Division for Planetary Sciences and the European Planetary Science Congress held in Pasadena, California. Research papers describing the observations will be published in the journal Icarus.

A Pepperoni Pizza Moon

On the night of January 10, 1610, Galileo looked up at the clear winter sky with his tiny telescope, and located the giant planet Jupiter. He then spotted, to his surprise, a dancing trio of fuzzy, bright balls bouncing around very close to it. A few nights later, he spotted yet a fourth blurry, bright sphere. Galileo then went on to carefully describe his discoveries in The Starry Messenger.

As the banded behemoth of our Solar System, Jupiter certainly stands out majestically in the madding crowd. Jupiter, the fifth planet from our Sun, is twice as massive as all of the other planets in our Sun’s family combined and, as such, it was appropriately named after the king of the gods in ancient Roman mythology. The quartet of Galilean moons carry the name of their discoverer, and are individually named–from innermost to outermost–Io, Europa, Ganymede, and Callisto. Of the four, Io shows itself to be different from its sibling moon-worlds. As the innermost Galilean moon, Io sports a bizarre surface that has been compared to a “pepperoni pizza”–or, alternatively, to something that looks “diseased”. This is because of its highly volcanic nature–it is a fiery small world, liberally smeared with yellow sulfur.

The four Galilean moons are a diverse bunch, ranging from volcano-tormented Io–which shoots out more lava per unit than any other known body in our Sun’s family–to the outermost of the four sibling moons, Callisto, which is a frozen sphere of icy mud. Callisto, the third largest moon in our Solar System, is almost as large as the planet Mercury, and has a heavily cratered and very ancient surface. In stark contrast to Io, Callisto is inactive, desolate, and dead. However, more recent studies have proposed that Callisto might have more going on beneath its surface than originally supposed–as dead as it looks at first glance, Callisto may very well possess a subsurface global ocean of sloshing liquid water. It is unlikely, however, that tidbits of life could emerge and evolve in Callisto’s strange, well-hidden sea.

In between Io and Callisto are the two other Galilean moons, Europa and Ganymede. Europa displays a cracked icy surface that resembles a shattered egg shell, and it is thought that Europa harbors a hidden ocean of life-loving liquid water beneath its cracked shell of ice. Indeed, Europa’s hidden global subsurface ocean could potentially host primitive life forms that swim around in its cold dark sea under ice.

Ganymede is the largest moon in our Solar System, and it shows a weird patchwork surface that is scarred by many faults, fractures, cracks, and grooves. This diverse surface indicates that this gigantic moon probably had an extremely violent youth. Like its sibling-moons, Europa and Callisto, Ganymede is thought to also possess a subsurface global ocean of liquid water, hidden beneath its complicated crust of ice.

Europa, Ganymede, and Callisto are all well-coated with shells of ice that have been deformed, twisted, fragmented, and cracked by processes that are still only partly understood by planetary scientists. However, Io is not icy at all. In contrast to its three Galilean siblings, it bears a strong and disturbing resemblance to literary and bibilical descriptions of hell.

Io’s fiery volcanic plumes and rivers of lava produce dramatic surface changes. The many extensive lava flows, some more than 300 miles in length, badly scar its battered surface. The materials manufactured by volcanoes-gone-wild make up Io’s etherial and patchy atmosphere–as well as Jupiter’s extensive magnetosphere (its region of magnetic influence). Io’s volcanic emissions also create a large plasma torus around its giant parent-planet.

Io remained a tiny, faint point of light until the late 19th and early 20th centuries, when it finally became possible for astronomers to resolve its large-scale surface features–such as its dark red polar and bright equatorial areas. The first spacecraft to zip past Io were Pioneer 10 and Pioneer 11, on December 3, 1973 and December 2, 1974, respectively.

When the twin Voyagers 1 and 2 spacecraft passed by Io in 1979, their comparatively sophisticated imaging systems provided more detailed images than the earlier Pioneers. The Voyagers revealed to astronomers that Io is a very active moon-world geologically. The two spacecraft discovered that Io had many volcanic features, tall mountains, and a young surface that showed no obvious impact craters. Youthful surfaces usually are smooth and bereft of craters, in contrast to older surfaces that show a large number of craters. The lack of craters on Io suggests that its surface is constantly being resurfaced as a result of its active geology.

NASA’s Galileo spacecraft, when it explored the Jupiter-system, performed a number of close flybys of Io in the 1990s and early 2000s. The flybys obtained important data concerning Io’s interior structure and surface composition. The revelations derived from the two Voyagers and Galileo showed the relationship between Io and Jupiter’s magnetosphere and the existence of a belt of high-energy radiation centered on Io’s orbit.

Additional observations were made by Cassini-Huygens –when it was enroute to its rendezvous with Saturn–and, in 2007, NASA’s New Horizons spacecraft on its way to Pluto and our Solar System’s outer limits, sent back to Earth precious information about Io as it made its treacherous journey to previously unexplored regions far, far away. Also, Earth-based telescopes and the Hubble Space Telescope (HST) contributed important information about this fiery little moon.

The old Voyager images revealed more than 100 volcanic craters (calderas) on Io, and some of them pictured flows of a strange material that makes up Io’s very alien lava. Io’s odd lava flows appeared to be both thin and runny.

Voyager’s sensors also detected the presence of sulfur dioxide gas. It seemed to be erupting from Io’s volcanoes. However, there are likely other substances contributing to this bizarre brew, as well. Water is surprisingly absent on Io, indicating that it has been very active (and throwing its water away) for much of its existence. Once the gases are thrown out into the open, there are striking suggestions that they may partly condense and then tumble back down to the moon’s surface. Ultraviolet data suggest the existence of small-grained solid particles, perhaps less than about.00004 inches across, as well as some bizarre bright blue splotches on Io’s surface. Some astronomers propose that these brilliant blue splotches are regions of fallen “blue snow”–and they dramatically stand out as an obvious contrast to the rest of Io’s primarily yellow and red surface. The immense volcano, Pele, is approximately 600 miles across, and it revealed itself for the very first time in an image taken of Io on March 5, 1979 by the traveling Voyager. This very revealing picture was obtained at a distance of approximately 240,000 miles above the moon’s surface. In addition, many of the surface features on Io may be composed of sulfur.

Volcanic Eruptions On A Fiery Little Moon

Io’s extremely active volcanoes are powered by tidal heating caused by the friction that is generated deep within Io’s interior. The friction itself is caused by Jupiter’s relentless and powerful gravitational pull on Io that alters its orbit by small amounts. Models devised to explain how tidal heating occurs predict that most of Io’s total volcanic power should be emitted either near the poles or near the equator, depending on the model. Furthermore, the models suggest that the pattern should be symmetric between the forward-and backward-facing hemispheres in Io’s orbit (that is, at longitudes 0-180 degrees versus 180-360 degrees).

However, this is not what the planetary scientists saw. Over the observational period that began in August 2013, and ended in December 2015, the team collected images of Io on 100 nights. Even though they spotted a surprising number of intense, albeit short-lived, eruptions that appeared suddenly and became dimmer in only a matter of days, every single one occurred on the trailing face of Io (180-360 degrees longitude)–instead of the leading face, and at higher latitudes than more commonplace eruptions.

“The distribution of the eruptions is a poor match to the model predictions, but future observations will tell us whether this is just because the sample size is too small, or because the models are too simplified. Or perhaps we’ll learn what local geological factors play a much greater role in determing where and when the volcanoes erupt than the physics of tidal heating do,” explained Ms. de Kleer in the October 20, 2016 Berkeley Press release.

Io’s most powerful and persistent volcano, Loki Patera, was a particularly important target for the team of astronomers. This is because it brightens by more than a factor of 10 every 1 to 2 years. A patera is an irregular crater that is usually volcanic.

Many planetary scientists propose that Loki Patera is actually an enormous lava lake, and that these episodes of dazzling brightness herald the overturning of its crust–in a way that is frequently observed to occur in lava lakes on Earth. Indeed, the heat emissions emanating from Loki Patera seem to move around the lake during each brightening event, resembling a wave traveling around a lake that results in the destabilization and sinking of regions of crust. Before 2002, this front appeared to circle the cool island that exists at the center of the lake in a counter-clockwise direction.

Following the apparent halt of brightening episodes after 2002, Dr. de Pater observed renewed activity in 2009.

“With the renewed activity, the waves traveled clockwise around the lava lake,” she explained to the press on October 20, 2016.

Another of Io’s numerous volcanoes, dubbed Kurdalagon Patera, produced especially searing-hot eruptions twice in the spring of 2015. The two eruptions coincided with the brightening of an extended cloud of neutral material that orbits Jupiter. This provides circumstantial evidence that eruptions on Io’s surface are the origin of variability observed in this neutral cloud. However, it remains unclear why other eruptions were not also associated with the brightening, Dr. de Pater added.

The Keck and Gemini telescopes complement one another. Gemini North’s queue scheduling allowed for more frequent observations–often several a week–while the Keck instruments are sensitive also to longer wavelengths (5 microns), revealing cooler features such as older lava flows that cannot be seen in the Gemini observations.

The team of planetary scientists are still in the process of observing Io, thus providing a long-term database of high spatial resolution images that not even Galileo–which orbited Jupiter for eight years–was able to produce.

Dr. Chris Davis, who is program director for the Gemini Observatory at the National Science Foundation, told the press on October 20, 2016 that “These remarkable images illustrate the great strides that have been made in high-resolution imaging from the ground over the past decade. It is amazing to think that, with adaptive optics on 8-to 10-meter-class telescopes like Gemini and Keck, we are now able to resolve features on the surfaces of not just neighboring planets, but their moons as well.”

The National Science Foundation contributes the bulk of the operating costs of the Gemini Observatory.

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