Surf’s up on Jupiter’s moon. Magma waves travelling both clockwise and anticlockwise have been spotted on the surface of a lava lake on Io, the most volcanically active body in the solar system.
The lake, called Loki Patera, is a bowl-shaped volcanic crater on Io, Jupiter’s innermost moon. It is roughly 200 kilometres across, and responsible for 10 to 20 per cent of the heat that the jovian moon puts out.
We’ve known that Loki periodically brightens and dims since the 1970s. Previous observations suggested that these changes are due to the lake recycling itself. As the top layer of lava cools, it solidifies and grows dense, until eventually it sinks beneath the underlying magma and pulls nearby crust with it in waves moving across the surface.
But most of those observations, based on a technique for reducing atmospheric blurring called adaptive optics, were only sharp enough to tell which direction the waves were moving, not how fast or where they started.
Now, Katherine de Kleer at the University of California, Berkeley and her colleagues have taken advantage of a rare collusion between Jupiter’s moons to get a high-quality time lapse of the lava lake’s surface.
Every six years, the orbits of Io and Europa – a moon of Jupiter best known for its ice shell covering a liquid water ocean – align, then cross one another from the point of view of Earth.
On 8 March 2015, de Kleer and her colleagues turned the Large Binocular Telescope Observatory in Arizona on the criss-crossing moons to observe the heat coming from Loki Patera in unprecedented detail.
By combining adaptive optics with the binocular observations, they were able to make a map of changing temperatures over time across the lava lake surface with 10 times better spatial resolution than previously possible. “People have looked at Io with each of these methods, but not together,” de Kleer says.
Knowing the temperature of different parts of the lake and how fast the magma cooled and sank helped de Kleer’s team decipher which parts of the surface recycled at which times.
Surprisingly, the temperature map revealed not one, but two waves, one clockwise and the other anticlockwise, moving from the west to the southeast of the lake. The waves started at different times and ran around a cool island in the lake’s centre.
“It’s a giant bowl of molten rock; it should all be behaving the same,” says Julie Rathbun at the Planetary Sciences Institute in Tuscon, Arizona. “But having two waves suggests there are compositional differences within the lake, and that’s strange.”
De Kleer thinks understanding how new magma is exposed on Loki Patera’s surface can offer insight into volcanism on planets and moons that are different from Earth. Io is in an almost constant state of eruption, but it lacks the plate tectonics that are responsible for much of our own planet’s volcanic activity. Instead, its volcanoes are largely driven by tidal heating from Jupiter’s enormous gravity.
It could also shed light on subsurface oceans on moons like Europa and Saturn’s Enceladus, which are also probably kept warm by tidal heating.
“The same process might lead to volcanic activity at the bottom of those oceans that injects the raw materials that would make these systems able to host life,” de Kleer says. “Understanding how heat is deposited in and transported through satellite interiors is therefore important for understanding the potential habitability of these other worlds.”
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.
Binocular telescope turns two eyes on Io
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.”
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.
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.”
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.
Quelle: Berkeley University