Images reveal new insights about Jupiter's volcanic moon Io

This composite image shows Jupiter's moon Io in a combination of radio and optical light, with a separate image of Jupiter behind it. Data from ALMA show for the first time plumes of sulfur dioxide (in yellow) that rise up from its volcanoes.

Jupiter's third-largest moon Io is covered by more than 400 active volcanoes, and it's the most volcanically active world in our solar system.

New radio images collected by an array of telescopes on Earth have observed the direct effect of this volcanic activity on the moon's thin atmosphere for the first time.

A study including the data gathered from these images has been accepted for future publication in the Planetary Science Journal.

The images captured by ALMA, or the Atacama Large Millimeter/submillimeter Array of telescopes in Chile, provide a new perspective on the moon and its signature color palette of yellow, white, orange and red. These colors are due to the sulfurous gases spewing from the moon's many volcanoes that freeze when they meet the cold temperatures of the icy surface.

Although the idea of a moon covered in volcanoes suggests Io would be a hot celestial body, Io's surface is always cold at about negative 230 degrees Fahrenheit.

Io's atmosphere is so faint that it's about a billion times thinner than Earth's. Previous observations and studies of the moon revealed that this atmosphere is largely comprised of sulfur dioxide gas.

"However, it is not known which process drives the dynamics in Io's atmosphere," said study author Imke de Pater, a professor of astronomy, Earth and planetary science at the University of California, Berkeley, in a statement. "Is it volcanic activity, or gas that has sublimated (transitioned from solid to gaseous state) from the icy surface when Io is in sunlight?"

Researchers used ALMA to capture images of the moon as it moved into and out of Jupiter's shadow to understand more about the moon's atmosphere.

"When Io passes into Jupiter's shadow, and is out of direct sunlight, it is too cold for sulfur dioxide gas, and it condenses onto Io's surface. During that time we can only see volcanically-sourced sulfur dioxide. We can therefore see exactly how much of the atmosphere is impacted by volcanic activity," said Statia Luszcz-Cook, study coauthor and observational astrophysicist at Columbia University, in a statement.

The clarity of the ALMA images revealed distinct plumes of sulfur dioxide and sulfur monoxide coming from the volcanoes, contributing between 30% to 50% of the moon's atmosphere. The scientists also saw potassium chloride gas, a common component of magma, emerging from the volcanoes. The researchers believe that this suggests that the magma reservoirs differ between volcanoes.

Io is only slightly larger than our moon, but it couldn't be more different. What's more, its environment is unlike anything found on Earth.

The moon, named for a mortal woman who is transformed into a cow during a fight between Zeus and Hera in Greek mythology, has lava fountains that can erupt to reach dozens of miles high.

Some of Io's volcanoes are so powerful that their eruptions can be seen using large telescopes on Earth.

The surface is also covered with lakes of molten silicate lava. With such a dramatic landscape it would be an adventure to fly by Io, but you wouldn't want to live there.

Io is caught between Jupiter's massive gravity and the tug of orbits from the planet's other moons like Europa and Ganymede, which contributes to the activity on Io. Some of its volcanoes are massive, like Loki Patera, which is 124 miles across.

The moon is in a tidally locked orbit around Jupiter, meaning that the same side of the moon always faces the planet.

Hot volcanoes on a cold moon

The ALMA images revealed that Io's atmosphere becomes incredibly unstable when it passes through Jupiter's massive shadow. This occurs every 42 hours during Io's orbit around Jupiter.

During these "eclipses," Io's sulfur dioxide gas drops, which suggests that the lower atmosphere of the moon basically collapses and freezes onto the surface. But as Io emerges from Jupiter's shadow and receives sunlight, the gas returns.

"As soon as Io gets into sunlight, the temperature increases, and you get all this (sulfur dioxide) ice subliming into gas, and you reform the atmosphere in about 10 minutes' time, faster than what models had predicted," de Pater said.

But the researchers' data showed that not all of the sulfur dioxide gas freezes during the temperature drop Io experiences while in Jupiter's shadow. In fact, ALMA was able to detect global radio emissions of sulfur dioxide from what the researchers call stealth volcanoes, which don't emit smoke or detected particles, but release the gas into the atmosphere that is warm enough to keep from condensing and freezing.

How do these hot processes unfold on such a cold moon? The tug of Jupiter, Ganymede and Europa heat the interior of Io, which creates volcanoes that release hot sulfur dioxide gas.

Eventually, the gas condenses and freezes in a thick layer of sulfur dioxide ice on Io's surface. That layer is covered over by volcanic dust, which creates Io's signature colors.

"By studying Io's atmosphere and volcanic activity we learn more about not only the volcanoes themselves, but also the tidal heating process and Io's interior," Luszcz-Cook said.

Future observations and studies will allow researchers to determine the temperature of Io's lower atmosphere, which remains unknown for now.

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