Adaptive Optics Allows Earth-Based Monitoring of Io’s Fiery Show

W. M. Keck Observatory press release

Watching active volcanic eruptions should be done from a safe distance, and a group of California researchers has figured out how to do it from, ironically, Mauna Kea – one of Earth’s tallest volcanoes – using the W. M. Keck Observatory. Employing an ingenious combination of telescopic surveys and archival data, they have gathered nearly 40 distinct snapshots of effusive (slow) volcanic eruptions and high temperature outbursts on Jupiter’s tiny moon, Io, showing details as small as 100 km (60 miles) on the moon’s surface.

While space-based telescopes were once required for viewing surface details on Io – similar in size to our Moon, but more than 1,600 times distant – adaptive optics (AO), pioneered at Keck, allows teams like that led by Franck Marchis, a researcher at the Carl Sagan Center of the SETI Institute, to collect fascinating data on the wild show from Earth. Marchis presented results from ground-based telescopic monitoring of Io’s volcanic activity over the past decade this week, at the 2012 Division of Planetary Sciences Meeting of the American Astronomical Society.

Erupting volcanoes on Io cannot be seen well from beneath the Earth’s atmosphere using classical astronomical techniques. Io is a relatively small satellite with a 3,600 km diameter, more than 630 million kilometers away. In 1979, Voyager 1 visited the Jovian system, revealing Io’s dynamic volcanic activity from the first close-up pictures of its surface, capturing bizarre volcanic terrains, active plumes and hot spots. The Galileo spacecraft remained in orbit in the Jovian system from 1995 to 2003 and observed more than 160 active volcanoes and a broad range of eruption styles. Several outstanding questions remained in the post-Galileo era, and the origin and long-term evolution of Io’s volcanic activity is still not fully understood.

IO with Keck AO
Quiescent activity of Io observed in 2010 and 2011 showing several quasi-permanent eruptions at 3.8 microns [bottom] and the absence of bright, hotter outbursts at 2.1 microns [top]. Credit: Franck Marchis, SETI Institute

In the meantime, astronomers designed instruments to break the “seeing barrier” and improve the image quality of ground-based telescopes. The blurring (“seeing”) introduced by the constant motion of the Earth’s atmosphere can be measured and corrected in real time using adaptive optics (AO), providing an image with a resolution close to the theoretical “diffraction limit” of the telescope. The W. M. Keck Observatory has used adaptive optics since 1999.

“Since our first observation of Io in 2001 using the Keck II 10-meter telescope and its AO system from Mauna Kea in Hawaii, our group became very excited about the technology. We also began using AO at the Very Large Telescope in Chile, and at the Gemini North telescope in Hawaii. The technology has improved over the years, and the image quality and usefulness of these AO systems have made them part of the essential instrument suite for large telescopes,” said Marchis.

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