This Voyager 1 image mosaic shows a large area of Io’s volcanic plains, with numerous volcanic calderas and lava flows. Loki Patera, an active lava lake 1,000 times large than KÄ«lauea Volcano’s summit lava lake, is the black horseshoe-shaped feature in the lower part of the image. Credit: NASA/JPL/USGS

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Exploration of volcanoes within our solar system has been much like the exploration of Hawaiian volcanoes in the 19th century: sporadic. We now know where most of the volcanoes beyond Earth are, but know little about how they work. But bits of information are being collected, and whether they are from Earth-based observations, satellite imagery, or fly-by missions, every piece is important, because our best understanding will come from compiling all available data from every source.

Even though exploration of planetary volcanoes has been sporadic, exciting discoveries have been made. And some of these discoveries have revealed volcanic features similar to those found on Hawaiian volcanoes.

Active volcanism on Io, one of Jupiter’s moons, was first discovered by the Voyager 1 spacecraft during a fly-by mission in March 1979. Since then, more information has been obtained from limited observations by four additional NASA spacecraft as they passed through the solar system and from more frequent observations from the Hubble Space Telescope. A complete map of Io’s volcanoes was published by the U.S. Geological Survey in 2011 (http://pubs.usgs.gov/sim/3168).

Combining these data, planetary volcanologists have hypothesized that Loki Patera, the largest depression on Io, hosts an active lava lake. The temperatures of this lava lake are in the right range for a molten silicate, but scientists can’t differentiate exactly what type of lava is in the lake by temperature alone.

Loki Patera and the lava lake it contains are huge by Earth standards—about 200 km (125 mi) in diameter, which is larger than the entire Island of Hawai‘i! Detailed analyses of all the data show that the lava lake is probably horseshoe-shaped.

During the 1990s, the Loki Patera lava lake displayed periodic bursts of thermal energy every 18 months, suggesting that the lake was resurfaced by a new solidified crust during each burst. The areas of highest temperature within the lake seem to move during these cycles, so the speed of lake resurfacing appears to be about 2.3 cm per second (2.3 cm/s, or 1 in/s). These bursts have been irregular in the last decade.

In the past few years, Earth-based Extremely Large Telescopes (ELTs) have been designed to achieve enough spatial resolution to obtain details of volcanic features on Io, as well as a more continuous record of volcanic activity. The first Earth-based ELT is the Large Binocular Telescope located at an elevation of 3,200 m (10,500 ft) on Emerald Peak in the Pinaleno Mountains of Arizona. It consists of two identical 8.4 m (27.6 ft) telescopes mounted side-by-side for a combined collecting area of a single 11.8 m (38.7 ft) telescope.
Using an infrared camera with this telescope, scientists were able to achieve a final spatial resolution of less than 20 km (12 mi) on the surface of Io during a one-hour observation on December 24, 2013. At the time, Loki Patera had just finished one of its brightening bursts. The 2013 infrared image also showed two hotspots. These two locations are interpreted to be a persistent hotspot in the southwest portion of the lake (seen before) and another hotspot to the east, which might be the leading edge of the recent lake-resurfacing burst.

Back on Earth, the Kīlauea summit lava lake within Halema‘uma‘u Crater is 1,000 times smaller than Io’s Loki Patera lava lake, and contains molten basalt. The Halema‘uma‘u lava lake continuously circulates, with new crust generated at one side of the lake, then moving across the lake surface at speeds of around 15 cm/s (6 in/s) before being consumed at the opposite side. It rarely resurfaces in the way envisioned for the Loki Patera lava lake, but, when it does, the Halema‘uma‘u lake resurfaces at about 7 cm/s (3 in/s).

More sophisticated analyses of existing Io data, as well as more imagery data from spacecraft and Earth-based ELTs, will yield additional details of the Loki Patera lava lake. Studies on the behavior of the Halemaʻumaʻu lava lake are also underway. As scientific papers are published, perhaps comparisons between Earth’s and Io’s lava lakes can be made and will reveal more about planetary volcanoes in the far reaches of our solar system.