Jupiter’s Shadow Freezes Io’s Atmosphere Solid Every Day

Io, Jupiter's innermost moon, is the most volcanically active body in the solar system, and its molten eruptions belch out enough sulfur dioxide gas to envelop this little world in a thin atmosphere. Scientists now say this atmosphere freezes solid and thaws again, every day.

New research published Tuesday in the Journal of Geophysical Research reveals that Io's atmosphere genuinely collapses whenever Jupiter's shadow falls on the moon, which happens every time Io passes behind the massive gas giant's frame, causing it to temporarily block out the Sun.

These eclipses occur every 1.7 Earth days, and last about two hours each, long enough to cool Io's surface temperature from a "sunny" minus 235 degrees Fahrenheit (minus 148 degrees Celsius) to minus 270 degrees Fahrenheit (minus 168 degrees Celsius) when Jupiter throws literal shade on it. (It's worth noting that these are average surface estimates, and Io can get as hot as 3,000 degrees Fahrenheit, or 1,649 degrees Celsius, around its volcanos and lava flows.)

These ecliptic drops in temperature are sharp enough to freeze most of Io's atmospheric gas, solid on the ground, as frost and ice. When Io peeks out from behind Jupiter once more, the frozen sulfur dioxide warms and sublimes back to its gaseous state.

This clockwork cycle of atmospheric collapse and rebirth has been speculated about for years, but Jupiter's shadow renders Io too dark to study with today's optical telescopes.

By using the Texas Echelon Cross Echelle Spectrograph (TEXES) instrument mounted on Hawaii's Gemini North telescope, the team was able to instead pick up the moon's heat radiation signatures as it passed behind Jupiter, providing the first direct evidence that this longstanding hunch might really be true.

"This confirms that Io's atmosphere is in a constant state of collapse and repair, and shows that a large fraction of the atmosphere is supported by sublimation of [sulfur dioxide] ice," said John Spencer, a space scientist at the Southwest Research Institute and a co-author of the study, in a statement.

"Though Io's hyperactive volcanoes are the ultimate source of the [sulfur dioxide], sunlight controls the atmospheric pressure on a daily basis by controlling the temperature of the ice on the surface. We've long suspected this, but can finally watch it happen."