Researchers catch first-ever glimpse of Io’s atmospheric collapse

For the first time, scientists have directly observed Io’s atmosphere during an eclipse, getting an up-close look at the atmosphere of the Jovian moon freezes onto the surface when it is shaded by Jupiter, according to research published Tuesday in the Journal of Geophysical Research.

As lead author Constantine Tsang of the Southwest Research Institute (SwRI) and his colleagues explained, each day the gas giant’s shadow moves stretches across Io’s surface during an eclipse, causing the sulfur dioxide gas emitted by volcanoes on the moon to freeze and fall to the surface. When the eclipse ends, the ice warms and the atmosphere reforms through sublimation.

“This research is the first time scientists have observed this phenomenon directly, improving our understanding of this geologically active moon,” Tsang, a senior research scientist in the SwRI’s Space Science and Engineering Division, explained in a statement.

“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 SO2 ice,” added fellow SwRI scientist John Spencer. “Though Io’s hyperactive volcanoes are the ultimate source of the SO2, 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.”


Credit: Southwest Research Institute

Detecting, monitoring heat signatures the key to collecting eclipse data

Tsang, Spencer and their colleagues monitored Io using the Gemini North telescope in Hawaii and the Texas Echelon Cross Echelle Spectrograph (TEXES), and found that the atmosphere of the moon began to “deflate” when temperatures fell from -235 degrees Fahrenheit in the sun to -270 degrees Fahrenheit during an eclipse.

Each eclipse lasts for two hours every Io day (1.7 Earth days), and in a full eclipse, the moon’s atmosphere effectively collapses due to the freezing of the sulfur dioxide, which settles onto the surface in the form of frost, according to the researchers. When the moon re-enters full sunlight, the frost is heated and sublimates, or converts directly back into an atmospheric gas.

The new study is the first to conduct direct observations of the moon’s atmosphere, as previously scientists had been unable to see through the darkness created by Jupiter’s shadow. However, by using TEXES’ ability to use heat radiation instead of sunlight to measure atmosphere, as well as the Gemini telescope’s ability to detect very faint heat signatures, the SwRI-led team was able to observe Io’s atmosphere for a total of 40 minutes over two nights in November 2013.

“No direct observations of Io’s atmosphere in eclipse have previously been possible, due to the simultaneous need for high spectral and time sensitivity, as well as a high signal-to-noise ratio,” the study authors wrote. “Here we present the first ever high-resolution spectra at 19 µm of Io’s SO2 atmosphere in Jupiter eclipse from the Gemini telescope.”

“The strongest atmospheric band depth is seen to dramatically decay from 2.5 ± (0.08)% before the eclipse to 0.18 ± (0.16)% after 40 min in eclipse,” they added. “Further modeling indicates that the atmosphere has collapsed shortly after eclipse ingress, implying that the atmosphere of Io has a strong sublimation-controlled component. The atmospheric column density – from pre-eclipse to in-eclipse – drops by a factor of 5 ± 2.”


Image credit: Southwest Research Institute