THE SOUTHWORTH PLANETARIUM
207-780-4249      www.usm.maine.edu/planet
70 Falmouth Street     Portland, Maine 04103
43.6667° N                   70.2667° W
Founded January 1970
Julian date:  2457676.16

              "We cannot recapture past moments anymore than we can inhale yesterday's breaths or revisit last night's dreams. In fact, any attempt to re-live the irretrievable past will make one feel its loss all the more keenly.     Only by engaging in the present and striving for the future will the most agreeable memories remain vibrant and fresh, instead of toxic and musty."   

THE DAILY ASTRONOMER

Thursday, October 13, 2016

Atmosphere Collapse

We hinterland inhabitants all know that all the foliage-brightened trees adorning our sidewalks are like dapper sentries poised at winter's crystalline gates, preparing to permit us entry.  Mother Nature, blithely indifferent to the democratic processes that should decide such matters,  will soon invite us to experience winter's onslaught whether we want to or not.     One way to seek solace in such dark times is through the black magic of schadenfreude, a German term referring to the depraved pleasure one derives through the contemplation of another person's misery.   (The Yankees are likely drinking their fill of it right now.)       So, today we seek to mitigate our own despair by visiting a world that grows so cold its atmosphere literally freezes and collapses to its surface. Does this sound impossible?     We assure you it isn't.  

Io, a moon of Jupiter, is just such a world.     Its weather changes sharply whenever Io passes into Jupiter's shadow.  The resultant temperature drop is so dramatic, its  atmosphere solidifies and falls to the surface.    Once the Jovian shadow passes, the atmosphere reheats under the distant Sun and becomes gaseous.  It sublimates, meaning that the material turns from solid to gas without passing through the intervening liquid phase.     Io's atmosphere consists primarily of sulfur dioxide, a substance that remains gaseous at -235 degrees F, Io's typical daytime temperature.   However, when the surface temperature drops to -270 degrees F,that sulfur dioxide becomes a solid. 

​"The Freezing Shadow."     Whenever Io passes into Jupiter's shadow, its atmosphere solidifies and settles onto the moon's frigid surface.   However,the geologically dynamic moon constantly spews more gases into its atmosphere though its active volcanoes.     Io's atmosphere is therefore in a constant state of flux: collapsing and then rising.  

Io's atmosphere is therefore collapsing and then reconstituting itself.  Moreover, Io's many active volcanoes spew more sulfur dioxide into the atmosphere.   These gases burn hot, with temperatures exceeding 3000 degrees!    Soon after emission, the gases that remain in the atmosphere become incorporated into the Ionian atmosphere.   

Io is one the solar system's four volcanic worlds, defined as those that currently have active volcanoes.*   Io remains volcanically active because of tidal stresses induced on it by both Jupiter and Europa, another Galilean moon.   Io and Europa experience an orbital "resonance," meaning that their orbit periods have an integer ratio.     Io, which is closer to Jupiter than Europa, completes two orbits in the time Europa requires to complete one orbit.     Therefore, Io is often "squeezed" between Jupiter and Europa, which induces heating on both moons. It is for this reason many astronomers suspect than a large liquid ocean -a possible life-rich pool- is concealed beneath Europa's ices.  

Other worlds experience weather that is just as wild as Earth's. Io, to cite an extreme case, has a surface steeped in deep cold temperatures that sometimes sink so low, the atmosphere literally falls to the ground.    So, this winter, we can take solace in the knowledge that some worlds have it worse...barely. 

*The four volcanic worlds are Earth, Io, Triton (Neptune's largest moon) and Enceladus (a moon of Saturn.)