Life in the Universe
Satellites of JupiterLecture 23
Galilean SatellitesIo, Europa, Ganymede, CallistoOrbits of the four Galilean satellites as seen from Earth. All four orbits lie in nearly the same plane as Jupiters equator.
Heat from the protosun made it impossible for icy grains to survive within the innermost 2.5 to 4 AU of the solar nebula. In the same way, Jupiters heat evaporated any icy grains that were too close to the center of the Jovian nebula. Hence, the two inner Galilean satellites were formed primarily from rock, while the outer two incorporated both rock and ice.
Formation of Galilean Satellites
Io : most active volcanic site in the Solar System
The unique coloration of Io is a result of its tremendous volcanic activity. The brilliant orange is due to sulfur ejected from the volcanoes. The white is probably sulfur snow.Ios Active Volcanoes
Io seen from Galileo SpacecraftLave FlowIR view of volcanoes
Magnetic field interaction with IoWhen charged particles in Jupiter's magnetosphere collide with Ios volcanic plumes and with Io's surface, ions are knocked out of the plumes and off the surface. The result is a huge doughnut-shaped ring of electrons and ions, called the Io torus, that encircles Jupiter.
Tidal Friction Synchronous RotationEven a larger effect on the MoonAdjusting rotational+orbital motions until bulges on the Moon always face to the Earth Synchronous Rotation = Tidal LockingTidal locking takes a short time (a few Myrs!)All close-in objects to a larger objects are tidally locked-in!
We know that Mercury and Moon had lost their internal energy1979 Voyager 1 image of Io active volcano!! But Io is smaller than Mercury!Io is the most volcanically active place in the solar system.If Io was completely tidally locked, there shouldnt be on-going heating..
Io : Continuing tidal heating
Due to its elliptical orbit, bulges are misaligned w.r.t. Jupiter!continuing tidal friction increased internal heat!This internal heat is ~200 times more than Earths radioactive decay heat energy!But, tidal locking forces the orbit to be circular. ? Why Ios orbit is elliptical still? Move in Harmony : Orbits of Galilean Satellites
Three inner most Galilean moons orbit Jupiter in a resonant wayPeriodic alignments exert force on each other smallest, shortest orbit got influenced the most Ios orbit is distorted to be elliptical!Initially, Ios rotation was faster and it was located closer to Jupiterbeing tidally lockedmoving outwardmeet Europa and formed 1:2 resonance.Io+Europa being tidally locked together and moving outwardThey meet Ganymede and formed a resonance 1:2:4Three of them are being moving outward to meet Callisto nowEuropaDiameter 3138 km (slightly smaller than Earths Moon)Mass less than Earths moon ( 1/125th of Earths mass, 65% of lunar mass)Distance from Jupiter ~ 410,000 miles ( more than Earth - Moon), period=3.55 daysHas a very weak magnetic fieldHas a very tenuous atmosphere about 10-11 bar of mostly oxygen gas.Surface is exceedingly smooth with highest elevations of a few hundred meters high.The smooth surface has few craters but lots of cracksExistence of water was already known from the ground-based spectrum of Europa
A-D : different parts of Europa, E:Ganymede. F: pure lab-made ice13Surface feature of EuropaChaotic terrain : a surface that looked as if it had been clawed by a tiger with talons several kilometers wide,Closer views resolved each line into a groove flanked by ridges. The larger channels travel thousands of kilometers along great circles without being diverted by the terrain. Whatever mechanism formed them must explain this tendency.Repeated tidal cracking and compression of ice is too chaotic a process to explain
If true, we should be able to findsome single ridges. But, all Europanridges are double ridges!
Fissures, cracks, domes, and pits
Broken Ice and Refrozen Waterchaotic terrain (top left); an enigmatic dark spot nicknamed The puddle (bottom left); cycloidal ridges (right); and a shallow impact crater (bottom right).
cycloidal ridges might have been generated by the obliquity change of nearly 80 degrees! build up of ice at the pole became unstable17Moving Ice on Europa and Earth Some time after a series of ridges formed in this region of Europas surface, the icy crust broke into rafts that were moved around by an underlying liquid or plastic layer.
Europa Craters140 km wide crater formed from a mountain size asteroid or cometsimilar to a gunshot glass structureThickness of the ice crust is estimated to be ~20km
19How do we measure the thickness of the ice crust?Compare morphology of various impact craters.
Top: impact craters on Ganymede and CallistoBottom: impact craters on Europa
EuropaGalileo spacecraft measured gravitational field of Europa dense core + low density (1 g/cm3; some sorts of water) material near the surface central metallic core, thick rocky interior, 80-170km thick water layer, very cold (-150C) surface ice crust.Galileo orbiter image = very few impact craters young surface ( 100 Myr)
Severe lacking of impact craters (only a few) surface younger than 100 Myr resurfacing by occasional breakthrough of subsurface water21
Subsurface Ocean Life in Europa
The global ocean on Europa contains about twice the liquid water of all the Earths oceans combined. There may be plenty of oxygen available in that ocean to support life, a hundred times more oxygen than previously estimated.Ganymedethe largest moon in the solar systemsurface of hard iceold + young surfacesgrooves tectonic stressesunderground ocean weak induced magnetic field (+ its own) & surface salt.lesser tidal heating. But with its larger size, enough to maintain an oceanVery thick ice crust (>150km) life less likely or harder to detect!
23Ganymedewhite craters and rims = impact exposes mantle icethin oxygen atmosphere, possible aurora at its polesghost craters = smoothed by ice flow
very diversified surface with dark regions, valleys, mountains, evidence of past tectonic activity and lots of vertical reliefCallisto
pockmarked surface as old as late heavy bombardmentdark powder at the low-lying areas dusts after sublimated ice?Gravity a ball of mixed rock and ice + hundred km of water iceUndifferentiated interior interior was never warm enough!Induced magnetic field subsurface salty ocean!!
Meager amount of radioactive decay heat + thick icy skin as an insulation + salts and ammonia as antifreeze liquid ocean!?! 25Interior of Galilean Satellitesprobable internal structures of the four Galilean satellites of Jupiter, based on information from the Galileo mission.
Galilean Moons Too hot Maybe less likely Too cold, no? Io Europa Ganymede Callisto recent < 60 Myr 2-3 Gyr? 4+ Gyr
In summaryImportant ConceptsFormation of Galilean satellites (scaled-down version of Solar System formation)Ios prolonged volcanismSubsurface oceans in Europa and GanymedeGeological activities on these moonsImportant TermsOrbital resonanceIo torusTidal heating
Chapter/sections covered in this lecture : 13.1 through 13.728