Satellites of Jupiter Lecture 23

Galilean Satellites Io, Europa, Ganymede, Callisto Io, Europa, Ganymede, Callisto Orbits of the four Galilean satellites as seen from Earth. All four orbits lie in nearly the same plane as Jupiter’s equator. Orbits of the four Galilean satellites as seen from Earth. All four orbits lie in nearly the same plane as Jupiter’s 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. 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, Jupiter’s 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. In the same way, Jupiter’s 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. 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.

Io’s Active Volcanoes

Io seen from Galileo Spacecraft Lave Flow IR view of volcanoes

Magnetic field interaction with Io When charged particles in Jupiter's magnetosphere collide with Io’s 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. When charged particles in Jupiter's magnetosphere collide with Io’s 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 Rotation Even a larger effect on the Moon Even a larger effect on the Moon Adjusting rotational+orbital motions until “bulges” on the Moon always face to the Earth  Synchronous Rotation = “Tidal Locking” Adjusting rotational+orbital motions until “bulges” on the Moon always face to the Earth  Synchronous Rotation = “Tidal Locking” Tidal locking takes a short time (a few Myrs!) Tidal locking takes a short time (a few Myrs!) All close-in objects to a larger objects are tidally locked-in! All close-in objects to a larger objects are tidally locked-in!

Tidal Heating We know that Mercury and Moon had lost their internal energy… We know that Mercury and Moon had lost their internal energy… 1979 Voyager 1 image of Io  active volcano!! But Io is smaller than Mercury! 1979 Voyager 1 image of Io  active volcano!! But Io is smaller than Mercury! Io is the most volcanically active place in the solar system. Io is the most volcanically active place in the solar system. If Io was completely “tidally locked”, there shouldn’t be on-going heating.. If Io was completely “tidally locked”, there shouldn’t be on-going heating..

Io : Continuing tidal heating… Due to its elliptical orbit, bulges are “misaligned” w.r.t. Jupiter! 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 Earth’s radioactive decay heat energy! This internal heat is ~200 times more than Earth’s radioactive decay heat energy! But, tidal locking forces the orbit to be circular. ? Why Io’s orbit is elliptical still?

Move in Harmony : Orbits of Galilean Satellites

Orbital resonance Three inner most Galilean moons orbit Jupiter in a resonant way… Three inner most Galilean moons orbit Jupiter in a resonant way… Periodic alignments exert force on each other  smallest, shortest orbit got influenced the most  Io’s orbit is “distorted” to be elliptical! Periodic alignments exert force on each other  smallest, shortest orbit got influenced the most  Io’s orbit is “distorted” to be elliptical! Initially, Io’s rotation was faster and it was located closer to Jupiter Initially, Io’s rotation was faster and it was located closer to Jupiter  being tidally locked  moving outward  “meet” Europa and formed 1:2 resonance.  Io+Europa being tidally locked together and moving outward  They “meet” Ganymede and formed a resonance 1:2:4  Three of them are being moving outward to meet Callisto now…

Europa Diameter – 3138 km (slightly smaller than Earth’s Moon) Diameter – 3138 km (slightly smaller than Earth’s Moon) Mass – less than Earth’s moon ( 1/125th of Earth’s mass, 65% of lunar mass) Mass – less than Earth’s moon ( 1/125th of Earth’s mass, 65% of lunar mass) Distance from Jupiter ~ 410,000 miles ( more than Earth - Moon), period=3.55 days Distance from Jupiter ~ 410,000 miles ( more than Earth - Moon), period=3.55 days Has a very weak magnetic field Has a very weak magnetic field Has a very tenuous atmosphere – about bar of mostly oxygen gas. Has a very tenuous atmosphere – about bar of mostly oxygen gas. Surface is exceedingly smooth with highest elevations of a few hundred meters high. Surface is exceedingly smooth with highest elevations of a few hundred meters high. The smooth surface has few craters but lots of cracks The smooth surface has few craters but lots of cracks Existence of water was already known from the ground-based spectrum of Europa Existence of water was already known from the ground-based spectrum of Europa

Surface feature of Europa Chaotic terrain : Chaotic terrain : “…a surface that looked as if it had been clawed by a tiger with talons several kilometers wide,…” “…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. 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. 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 Repeated tidal cracking and compression of ice is too chaotic a process to explain

Enigma… Conventional explantion  Conventional explantion  If true, we should be able to find some single ridges. But, all Europan ridges are double ridges!

Fissures, cracks, domes, and pits…

Broken Ice and Refrozen Water chaotic terrain (top left); an enigmatic dark spot nicknamed “The puddle” (bottom left); cycloidal ridges (right); and a shallow impact crater (bottom right). chaotic terrain (top left); an enigmatic dark spot nicknamed “The puddle” (bottom left); cycloidal ridges (right); and a shallow impact crater (bottom right).

Moving Ice on Europa and Earth Moving Ice on Europa and Earth Some time after a series of ridges formed in this region of Europa’s surface, the icy crust broke into “rafts” that were moved around by an underlying liquid or plastic layer. Some time after a series of ridges formed in this region of Europa’s surface, the icy crust broke into “rafts” that were moved around by an underlying liquid or plastic layer.

Europa Craters 140 km wide crater formed from a mountain size asteroid or comet… 140 km wide crater formed from a mountain size asteroid or comet… similar to a gunshot glass structure… similar to a gunshot glass structure… Thickness of the ice crust is estimated to be ~20km Thickness of the ice crust is estimated to be ~20km

How do we measure the thickness of the ice crust? Compare morphology of various impact craters. Compare morphology of various impact craters. Top: impact craters on Ganymede and Callisto Top: impact craters on Ganymede and Callisto Bottom: impact craters on Europa Bottom: impact craters on Europa

Europa Galileo spacecraft measured gravitational field of Europa  dense core + low density (≈1 g/cm 3 ; some sorts of water) material near the surface Galileo spacecraft measured gravitational field of Europa  dense core + low density (≈1 g/cm 3 ; some sorts of water) material near the surface  central metallic core, thick rocky interior, km 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 water

Subsurface Ocean Life in Europa The global ocean on Europa contains about twice the liquid water of all the Earth’s oceans combined. There may be plenty of oxygen available in that ocean to support life, a hundred times more oxygen than previously estimated.

Ganymede the largest moon in the solar system the largest moon in the solar system surface of hard ice surface of hard ice old + young surfaces old + young surfaces grooves  tectonic stresses grooves  tectonic stresses underground ocean  weak induced magnetic field (+ its own) & surface salt. underground ocean  weak induced magnetic field (+ its own) & surface salt. lesser tidal heating. But with its larger size, enough to maintain an ocean lesser tidal heating. But with its larger size, enough to maintain an ocean Very thick ice crust (>150km)  life less likely or harder to detect! Very thick ice crust (>150km)  life less likely or harder to detect!

Ganymede white craters and rims = impact exposes mantle ice white craters and rims = impact exposes mantle ice thin oxygen atmosphere, possible aurora at its poles thin oxygen atmosphere, possible aurora at its poles ghost craters = smoothed by ice flow ghost craters = smoothed by ice flow very diversified surface with dark regions, valleys, mountains, evidence of past tectonic activity and lots of vertical relief

Callisto pockmarked surface pockmarked surface  as old as late heavy bombardment dark powder at the low-lying areas  dusts after sublimated ice? dark powder at the low-lying areas  dusts after sublimated ice? Gravity  a ball of mixed rock and ice + hundred km of water ice… Gravity  a ball of mixed rock and ice + hundred km of water ice… Undifferentiated interior  interior was never warm enough! Undifferentiated interior  interior was never warm enough! Induced magnetic field  subsurface salty ocean!! Induced magnetic field  subsurface salty ocean!!

Interior of Galilean Satellites probable internal structures of the four Galilean satellites of Jupiter, based on information from the Galileo mission. probable 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? Too hot Maybe less likely Too cold, no? Io Europa Ganymede Callisto Io Europa Ganymede Callisto recent < 60 Myr 2-3 Gyr? 4+ Gyr recent < 60 Myr 2-3 Gyr? 4+ Gyr

In summary… Important Concepts Formation of Galilean satellites (scaled-down version of Solar System formation) Io’s prolonged volcanism Subsurface oceans in Europa and Ganymede Geological activities on these moons Important Terms Orbital resonance Io torus Tidal heating Chapter/sections covered in this lecture : 13.1 through 13.7