Life on Jovian Moons ASTR 1420 Lecture 14 Sections 9.1 & 9.2

Background… Jovian planets themselves are unlikely habitable (Do you remember why?) Jovian planets themselves are unlikely habitable (Do you remember why?) There are many moons orbiting these planets  Jovian Moons. There are many moons orbiting these planets  Jovian Moons. Some jovian moons are potentially habitable (i.e., with liquid water). Some jovian moons are potentially habitable (i.e., with liquid water). If any one of them is habitable, this will greatly increase the chance of finding life in the Universe If any one of them is habitable, this will greatly increase the chance of finding life in the Universe  many more jovian moons than jovian planets! Let’s explore the mechanisms that make some jovian moons habitable. Let’s explore the mechanisms that make some jovian moons habitable.

Galileo, the inventor of telescope? Did Galileo invent the telescope? Did Galileo invent the telescope?

Discovering the Moons Soon after the news on the invention of telescope (1608), Galileo built his own one in  A page from Gelileo’s notebook written in ‘*’ are Galilean Moons! Wood strips, leather cover, objective & eyepiece lenses

Four Galilean Moons On January 7th 1610, gazing at Jupiter, Galileo for the first time saw three moons of Jupiter On January 7th 1610, gazing at Jupiter, Galileo for the first time saw three moons of Jupiter March 1610, Galileo published his results in a pamphlet called The Starry Messenger claiming to have found four bodies moving around the giant planet “as Venus and Mercury around the Sun” March 1610, Galileo published his results in a pamphlet called The Starry Messenger claiming to have found four bodies moving around the giant planet “as Venus and Mercury around the Sun” The four moons are now called the “Galilean Moons” - Io, Europa, Ganymede and Callisto The four moons are now called the “Galilean Moons” - Io, Europa, Ganymede and Callisto Currently, 63 moons were discovered orbiting Jupiter. Currently, 63 moons were discovered orbiting Jupiter.

Galileo Galilei Galileo Telescope’s Objective Lens In 1677, lens mounted in an ebony frame to preserve it. Galileo’s Finger On March 12, 1737 Galileo's remains were transferred to the church of Santa Croce. During the transfer, a devotee cut off Galileo's middle finger of his right hand. Today, Galileo's finger can be found on display in the Florence Institute and Museum of the History of Science.

Discovering Moons In 1656, Christiaan Huygens found the largest of Saturn’s moon – Titan. Huygens was also the first one to realize that the rings around Saturn do not touch the planet. In 1656, Christiaan Huygens found the largest of Saturn’s moon – Titan. Huygens was also the first one to realize that the rings around Saturn do not touch the planet. Cassini discovered four more moons of Saturn and showed that the ring of Saturn is not solid but instead of a collection of smaller rocks with a division (“Cassini division”) Cassini discovered four more moons of Saturn and showed that the ring of Saturn is not solid but instead of a collection of smaller rocks with a division (“Cassini division”) Saturn has about 62 moons that we currently know of. Saturn has about 62 moons that we currently know of. HST near infrared light image

Jupiter has a ring, too! The ring of Jupiter was discovered by Voyager 1 in March of The ring of Jupiter was discovered by Voyager 1 in March of All four Jovian planets have rings!! All four Jovian planets have rings!!

Larger Moons Ganymede and Titan are larger than Mercury Ganymede and Titan are larger than Mercury Big2 + Io, Europa, Callisto, Triton, and our Moon are larger than Pluto Big2 + Io, Europa, Callisto, Triton, and our Moon are larger than Pluto smallest moons are about the size of a single mountain on Earth smallest moons are about the size of a single mountain on Earth Moderate—large moons = miniature solar system Moderate—large moons = miniature solar system spherical shape spherical shape orbit in the same plane orbit in the same plane Orbit in the same direction to their planets Orbit in the same direction to their planets

Schematics of Jovian Moons

Synchronous Rotation keeping the same face turned toward the Earth  the Moon completes exactly one rotation around its axis while it makes one orbit.

Synchronous Rotation Nearly all jovian moons show synchronous rotation similar to our Moon. Nearly all jovian moons show synchronous rotation similar to our Moon. keep the same face turned toward their host planets  a moon completes exactly one rotation around its axis while it makes one orbit. This is due to the same gravitational effect for the tides on Earth! This is due to the same gravitational effect for the tides on Earth!

Tides on Earth Tides on Earth are due to the differential gravity. Tides on Earth are due to the differential gravity. Two tidal bulges  two high tides per day. Why two bulges - one facing the Moon and the other opposite?? Why two bulges - one facing the Moon and the other opposite??

Tidal Friction Moon’s orbital period = ~29 days Moon’s orbital period = ~29 days Earth’s rotation period = 1 day Earth’s rotation period = 1 day  Earth needs to rotate through two tidal bulges  tidal friction!  Bulges are slightly pulled forward w.r.t. the line connecting Earth—Moon  Earth rotation gets slower  while Moon’s orbit gets larger (due to angular momentum conservation)

Tidal Friction  Synchronous Rotation A larger tidal effect on the Moon than on the Planet A larger tidal effect on the Moon than on the Planet 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”  the orbit will get ‘circularized’!  the orbit will get ‘circularized’! 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 object are tidally locked-in! All close-in objects to a larger object are tidally locked-in!

Tidal Friction  Synchronous Rotation When a satellite rotates too slowly When a satellite rotates too rapidly A lot of words with ‘tidal’ adjective tidal locking tidal friction tidal heating …

Io : Excellent Example of 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 should be no more on-going heating because of no more tidal friction. If Io was completely “tidally locked”, there should be no more on-going heating because of no more tidal friction.

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?

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…

Io Europa Ganymede Callisto Life on Galilean Moons?

Too strong tidal heating  lack of water + extreme volcanism “Just right” tidal heating? ?? ??

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

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

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

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).

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…

Tracking Evolution Youngest YoungestEDCBA Oldest Oldest

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

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!!

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 Tidal Effects Evidence of subsurface ocean in Europa Relative age-dating of surface features Important Terms Jovian Moons Synchronous rotation Tidal friction Tidal heating Orbital resonance Induced magnetic field Chapter/sections covered in this lecture : 9.1 & 9.2 Life on Titan and other places in SS : next class!!