Life on Jovian Moons

ASTR 1420

Lecture 14

Sections 9.1 & 9.2

Background…• Jovian planets themselves are unlikely habitable (Do you remember why?)

• There are many moons orbiting these planets Jovian Moons.

• 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 many more jovian moons than jovian planets!

• Let’s explore the mechanisms that make some jovian moons habitable.

Galileo, the inventor of 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 1609.

A page from Gelileo’s notebook written in 1610. ‘*’ 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

• 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

• Currently, 63 moons were discovered orbiting Jupiter.

Galileo Galilei

Galileo Telescope’s Objective LensIn 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.

• 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.

HST near infrared light image

Jupiter has a ring, too!

• The ring of Jupiter was discovered by Voyager 1 in March of 1979.

• All four Jovian planets have rings!!

Larger Moons

• Ganymede and Titan are larger than Mercury• Big2 + Io, Europa, Callisto,

Triton, and our Moon are larger than Pluto• smallest moons are about

the size of a single mountain on Earth• Moderate—large moons =

miniature solar system• spherical shape• orbit in the same plane• 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.

Far side of the Moon Near side of the Moon

Synchronous Rotation

• 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!

Tides on Earth

• 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??

Tidal Friction

• Moon’s orbital period = ~29 days• Earth’s rotation period = 1 dayEarth needs to rotate through two tidal bulges tidal friction!Bulges are slightly pulled forward w.r.t. the line connecting Earth—MoonEarth 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• Adjusting rotational+orbital motions until “bulges” on the Moon

always face to the Earth Synchronous Rotation = “Tidal Locking” the orbit will get ‘circularized’!

• Tidal locking takes a short time (a few Myrs!)• 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’ adjectivetidal lockingtidal frictiontidal heating…

Io : Excellent Example of Tidal Heating

• 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!• 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.

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 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…• 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

being tidally lockedmoving outward“meet” 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 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)• 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• Has a very weak magnetic field• Has 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 cracks

• 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/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 water

Surface feature of Europa

• Chaotic 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

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).

Europa Craters• 140 km wide crater formed from a mountain size asteroid or comet…• similar to a gunshot glass structure…

Tracking Evolution

Youngest

EDCBA Oldest

A

BC

D

E

Enigma…

• Conventional explantion

If true, we should be able to findsome single ridges. But, all Europanridges are double ridges!

Ganymede

• the largest moon in the solar system

• surface of hard ice• old + young surfaces• grooves tectonic stresses• underground ocean weak

induced magnetic field (+ its own) & surface salt.

• 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!

Ganymede• white craters and rims = impact exposes

mantle ice

• thin oxygen atmosphere, possible aurora at its poles

• 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 as old as late heavy bombardment

• dark powder at the low-lying areas dusts after sublimated ice?

• Gravity a ball of mixed rock and ice + hundred km of water ice…

• Undifferentiated interior interior was never warm enough!

• Induced magnetic field subsurface salty ocean!!

Galilean Moons

Too hot Maybe less likely Too cold, no? Io Europa Ganymede

Callisto 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 & 9.2Life on Titan and other places in SS : next class!!