Life in the Terrestrial Planet Region:Life in the Terrestrial Planet Region:

– MERCURYMERCURY

– MOONMOON

– VENUSVENUS

– MARSMARS

XX Terrible Extremes of Temperature, No Atmosphere, Terrible Extremes of Temperature, No Atmosphere, UV, Cosmic Rays, Little or No Volatiles, No LiquidsUV, Cosmic Rays, Little or No Volatiles, No Liquids

Terrible Extremes of Temperature, No Atmosphere, Terrible Extremes of Temperature, No Atmosphere, UV, Cosmic Rays, Little or No Volatiles, No LiquidsUV, Cosmic Rays, Little or No Volatiles, No LiquidsXX

Very High Temperatures, No or Little Water Very High Temperatures, No or Little Water Young Surface Young Surface No Fossil Record No Fossil RecordXX

?? Evidence for Liquid Water in PastEvidence for Liquid Water in PastPossible Environments for Life to Survive?Possible Environments for Life to Survive?Volatiles and Water Present NowVolatiles and Water Present Now

Moving Moving OutwardsOutwards

Gas and Ice Giant Gas and Ice Giant Planets:Planets:

Prospects for Life?Prospects for Life?

– Ingredients for organic chemistry– Atmospheric layers with roughly Earth-like Temperature &

Pressure;– ButBut no solid surfaces (except ice crystals) and no liquid water

(except veryvery deep in UranusUranus and NeptuneNeptune),– Violent winds and convective turbulence would mix any life-

bearing gas quickly over extremes of temperasture & pressure.– Although the sunlight is very weak, internal heat is available.– They are veryvery difficult to explore!

Life on Jupiter?Life on Jupiter?

•All the basic molecular ingredients are All the basic molecular ingredients are present in the atmospherespresent in the atmospheres

•But But convectionconvection will ultimately (and quickly) will ultimately (and quickly) bring any organic molecules down to regions bring any organic molecules down to regions where T~ 700 C (over 1200 F!) where they will where T~ 700 C (over 1200 F!) where they will be destroyedbe destroyed

Sagan & Salpeter

“speculations”

“floaters” “hunters”

Life on Moons of Planets?Life on Moons of Planets?

For instance, the GalileanFor instance, the GalileanSatellites of JupiterSatellites of Jupiter

IoIo

EuropaEuropa

GanymedeGanymede

CallistoCallisto

Medium & large moons

• Enough self-gravity to be spherical

• Are, or were, geologically active.• Have substantial amounts of ice.• Roughly circular, equatorial orbits

in same direction as planet rotation.

Small moons• Far more numerous than the medium and large moons.• Not enough gravity to be spherical: “potato-shaped”• Captured asteroids, so orbits do not follow patterns.• Orbits can be tilted, elliptical, and even backwards!• No atmospheres or liquid water – not suitable for life

Focus on the large & Focus on the large & medium sized moonsmedium sized moons

Sizes and orbits

Diameter

(km)

Orbit Period

(days) comments

Earth’s Moon 3,474 28 orbit stabilizes Earth’s tilt & seasons

Io (Jupiter) 3,640 1.77 orbit perturbed by Ganymede, Europa, eccentric irregular orbit, extremely variable tidal force

Europa (Jupiter) 3,130 3.55 eccentric orbit

Ganymede (Jupiter) 5,268 7.15

Callisto (Jupiter) 4,806 16.69

Enceladus (Saturn) 494 1.37

Titan (Saturn) 5,150 15.94Largest in solar system, substantial atmosphere

What about…

- a source of energy (Sun is too faint)?

- presence of liquid water?

Tidal Forces

Because the gravitational force decreases with (distance)2, the attractive force experienced by one object (e.g., the Earth) due to the gravitational field of a second object (e.g., the Moon) varies with position (closest parts attracted most strongly).

Tidal forces are difference forces.

Synchronous Rotation

• …is when the rotation period of a moon, planet, or star equals its orbital period about another object.

• Tidal friction on the Moon (caused by Earth) has slowed its rotation down to a period of one month.

• The Moon now rotates synchronously.– We always see the same side of the Moon.

• Tidal friction on the Moon has ceased since its tidal bulges are always aligned with Earth.

• All of the large moons in the solar system are in synchronous rotation.

Tidal Heating in Jovian Moons

The four inner moons of Jupiter - Io, Europa, and Ganymede - all show evidence of geological activity - indicators of molten interiors. The heat source is tidal heating.

Moons have elliptical orbit and synchronous rotation - one side always faces Jupiter- as Ganymede completes one orbit, Europa completes exactly two orbits, and Io completes exactly four orbits -

moons periodically line up - causes orbital ellipticity.- tidal bulges are constantly being flexed in different directions - generates friction inside

Effects of Tidal InteractionsEffects of Tidal Interactions• RotationRotation

– Rotation of moons become synchronizedsynchronized with their orbits.– They keep the same face toward the planet.– The rotation of the planet is slowed down.

• OrbitsOrbits– Orbits of moons mostly evolve outwardoutward.

• Internal “Tidal Heating”Internal “Tidal Heating”– Eccentric orbits lead to periodic flexing of the moon’s shape which

heats the interior.– Orbital resonances Orbital resonances with other moons can maintain eccentric orbits

and tidal heatingtidal heating.

Jupiter’s Galilean Satellite’s

Io Europa Ganymede Callisto

Io

Io’s Volcanoes

Io is the most volcanically active world in the solar system.

Io Jupiter’s tidal forces flex Io like a ball of silly putty.

- friction generates heat- interior of Io is molten

Volcanoes erupt frequently.- sulfur in the lava accounts for yellow

color- surface ice vaporizes and jets away

-Thin atmosphere made up mainly of sulfur dioxide, produced by volcanic activity and temporarily retained by the moon’s gravity.- Evidence of tectonics and impact cratering is covered.

Volcanic Plumes

Lava fountain - active lava hot enough to cause "bleeding" in Galileo's camera - overloading of camera by the brightness of the target

Newly erupted hot lava flow. Dark, "L"-shaped lava flow marks the location of the November 1999 eruption.

A broad plume of gas and dust about 80 km high above a lava flow

Gas and Dust Plume

Europa

Jupiter’s EuropaJupiter’s Europa– Has similar but weaker tidal weaker tidal

heating,heating,

– Has a young cracked water ice water ice crustcrust perhaps only a few kilometers thick, and

– May have a warm ocean of warm ocean of liquid waterliquid water below the crust.

– Could there be life?life?

EuropaIcy surface - “fresh” - almost no craters

Ocean under the ice?• Evidence

– Gravity measurements: central metallic core surrounded by 80170 km of water/ice

– Lack of craters ice tectonics liquid below (but could be “fluid” ice, like glaciers)

– Chaotic terrain: like arctic ice pack, with separating pieces

– Magnetic field: conducting liquid for internal dynamo & metallic core too cold brine ocean

– Tidal heating: computations show it can do the job

• Estimated size

– Crust depth: 525 km, based on flooded impact crater

– Ocean 50150 km deep (< 11 km on Earth)

Surface of Jupiter’s Moon: :

EuropaEuropa

Fractures inFractures in Floating IceFloating Ice

IcebergsIcebergs

Evidence of a Subsurface ocean

Jumbled crust with icebergs and surface cracks with double-ridged pattern - caused by tidal flexing of thick layer of ice on top of liquid ocean of water.

Europa’s interior also warmed by tidal heating

Salty - Europa has a magnetic field

Sub-CrustSub-CrustOceanOcean ....

Life in the Ocean?

..

Hydrogen-Carboncompounds likely:

Amino acids

First New Ocean

Since Balboa

Missions to Europa

http://www.jpl.nasa.gov/europaorbiter/

Ganymede

Ganymede• Largest moon in the solar system• Clear evidence of geological activity • Tidal heating expected - but is it enough?

Ganymede

Wrinkles due to tectonic movement in ice crust in (distant) past - possible water deep below?

Ganymede• Cratering

– Dark areas: cratering upon cratering several byr old– Bright areas: far fewer craters and grooves– Explanation: “lava” (i.e., water) eruptions followed by freezing

• Ocean?– Magnetic field convecting core– Part of magnetic field varies with Jupiter’s rotation electrically

conducting interior (brine?)– Salts found on the surface

• Heat source– Less tidal heating than Europa (larger distance from Jupiter)– Large mass more radioactivity– Much less heat than in Europa thick crust (>150 km?) Much harder to prove the existence of life never mind finding it

Callisto

Callisto• “Classic” cratered iceball.

• No tidal heating - no orbital resonances.

• But it has magnetic field !?

Callisto Scarp close up

Possible water deep?

Callisto• Cratering

– Heavily cratered everywhere no water gushing to the surface

• Gravity– Undifferentiated: mix of ice and rock throughout

• Induced magnetic field– Exists underground ocean?

• Heat source?– Does not participate in the tidal resonance– Radioactive decay: only possibility

Life on Galilean Moons?Life on Galilean Moons?

• IoIo

• EuropaEuropa

• GanymedeGanymede

• CallistoCallisto

XX Very active volcanically. Very active volcanically. Hostile environmentHostile environment

Subsurface saline Subsurface saline ocean, hydrothermal ocean, hydrothermal vents?vents?

??

Subsurface saline Subsurface saline ocean? hydrothermal ocean? hydrothermal vents?vents?

??

Subsurface saline Subsurface saline ocean? ocean?

????