Phys 214. Planets and Life

Dr. Cristina Buzea

Department of Physics

Room 259

E-mail: cristi@physics.queensu.ca

(Please use PHYS214 in e-mail subject)

Lecture 29. Search for life on jovian moons.

Habitability.

March 26th, 2008

Contents

Textbook pages 304-347

Search for life on:

- Jupiter’s moons Europa, Ganymede, & Callisto

- Saturn’s moons Enceladus & Titan

- Neptune’s moon Triton

- Life in our Solar System - Conclusions

Europa – evidence of subsurface oceans

The level of tidal heating on Europa andGanymede might be just right for life.

Surface: bright white - entirely coveredwith ice; smooth, giant cracks in the ice– crisscross the surface.

few impact craters -> very young surface.

Evidence for a subsurface ocean of water:

1) the MOST convincing evidence - themagnetic field of Jupiter is able toinduce a magnetic field in Europaconsistent with a salty ocean beneath itscrust

2) The lack of large impact craters on thesurface - large impacts will break thethin crust causing water and slushy icebelow to flood out and resurface thecrust.

3) Much of Europa’s surface appearschaotic and clogged with huge iceberg-like blocks. This is consistent with athin icy crust that has been broken intopieces by tidal forces below which is asubsurface ocean of water.

Landscape suggest that liquid water or slush ice has

welled up from below, breaking apart the

surface and then freezing in place.

Europa – evidence of subsurface oceans

The closest photograph ever taken of Europa.Fractured ice planes

Europa – models

The internal structure of Europa fromgravitational field measurements:

Model 1.

thin icy crust (up to 3 km),

warm convecting ice layer,

thick rocky mantle,

central iron core.

Model 2.

thin icy crust,

100 km deep subsurface ocean of water,

thick rocky mantle,

central iron core.

Model 3.

thin icy crust,

warm convecting ice layer,

subsurface ocean of water,

thick rocky mantle,

central iron core.

Jupiter’s moon Europa

The floating ice shell is heated by flexing and squeezing of Europa’s tides.

This tidal heat keeps the ocean liquid at a temperature near 0°C, even though the surface is very cold at

a temperature of about -170°C.

The cold surface ice can

crack, while the ice below

is hotter due to the tidal

heat, so it can slowly flow

like a glacier.

Life on Europa?

Life in the subsurface ocean -would most likely

obtain energy from tidal heating.

In the subsurface ocean beneath Europa’s icy crust,

if life exists, it most likely originated close to

volcanic vents on its ocean floor.

The complexity of any life present in Europa’s

subsurface ocean is mainly limited by the

amount of available energy to sustain it.

Recent research indicates that enough carbon

exists to support an underwater biosphere.

Europa movie (4 minutes)

A possible scenario for life on Europa. Image credit: Richard Greenberg

Estimated concentrations of major elements in Europan oceanic

water compared with seawater on Earth.Artist's concept of the cryobot and hydrobot.

Jupiter’s moons with subsurface oceans of water

Europa, Ganymede, and Callisto

- moons of Jupiter that show evidence for subsurface oceans of water beneath their icy crusts.

Jupiter’s moon Ganymede

Ganymede -the largest moon in the Solar System

1) the largest magnetic field of any moon.

2) the only moon to have its own internal

magnetic field – indicative of a molten

convecting core (radioactive decay?).

3) a small part of its magnetic field varies with

Jupiter’s rotation – indicates an electrically

conducting material under the surface – salty

ocean?

Less tidal heating on Ganymede suggests a thicker

ice cover than on Europa- at least 150 km.

The higher pressure in Ganymede’s interior is

high enough to allow high-density forms of

ice beneath any liquid water ocean - > no rock

–water boundary and less energy for life than

on Europa.

Magnetic field of the

Jovian satellite

Ganymede embedded

into the magnetosphere

of Jupiter. The image is

based on the reported

Galileo

measurements.The green

color denotes closed

field lines.

Jupiter’s moon Ganymede

Left. Fresh craters

Right.Parallel ridges and troughs that are the principalfeatures in the brighter regions of Ganymede.Resolution of the Galileo images is 74 meters.

Ganymede has both young and old surface regions, separated

by sharp boundaries. The young surfaces were created by

water eruptions with subsequent freezing.

Jupiter’s moon Callisto

Entire surface packed with craters dating probably from the heavy bombardment.

Dark dust covering the low-lying areas, with ridges and crests bright white.

Its interior does not seems to be fully differentiated – probably very few radioactive decay

(it has never been heated enough to melt its ice component).

Magnetic measurements made during Galileo flybys indicate Callisto's magnetic field is

variable – because Callisto too has a subsurface liquid layer (water contains a small

amount of ammonia or other antifreeze, up to 5% by weight.).

Tidal heating might is probably responsible for the subsuraface liquid.

200 kilometer

thick band of ice

just beneath the

moon's surface

light blue stripe -

potentially a

salty layer of

liquid water up

to 10 km thick

interior of rock

and ice

Jupiter’s moon Callisto

Magnetic field around

Callisto. The bending

of the field lines

indicates the existence

of an electrically

conducting layer in the

interior. The red line

shows a trajectory of

the Galileo spacecraft

during a typical flyby

The conditions for life appear to be less favourable on Callisto

than on Europa because of:

-the lack of contact with differentiated rocky material and

-the lower heat flux from the interior of Callisto.

Saturn’s moons

Mimas Enceladus Thetys Dione Rhea Titan Iapetus Earth’s Moon

Radius (km) 199 249 530 560 764 2575 718 1738

Mass (1020 kg) 0.4 0.7 6 10 23 1346 16 730

Density (g cm-3) 1.14 1.21 1 1.44 1.24 1.88 1.02 3.3

Orbit period (days) 0.9 1.4 1.9 2.7 4.5 16 79 27

Enceladus size Earth Titan Moon

Saturn’s moons

Mimas Enceladus Thetys Dione

Rhea Titan Iapetus (Movie) Iapetus

Saturn’s moon Enceladus

Cryovolcanism - ice geysers erupt on Enceladus

along surface fractures in the moon's south

polar region.

Geysers arise from near-surface pockets of liquid

water with temperatures near 0oC compared to

moon's surface temperature of -200oC.

The ice geysers also likely produce Saturn's faint

but extended E ring.

Enceladus seems to have a substantial subsurface

liquid – probably ammonia/water mixture.

Surprisingly, small Enceladus may have subsurface

habitable zones. Enceladus movie 1, 2

Saturn’s moon Titan

Titan - the second-largest moon in the solar system

and the only moon with its own atmosphere

(pressure ~ 1.6 times the one of Earth’s; thickness

of atmosphere 200-800 km).

Outgassing - the main source of its atmosphere

Like the Earth, Titan has an atmosphere made mostly

of molecular nitrogen. Other components are

hydrocarbons like methane and ethane.

The origin of N2 in Titan’s atmosphere is the

breakdown of ammonia (NH3) by ultraviolet light

from the Sun.

Methane should be rapidly destroyed in Titan’s

atmosphere, yet it is still present in appreciable

amounts. Probably because is continually

evaporating from the surface and the interior.

Hydrocarbons rain down on the surface, forming

enclosed seas, lakes, and ponds.

Titan is roughly the same size as Mercury, yet Titan

has an atmosphere while Mercury does not. This

is because Titan is much colder, allowing

molecules to be trapped in its atmosphere.

UV and infrared

image of Titan

Saturn’s moon Titan

In 2005 Huygens probe

landed on Titan,

showing a world

similar to the Earth in

many respects.

Landscape shows

strong evidence that a

liquid, possibly

methane, has flowed

on the surface, causing

erosion.

Liquid seas and lakes on Titan. Titan.

Internal water-ammonia ocean on Titan

The Huygens probe showed most of it to be solid. Since

then, geological features such as dunes, channels,

lakes, impact craters have been documented.

Three years after their discovery, Cassini observed these

features for a second time and reported a systematic

drift (by 31 km) of these features compared to their

expected position.

Conclusion - Titan has an internal water-ammonia

ocean buried below several tens of kilometers of ice

that mechanically decouples the crust from the

interior.

Titan internal structure:

Atmosphere

Water Ice & methane

compounds

Liquid water

High-pressure ice

Rocky core

Life on Titan?

The atmosphere of early Earth was probably similar

in composition to the current atmosphere on Titan.

The Miller-Urey experiment has shown that with an

atmosphere similar to that of Titan and the addition

of UV radiation, complex molecules and polymer

substances can be generated.

Enough organic material exists on Titan to start a

chemical evolution analogous to what is thought to

have started life on Earth.

Even though Titan has liquid methane on its surface,

some internal heat, and plenty of carbon-containing

compounds, it is not a suitable place for life as we

know it because it is far too cold, and methane is not

a very good biological solvent.

However, life could exist in the subsurface ocean of

ammonia/water mixture..

Movie 1, Movie 2, Movie 3, Movie 4

Temperature

profile derived

from Voyager

data.

The temperature

of Titan's surface

is -179C.

Dunes on Titan

Dunes on Earth

Neptune’s moon Triton

Triton orbits in the opposite direction toNeptune’s rotation - probably captured byNeptune’s gravity.

Triton’s source of internal heat is mostlyassociated with tidal heat.

Voyager 2 images showed active geyser-likeeruptions of nitrogen gas and dark dustparticles several kilometers into theatmosphere.

Triton is one of only three objects in the SolarSystem known to have a nitrogen-dominated atmosphere (Earth & Titan).

Triton has the coldest surface known in theSolar System - that most of its nitrogen iscondensed as frost, making it the onlysatellite in the Solar System known to havea surface made mainly of nitrogen ice.

Triton's surface indicates a long history ofmelting - differentiated core - radioactivedecay - heat sufficient to maintain anunderground ocean.

If Triton has a subsurface ocean, it will mostlikely consist of water mixed withammonia, methane, or other melted ices.

Triton Earth’s Moon

Radius (km) 1353 1738

Mass (1020 kg) 214 730

Density (g cm-3) 2 3.3

Orbit period (days) 6 27

Surface T (38K) -235C 250K (-23C)

Jovian Moons habitability- conclusions

Sequence of Jovian moons in the most likely order of decreasing habitability:

Europa, Ganymede, Callisto, Titan, Enceladus, Triton.

Life might exist on 6 jovian moonss in our Solar System that might have liquids on

their surface or beneath the surface!

Europa Ganymede Callisto

Titan Enceladus Triton

Life in our Solar system - Conclusions

In clouds? In subsurface In the water-ammonia

oceans beneath the

surface

In clouds?

In subsurface oceans

In the surface seas and lakes of methane?

Planetary bodies that might harbour life (in the most likely

order of decreasing habitability):

Mars, Europa, Ganymede, Callisto, Titan, Enceladus,

Triton, Venus, Uranus, Neptune, Jupiter, Saturn.

Next lecture

• Habitability & Extrasolar planets