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Phys 214. Planets and Life
Dr. Cristina Buzea
Department of Physics
Room 259
E-mail: [email protected]
(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