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 and
Ganymede might be just right for life.
Surface: bright white - entirely covered
with 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 - the
magnetic field of Jupiter is able to
induce a magnetic field in Europa
consistent with a salty ocean beneath its
crust
2) The lack of large impact craters on the
surface - large impacts will break the
thin crust causing water and slushy ice
below to flood out and resurface the
crust.
3) Much of Europa’s surface appears
chaotic and clogged with huge iceberglike blocks. This is consistent with a
thin icy crust that has been broken into
pieces by tidal forces below which is a
subsurface 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
Fractured ice planes
The closest photograph ever taken of Europa.
Europa – models
The internal structure of Europa from
gravitational 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
Artist's concept of the cryobot and hydrobot.
Estimated concentrations of major elements in Europan oceanic
water compared with seawater on Earth.
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
Ganymede has both young and old surface regions, separated
by sharp boundaries. The young surfaces were created by
water eruptions with subsequent freezing.
Left. Fresh craters
Right.Parallel ridges and troughs that are the principal
features in the brighter regions of Ganymede.
Resolution of the Galileo images is 74 meters.
Jupiter’s moon Callisto
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
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.
Jupiter’s moon Callisto
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.
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
Saturn’s moons
Enceladus size
Mimas
Radius (km)
199
20
Mass (10 kg)
0.4
-3
Density (g cm )
1.14
Orbit period (days) 0.9
Enceladus
249
0.7
1.21
1.4
Earth
Titan
Moon
Thetys
Dione
Rhea
Titan
Iapetus
Earth’s Moon
530
6
1
1.9
560
10
1.44
2.7
764
23
1.24
4.5
2575
1346
1.88
16
718
16
1.02
79
1738
730
3.3
27
Saturn’s moons
Mimas
Rhea
Enceladus
Titan
Thetys
Iapetus (Movie)
Dione
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?
Dunes on Earth
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.
Dunes on Titan
Enough organic material exists on Titan to start a
chemical evolution analogous to what is thought to
have started life on Earth.
Temperature
profile derived
from Voyager
data.
The temperature
of Titan's surface
is -179C.
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
Neptune’s moon Triton
Triton orbits in the opposite direction to
Neptune’s rotation - probably captured by
Neptune’s gravity.
Triton’s source of internal heat is mostly
associated with tidal heat.
Voyager 2 images showed active geyser-like
eruptions of nitrogen gas and dark dust
particles several kilometers into the
atmosphere.
Triton is one of only three objects in the Solar
System known to have a nitrogendominated atmosphere (Earth & Titan).
Triton has the coldest surface known in the
Solar System - that most of its nitrogen is
condensed as frost, making it the only
satellite in the Solar System known to have
a surface made mainly of nitrogen ice.
Triton's surface indicates a long history of
melting - differentiated core - radioactive
decay - heat sufficient to maintain an
underground ocean.
If Triton has a subsurface ocean, it will most
likely consist of water mixed with
ammonia, methane, or other melted ices.
Triton
Radius (km)
Mass (1020 kg)
Density (g cm-3)
Orbit period (days)
Surface T
Earth’s Moon
1353
214
2
6
(38K) -235C
1738
730
3.3
27
250K (-23C)
Jovian Moons habitability- conclusions
Europa
Titan
Ganymede
Enceladus
Callisto
Triton
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!
Life in our Solar system - Conclusions
In clouds?
In subsurface
In clouds?
In the water-ammonia
oceans beneath the
surface
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