Download Phys 214. Planets and Life

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 28. Search for life on jovian moons.
March 24th, 2008
Contents
Textbook pages 295-312
First several slides from life on Mars, Neptune and Uranus.
Jovian moons
Formation of jovian moons
Composition of jovian moons
Synchronous rotation of the moons
Tidal heating
Orbital resonance
Jupiter’s moons Io
Life on Jovian moons
The largest Jovian moon Ganymede and Titan are larger than Mercury but smaller than the Earth.
Three of Jupiter’s moons – Io, Europa, and Callisto, and Neptune’s moon Triton are bigger than Pluto.
The formation of Jovian moons
Most of the smallest Jovian moons are most
likely captured asteroids and comets.
The largest Jovian moon that appears to have
been captured is Triton, the moon of
Neptune – it orbits backwards relative to its
planet’s rotation.
The fact that many of the larger Jovian moons
orbit nearly in the equatorial plane of their
host world, moving in the same direction as
their planet’s spin, suggest that they formed
from a rotating disk of gas and dust like a
miniature solar system
Triton
Composition of Jovian moons
Planetary objects fall into several groups:
- Giant planets with radii greater than 10,000
km and low densities
- Terrestrial planets and terrestrial-like bodies
- Icy satellites
- Small asteroids
Jovian moons are typically made of ice and
rock. The outer Solar System was cold
enough to allow ices to condense along
with metal and rock.
The average densities of most jovian moons are
significantly lower than that of Earth – they
contain a lot of water ice.
Within individual moons we see variation in
composition reflecting the fact that the
moons formed at different distances from a
hotter, central planet.
Jupiter’s moons show a decrease in density
with distance from Jupiter – formed of a
cloud of gas that was hotter in the center
than in its outer regions.
Jupiter’s satellites
Io
Radius (km)
Mass (1020 kg)
Density (g cm-3)
Escape velocity (km/s)
Average surface T (K)
Orbit period (days)
Europa
Ganymede
Callisto
Io
Europa
Ganymede Callisto
Earth’s Moon
1822
893
3.5
2.6
118
1.7
1561
480
3
2
103
3.5
2631
1480
1.9
2.7
113
7
1738
730
3.3
2.4
253
27
2410
1076
1.8
2.4
118
16
Io’s density indicates it has no water, Europa is mostly rock with water ice, Ganymede and
Callisto have more significant amounts of water ice compared to rock.
Composition of Jovian moons
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In addition to compositional variation within individual moons, there is also variation in
composition of moons as we move from one planet to the next – because of temperature
differences in the solar nebula.
Water ice condenses easily at temperatures near Jupiter.
Methane and other ices condensed only at colder temperatures – farther away from the
Sun.
Jupiter’s moon contains significant quantities of water ice, but no other ices.
Moons of more distant planets contain higher proportion of water ice compared to
rocks, and contain other ices (methane in addition to water ice).
Synchronous rotation of the Moons
Like our own moon, most of Jovian moons exhibit synchronous rotation they keep the same
face turned toward their planet (they rotate at the same rate that they orbit their host planet)
Synchronous rotation develops naturally for any moon that orbits close to its parent, and is the
consequence of the same gravitational effect that lead to tides on Earth.
Tidal force = the force of gravity exerted by one object on another is greater on the near side
than the far side.
Tidal forces stretches the Earth and creates tidal friction. Tidal friction allows Earth’s rotation
to pull the bulge slightly ahead of the Earth-Moon line =>
1) The moon gravity is pulling back causing earth’s rotation to slow down (1 second every
50,000 years)
2) The gravity of the bulge pulls the Moon ahead in its orbit – causing the moon to move
farther away from Earth.
Synchronous rotation of the Moons
Calculation. Calculate the the tidal force
that a planet exerts on its moon (per
kg).
Fg = G
M 1M 2
d2
Ftidal = Fg (near side ) ! Fg (on far side ) =
=G
M planet M rock
2
(d planet !moon ! rmoon )
!G
M planet M rock
(d planet !moon + rmoon )2
Tidal heating - Jovian moons
Planets gradually loose their internal heat
(accretion, differentiation, radioactive decay),
the smaller the planet – the faster it cools.
Several of jovian moons still have a source of
internal heat, despite their smaller size.
Io (one of Jupiter’s moons) is the most volcanically
active body in our solar system! Its internal
source of heat is very different from that of
planets = tidal heat due to tidal forces.
Even though Jupiter’s moon Io is similar in size to
our geologically dead Moon, it is more
geologically active than the Earth.
This is because Io experiences strong internal
tidal heating because of the strong tidal force
from the massive Jupiter combined with its
elliptical orbit which causes the strength and
direction of the force to constantly change.
We would expect most large satellites to have
nearly circular orbits, and not elliptical. Why is
Io’s orbit elliptical?
Orbital resonance and Jupiter's moons
Because of orbital resonance between the three of
the moons of Jupiter (I = Io, E = Europa, G =
Ganymede).
Sequence of conjunctions as they orbit about
Jupiter (J). The seventh configuration would
complete one period of Ganymede, which is
two periods of Europa and four periods of Io
(Murray and Dermott, 2001)
Jupiter’s moon Io
The resulting tidal friction greatly heats Io's
interior, causing molten rock to explode
through the surface. Io's volcanoes are so
active that they are effectively turning the
whole moon inside out.
Io's colors derive from sulfur and molten
silicate rock - 140 kilometers plume.
Io lacks water and extreme volcanic activity
makes is improbable for hosting life.
Next lecture
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Searching for life on Jupiter’s moons Europa, ganymede, Callisto,
Saturn’s moon Titan, Neptune’s moon Triton
The nature and evolution of habitability