Transcript Convection is the main cooling process for planets with warm interiors.

Building the Planets. IV. Nebular Capture
Nebular capture – growth of icy
planetesimals by capturing
larger amounts of hydrogen and
helium. Led to the formation of
the Jovian planets
Numerous moons were formed by the same processes
that formed the proto-planetary disk
Condensation and accretion created “mini-solar systems”
around each Jovian planet
Building the Planets. V.
Expulsion of remaining gas
The Solar wind is a flow of charged particles ejected by the
Sun in all directions. It was stronger when the Sun was young.
The wind swept out a lot of the remaining gas
Building the Planets. VI.
Period of Massive Bombardment
Planetesimals remaining after the
clearing of the solar nebula
became comets and asteroids
Rocky leftovers became asteroids
Icy leftovers became comets
Many of them impacted on objects
within the solar system during first
few 100 million years (period of
massive bombardment - creation
of ubiquitous craters).
Brief
Summary
Three outstanding issues:
1.Outer Jovian planets may not have had enough time to
formed in their current locations
2.Rocks returned by astronauts from the heavily cratered
lunar highlands are ~ 3.9 million yrs old – younger than
solar system
3.There were no icy planetesimals in the inner solar
system. Where did the Earth’s water come from?
Solution? Late Heavy Bombardment (LHB)
1.
All Jovians planets formed in orbits closer to that of Jupiter
2.
Orbital resonances between Jupiter and Saturn caused outer Jovians to
move suddenly to larger orbits
3.
Uranus and Neptune interacted with Kuiper Belt objects, scattering large
numbers of them into the inner solar system
4.
This lead to heavy bombardment & delivery of ices to Terrestrial planets.
The oldest biological markers known to
scientists date precisely to the end of the LHB.
The Origin of the Earth’s Moon
The Earth-Moon
double planet does not fit
well into the nebular theory
planetesimal accretion
predicts both should have
the same chemical
composition.
They don’t - there are
subtle but significant
differences
Moon is composed of less dense material than Earth
The general view is that
the Earth’s moon was
created as a result of the
impact of a large object,
perhaps as large as
Mars, with the Earth very
early in its existence.
The moon was formed
from the debris of this
collision, which included
lower density “mantle”
material from the Earth.
The Moon stabilizes the tilt of
Earth's spin axis, leading to
stable seasonal changes.
This is not the case for the
other terrestrial planets.
WE NOW UNDERSTAND HOW THE
LOCATIONS AVAILABLE FOR LIFE WERE
FORMED.
NOW, WHAT ARE THE REQUIREMENTS
FOR LIFE?
Life depends critically on environment.
We will examine how life-friendly
environments can form in the universe.
Fundamentals: Temperature
Liquids (particularly H2O)
Sources of Energy
Chemical environment
Radiation environment
What determines the environments of
terrestrial-like planets? A look at:
interiors
surfaces
atmospheres
(much of what follows also applies to Jovian planets & moons)
Terrestrial planets are mostly made of rocky materials (with
some metals) that can deform and flow.
Likewise, the larger moons of the Jovian planets are made
largely of icy materials (with some rocks and metals) that can
deform and flow.
The ability to deform and flow has many consequences.
Weight of mountain is
determined by its mass &
the strength of the
gravitational acceleration,
Fg = mg
weight of
mountain
If this force exceeds the
ability of the underlying
rock/ice to support it, the
mountain will sink into the
crust.
The ability to deform and flow leads every object with
diameters greater than a few hundred km to become
spherical under the influence of gravity.
The ability to deform and flow also
created structure in the interiors of
planets
Early in their existence, the Terrestrial
planets and the large moons had an
extended period when they were
mostly molten.
The heating that led to this condition
was caused by impacts, where the
kinetic energy of the impacting
material was converted to thermal
energy.
Today, the interiors of planets are heated mainly by
radioactive decay.
Differentiation – the
process by which
gravity separates
materials according to
their densities
Denser materials sink,
less dense material
“float” towards top
DIFFERENTIATION: During the time when interiors were
molten, denser material sank towards the center of a
planet/moon while less dense material “floated” towards top.
This created density layers: core, mantle, crust
Terrestrial planets have metallic
cores (which may or may not be
molten) & rocky mantles
Earth (solid inner,
molten outer core)
Earth’s interior structure
Mercury (solid core)
Differentiated Jovian moons have
rocky cores & icy mantles
Europa
Ganymeade
Io
Callisto
Interior structure of the Terrestrial planets:
The Lithosphere…
Layer of rigid rock (crust plus upper mantle) that
floats on softer (mantle) rock below
While interior rock is mostly solid, at high
pressures stresses can cause rock to deform and
flow (think of silly putty)
This is why we have spherical planets/moons
The interiors of the terrestrial planets slowly cool
as their heat escapes.
Interior cooling gradually makes the lithosphere
thicker and moves molten rocks deeper.
Larger planets take longer to cool,
and thus:
1) retain molten cores longer
2) have thinner (weaker) lithospheres
Geological activity is driven by the thermal energy of the
interior of the planet/moon
The stronger (thicker) the lithosphere, the less geological
activity the planet exhibits.
Planets with cooler interiors have thicker lithospheres!
Earth has lots of geological activity today, as
does Venus. Mars, Mercury and the Moon
have little to no geological activity (today)
This has important repercussions for life:
1) Outgassing: produces atmosphere
2) Magnetic fields (need molten cores):
protect planet surface from high energy
particles from a stellar wind.
Larger planets stay hot longer.
Earth and Venus (larger) have continued to cool
over the lifetime of the solar system  thin
lithosphere, lots of geological activity
Mercury, Mars and Moon (smaller) have cooled
earlier  thicker lithospheres, little to no
geological activity
Initially, accretion provided the dominant
source of heating.
Very early in a terrestrial planet’s life, it is
largely molten (differentiation takes place).
Today, the high temperatures inside the
planets are due to residual heat of
formation and radioactive decay heating.
Stresses in the lithosphere lead to “geological activity”
(e.g., volcanoes, mountains, earthquakes, rifts, …)
and, through outgassing, leads to the formation and
maintenance of atmospheres.
Cooling of planetary interiors (energy transported
from the planetary interior to the surface)
creates these stresses
Convection is the main cooling process for
planets with warm interiors.
Convection - the transfer of thermal energy in which
hot material expands and rises while cooler material
contracts and falls (e.g., boiling water).
Convection is the
main cooling
process for planets
with warm interiors.
Side effect of hot interiors - global planetary
magnetic fields
Requirements:
•
Interior region of electrically conducting fluid
(e.g., molten iron, salty water)
•
Convection in this fluid layer
•
“rapid” rotation of planet/moon
Earth fits
requirements
Venus rotates too
slowly
Mercury, Mars &
the Moon lack
molten metallic
cores
Sun has strong
field