Transcript Tectonics

Homework #5 due next Tuesday, 4:00 pm
Interactions between the surfaces of
planets and moon and their interiors
play a large role in determining their
habitability
Four major processes affect planetary surfaces:
Impact cratering – from collisions
with asteroids and comets
Erosion – wearing down or
building up geological feature by
wind, water, ice, etc.
Volcanism – eruption of molten
rocks & outgassing
Tectonics – disruption of a planet's
surface by internal stresses (upper
mantle and lithosphere)
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
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.
Initially, accretion provided the dominant
source of heating of the interiors of planets.
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.
Convection - the transfer of thermal energy in which
hot material expands and rises while cooler material
contracts and falls (e.g., boiling water).
Tectonics: refers to the action of internal forces
and stresses on the lithosphere to create
surface features, i.e., “geological processes”
Can only occur on planets
or moons with convection
in the mantle:
Earth & Venus
Europa
Ganymede
Enceladus
Tectonics…
•can move large segments of the lithosphere
(plate tectonics)
Tectonics…
• raise mountains
Tectonics…
create huge valleys (rifts)
and cliffs
Tectonics…
• create new crust
Tectonics…
generate volcanoes
=> maintain atmospheres!
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)
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
A magnetic field creates
“magnetosphere” that
deflects away solar wind
particles.
In the absence of a “magnetosphere”,
the solar wind will slowly strip away an
atmosphere and will bombard its
surface with high energy particles from
the solar wind.
ATMOSPHERES
Atmospheric Basics
 Layer of gas surrounding a planet.
 Usually very thin for terrestrial
planets (exception Venus).
 Affects conditions on the planet.
 We would like to understand how
each of the terrestrial planets
ended up having such different
atmospheres.
Venus’s thick atmosphere
Interlude:
thermal emissions and interactions
All terrestrial planets probably had minimal atmospheres at
some point after they formed: “primary” atmosphere of H,He
These original atmospheres were swept away from the
terrestrial planets early in their life.
Current atmospheres are
“secondary” atmospheres,
formed primarily by
outgassing
(mostly carbon dioxide - CO2)
Holding onto an atmosphere requires gravity
 The strength of gravity determines the escape
velocity from the planet.
 The temperature and composition of an atmosphere
determines the velocities of atoms and molecules in
the atmosphere; lighter molecules will move faster.
At a given temperature, H and He will have higher
velocities than more massive elements or
molecules (recall that K.E. = 1/2 mv2)
Holding onto an atmosphere requires gravity
 The strength of gravity determines the escape
velocity from the planet.
 The temperature and composition of an atmosphere
determines the velocities of atoms and molecules in
the atmosphere.
 If the constituents of an atmosphere are moving
faster than escape velocity, then a planet or moon
will be unable to hold onto an atmosphere.
Larger (stronger gravity),
cooler (slower moving molecules) planets
can hold onto atmospheres better than
smaller (weaker gravity),
hotter (faster moving molecules) planets
●
Moon and Mercury are “Airless” worlds
 gravity too weak to hold onto an
atmosphere
“black sky”
 The little atmosphere that exists
consists of particles of the solar wind
that are temporarily trapped.
●
Mars

Very little atmosphere today (mainly CO2)
Mars had standing and running water on its
surface in the past.
 Therefore, it must have had a more
substantial atmosphere in the past
 Does it have water today? Yes - frozen in
polar ice caps and beneath its soil
●
Venus
 Densest atmosphere of all Terrestrials
 Mostly CO2
 Temperature at surface hot enough to melt lead
 Pressure at the surface ~ 90 times that on Earth
 Perpetual cloud cover, sulfuric acid rain
 Weather forecast “awful” all the time.
●
Earth
 A moderate atmosphere today
 Mostly nitrogen (N2), with some oxygen (O2:
arises from photosynthetic life), carbon
dioxide (CO2), etc.
 Enough to enable liquid water to exist
(temperature and pressure adequate)
 Together the air & water produce erosion
The Jovian planets (high gravity, cool/cold
atmospheres) have very substantial
atmospheres, primarily H & He.
We see
the tops
of clouds
LAYERING OF ATMOSPHERES
Structure is created
within an atmosphere
through interactions of
atmospheric gasses
with light
Exosphere
hottest layer, v. rarified
Thermosphere
absorbs X-rays, ionized,
ionosphere, reflects
some radio, aurora
Mesosphere
•
weakly absorbs UV
Stratosphere
•
strongly absorbs UV, ozone
(O3), stratified (no
convection), Earth only
Terrestrial planet with one.
Troposphere
•
absorbs IR (greenhouse);
convective; weather
From the perspective of
life, stratosphere &
troposphere are the
most important
Stratosphere absorbs
harmful UV
Troposphere provides
greenhouse effect
The greenhouse effect

Planets heat up by absorbing the Sun’s visible light
 Planets cool off by radiating infrared out to space
 Greenhouse gasses trap infrared radiation in
troposphere (lowest level of atmosphere), thereby
heating the lower atmosphere.
●
greenhouse gasses (e.g., H2O, CO2, CH4 - methane)
transmit visible light but absorb infrared light
●
●
Greenhouse effect raises
temperature of lower
atmosphere
Greenhouse effect is critical
to the existence of life on
Earth – it raises temperatures
to “habitable” level, permits
liquid
water