Transcript NATS1311_120208_bw

Weather and Climate
weather – short-term changes in wind, clouds, temperature, and
pressure in an atmosphere at a given location
climate – long-term average of the weather at a given location
These are Earth’s global wind patterns
or circulation
- local weather systems move along
with them
- weather moves from W to E at midlatitudes in N hemisphere
Two factors cause these patterns
- atmospheric heating
- planetary rotation
Global Wind Patterns
air heated more at equator
- warm air rises at equator;
heads for poles
- cold air moves towards equator
along the surface
two circulation cells are created in
each hemisphere
cells do not go directly from pole to
equator; air circulation is diverted by
the Coriolis effect
- moving objects veer right on a
surface rotating
counterclockwise
- moving objects veer left on a
surface rotating clockwise
Coriolis Force and Merry-go-Round Animation
Coriolis Force on Earth Animation
Four Major Factors Which Affect Long-Term Climate
Change
Gain/Loss Processes of Atmospheric Gas
Unlike the Jovian planets, the terrestrials were too small to capture significant gas
from the Solar nebula.
- what gas they did capture was H and He, and it escaped
- present-day atmospheres must have formed at a later time
Sources of atmospheric gas:
- outgassing – release of gas trapped in interior rock by volcanism
- evaporation/sublimation – surface liquids or ices turn to gas when heated
- bombardment – micrometeorites, Solar wind particles, or high-energy
photons blast atoms/molecules out of surface rock
- occurs only if the planet has no substantial atmosphere already
Gain/Loss Processes of Atmospheric Gas
Ways to lose atmospheric gas:
- condensation – gas turns into liquids or ices on the surface when
cooled
- chemical reactions – gas is bound into surface rocks or liquids
- stripping – gas is knocked out of the upper atmosphere by Solar
wind particles
- impacts – a comet/asteroid collision with a planet can blast
atmospheric gas into space
- thermal escape – lightweight gas molecules are lost to space when
they achieve escape velocity
gas is lost
forever!
Origin of the Terrestrial Atmospheres
Venus, Earth, and Mars received their atmospheres through outgassing.
- most common gases: H2O, CO2, N2, H2S, SO2
Chemical reactions caused CO2 on Earth to dissolve in oceans and go into
carbonate rocks (like limestone.)
- this occurred because H2O could exist in liquid state
- N2 was left as the dominant gas; O2 was exhaled by plant life
- as the dominant gas on Venus, CO2 caused strong greenhouse effect
Mars lost much of its atmosphere through impacts
- less massive planet, lower escape velocity
Origin of the Terrestrial Atmospheres
Lack of magnetospheres on Venus and Mars made stripping by the Solar wind
significant.
- further loss of atmosphere on Mars
- dissociation of H2O, H2 thermally escapes on Venus
Gas and liquid/ice exchange occurs through condensation and
evaporation/sublimation:
- on Earth with H2O
- on Mars with CO2
Since Mercury and the Moon have no substantial atmosphere, fast particles and highenergy photons reach their surfaces
-bombardment creates a rarified exosphere
Ice recently discovered on Moon in craters near the poles
- perpetually in shadow and frozen
- probably came from impacts of ice-rich comets
- possibly on Mercury too
Martian Weather Today
Seasons on Mars are more extreme than on Earth
- Mars’ orbit is more elliptical
CO2 condenses and sublimes at opposite poles
- changes in atmospheric pressure drive pole-to-pole winds
- sometimes cause huge dust storms
Martian Weather: N Polar Ice Cap and Dust Storm
Climate History of Mars
More than 3 billion years ago, Mars must have had a thick CO2
atmosphere and a strong greenhouse effect.
- the so-called “warm and wet period”
Eventually CO2 was lost to space.
- some gas was lost to impacts
- cooling interior meant loss of magnetic field
- Solar wind stripping removed gas
Greenhouse effect weakened until Mars froze.
Venusian Weather Today
Venus has no seasons to speak of.
- rotation axis is nearly 90º to the ecliptic plane
Venus has little wind at its surface
- rotates very slowly, so there is no Coriolis effect
The surface temperature stays constant all over Venus.
- thick atmosphere distributes heat via two large circulation cells
There is no rain on the surface.
- it is too hot and Venus has almost no H2O
Venusian clouds contain sulfuric acid!
- implies recent volcanic outgassing?
Climate History of Venus
Venus should have outgassed as much H2O as Earth.
- Early on, when the Sun was dimmer, Venus may have had oceans of water
Venus’ proximity to the Sun caused all H2O to evaporate.
- H2O caused runaway greenhouse effect
- surface heated to extreme temperature
- UV photons from Sun dissociate H2O; H2 escapes, O is stripped
If Earth Moved to Venus’ Orbit Today
Shaping Planetary Surfaces
Four major geological processes that shape planetary surfaces:
- impact cratering: excavation of surface by asteroids or comets striking
the planet
- volcanism: eruption of lava from interior
- tectonics: disruption of lithosphere by internal stresses
- erosion: wearing down by wind, water, ice
Terrestrial Planet Surfaces
Comparison of Planetary Surfaces
Mercury & the Moon
- heavily cratered {scars from the heavy bombardment}
- some volcanic plains
Venus
- volcanoes and bizarre bulges
Mars
- volcanoes and canyons
- apparently dry riverbeds {evidence for running water?}
Earth
- all of the above plus liquid water and life
Inside the Terrestrial Worlds
After they formed, the molten planets differentiated into three zones:
- core - made of metals
- mantle - made of dense rock
- crust - made of less dense rock
Most of Earth’s interior is rock - only narrow region of upper mantle is molten rock - where lava comes from
Interior layers also categorized by strength of rock which depends on composition, temperature, and
surrounding pressure. Weaker rock can slowly deform and flow over millions of years. Why asteroids are
irregularly shaped - weak gravity unable to overcome rigidity of rock. Gravity of larger world can overcome
strength of solid rock, shaping it into a sphere - will shape anything over about 500 km in diameter into a
sphere in about 1 billion years
Lithosphere - the rigid, outer layer of crust and part of the mantle which does not deform easily - “floats” on
softer rock beneath.
Inside the Terrestrial Worlds
Comparison of Terrestrial World Interiors
active geologyEarth and Venus(?) still
have molten cores
inactive geology - the
cores of Mercury, mars
and the Moon have long
since cooled and
solidified
Impact Cratering
objects hit planet at 10 – 70
km/s (30,000 - 250,000 km/hr)
- solid rock is vaporized
- a crater about 10 times the
size of the object and one to
two times as deep is
excavated
Impact Cratering
matter is ejected in all directions
- craters are circular
- large craters have a central peak like the way water rebounds in the
center when you drop a pebble in it
Production of a Crater Animation
Counting Craters to find Surface Age
Lunar highlands - crowded areas of cratering - rocks date to 4.4 billion years
Lunar maria - huge impact basins filled in by lava flow - relatively few craters - rocks date to 3 - 3.9 billion years
Heavy bombardment must have subsided very early in solar system history
Cratering rate decreased as Solar Systems aged.
The older the surface, the more craters are present.
History of Cratering Animation