Transcript The Milky Way

Lecture 14
Venus, Earth and Mars
Announcements
•Telescope Observing lab following class
•Maps are available at the front of the classroom
•You may want to stop for a snack on the way, we
probably won’t do a lot of observing before about
7:30 due to sunlight (although we may look at the
Sun if we get set up early enough).
• If weather (ie, clouds) prevents us from seeing
enough tonight, we will likely try again later in the
semester.
Comparative Planetology
•Comparative Planetology is the study of planets as
groups, comparing their similarities and differences. We
will take this approach to studying the solar system,
starting with the planets most similar to our own.
•We Will compare several features of these planets:
• Interiors
• Surfaces
• Atmospheres
• Magnetospheres
• Moons
The Early History
The terrestrial planets formed 4.6
billion years ago from the inner
solar nebula.
Four main stages of evolution:
Two sources of heat in planet’s interior:
• Potential energy of infalling material
• Decay of radioactive material
Most traces of bombardment
(impact craters) now destroyed
on Earth by later geological
activity
Earth’s Interior
Direct exploration of Earth’s interior (e.g. drilling) is impossible.
Earth’s interior can be explored through seismology:
earthquakes produce seismic waves.
Two types of seismic waves:
Pressure waves:
Shear waves:
Particles
vibrate back
and forth
Particles
vibrate up
and down
Seismology
Seismic waves do not
travel through Earth in
straight lines or at
constant speed.
They are bent by or
bounce off transitions
between different
materials or different
densities or
temperatures.
Such information can
be analyzed to infer
the structure of
Earth’s interior.
Venus and Mars
Two most similar planets to Earth:
• Similar in size and mass
• Same part of the solar system
• Atmosphere
• Similar interior structure
Yet, no life possible on either one of them.
Interior
Basic structure:
Solid crust
Solid mantle
Liquid core
Solid inner core
interior gets hotter towards the center.
Earth’s core is as hot as the sun’s surface; metals are liquid.
Melting point
increases with
increasing pressure
Melting point =
temperature at which an towards the center
element melts (transition
from solid to liquid)
=> Inner core
becomes solid
The Rotation of Venus
• Almost all planets rotate
counterclockwise, i.e. in the
same sense as orbital motion.
• Exceptions: Venus, Uranus
and Pluto
• Venus rotates clockwise,
with period slightly longer
than orbital period.
Possible reasons:
• Off-center collision with
massive protoplanet
• Tidal forces of the sun on molten core
Mars
• Diameter ≈ 1/2 Earth’s
diameter
• Axis tilted against
orbital plane by 25o,
similar to Earth’s
inclination (23.5o)
• Seasons similar to
Earth  Growth and
shrinking of polar ice
cap
• Crust not broken into
tectonic plates
• Volcanic activity
(including highest
volcano in the solar
system)
• Very thin
atmosphere, mostly
CO2
• Rotation period
= 24 h, 40 min.
The Active Earth
Earth’s surface
geology is much
more dynamic than
Venus and Mars.
About 2/3 of Earth’s
surface is covered
by water.
Mountains are
relatively rapidly
eroded away by the
forces of water.
Tectonic Plates
Earth’s crust is composed of several distinct tectonic plates, which
are in constant motion with respect to each other  Plate tectonics
Evidence for plate tectonics can
be found on the ocean floor
… and in geologically active
regions all around the Pacific
Plate Tectonics
Tectonic plates move with respect to each other.
Where plates move toward
each other, plates can be
pushed upward and
downward  formation of
mountain ranges, some with
volcanic activity, earthquakes
Where plates move away
from each other, molten
lava can rise up from
below  volcanic activity
Earth’s Tectonic History
History of Geological Activity
Surface formations visible today have emerged
only very recently compared to the age of Earth.
The Surface of Venus
Early radar images already revealed mountains, plains, craters.
More details from orbiting and landing spacecraft:
Venera 13 photograph of surface of Venus:
Colors modified
by clouds in
Venus’s
atmosphere
After correction
for atmospheric
color effect:
Radar Map of Venus’s Surface
Surface features
shown in
artificial colors
• Scattered
impact craters
• Volcanic
regions
• Smooth lava
flows
Lava Flows
Young, uneven lava flows (shown: Lava flow near
Flagstaff, AZ) show up as bright regions on radar
maps.
Surface Features on Venus
Smooth
lowlands
Highland Maxwell Montes
regions: are ~ 50 % higher
than Mt. Everest!
Craters on Venus
Nearly 1000 impact
craters on Venus’s
surface:
 Surface
not
very old.
No water on the
surface; thick,
dense atmosphere
 No
erosion
 Craters
appear
sharp and fresh
Volcanism on Earth
Volcanism on Earth is commonly
found along subduction zones
(e.g., Rocky Mountains).
This type of volcanism is not found on Venus or Mars.
Shield Volcanoes
Found above
hot spots:
Fluid magma
chamber, from
which lava erupts
repeatedly through
surface layers
above.
All volcanoes on Venus and Mars are shield volcanoes
Shield Volcanoes
Tectonic plates moving over hot spots producing
shield volcanoes  Chains of volcanoes
Example: The
Hawaiian Islands
Volcanism on Venus
Sapas Mons (radar image)
~ 400 km (250 miles)
2 lava-filled calderas
Lava flows
Volcanic Features on Venus
Baltis Vallis: 6800 km long
lava flow channel (longest
in the solar system!)
Some lava flows collapsed
after molten lava drained away
Aine
Corona
Coronae: Circular bulges formed by
volcanic activity
Pancake
Domes:
Associated
with volcanic
activity forming
coronae
Lakshmi Planum and Maxwell Mountains
Radar image
Wrinkled mountain formations indicate compression
and wrinkling, though there is no evidence of plate
tectonics on Venus.
Tales of Canals and Life on Mars
Early observers (Schiaparelli, Lowell) believed to see
canals on Mars
This, together with
growth/shrinking of
polar cap, sparked
imagination and sci-fi
tales of life on Mars.
We know today:
“canals” were optical
illusion; do not exist!
No evidence of life on
Mars.
The Geology of Mars
Giant volcanoes
Valleys
Impact craters
Reddish deserts of broken
rock, probably smashed by
meteorite impacts.
Vallis
Marineris
The Geology of Mars
Northern Lowlands: Free of craters; probably
re-surfaced a few billion years ago.
Possibly once
filled with water.
Southern Highlands: Heavily cratered; probably
2 – 3 billion years old.
Volcanism on Mars
Volcanoes on
Mars are shield
volcanoes.
Olympus Mons:
Highest and
largest volcano
in the solar
system.
Volcanism on Mars
Tharsis rise
(volcanic bulge):
Nearly as large as
the U.S.
Rises ~ 10 km
above mean
radius of Mars.
Rising magma has
repeatedly broken
through crust to
form volcanoes.
Hidden Water on Mars
No liquid water on the surface:
Would evaporate due to low pressure.
But evidence for liquid water in the past:
Outflow channels from sudden,
massive floods
Collapsed structures after
withdrawal of sub-surface water
Splash craters and valleys
resembling meandering river beds
Gullies, possibly from debris flows
Central channel in a valley
suggests long-term flowing water
Hidden Water on Mars
Gusev Crater and Ma’adim Vallis:
Giant lakes might have drained repeatedly
through the Ma’adim Vallis into the crater.
Ice in the Polar Cap
Polar cap contains
mostly CO2 ice,
but also water.
Multiple ice regions
separated by valleys
free of ice.
Boundaries of
polar caps
reveal multiple
layers of dust,
left behind by
repeated growth
and melting of
polar-cap
regions.
Evidence for Water on Mars
Galle,
the “happy face crater”
Meteorite ALH84001:
Identified as ancient rock from Mars.
Large impacts may have
ejected rocks into space.
Some minerals in this meteorite were
deposited in water  Martian crust must
have been richer in water than it is
today.
Atmospheres
Terrestrial planets had primeval atmospheres from
remaining gasses captured during formation
Atmospheric
composition
severely
altered (
secondary
atmosphere)
through a
combination of
two processes:
1) Outgassing: Release of
gasses bound in compounds in
the interior through volcanic
activity
2) Later bombardment with icy
meteoroids and comets
The Structure of Earth’s Atmosphere
Composition of Earth’s
atmosphere is further
influenced by:
• Chemical reactions
in the oceans,
• Energetic radiation
from space (in
particular, UV)
The ozone
layer is
essential for life
on Earth since
it protects the
atmosphere
from UV
radiation
• Presence of life on Earth
The temperature of the atmosphere depends critically on its
albedo = percentage of sun light that it reflects back into space
Depends on many factors, e.g., abundance of water vapor in
the atmosphere
Human Effects on Earth’s Atmosphere
1) The Greenhouse Effect
Earth’s surface is heated by the
sun’s radiation.
Heat energy is re-radiated from
Earth’s surface as infrared radiation.
CO2, but also other gases in the
atmosphere, absorb infrared light
 Heat is trapped in the
atmosphere.
This is the Greenhouse Effect.
The Greenhouse Effect occurs naturally
and is essential to maintain a
comfortable temperature on Earth,
but human activity, in particular
CO2 emissions from cars and
industrial plants, is drastically
increasing the concentration of
greenhouse gases.
Global Warming
• Human activity (CO2 emissions + deforestation) is
drastically increasing the concentration of
greenhouse gases.
• As a consequence, beyond any reasonable doubt,
the average temperature on Earth is increasing.
• This is called Global Warming
• Leads to melting of glaciers and polar ice caps
( rising sea water levels) and global climate
changes, which could ultimately make Earth
unfit for human life!
Human Effects on the Atmosphere
2) Destruction of the
Ozone Layer
Ozone (= O3) absorbs UV radiation,
(which has damaging effects on
human and animal tissue).
Chlorofluorocarbons (CFCs) (used,
e.g., in industrial processes,
refrigeration and air conditioning)
destroy Ozone.
Destruction of the ozone
layer as a consequence of
human activity is proven
(e.g., growing ozone hole
above the Antarctic);
The Atmosphere of Venus
UV
UV image
image
4 thick cloud layers ( surface
invisible to us from Earth).
Very stable circulation patterns with
high-speed winds (up to 240 km/h)
Extremely inhospitable:
96 % carbon dioxide (CO2)
Very efficient “greenhouse”!
3.5 % nitrogen (N2)
Rest: water (H2O), hydrochloric
Extremely high surface
acid (HCl), hydrofluoric acid (HF)
temperature up to 745 K (= 880 oF)
The Atmosphere of Mars
Very thin: Only 1% of pressure on Earth’s surface
95 % CO2
Even thin Martian
atmosphere
evident through
haze and clouds
covering the
planet
Occasionally:
Strong dust storms
that can enshroud
the entire planet.
The Atmosphere of Mars
Most of the Oxygen bound in oxides in rocks
 Reddish
color of the surface
History of Mars’s Atmosphere
Atmosphere probably
initially produced
through outgassing.
Loss of gasses from a
planet’s atmosphere:
Compare typical velocity
of gas molecules to
escape velocity
Gas molecule
velocity greater than
escape velocity 
gasses escape into
Mars has lost all lighter gasses;
space.
retained only heavier gasses (CO ).
2
Earth’s Magnetic Field
• Earth’s core consists
mostly of iron +
nickel: high electrical
conductivity
• Convective motions
and rotation of the
core generate a
dipole magnetic field
The Role of Earth’s Magnetic Field
Earth’s magnetic field protects Earth from high-energy
particles coming from the sun (solar wind).
Surface of first
interaction of solar
wind with Earth’s
magnetic field =
Bow shock
Region where
Earth’s magnetic
field dominates =
magnetosphere
Some high-energy particles leak through the magnetic field and produce
a belt of high-energy particles around Earth: Van Allen belts
The Aurora (Polar Light)
As high-energy particles leak into the lower magnetosphere,
they excite molecules near the Earth’s magnetic poles,
causing the aurora
A History of Venus
Complicated history; still poorly understood.
Very similar to Earth in mass, size, composition, density,
but no magnetic field  Core solid?
Heat transport from core mainly through magma flows
close to the surface ( coronae, pancake domes, etc.)
Solar Wind Interaction
Solar wind interacts directly with the atmosphere, forming a
bow shock and a long ion tail.
CO2 produced
during outgassing
remained in
atmosphere (on
Earth: dissolved in
water).
Any water present
on the surface
rapidly evaporated
→ feedback
through
enhancement of
greenhouse effect
The Moons of Mars
Two small moons:
Phobos and
Deimos.
Too small to pull
themselves into
spherical shape.
Typical of small,
rocky bodies: Dark
grey, low density.
Phobos
Very close to Mars; orbits around
Mars faster than Mars’ rotation.
Probably captured from outer
asteroid belt.
Deimos
Earth’s Moon
Earth has an unusually large moon… details next time.
For Next Time
We will discuss the Moon and Mercury
Read Units 37 and 38