Download Terrestrial Planets

Reminders!
Website: http://starsarestellar.blogspot.com/
Lectures 1-6 are available for download as study aids.
Reading: You should have Chapters 1-5 read, and
Chapters 6 and 7 by the end of this weekend.
Homework: Homework #2 is due this Wednesday at the
BEGINNING of class.
Discussion: Next week, we will be reviewing for the
midterm.
Midterm!!!
The midterm is Thursday, June 11th in class. Don’t be late!
The test will cover Chapters 1-8.
It will consist of 40 to 50 questions and you will have until 12
o’clock to complete it.
The test is true-false and multiple choice. Make sure you have a
SCAN-TRON 882 form and a #2 pencil. I will NOT have these
available in class.
Bring your student ID.
The test is closed book, closed notes, and no calculators.
There will be a sheet of equations available.
A practice midterm is available. Take the practice test for 1.5
hours, and see how you do. If you have any questions, you can
ask about them in discussion next week.
Terrestrial Planets
Today’s Lecture:
• Solar System Formation (chapter 5, pages 105-108)
• Terrestrial Planets (chapter 6, pages 110-151)
Earth and Moon
Mercury, Venus, and Mars
The Solar System
Top view of the
solar system
• All the planets orbit
the Sun in the same
direction.
• Most of the planets
rotate in the same
direction too.
Sun
• And so does the Sun.
• Orbits are all nearly in
the same plane.
Side view is
a thin disk
Origin of the Solar System
“Nebular Hypothesis” (Kant, Laplace, 18th century)
• Before the Sun was born, there was a gravitationally
contracting, slowly spinning cloud of dust.
• As it contracts due to gravity, is spins up to conserve
angular momentum (figure skater effect)
• Sun forms in the center, and the outer parts make a
disk.
• Planets form out of the disk.
• Extra gas is blown away by the Sun’s winds.
Terrestrial Planets
• The inner planets: Mercury,
Venus, Earth, and Mars
• All have a smaller radius than
the Earth
• Not very massive (less than
Earth), but dense (>1 g/cm3,
which is the density of water)
• Close to the Sun and few
moons
• Rocky outer parts and iron
cores.
Earth: 3rd Rock from the Sun
• Upper mantle and crust float on a soft, churning layer (a
convecting layer)
• This causes continental drift (plate tectonics)
• Mountains, volcanoes, and earthquakes all occur at the plate
boundaries.
The Earth (cont.)
• Earth’s atmosphere: 79% N2, 20% O2 (so much oxygen is due to
life, photosynthesis).
• The atmosphere is very thin < 100 km
• Earth’s magnetic field looks like a giant bar magnet (dipole).
Molten interior sustains a field which reverses every ~3 x 105 yrs
Tides: Differential forces from the Moon (mostly)
• The force by the Moon on the Earth is greatest on the near side
and smallest on the far side.
• This causes two high tides at each of these points.
• There are TWO high tides per 24 hours!
The Sun and Moon and Tides
• What about the Sun? The Sun’s differential force is only 1/2 of
the Moon’s because the Sun is so far away.
Tides very high
S, E, M aligned
Tides not so high
S, E, M perpendicular
• In the past, the Moon rotated faster, but gradually lost its energy
due to tidal friction.
• Hence, the Moon’s rotation is now locked to its revolution: both
periods are the same (“synchronous rotation”)
• We always see the SAME SIDE OF THE MOON!
We see one side of our Moon
It’s orbit and rotation are in synch
Moon
Earth
Non-synchronous rotation
Moon
Earth
Synchronous rotation
The faces of the Moon
• The Moon is heavily
cratered due to impacts.
The most heavily cratered
parts are the oldest.
Maria
• Lava flows produced
maria (“seas”) which are
seen as dark regions
• Very little erosion on the
Moon: can get relative ages
of features (craters on
maria must be younger
than the maria)
Craters
The Moon’s Surface
• Derive the absolute ages of rocks (since last molten)
by radioactive dating (for example, Uranium) of Moon
rocks brought back by astronauts (Apollo 11-17; 19691972)
• Heavily cratered areas are 3.9-4.3 billion years old.
• Maria areas are 3.1-3.9 billion years old.
• There are few craters in the maria; thus, intense
bombardment of the Moon happened very early after
Solar System formation
Formation of the Moon
• The Earth-Moon system is almost like a “double
planet” - the Moon is quite large compared to other
Moons that we know about.
• The composition of the Moon is very similar (but not
exactly so) to the Earth.
• The Moon probably coalesced from debris ejected by
a collision of a Mars-sized object with the Eath, early
in the history of the Solar System.
Mercury: The Innermost Planet
• Difficult to study: always
near the Sun in our sky
Mercury
• Sets or rises within 1 to 2
hours of the Sun (near
horizon so images blurred)
• Rotation rate is known
from Doppler effect with
radar.
Sun
Venus
Earth
Mercury: The Innermost Planet
• Day and night cycle on Mercury
= 176 days (88 days of sunshine, then 88 days
of darkness!)
• Almost no atmosphere
Daytime T = 430 °C, night T = -170 °C!!!
• Mariner 10 flew past 3 times (1974/1975)
• Heavily cratered like the Moon
• No evidence for plate tectonics
• MESSENGER satellite is currently studying Mercury.
Found evidence for water in the atmosphere and a
liquid (molten) core.
Venus: The Evening
(or Morning) Star
• Easy to see in the evening
or early morning
• Shrouded in highly
reflective clouds
• NASA “Magellan”
spacecraft (1990-1993);
detailed radar maps. Saw
craters, volcanoes, and
large plains.
• Crust consists of only 1
thick plate.
Mercury
Sun
Venus
Earth
Venus (cont.)
• Thick atmosphere:
90 times the Earth’s surface pressure!
• 96% CO2 and less than 4% N2
Earth 79% N2 and 20% O2
Earth’s CO2 is trapped in rocks and oceans
• Temperature of surface is 480 degrees Celsius. It is the
hottest planet, due to the “greenhouse effect.”
• As CO2 increases on the Earth, we similarly get a
greenhouse effect, contributing to global warming
Venus: the hottest planet
atmosphere
blocks
infrared
Surface heats up due to visible light. The surface then
radiates in the infrared which is absorbed by the
atmosphere (clouds) ---> temperature increases!
Mars: The Red Planet
• Orange (reddish) due to
rust (iron oxide)
• 2 tiny, irregular moons
• Mars has polar ice caps
(mostly CO2) that grow and
shrink with the seasons
Mercury
Sun
• Thin atmosphere (1% of
Earth’s) 90% CO2.
• Windy: major dust storms
• T = -130 °C to 30 °C, but
generally cold
• No plate tectonics
Venus
Earth
Mars
Mars: Evidence for water
• One of the first major discoveries: ancient river
beds and flood plains! Water was probably
abundant long ago; now locked in permafrost
and polar caps.
• Several recent spacecraft have indirectly
detected water.
• “Spirit” and “Opportunity” rovers (2004):
additional evidence for the presence of liquid
water long ago on the Martian surface.
Mars: The Search for Life
• 1976: Viking lander: tests for life were negative
• 1996: Martain meteorite (found in Earth’s
Antartic) shows possible signs of primitive life
from 3.6 billion years ago
• Tube-like structures (tiny bacteria?) and several
lines of chemical evidence. No smoking gun
evidence that this was primitive life, but it is very
intriguing!
• The ultraviolet light hitting Mars’ surface may
breakdown any life on the surface, but may still
exist below surface or long ago.