Mercury

Closest planet to sun
 Surface similar to Moon



heavily cratered
no atmosphere
Only one spacecraft flyby


Mariner 10 flew by 3 times
in 1974-75
New mission Messenger
is in planning stages
Mercury
Mass = 0.055 MEarth
Radius = 0.38 REarth
Surface Temp: -300 - +800 C
Orbital period = 88 days
Rotation period = 59 days
(Exactly 2/3 of Mercury’s
year!)
Mercury - Structure

Smallest of planets by
mass and size



Density 5.4 g/cm3



Mass 1/18 of Earth’s mass
Radius less than half of
Earth’s
denser than Earth’s mantle
quite different from Moon
Dense iron-nickel core


60% of total mass
almost size of Moon!
Mercury - Surface



Moonlike: Surface craters, no
atmosphere
No evidence of tectonic activity
Scarps (cliffs) in middle of
craters

crust shrank and cracked
(after craters formed)
Question
 How
do we determine the rotation rate of
a planet?
Determining rotation rate of a planet
•
Use reflected radio waves to determine
line of sight doppler shifts.
We always see the same face of the Moon.
So period of orbit = period of rotation
Why?
Tidal Locking
If orbit period faster than spin
period, bulge moves around
surface of Moon => friction
Moon
Top view of Moon
orbiting Earth
Earth
What does this have to do with
Mercury?
Mercury’s Rotation
•
Highly eccentric orbit => Mercury’s speed
changes as it orbits the sun => no synchronous
rotation
•
Next best thing - presents same side to sun
every other time around.
Earth-Like Planets:
Venus and Mars
Chapter 7
View from Earth
 Venus

shows phases
otherwise featureless
• Obscured by clouds
 Mars



appears red
due to iron oxides (rust)
polar ice caps
surface markings
• Seasonal changes
Highlights

Venus:


seems a “twin” of Earth (but not!)
massive atmosphere
• large greenhouse effect due to high CO2
• surface temp about 750 K

Mars:

quite cold
• once much warmer

very thin atmosphere
• once much thicker

had flowing water in past
• possibility of life?
Radar Map of Venus
Venus: General Properties
 Mass

82% of Earth’s mass
 Radius

about same as Earth
 Density

(5.3 g/cm3)
about same as Earth
Surface of Venus

Problem: how to see
through cloud cover?
 Spacecraft exploration

Venera 7 (USSR)
• landed on surface (1970)
• lasted 23 minutes!

Magellan Orbiter (USA)
• use radar imaging (from orbit)
• mapped surface at 100m
resolution
Surface of Venus
 Surface



features
produced by volcanic and
tectonic activity
but no plate motion
75% low lava plains
• produced like lunar maria
• very “young” surface (few
craters)

25% mountains/mountain
ranges
Craters on Venus

Few small craters


small objects burn up in
dense atmosphere
Use large (>30 km) craters
to estimate age

lava plains 500-600 million yrs
• vast geologic activity then
• not much since
Atmospheric Conditions

Extreme atmospheric
pressure / density


100x greater than
Earth
Very high surface
temps


around 750 K
little day/night variation
• due to thick atmosphere

Generally hot and dry


no water
Occasional “acid rain”
Atmosphere of Venus

Layers of sulfuric acid clouds


Composition




30-60 km above surface
96% carbon dioxide
3% nitrogen
remove CO2, then atmos. like Earth’s
Runaway greenhouse effect




Greenhouse gasses -> high temps
water evaporates
carbon dioxide CO2 released from rocks
increases greenhouse
Venus
 About
how far from the Sun (in AU) is
Venus?
 How
long is its orbital period (Venutian
year)?
 How
long is its rotational period
(Venutian day)?
Venus
Mass = 0.82 MEarth
Radius = 0.95 REarth
Average distance from Sun = 0.72 AU
Orbital period = 225 days
Rotation period = 243 days (longer
than orbital period, and retrograde!)
Anomalous rotation of Venus
•
•
•
Extremely slow - Venutian day longer
than Venutian year!
Retrograde - Sun rises in the west and
sets in the east!
Most likely due to a collision during solar
system formation
Venus
•
Thick clouds prevent
viewing of surface. (UV
Image)
•
High temperatures and
pressures, acidic gases
•
Led to much speculation.
•
How did we get info
"Radar Echo" technique measures altitude
space probe
time for signal to return tells you the
altitude of surface feature.
Planet Surface
Venera 14 photo of
surface. Lander
destroyed after
about an hour!
Venus' Atmosphere
- Hot at surface - 750 K! (Room temp. on Earth about 300K)
- Why so hot?
Runaway Greenhouse Effect
1) Water and CO2 evaporate from oceans into atmosphere.
2) Greenhouse effect more efficient.
3) Temperature rises.
4) More evaporation (back to #1).
=> complete evaporation of oceans. Thick atmospheric blanket.
Atmospheric Comparison
•
How does Venus lose its interior
heat?
We don’t know!
No plate tectonics => no interior convection
• Possible volcanic activity, but …
•
•
•
Distribution of craters on surface is uniform
• Surface of Venus is same age everywhere: 500 Million
yrs. old
Cataclysmic resurfacing?
• Geologically dead rest of the time
Mars: General Properties

radius


density 3.9 g/cm3




less than Earth; more than Moon
mostly silicates
possible metal core
Rotation


about 1/2 of Earth’s
period 24 hours, 37 min (like Earth)
tilt of axis about 25º; orbital period 1.88 years


seasons similar to Earth’s
duration ~ 6 months (instead of 3)
Surface Conditions

Temperatures

Summer:
•
•
•
•

Day 240 K (-33 C)
Night 190 K (-83 C)
Coldest 173 K (-100 C)
Water frost deposits
Surface winds mostly
moderate

but giant dust storms can occur
Mars Atmosphere & Climate

Composition:
95% carbon dioxide (CO2)

3% nitrogen (N2)
similar to Venus!


Atmospheric pressure




100x smaller than Earth’s
Surface Pressure 0.006 that of Earth's
atmosphere (thin air!)
equiv. to 30 km above Earth surf.
Clouds



dust clouds
water ice clouds
dry ice (CO2) crystals
Polar Caps

At both N and S poles


change with seasons
seasonal ice caps
• composed of dry ice
(frozen CO2)

permanent ice caps
• composed of water ice
Seasonal Changes
Channels and Flood Plains

Evidence liquid water existed
on Mars

highlands runoff channels
• from ancient rainstorms?
• older than 3.9 million yrs

outflow channels
• much larger
• carved by huge floods
Martian
Canals

Schiaparelli (1877)
reported canale on
Mars



Led to suggestion of intelligent
Martian civilization



Italian = “channels”
English = “canals”
War of the Worlds (H.G. Wells)
Percival Lowell primary American
proponent
Eventually shown to be optical
illusion
Climate Change

Evidence indicates




Mars had liquid water (warmer)
much denser atmosphere
What happened?
“Runaway refrigerator effect”

atmosphere began to escape into space (low gravity)
•
•
•
•
•

less dense atmosphere -> less greenhouse
surface cools
water freezes -> less greenhouse
more cooling
carbon dioxide freezes -> less greenhouse
happened over 3 billion yrs ago
Mars
Mass = 0.11 MEarth
Radius = 0.53 REarth
Average distance from Sun = 1.52 AU
Rotation Period = 24.6 hours
Orbital Period = 687 days
Olympus Mons
Largest known volcano in Solar System – 3X the height of
Everest!
Question
 What
is the main reason that many
scientists think Mars may have once
harbored life?
Evidence for Past Surface Water
"runoff channels" or
dry rivers
"outflow channels"
standing water erosion in craters?
teardrop "islands" in
Mars' Moons Phobos and Deimos
Deimos: 16 x 10 km
Phobos: 28 x 20 km
Properties similar to asteroids. They are probably asteroids
captured into orbit by Mars' gravity.