1. Terrestrial Planets

2. The Earth-Moon System Earth/Moon radius: ¼ Earth/Moon mass: 1/81
Earth-Moon distance:
384,000 km
Mass known from acceleration of bodies near the earth’s surface
Size known since Eratosthenes; modern measurements from satellites
Center must be very dense
Dist to moon from radar ranging; 2.56 sec for radar signal to return. Dist is about 10 x the circumference of the earth

3. Features of the Earth & Moon
Mass: Earth: 6  1024 kg Moon: 1/81 Earth’s
Radius: Earth: 6400 km Moon: 1/4 Earth’s ra
Density: Earth: 5500 kg/m Moon: 3300 kg/m3
5.5 times that of water
About 2 times that of a rock
Gravity: Earth: 9.8 m/s Moon: 1/6 Earth’s gravity
(about the same as in water)
Mass known from acceleration of bodies near the earth’s surface
Size known since Eratosthenes; modern measurements from satellites
Center must be very dense
Dist to moon from radar ranging; 2.56 sec for radar signal to return. Dist is about 10 x the circumference of the earth

4. Moon: Large-Scale Features
“Maria”
Dark areas resembling oceans
Plains of solidified lava
Part of the lunar mantle
About 3.2–3.9 billion years old
Highlands (“Terrae”)
Light-colored, resemble continents
The lunar crust
More than 4 billion years old
highlands: oldest parts of the moon
maria: seas of lava left from partial melting of the moon in its early days (melted by bombardment)
Mare Imbirum, ~700 mi across
craters: caused by impact of meteorites (maria are less cratered, hence younger)
ATMOSPHERE – none
Water -- water ice detected near poles by Clementine in 1996 (confirmed by Lunar Prospector in 1998)

5. The Moon – Far Side
Can be seen by satellites only

6. The Mountains of the Moon
Especially well visible near the terminator – the borderline between light and shadow

7. Structure of the Moon Also consists of crust, mantle and core
No hydrosphere, magnetosphere or atmosphere
Little seismic action
Determined by seismographs left by the Apollo astronauts. Moonquakes are very weak: there is little geological activity left in the moon.
Density fairly uniform, but chemically differentiated.
iron rich core, not as dense as earth's; possibly molten
crust is thicker than earth's (thicker on far side)
mantle is more rigid than earth's. plate tectonics requires thin crust and ductile mantle, so moon is inactive

8. Tides Daily fluctuations in the ocean levels
Two high and two low tides per day
A result of the difference in gravitational pull from one side of the Earth to the other
F = G M m / R2

9. Lunar Craters Old scars from meteoroid impacts Lots of them; all sizes
Copernicus ~ 90 km across
Reinhold ~ 40 km across
Also craters as small as 0.01 mm!
A planet or satellite with lots of craters has an old surface and little or no geological activity
Meteoroid impact the only source of surface erosion on the moon

10. Ages of the Earth and Moon
Determined by radioactive dating
Compare amount of radioactive material with amount of decay product
Useful isotopes:
Uranium-238 (half-life 4.5 billion years)
Uranium-235 (half-life 0.7 billion years)
For shorter time scales, Carbon-14 (5730 years)
Oldest surface rocks on Earth (Greenland, Labrador) about 3.9 billion years old
When rocks solidified
Lunar highlands: 4.1–4.4 billion years old
Rocks from lunar maria slightly younger, more recently melted
Meteorites: 4.5 billion years old
Date to origin of solar system
Very large number of decays/isotopes in lots of rocks; good agreement in the ages obtained

11. Mercury Small, bright but hard to see About the same size as the moon
Density about that of Earth
Day ~ 59 Earth days
Year ~ 88 Earth days
Never far from the sun – said that Copernicus never saw it!
Mass deduced from gravitational effects on other planets or, more recently, space probes
Most of what we know about surface, composition from probes (also true of other planets)
Large metallic core (iron-rich)
How do we know rotation? (next…)

12. Venus Bright, never very far from the sun
“Morning/Evening star”
Similar to Earth in size and density
Day ~ 243 Earth days (retrograde!)
Year ~ 225 Earth days
“Morning/evening star”
Hit by something to cause its unusual rotation?

13. Venus Very thick atmosphere, mostly CO2
Heavy cloud cover (sulfuric acid!)
About 90 times the pressure of Earth’s atmosphere
Very strong greenhouse effect, surface temperature about 750 K
No magnetic field
Little or no water; boiled off due to hi temp
To have a magnetic field, a planet probably must have a liquid metallic core that is being stirred by planetary rotation. Venus rotates too slowly.
No seismic data – little hard information on Venus’s interior
On Earth, most of the CO2 dissolved in the oceans or combined in surface rocks. If it were released we’d have about the same atmosphere as Venus
Question is really “Where is Earth’s CO2?” not “Why does Venus have so much?”
Also “Where is the H2O on Venus?” not “Why does Earth have so much?”
Importnt to know the right questions!

14. Surface Features Two large “continents”
Aphrodite Terra and Ishtar Terra
About 8% of the surface
Highest peaks on Aphrodite Terra rise about 14 km above the deepest surface depression
Comparable to Earth’s mountains
Ishtar (~Australia) – top center, Aphrodite (~Africa) right side along equator
Less differentiation between hi and low as on Earth

15. View from Russian probe Venera 14 (1975)
Venus - Touchdown
View from Russian probe Venera 14 (1975)

16. Hothouse Venus: 850 °F

17. Mars Fairly bright, generally not too hard to see Smaller than Earth
Density similar to that of the moon
Surface temperature 150–250 K
Day ~ 24.6 hours
Year ~ 2 Earth years
Thin atmosphere, mostly carbon dioxide
1/150 the pressure of Earth’s atmosphere
Tiny magnetic field, no magnetosphere
About the same axial tilt as the Earth (24 deg) – seasonal variations as on Earth (polar ice caps grow and shrink, e.g.)

18. Mars Northern Hemisphere basically huge volcanic plains
Similar to lunar maria
Valles Marineris – Martian “Grand Canyon”
4000 km long, up to 120 km across and 7 km deep
So large that it can be seen from Earth
As on the moon, maria material less cratered, younger (~3Gyr, compared to ~4Gyr for the south)

19. Martian Volcanoes Olympus Mons
Largest known volcano in the solar system
700 km across at base
Peak ~25 km high (almost 3 times as tall as Mt. Everest!)
Rhode Island could fit in the crater
Shield volcanoes, like on Venus
Often bluish-white clouds above them!
Lots of them, probably active in the not-too-distant past (100 Myrs ago?)
No plate tectonics evident

20. Martian Seasons: Icecaps & Dust Storms
Giant dust storms on Mars can engulf the entire planet and may last for a few months
Often occur when it’s summer in the southern hemisphere
The airborne gas is warmed by the Sun and it raises temperatures of upper Martian atmosphere while the planet’s surface is being chilled.

21. Martian Surface Iron gives the characteristic Mars color: rusty red!
View of Viking m rock Sojourner
Iron gives characteristic red color – rust

22. Water on Mars? Mars Louisiana Runoff channels Outflow Channels
Runoff channels in southern highlands, like river systems on Earth
Outflow channels appear in equatorial regions, probably the result of huge volumes of water flooding into the northern plains (which are at a lower elevation, ~ several km)
Possibly volcanic activity caused massive flash floods (after most of the water was frozen in permafrost)

23. Life on Mars? All an illusion! There are no canals…
Giovanni Schiaparelli (1877) – observed “canali” (channels) on Martian surface
Interpreted by Percival Lowell (and others) as irrigation canals – a sign of intelligent life
Lowell built a large observatory near Flagstaff, AZ
(Incidentally, this enabled C. Tombaugh to find Pluto in 1930)
Speculation became more and more fanciful
A desert world with a planet-wide irrigation system to carry water from the polar ice caps?
Lots of sci-fi, including H.G. Wells, Bradbury, …
All an illusion! There are no canals…

24. Viking Lander Experiments (1976)
Search for bacteria-like forms of life
Results inconclusive at best
In 2003 Beagle 2 (UK) will carry out a different set of tests