1. MARS

Viking orbiters and landers (1976)

Once liquid water was abundant!

billions of years ago (4 Gyr)

Where did it go?

Evaporated? (Mars is less massive than Earth)

Absorbed? (still there!?)

Temperature: cold!

seldom above freezing, usually much colder

Polar 'ice' caps of mostly frozen C0₂, some water ice

Impact craters (meteorites) - 3-4 Gyr old

Volcanos -- largest in the solar system (Olympus Mons),

2.5 times higher than the largest on Earth (Mauna Loa, Hawaii)

Canyons

- largest canyons due to crust fractures

- also due to liquid erosion

No plate tectonics like on Earth

What makes it red?

High iron content, and also oxygen rust!

2. VENUS

Magellan spacecraft (1989) - radar mapping

Venera landers (1970's and 1980's)

several made it to the surface for 1 hour before melting!

Pioneer orbiter and probes (1978)

Nearly same size and mass as Earth.

- but no twin otherwise!

Extremely dense atmosphere

- optically opaque

- but transparent at longer wavelengths

- hence radar mapping

243 Earth-day rotation period

Atmosphere rotates in 6 days!

Volcanos - same size as Earth

Impact craters due to meteorite impacts < 1 Gyr (young!)

Run-away greenhouse effect due to an abundance of CO₂ in atmosphere

Temperature: HOT! enough to melt lead.

No plate tectonics

3. MERCURY

Mariner 9 and 10 (1973-1974)

Estimated formation: 4.6 Gyr ago

Violent 'early' cratering - early in age of solar system

2nd era of cratering - mostly smaller craters

No plate tectonics

No volcanos

Similarities and differences with Moon:

- craters, craters, craters!

- no atmosphere

- no lava maria as seen on Moon

- crater walls on Mercury not as high as on Moon due to molten back-fill during first bombardment

Because of no weathering nor volcanic activity, Mercury is 'pristine' and gives us a fossil record of:

(i) early solar system

(ii) solar system bombardment history

Temperature extremes: (greatest of all planets)

700 K towards Sun

100 K away from Sun

Why such extremes?

- proximity to sun (hot)

- orbital period (59 days -- nights are long!)

- lack of atmosphere (nights are cold!)

4. Planetary Evolution

The solar system is constantly changing

Example 1: Earth's spin rate ...

... slowing down due to gravitational torque from moon

Why: Lag of tidal bulge behind Earth-Moon line, due to friction, produces 'handles' which allows Moon's gravity to 'torque down' the Earth's spin.

Example 2: Cratering

Cratering used as dating.

Accurate?

What about rate of cratering?

What about 'weathering' or volcanic activity?

What are the other hidden assumptions?

sizes (why does big=old?)

back-fill from molten material (why old?)

Moon vs. Mercury - crater differences

Northern vs. Southern Martian hemispheres

Northern surface age estimated at 3 Gyr

Southern surface age estimated at 4 Gyr

Example 3: Atmospheres

Light elements 'boiled' off early from terrestrial planets' atmospheres. At formation they were most common gases.

Why?

- Recall helium -- a light element -- rises!

- Sun made mostly of hydrogen and helium.

Role of CO as the primary green-house gas:

- CO traps heat (near-infrared wavelengths)

Role of liquid water in 'trapping' CO

Mars:

- cool temperatures allows for abundant liquid water, efficient CO trapping further cooling

- low mass allows for more of atmosphere to escape

Venus:

- initial hot temperatures prevent much liquid water

from forming; CO isn't trapped

- temperature goes up; less water; system out of control green-house effect

Earth: just right!

Just how close a call? Maybe a few tenths of an A.U.!

But there are other differences too:

plate tectonics, composition, bombardment history, tilt, spin rate

(a) Colder temperatures makes it easier for volcanos to grow larger.

(b) Mars is 40% less massive, and so the gravitational force is also less than the gravitational force on Earth.

(c) The thinner Martian atmosphere provides less resistance for volcanos to grow

(d) There has never been any rain fall on Mars to wear away at the structures.

(e) Mars spins faster.

(a) The cloud cover on Venus would remain the same.

(b) Venus would have a hotter atmosphere and surface.

(c) Mars current conditions would be little changed.

(d) Mercury would still have some atmosphere.

(e) Earth's temperature would be higher.