1. Goals
Explain how Mercury's rotation has been influenced by its orbit around the Sun.
Describe how the atmospheres of Venus and Mars differ from one another and from Earth's.
Compare the surface of Mercury with that of the Moon and the surfaces of Venus and Mars with that of Earth.
Describe how we know that Mars once had running water and a thick atmosphere.
Discuss the similarities and differences in the geological histories of the four terrestrial planets.
Explain why the atmospheres of Venus, Mars, and Earth are now so different from one another.

3. Rotation Rates
Ground based telescopes cannot see much details because of its small size and large distance.
So astronomers have used the Doppler effect with radar pulses to measure the rotation rate.
They find that one Mercury day lasts two Mercury years.
This odd synchronous rate is due to Mercury's eccentric orbit. It presents the same face to the Sun every other time around.

4. After the Sun and Moon, Venus is the brightest object in the sky
After the Sun and Moon, Venus is the brightest object in the sky. The same clouds that make it bright also prevent us from seeing surface detail and measuring its rotation rate.
Once again Doppler radar revealed the answer, a 243-day rotation rate and in the wrong direction! Its rotation axis is almost perpendicular to its orbital plane.
The only explanations for this is that Venus had a close encounter of the planetary kind early in its history.

5. Mars, on the other hand, has very little atmosphere and we can visually measure a rotation rate of 24.6 hours with an inclination to its orbit of 25.2o. Very similar to earth!
With its polar ice caps and dark regions near the equator, we can see seasonal changes as the planets orbits the Sun

7. Atmospheres (Mercury)
Astronomers have never observed, either from earth or from spacecraft, any atmosphere on Mercury.
Because Mercury is so close to the Sun it has a high temperature (700K) which, combined with its low gravity (1/3 Earth), means Mercury has troubles holding onto an atmosphere.
Because it has no atmosphere it has no protection from the Sun and its 600K temperature range is the greatest for any planet in the solar system.

8. Atmospheres (Venus)
While the cloud tops are a comfortable 240K, radio observations of the surface reveal temperatures of 600K!
Venus has the most massive atmosphere of all the terrestrial planets, 90 times the pressure of Earth’s.
Venus’ atmosphere is made of 96.5% CO2 and 3.5% nitrogen.
Given its similar size and distance from the Sun as Earth, it must have started out similar to Earth.
Astronomers believe a run away greenhouse effect caused the high temperatures which destroyed the water and the oxygen combined with sulfur to form sulfuric acid clouds.

9. Atmospheres (Mars)
The Martian atmosphere is only 1/150 that of Earth’s.
It contains 95.3% CO2, 2.7 % nitrogen, 1.6% argon plus small amounts of H2O, O2 , and CO
Average temperature on Mars is 50K cooler than on Earth.

10. Surfaces (Mercury)
The best comparison of the surface of Mercury is the Moon
Heavily crated, no lava flows, or evidence of clouds, water, or dust storms
Craters on Mercury tend to have double rings.

11. Mercury is geologically dead.
As the planet cooled, it shrake and cracked producing large scarps.
Evidently Mercury small size allowed to to cool quickly and its thick crust prevented any plate tectonic formation
This impact was so large that it disturbed the back side of Mercury producing the “weird terrain”.

12. Surfaces (Venus)
Because the thick clouds abscise the surface, most information has been obtained by radar mapping by US spacecraft and by landers from the Soviets.

13. Surfaces (Venus)
Volcanism appears to resurface Venus every few hundred million years. Thus we see no craters.
Instead we see lots of volcanic formations.
The most common are Shield volcanoes much like Hawaii.

14. Surfaces (Mars)
From the Earth and space Mars shows many interesting surface features. Polar ice caps which grow and shrink with the seasons, and dark patches which also appear to wax and wane with the seasons.
So convinced were some astronomers that life existed on Mars that observatories were built and detailed maps produced to study the canals.

15. Percival Lowell built an observatory in Flagstaff, Arizona where he mapped out several canals.

16. Surfaces (Mars)
(a) Mars's northern hemisphere consists of rolling volcanic plains
(false color image.)
(b) The southern Martian highlands are heavily cratered
(true color). Both photographs show roughly the same scale, nearly 1000 km
across.

17. Most of our information about the surface comes the Viking landers and Mars Pathfinder (Sojourner)
They reveal a dry, rocky desert plain.
The red color is due to iron oxide (rust) on the surface.

18. Surfaces (Mars)
While Mars may now be geologically dead, in the past volcanoes played an important role
Olympus Mons is the largest volcano. At 700 km across and 25 km tall it is about three times larger than similar features on Earth.
There is evidence that water once flowed on Mars.

19. Surfaces (Mars)
Just beneath the surface of Mars may be extensive permafrost. Evidence includes the mud-like response to meteor impacts.

20. Geology (Mercury)
Mercury’s magnetic field is 100 times smaller that Earth’s
Still, we were surprised because Mercury does not rotate very fast (one of the necessary conditions). On the other hand, its density implies lots of iron/nickel in the core (the other condition).
Its core is much bigger (in a relative sense) than Earth’s
Perhaps the magnetic field is a left over frozen from when it was created.

21. Geology (Venus)
Has no detectable magnetic field. Since it is other wise very Earthlike this must be caused by the slow rotation rate.
We have no seismic data to examine the interior.
While Venus is geologically active it does not have any plate tectonics.
It may be that the high temperatures have prevented the crust from being solid enough to support plates.

22. Geology (Mars)
No Martian magnetic field has ever been detected.
Since Mars rotates as fast as the Earth, this must imply that Mars has very little molten iron/nickel core.
While Mars once had active volcanoes, because of its small size and lack of atmosphere, it has cooled to the point where it too has become dead.

23. Atmospheric Evolution of Venus, Earth, and Mars
Most planets start off with an initial or primary atmosphere which formed at the same time the planet did.
The atmospheres later evolve into a secondary atmospheres which may be caused by volcanoes, living systems, or changing temperatures.
The primary atmosphere is usually made up of light gasses like hydrogen, helium, methane, ammonia, and water vapor.

24. Earth’s Atmosphere
Most of the light gasses in Earth’s were lost into space because of its high temperatures and low gravity.
Earth’s secondary atmosphere came from volcanoes and perhaps comets.
Oxygen is so reactive that it must be replaced for it to be at its present concentration.
Green plants continually produce oxygen to keep the concentrations high.

25. Venus’ Atmosphere
Venus’ current atmosphere was caused by the run away greenhouse effect
Venus is too hot to maintain water and without green plants to produce oxygen, 99% of Venus is CO2.

26. Martian Atmosphere
While it appears that Mars once had large amounts of H2O probably produced by volcanoes, its temperature is too low to allow running water now.
Most of the CO2 was absorbed into the rocks and minerals with little left over to keep the planet warm.
Most of the water may be stored in permafrost just beneath the surface.