Part 5: The Terrestrial Planets
John P. Pratt
Introduction to the Planets
What are the Planets?
- "Planet" comes from the Greek for "wandering," meaning that planets wander though the stars.
- Planets are orbiting the sun, as is the earth, and are the largest of such bodies.
- Planets are visible because they reflect the sun's light back to the earth.
- There are nine such planets:
Comparative Planetology is the systematic study of the similarities and differences of the planets.
The Inner Planets are closer to the sun than is the earth.
The Outer Planets are all the rest, which are farther from the sun than is the earth.
- They are Mercury and Venus.
- They are both evening and morning stars because they cannot be opposite the sun in the sky.
The Terrestrial Planets are Mercury, Venus, Earth and Mars.
- They all Mars through Pluto.
- They have retrograde motion as the earth passes them in orbit.
The Giant Planets are Jupiter, Saturn, Uranus, and Neptune.
- They are so-named because they are similar to the earth in size and composition.
- They are also the first four planets.
- They are so-named because they are much larger than the earth.
- They have a lot of hydrogen compounds.
Synodic vs. Sidereal Orbital Periods
- The sidereal orbital period is the usual period, measured relative to the stars.
- The synodic orbital period is relative to the sun.
- The period of the moon's phases is the synodic period because they are caused by the sun.
- The synodic period is more important for the inner planets because they are near the sun.
- The sidereal period is more important for the outer planets because they are seen with stars.
- Its diameter is 40% of that of the earth, being smaller than two moons in the
solar system (Ganymede & Titan).
- Its distance from the sun is 40% of that of the earth.
- It looks like a bright star, but is so near the sun it is still hard to see.
- It is so close to the sun that it cannot be seen well in any telescope.
- It was first seen well when the Mariner 10 had a close fly-by.
- It looks a lot like the moon, covered with many craters, probably from intense
Rotation and Revolution
- Its orbital period is 88 days or about 1/4 year.
- Mercury must pass the earth 3 times each year in order to orbits 4 times per year.
- Hence, it's synodic period is about 1/3 of a year (116 days).
- It rotates on its axis in exactly 2/3 of 88-day sidereal period.
- The surface temperature is about 100 K in the dark night to 500 K in the hot day.
- Mercury has about the same density as the earth (5.5 x water), so it probably has iron core.
- It is so small, it has essentially no atmosphere.
- It has a small ice cap right at the north pole, where sun never rises.
- It is almost the same diameter as the earth.
- It is usually far brighter than any star.
- There are great variations in brightness as it comes near the earth, becoming a narrrow crescent.
- Because of thick cloud cover, little was known about it before space probes.
Rotation and Revolution
- Venus orbital period is 8/13 of a year (225 days), that is, it completes 13 orbits in 8 years.
- That means it has 13 - 8 = 5 evening/morning star cycles (synodic periods) every 8 years.
- It has a retrograde rotation on its axis, once every 243 days.
- Its atmosphere was discovered 200 years ago when it could be seen when nearly in front of the sun.
- Its thick atmosphere is near all carbon dioxide (CO2).
- The greenhouse effect causes the atmosphere to be 750 K, hotter than Mercury.
- The atmospheric pressure is over 90 times that on earth.
- The yellowish-orange clouds are made of sulfuric acid (car battery acid!).
- The upper clouds have 200 mph winds.
- The surface has almost no wind, and is free from clouds.
- The surface has been mapped by radar.
- It is covered by geologically young lava flows which resurfaced the planet recently.
- The surface features are named for women.
- It is red because of the high iron oxide (rust) content of the rocks (like Bryce Canyon).
- It sometimes looks like a bright red star, but can vary a lot in brightness.
- Surface markings and the ice cap were visible in earth-based telescopes.
- It has seasons like the earth, which are about twice as long as earth's seasons.
- People thought they saw canals on the surface of Mars.
Rotation and Revolution
- The sidereal period of Mars is a little less than 2 years.
- Hence, the synodic period of Mars is a little more than 2 years.
- Its rotational period on its axis is almost exactly the same as the earth's (1 day).
- It is tipped on its axis about the same amount that the earth is.
- The sky is reddish due to blowing dust.
- The atmosphere, like Venus, is nearly all carbon dioxide (CO2).
- The atmospheric pressure is only 1% of that on earth.
- Temperatures ranges from -120° at night to -20° in the daytime.
- The ice cap is mostly water, covered with dry ice in the winter time.
- There are many dry river beds.
- The canals turned out to be windblown dust deposits.
- It also has many large impact craters.
- It has the largest volcanoes in the solar system.
- Olympus Mons is 78,000 feet tall.
- The base is as large as Missouri.
- Its top is as large as Los Angeles.
No Life on Mars
- No life of any kind has been found on Mars.
- Those who thought that life evolves easily from water were disappointed.
- There is no evidence that there has ever been life of Mars.
- Mars has two tiny moons discovered by A. Hall in 1877.
- They are named Phobos and Deimos (Fear and Terror) after the chariot horses of the god Mars.
- The are so small (20 & 10 miles) that they can be potato-shaped.