Physics 101 - Astronomy - Spring 2019

Class notes for day 12, Feb. 26, 2019

Ch. 10: Earthlike Planets: Venus and Mars

Some of the key facts about the rotation of the terrestrial planets are:

Mercury’s rotation is tidally locked to 2/3 of an orbit.

Venus rotates very slowly backwards (clockwise as seen from above the north pole) compared to other planets (this is also called retrograde rotation).

Mercury and Venus both have almost no axial tilt, Earth and Mars both have similar axial tilts (which gives them seasons) and rotation rates.

The Martian "sol" is about 40 minutes longer than an Earth day.

The atmosphere of a planet has a big influence on the temperature.

Mercury has no atmosphere, just like our moon. Because there is no atmosphere to trap heat, the night side of Mercury gets very cold (100K or -280^oF). The day side of Mercury gets up to 700K (about 800^oF). This is the biggest variation of temperatures (from day to night) of any planet in the Solar System.

The atmosphere of Venus is made up of carbon dioxide, with clouds of sulfuric acid. At the surface of the planet, the atmosphere is some 90 times denser than Earth’s. The Greenhouse Effect causes the surface temperature of Venus to be close to 730K (over 850^oF) day or night. This is interesting, since Venus rotates so slowly. The night side of Venus doesn't cool off since the atmosphere is so thick it blocks the infrared radiation from escaping into space. Also, very strong winds distribute heat from the day side to the night side. This makes Venus even hotter than Mercury.

Mars has a very thin atmosphere (less than 1% of Earth’s) of mainly carbon dioxide. The surface temperature is around 50K lower than Earth’s. It only gets above the melting point of water ice for a period around noon, in the summer, near the equator. Most of the rest of the surface will be below freezing all of the time.

Venus is about the size of Earth. Because of Venus’s dense cloud cover, most of what we know about Venus’s surface and rotation comes from using radar. The radar pictures from orbit show a variety of unusual volcanoes and similar structures (see the book or my slides). Only two spacecraft have landed on Venus, but each survived for only a short time. Your book shows a picture of volcanic rocks, indicating ongoing volcanic activity.

Recall that we can learn something about the core of the Earth because we can have sensitive equipment (seismographs) which record the waves that travel through the Earth from distant earthquakes. These are refracted as they traverse the boundaries between the core and mantle, etc. and we can work backwards to deduce the internal structure of the Earth. We have not put similar types of equipment on other planets, so we have no direct evidence of the structure of their interiors.

Planetary interiors have not been studied directly, but we can make estimates based on density and magnetic measurements.

Mercury’s mantle is solid (not semi-molten like the Earth’s) and its iron core also may be solid as well.
Not much is known about the interior of Venus – but it is thought to be like a young Earth.
Mars is mostly solid and no longer geologically active in any way.

Magnetic fields give some clues about the interiors.

Mercury has a very weak magnetic field, probably due to its large iron core that solidified into a large magnet.

Venus has no measurable magnetic field, probably due to its very slow rotation rate.

Mars has a magnetic field weaker than Mercury, meaning that its core is either not liquid or not metallic.

The Earth’s magnetic field is formed because of the Earth’s rotation and because the Earth has a molten core.

There is no Lunar magnetic field!

So, among the terrestrial planets, only Earth has a significant magnetic field, which helps shield our atmosphere from the effects of the solar wind.

Mars

Mars’ orbital period is 1.88 Earth years, 687 Earth days or 669 Sols. (So about 2 years)
One solar day on Mars is called a Sol and is 24h 39m 35s long. (So 40 minutes longer than an Earth day)
Aphelion - 1.6660 AU - 249.2 million km
Perihelion - 1.3814 AU - 206.7 million km
So the orbit is more eccentric than Earth’s (e=0.0934 for Mars, and e=0.0167 for Earth)

Major geological features on Mars:

Plains, vast flat areas covered with sand and dust
Volcanoes, including one of the tallest in the solar system. They can be very tall due to the low gravity.
A huge canyon, Valles Marineris, much larger than our Grand Canyon in Arizona
Ice caps, containing solid carbon dioxide and water ice. The CO₂ partially evaporates in summer and so the pressure in the Martian atmosphere varies seasonally.
A few large craters and many smaller craters.
Almost the entire northern hemisphere is lower than the southern highlands and once contained an ocean.

I showed some images to reinforce the conclusions that there is water on Mars, mostly in the form of ice below the surface, and perhaps some liquid further below the surface.