29:50 Modern Astronomy
Fall 1999
Lecture 37...November 29, 1999
Mars I

Announcement: New homework problem set on the Web.

Watch the skies! Watch the skies!
The Mars Polar Lander is scheduled to land on Mars, near the South Pole, on Friday of this week.
There will be a total lunar eclipse on January 20. The Moon will enter the Earth's shadow at 9:01 PM CDT, and totality will commence at 10:05. Mid-eclipse is 10:44.

General Comments on the Solar System Today we continue with the exploration of the Solar System. Most of what we know about the planets comes from the unmanned program of exploration over the past 30 - 40 years.

We can classify the major planets into Terrestrial Planets, that is Earth-like, and Jovian Planets, or Jupiter-like. The Terrestrial planets are Mercury, Venus, Earth, and Mars. The Jovian planets, are Jupiter, Saturn, Uranus, and Neptune.

All of the terrestrial planets lie within 1.52 astronomical units (the semi- major axis of Mars). The closest Jovian planet to the Sun is Jupiter, with a semimajor axis of 5.2 astronomical units.

The two classes of planets also differ in their sizes and masses.
``Famous Transparency'' This shows that the Terrestrial planets are all comparable in their sizes, which are much smaller than the sizes of the Jovian planets. There is a similar separation in the masses of the two classes of planets. The Earth is the most massive of the Terrestrial planets. Jupiter has a mass of 318 , and Saturn has 95 .

Mars

Over the next couple of lectures I will be discussing the planet Mars. For the last century Mars has attracted a great deal of interest because it has been closely associated with the interesting topic of life elsewhere in the universe. This association has been greatly strengthened in the past few years with hotly-debated evidence for ancient life on Mars.

One thing I want to emphasize is the enormous accomplishment of our modern observational view of Mars. To grasp what we have learned, go outside tonight and look to the south-southwest as soon as the sky becomes dark. You will see a medium-brightness object. That is Mars. Tonight it is 1.675 astronomical units from Earth. That is the closest it will be for over the next year. During all of the year 2000 it will be more distant. Its last closest approach or ``opposition'' was in April of this year when it was 0.578 astronomical units.

First let's discuss some of the salient physical facts about Mars.

• diameter: 6378 kilometers, 53 % that of Earth
• mass: kilograms, 11 % mass of Earth
• major axis of orbit 1.52 a.u.
• eccentricity of orbit 0.093

The greater distance of Mars from the Sun means that the intensity of sunlight (technically the solar constant) will be less than half that of the Earth. The greater eccentricity means that the difference between closest approach to the Sun and most distant is greater than for the Earth.

By the middle of the 19th century, the technology of telescope manufacture had developed to the point where high quality instruments were being manufactured. It is worth emphasizing that Mars is usually a difficult object to observe. Those who went to the observatory a couple of weeks ago saw the tiny disk of the planet Uranus. At the present time, the disk of Mars is only slightly (about 50 %) larger. Clearly, it is difficult to say much about a planet under those circumstances. Nonetheless, at times of closest approach between Mars and the Earth, the 19th century telescopes were sufficient to show a fair amount of detail on the surface. These observations showed a good deal of apparent similarity to the Earth. First, Mars has north and south polar caps, which grow and retreat with the seasons, just like the polar caps on Earth. Second these observations showed surface features, such as Syrtis Major. This feature was observable in a small telescope last April. These features changed in appearance with the seasons. The most natural explanation was that we were seeing seasonal changes in vegetation (this is not the correct explanation).

The astronomer Percival Lowell believed he saw evidence of ``canals'' on Mars, and thought that they demonstrated the presence of intelligent life and civilizations on the planet. These were still represented in the 1960 Grade-B movie, Mars, The Angry Red Planet (Men of Earth!!!).

A number of finding in the 1950's considerably reduced our estimate of the habitability of Mars. Most importantly the atmospheric pressure is far too low to support liquid water. The pressure on the surface of Mars is only about 8-9 millibars; this compares with 1013 sea level pressure on Earth; a typical strong hurricane will have the pressure in the eye drop to around 950 millibars. The atmospheric pressure on Mars is about what one finds at 100,000 feet altitude on Earth. This is to be compared with a typical altitude of 35,000 feet for a commercial jetliner. At such low pressure, water would quickly boil, so large, permanent bodies of liquid water are an impossibility.

The composition of the atmosphere is also vastly different. On Earth, the atmosphere is 78 % Nitrogen and 21 % oxygen. The oxygen is crucial for life. On Mars, the atmosphere is 95 % Carbon Dioxide , with the rest being Nitrogen and Argon. Thus not only is the atmosphere thin, it is inhospitable for life.

Thus by the late 1950's things looked a bit grim for life on Mars, but there was still much to learn about the planet, and many people still hoped evidence for life would be found there. A crushing blow to the idea that Mars might be a hospitable planet for life came in 1965. At that time the Mariner 4 spacecraft flew by and photographed the planet. The picture returned is shown in Figure 11-3 of your textbook. I can remember this being on the front page of the Des Moines Register with the headline ``No Canals on Mars''.
Video disk, Chapter 17 picture 12496
Question for audience: why would this picture have been so depressing for someone hoping that there might be life on Mars?

Within a few years it was realized that the seasonal variations in the surface features of Mars were not due to vegetation, but rather planet-wide dust storms which occurred every summer.

Thus by the early 1970's it looked as though Mars more resembled the Moon than the Earth, and seemed to be extremely unpromising as an abode for life. As it turned out, however, the early Mariner spacecraft had painted too pessimistic a picture. Fortuitously, they had passed over the quite barren southern hemisphere of the planet, which appears most Moon-like. Around 1971 the spacecraft Mariner 9 went into orbit around Mars. It painted a more ``diverse'' planet, which once again kindled hopes that there might be restricted areas where life might be possible.

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