Exploration of the Solar System

Week 6, Topic 8

Mars

This week we begin several lectures on Mars. Mars is a terrestrial planet we know a lot about, and it has always been associated with the question of life elsewhere in the universe. Today that connection is closer and more compelling than ever. When you look up at Mars in the night you will see an object accompanied by 5 operating spacecraft, two of which are on the surface.

Unfortunately for the thrill of night-sky observing, Mars is right now in the worst possible place to see. The location of the planets in the inner solar system this week is shown in the diagram below, taken from the Jet Propulsion Laboratory Solar System Simulator.

You can see that Mars is on the opposite side of the Sun from this. As you look up in the sky, it will be behind the Sun in the daytime sky, and thus totally invisible.

Later in the semester, things will be better for early risers. By November, Mars will be visible in the early morning sky before dawn twilight, although you will be surprised at how faint it is. On November 30, Mars will be very close to Venus on the sky, and you will have a chance to see the two other major terrestrial planets “paired up” in the sky.

Now let’s talk about why the study of Mars is so important to modern astronomy, and why it is the object of intense activity in the space program.

The reason for this intense activity is to determine the geological history of Mars, and its implication for biology.

Orientation

First let’s start with some basic orientation information.

• View of the solar system http://space.jpl.nasa.gov/
• Right now, Mars is 2.65 astronomical units from us.

Just the facts, Maam

• Diameter: 6378 kilometers versus 12756 for Earth (53%). Mars is a significantly smaller “ball in space” than Earth.

• Mass 6.44 ‰ 10²³ kilograms (11 % that of Earth).
• Density 3.9 grams/cc . Think about what this means in terms of comparison to the density of the Earth, and what the densities of materials tell us.
• Major axis of orbit 1.52 au.
• Period of orbit 1.88 years.
• Eccentricity of orbit 0.093. This is rather high for a major planet; remember that the eccentricity of the Earth’s orbit is 0.017.
• See other facts in Table 11.1 of the book.

Basic Observations and History of Study of Mars

With telescopes on Earth you can make out surface features on Mars. A good illustration is in Figure 11.1 of the book. Some of the features you can see are

• Recurrent features such as Syrtis Major. With these you can tell the rotational period of Mars is almost exactly that of Earth, 24.6 hours.
• Polar caps, seemingly like Earth.
• Seasonal variations in the prominence of the surface features.
• All of these observations were available between 100 and 150 years ago, and led to the idea that Mars was an Earthlike planet.

Percival Lowell and the Martians

At the end of the last century and the beginning of this one, Percival Lowell set up an observatory in Arizona and began careful observations of Mars. He thought he saw the famous Martian Canals . These were believed to be the construction of intelligent Martians (see the 1960 movie Mars, the Angry Red Planet). This is an interesting topic, but one which is better suited to sociology than astronomy.

Mid Twentieth Century Physics and the Inhabitability of Mars

A number of discoveries were made with the improved knowledge of physics that we could bring to astronomical observations in the twentieth century. These made it clear that Mars could not really host intelligent life forms

• The atmospheric pressure is about 8-9 millibars, far too low to support liquid water. The atmospheric pressure here on Earth is about 1013 millibars at sea level. At the cruise altitude of commercial aircraft it is about 300 to 350. To reach

8-9 you have to go 20 miles up. A map of the structure of the Martian atmosphere is given in Figure 11.31 of the book.

• The chemical composition of the atmosphere of Mars is quite different. Carbon dioxide comprises 95 % of the atmosphere, and nitrogen 2.7 %. Argon, at 1.6 % is 5 times as abundant as it is in the atmosphere of the Earth (see Table 11.1). It turns out that this (apparently stultifyingly) dull detail is actually an extremely important clue to the ancient history of Mars.
• Because of the lack of an atmosphere, there are extreme variations in the surface temperature.

In spite of these discouraging properties, one of the biggest projects within NASA is the search for life on Mars. Even if it is not there now, it may have been present in the remote past. I will be describing the current status of this search in the next few lectures.