MARS

Key Concepts

Mars has a tenuous carbon dioxide (CO₂) atmosphere, with little water vapor and few clouds.
Mars has large volcanos and a gigantic rift valley, but no plate tectonics.
Mars has two small irregular moons, Phobos and Deimos.
Life on Mars ??

Mars, to the unaided eye, has a perceptibly reddish color. It was a natural series of associations that led ancient astronomers to associate the Red Planet, Mars, with the god of war. (Red = blood = war = war god = Mars). The ancient fascination with the planet Mars has lasted until the present day.

Information about missions to Mars, including the current Global Surveyor and Mars Odyssey missions and the upcoming Mars Exploration Rover mission, can be obtained from the NASA Mars Exploration Program web site.

Mars has an orbit with a semimajor axis of 1.5 A.U.; it is the terrestrial planet farthest from the Sun (and hence the terrestrial planet with the coldest surface).

Orbital period of Mars = 1.9 years
Rotation period of Mars = 24 hours, 37 minutes

The martian ``day'' is similar in length to the Earth day.

The rotation axis of Mars is tilted by 25 degrees relative to its orbit around the Sun. Hence, the martian seasons are similar to Earth seasons (only longer). This is in contrast to the case of Mercury and Venus, which have very small axial tilts, and negligibly small seasonal variations in their temperature.

(1) Mars has a tenuous carbon dioxide atmosphere, with little water vapor and few clouds.

The escape velocity on Mars is comparable to that of Mercury. However, Mars has an atmosphere where Mercury does not. This is because days on Mars are cooler than days on Mercury; hence, carbon dioxide molecules in the martian atmosphere are moving more slowly than the escape velocity.

The martian atmosphere is low in density and low in pressure. The air pressure on Mars is less than 1% of the air pressure on Earth. So low is the air pressure on Mars, in fact, liquid water cannot exist on the surface of Mars. Pour yourself a glass of water on Mars, and you can see it boil away into the air at a rate of about an inch per hour. The main components of the martian atmosphere:

Carbon dioxide (CO₂): 95%
Nitrogen (N₂): 3%

The martian atmosphere is similar in its composition to the atmosphere of Venus (but at a pressure 10,000 times lower); it is very different from the Earth's atmosphere. There is little water vapor in the martian atmosphere; this is because ultraviolet light breaks it up. (Mars, with no oxygen in its atmosphere, doesn't have an ozone layer to absorb ultraviolet light.) A water molecule (H₂O) is broken into a hydrogen molecule (H₂) and an oxygen atom. The lightweight hydrogen molecule flies off into outer space; the oxygen atom combines with iron in the martian soil, making rust.

The low density atmosphere of Mars has two implications:

Mars has only a small greenhouse effect (the small amount of CO₂ in its tenuous atmosphere raises the average temperature by only 5 degrees Kelvin).
Mars has large temperature swings between day and night.

To illustrate the weather of Mars, consider the weather forecast for a summer day on Mars:

Daytime high = 263 Kelvin = 14 Fahrenheit
Nighttime low = 197 Kelvin = -105 Fahrenheit

Now consider the equivalent forecast for a summer day on Earth:

Daytime high = 310 Kelvin = 99 Fahrenheit
Nighttime low = 290 Kelvin = 63 Fahrenheit

The average temperature during the martian summer is colder than winter temperatures on Earth; moreover, martian temperatures drop by 66 degrees at night, whereas they drop by only 20 degrees at night on Earth.

Martian clouds are thin and wispy, like cirrus clouds on Earth; they contain solid ice crystals, not liquid droplets. The martian clouds are made of both frozen water and frozen carbon dioxide. Similarly, the ice caps of Mars (the southern ice cap is shown below) are made of a mixture of frozen water and frozen carbon dioxide. Although Mars has no liquid water, and very little gaseous water, it does have frozen water, both in its clouds and in its polar ice caps.

[Image credit: NASA]

(2) Mars has large volcanos and a gigantic rift valley, but no plate tectonics.

The two hemispheres of Mars are very different, geologically speaking:

The Northern hemisphere shows signs of recent volcanic activity (like the larger terrestrial planets, the Earth and Venus).
The Southern hemisphere is heavily cratered and geologically dead (like the smaller terrestrial planet, Mercury, and like the Moon).

Why this difference? The crust is thinner in the northern hemisphere (just as the Moon's crust is thinner on the side facing the Earth). A thin crust makes it easier for lava to break through from the mantle to the surface, enhancing volcanic activity.

Since the southern hemisphere of Mars is simply a rust-colored, slightly eroded replay of Mercury and Venus, let's focus our attention on the more interesting northern hemisphere.

The northern hemisphere contains many large volcanos, including Olympus Mons, the largest volcano in the solar system. Olympus Mons (or Mount Olympus, in plain English) is 600 kilometers across at its base (about the distance from Cleveland to Washington, DC). Olympus Mons is a shield volcano, located over a hot spot in the martian mantle, just as Mauna Loa is located over a hot spot on the Earth's mantle. The height of Olympus Mons, from peak to base, is 25 kilometers, far higher than Mauna Loa. The greater size of Olympus Mons is due to the fact that the martian crust is stationary relative to the mantle beneath. Lava has been spewing upward to form Olympus Mons for most of the history of Mars. By contrast, the motion of the Pacific tectonic plate is dragging Mauna Loa past the hot spot beneath it. Judging from the number of impact craters on Olympus Mons, it last erupted about 300 million years ago. It is either completely extinct, or long dormant. A picture of Olympus Mons taken by the Mars Global Surveyor is shown below (click on it for a larger image).

[Image credit: NASA, Mars Global Surveyor]

The northern hemisphere of Mars also is home to Valles Marineris, the largest valley in the solar system. Valles Marineris (or Mariner Valley, in plain English) is 4000 kilometers long (about the distance from Los Angeles to Boston). Valles Marineris is a rift valley, caused by convection currents in the mantle pulling the crust apart. Unlike the much smaller Grand Canyon on Earth, Valles Marineris is not the result of erosion by water. A picture of Valles Marineris is shown below (click on the image for a much clearer enlarged version).

[Image credit: NASA, Viking]

(3) Mars has two small irregular moons, Phobos and Deimos.

Mercury and Venus have no moons; the Earth has only one (though it's a big one). Mars has two moons. The larger moon of Mars is called Phobos (the Greek word for ``fear''). Phobos, which is pictured below, is 28 kilometers across.

[Image credit: NASA]

The smaller moon of Mars is called Deimos (the Greek word for ``panic'' - fear and panic were thought to be appropriate companions for Mars, the god of war). Deimos, which is pictured below, is only 16 kilometers long.

[Image credit: NASA]

Note that Phobos and Deimos are both irregular in shape, and heavily pitted with craters, showing a history of collisions with other, smaller bodies. They are also undifferentiated; they have never been hot enough to melt. Phobos and Deimos strongly resemble asteroids. It is thought that they are both asteroids which have been captured by the gravity of Mars. Phobos is currently orbiting only 6000 kilometers above the surface of Mars.

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