----- The following copyright 1991 by Dirk Terrell ----- This article may be reproduced or retransmitted ----- only if the entire document remains intact ----- including this header Lecture #14 "The Red Planet" Observations of the phases of Venus by Galileo placed considerable doubt on the validity of the Ptolemaic solar system that had the Earth at the center. Kepler's detailed calculations of the orbit of Mars destroyed it for good. Since that time, Mars has been a source of fascination for us Earth-bound creatures. Why Mars? It isn't particularly bright in the sky most of the time, although at some oppositions (when the planet is directly opposite the Sun in the sky, like the moon when it is full)it can be brighter than Jupiter, such as the spectacular 1988 opposition. Mars is so popular, I think, because it is the most Earth-like of all the planets. Perhaps we are so enthralled by it because we know that someday we will live there. A day on Mars is 24 hours, 37 minutes, and 23 seconds long, which is very close to the Earth's day 23h 56m 4s. It has polar ice caps. There are seasons on Mars. There are clouds in the Martian sky (although Mars observers think that is an UNFORTUNATE similarity with Earth!). The surface has markings on it. Indeed Mars is similar to Earth in many ways. But, it is also different in some very dramatic ways. Mars has a rich observational history. The first person to turn a telescope on Mars was Galileo (I bet you were surprised by that!). In 1659, Christian Huygens, following surface features, measured the rotation period of Mars. About 100 or so years later, William Herschel measured the rotation period, and his value was different from the one accepted today by only 14 seconds. Around the turn of this century, Percival Lowell built an observatory, and dedicated himself to observing Mars. To date there have been 13 unmanned probes successfully sent to the planet by the US and USSR. They are as follows: Spacecraft Country Date Type of Mission Mariner 4 USA 7/65 Flyby Mariner 6 USA 7/69 Flyby Mariner 7 USA 8/69 Flyby Mariner 9 USA 11/71 Orbiter Mars 2 USSR 11/71 Orbiter/Lander Mars 3 USSR 12/71 Orbiter/Lander Mars 4 USSR 2/74 Flyby Mars 5 USSR 2/74 Orbiter Mars 6 USSR 3/74 Lander Mars 7 USSR 3/74 Lander Viking 1 USA 6/76 Orbiter/Lander Viking 2 USA 9/76 Orbiter/Lander Phobos 2 USSR 1/89 Orbiter/Lander Lowell was convinced that Mars was inhabited by intelligent beings who constructed canals to bring water from the polar ice caps to the dry deserts. Earlier, the Italian astronomer Giovanni Schiaparelli mapped what seemed to be straight lines on the surface. He called these "canali", which was subsequently mis-translated as "canals." The proper translation should have been "channels." I have on several occasions seen these straight lines myself. They do exist. Unfortunately, as Carl Sagan has said, they ARE a sign of intelligence, but the intelligence is on the eyepiece end of the telescope, not the objective. The eye and brain, straining at the limits of detection, will take scattered information and try to put it together into a more manageable form. The lines are created by our brains. Not that this is a bad thing. I mean, without this process, trying to watch television would be hopeless. All we would see is a bunch of red, green, and blue dots. The arrival of Mariner 4 showed that there were no canals on Mars. The problem was solved. But as has been the case with every mission I can remember, more questions were raised than were answered. Mariner discovered a cratered terrain very similar to the moon. Mariners 6 and 7 also showed such terrain, although the pictures were not terribly clear. Mariner 9, on the other hand, showed some amazing features very clearly. It took pictures of a large volcano (the largest in the solar system in fact), an immense canyon, and a vast network of dry riverbeds. The volcano, called Olympus Mons, has a base diameter of about 600 km and stands 27 km high. The volcanic crater (caldera) itself is 80 km across. No matter how you look at it, that's a big volcano! (Mt. Everest is only about 9 km high.) The canyon on Mars is pretty big too. The Grand Canyon is a hairline fracture compared to this. The canyon, called Valles Marineris, is about 4000 km long, up to 700 km across, and 7 km deep. The Grand Canyon lengthwise could fit across the width of Valles Marineris. The name of the canyon deserves some comment. Since it was discovered in photographs taken by Mariner, people thought that Valles Marineris, or "Mariner Valley" in Latin since Martian features are given Latin names, would be appropriate. The IAU agreed and the name was officially adopted. Sometime later, however, someone pointed out that Valles Marineris means "marinated valley" Latin, not "mariner valley." So I guess all the dry river beds were really carved by vinegar and melted butter. The atmosphere of Mars is much less dense than that of Earth, and, like Venus, is compsed primarily carbon dioxide. The surface pressure on Mars is about 150 times less than on Earth, while you will recall that Venus' atmosphere is about 90 times heavier. Thus, while Mars is similar to Venus in having a carbon dioxide-dominated atmosphere, its relative thinness makes the Martian greenhouse effect rather small. The global mean surface temperature is a rather chilly -53C (-63F). However, during the summertime, the local temperature can get as high as 22C (72F). The Martian atmosphere can also get quite active, with large dust storms that can be seen from Earth. Mars has two moons, which were discovered by Asaph Hall during the favorable opposition of 1877. Named after the attendants of Mars in the Illiad, Phobos ("Fear") and Deimos ("Dread"), these moons are mere rocks compared to our moon. On average, Phobos is 21 km across, while Deimos is 13 km across. Some planetary scientists think that these moons may, in fact, be captured asteroids.