| Mars, the "Red Planet", is named after the Roman god of war because it commonly appears with a reddish tinge when viewed in our sky. It has always held a fascination for those interested in the possibility of life on other planets. In 1895 a professor of astronomy, Samual Leland Phelps, wrote in a book called World Making that with a new 40 inch telescope being built by the University of Chicago, |
| "It will be possible to see cities on Mars, to detect navies in [its] harbors, and the smoke of great manufacturing cities and towns... Is Mars inhabited? There can be little doubt of it ... conditions are all favorable for life, and life, too, of a high order. Is it possible to know this of a certainty? Certainly." (quoted in Feb., 1973 National Geographic) |
| The adjacent image shows a Viking 2 image of Mars. There is no evidence for the things that Professor Leland thought we would see with an earth-based telescope, but Mars is still of great interest to us, not the least reason being that it may once have harbored conditions favorable to the evolution of life. |
| | Mariner 4, 6, 7, were flyby missions that could only photograph small regions of the Martian surface. They saw portions of the surface that suggested Mars was drab and cratered like the Moon, and geologically dead. This was changed profoundly by the orbiter Mariner 9, which went into orbit around Mars in late 1971. When it arrived the entire Martian surface was engulfed in a dust storm that left almost no surface features visible. |
| | When the dust storm finally subsided in early 1972, Mariner 9 discovered that we had been badly misled by the earlier flyby missions that had seen only small (in retrospect, unrepresentative) portions of the surface. Mariner 9 found evidence for a planet having many interesting geological features: |
| | No spacecraft in the history of space exploration has more profoundly changed our view of a planet than Mariner 9. |
| | Meteor craters and volcanic plains (the largest crater is Hellas, which is 2000 km across). |
| | Valleys that looked as if they could be water-formed (but these don't coincide with the "canals" that people erroneously thought they saw from Earth in earlier times). |
| | Huge volcanic cones (the three round features at the left of the adjacent image are volcanic cones). |
| | Vast sedimentary deposits in the Polar regions. |
| | Gorges larger than the Grand Canyon here on Earth (the feature in the center of the adjacent image is a canyon system, Valles Marineris, that extends over a region the width of the United States). |
| | êîíåöôîðìûíà÷àëîôîðìûThe atmosphere and (probably) the interior of Mars differ substantially from that of the Earth. The atmosphere is much less dense and of different composition, and it is unlikely that the core is molten. |
| | The density of Mars is about 25% less than that of the Earth, suggesting a proportionally larger concentration of lighter materials relative to the core. It is probably intermediate in composition between the Earth and the Moon. Though Mars is probably at least partially differentiated, there is little evidence for large-scale tectonic motion (but there is smaller scale motion such as that responsible for the Valles Marineris system). The core is thought to be iron sulphide; the absence of any detectable magnetic field even though the rotation period is comparable to that for Earth suggests that the core is probably not liquid. |
| | The atmosphere has a pressure at the surface that is only 1/200 that of Earth. The primary component of the atmosphere is carbon dioxide (95%), with the remainder mostly nitrogen. Seasonal heating drives strong winds that can reach 100 mph or more, stirring up large dust storms. Clouds form in the atmosphere, but liquid water cannot exist at the ambient pressure and temperature of the Martian surface: water goes directly between solid and vapor phases without becoming liquid. |
| | êîíåöôîðìûíà÷àëîôîðìûThe preceding image shows the variation of the surface temperature over a period of 50 Martian days at the Viking 1 landing site. Îáû÷íûéÒåðìèíÑïèñîêîïðåäåëåíèéÀäðåñÖèòàòûÔîðìàòèðîâàííûéêîíåöôîðìûíà÷àëîôîðìûNotice the large variation between night and daytime temperatures (associated with the low density of the atmosphere) and the almost constant high and low temperatures for this period. Compare this, for example, with the daily temperature variations for Nome, Alaska (note however that the Nome plot is in degrees Fahrenheit, not Celsius). |
| | Viking: The Search for Life |
| | In 1976 the Viking 1 and 2 landers undertook searches on the Martian surface for the chemical evidence of present or past life on Mars. The images shown below give a picture of one of the backup landers, and two different views of the Martian surface as photographed from Viking 1. |
| | In addition to photgraphing the surface, the Viking landers undertook a series of experiments at two points on the surface to find evidence for life. |
| | The results of these experiments were complex. The first three gave positive results, but the complete absence of any organic compounds in the Martian soil according to the mass spectrometer experiment suggests that the positive results for the first three were not evidence for life, but rather evidence for a complex inorganic chemistry in the Martian soil. Thus, the Viking verdict was that there was no evidence for present or past life on Mars. |
| | Renewed Interest in Martian Life |
| | êîíåöôîðìûíà÷àëîôîðìûThis issue has been given renewed impetus by the recent claim that a meteorite found on the Earth was once part of Mars (because of detailed chemical composition), and that there may be evidence in this rock for past organic activity. However, this is a very open topic at the moment, since there potentially are other explanations of the meteorite's content. We will have to wait on further evidence to clarify this issue. |
| | êîíåöôîðìûíà÷àëîôîðìûThe 4 basic experiments that the Vikings carried out to search for evidence of life were: |
| | These experiments were built around the hypothesis that if there were life on Mars it would have a similar metabolism to life on Earth, and that it would have a similar biochemistry based on the same organic compounds important to life on Earth. |
| | Gas Metabolism: look for changes in the atmosphere induced by metabolism in the Martian soil. |
| | Labeled Release: Look for release of radioactive carbon dioxide by metabolism from organic material labeled by radioactive carbon. |
| | Pyrolytic Release: Search for radioactive compounds in soil by heating soil exposed to radioactive carbon dioxide. |
| | Mass Spectrometer: Search directly in Martian soil for organic compounds known to be essential to Earth life. |
| | êîíåöôîðìûíà÷àëîôîðìûMars has two small moons that are illustrated in the following figures, Phobos and Deimos. |
| | These are examples of what are called minor satellites: small chunks of rock in orbit around planets as compared with large satellites like the Earth's Moon. As the adjacent images show, they are very irregular in shape. Phobos is 27 km long in its longest dimension and Deimos is 15 km long in its longest dimension. |
| | Îáû÷íûéÒåðìèíÑïèñîêîïðåäåëåíèéÀäðåñÖèòàòûÔîðìàòèðîâàííûéêîíåöôîðìûíà÷àëîôîðìûBoth are cratered and orbit the planet in rather low orbits. Phobos is only about 3000 miles above the Martian surface and orbits in a little over 7 hours (thus it makes more than 3 orbits in a single Martian day). Deimos is a little further out and orbits in about 30 hours. |
| | The figure to the right shows a rather spectacular image taken by the Viking 2 orbiter from an altitude of about 8000 miles above the Martian surface. The image looks down on a Martian shield volcano (Ascraeus Mons) which is about 200 miles across and in the center. The object down and to the left of the volcano is Phobos, which is 5000 miles below the orbiter, and 3000 miles above the Martian surface! (The regular horizontal rows of dots seen in the image are an artifact associated with the imaging; they don't correspond to real features). |
| | These moons of Mars were not formed in the same way as the Earth's Moon. They are probably fragments of larger objects broken apart in a collision. Such moons may be formed from collisions of objects originally in orbit around the planet, or they might also have been captured gravitationally at some point in the past. The adjacent image compares the appearance of several minor satellites of the Solar System (Phobos, Deimos, Gaspra, and Ida). We shall encounter many such small moons around the giant planets like Jupiter and Saturn. |
| | êîíåöôîðìûíà÷àëîôîðìûPhobos |
| | Mars has a rotational period of 24 hours and 37 minutes, a period for revolution about the sun of 687 days, and a diameter of 6800 km (about half that of Earth). Its average density is 3.9 g/cc, which is considerably less than the 5.2-5.5 g/cc characteristic of Mercury, Venus, and the Earth. This density gives it a mass about 11% of that for Earth. It is most easily observed from Earth when it is at opposition. Even then, it was difficult in the past to observe from Earth because of turbulence in the Martian atmosphere and ours. |
| | The animation to the right shows a sequence of Hubble Space Telescope images of the Martian surface. The image at the bottom of the page shows several still views from the same source. |
| | Earth based observations concluded that Mars |
| | Has a reddish hue over 3/5 of the planet, which we now known to be caused by red dust and rocks on the surface of the planet |
| | Has polar ice caps waxing and waning with the seasons that we now know to be composed both of dry ice (frozen carbon dioxide) and water ice |
| | Has surface markings that some originally thought looked like "canals" from Earth. These are now known to be features like the edges of mountain ranges |
| | Has areas of changing color that were once thought to correspond to vegetation. We now believe these regions of changing color to be due to blowing sand, not vegetation |
| | Has an atmosphere with clouds |
| | Mars has many interesting geological features on its surface that first became apparent with Mariner 9, were subsequently studied by the Viking missions, and many of which now are visible from the Hubble Space Telescope. |
| | Absence of Plate Tectonics |
| | There is no evidence on Mars for large-scale plate tectonics as we find on Earth. This is believed to be responsible for the different character of Martian and Terrestrial volcanoes. |
| | Îáû÷íûéÒåðìèíÑïèñîêîïðåäåëåíèéÀäðåñÖèòàòûÔîðìàòèðîâàííûéêîíåöôîðìûíà÷àëîôîðìûOn the Earth, as crustal plates move over subsurface chambers of molten rock the lava tends to come to the surface in a line of places, producing strings of volcanoes (for example, volcanic island chains like the Hawaiian Islands). On Mars, with no horizontal motion of crustal plates the same point in the crust sits over subsurface chambers of molten rock and a few very large volcanoes are built. Here is a more extensive discussion of volcanism on Mars. |
| | êîíåöôîðìûíà÷àëîôîðìûThe Martian surface has some large canyon systems. The largest is Valles Marineris, which extends for about 5000 km, is 500 km wide in the widest portions, and as much as 6km deep. The adjacent image shows a portion of the Valles Marineris (the full system is shown in the preceding section). This enormous system of connecting canyons appears to have been formed mostly by local tectonic activity (local motion of surface) rather than by erosion, though as we will discuss below there is some evidence for fluid erosion in portions of it. |
| | The atmosphere of Mars is thin (about 1/200 of the pressure of the Earth's atmosphere), but this atmosphere supports high velocity seasonal winds that are correlated with solar heating of the surface and that produce duststorms that lead to a lot of surface erosion. The following image shows a local dust storm on the Martian surface (lower left). |
| | êîíåöôîðìûíà÷àëîôîðìûHere is another example of a Martian dust storm. At times, such local dust storms grow and merge until essentially the entire surface of the planet is covered by a dust storm. These periods are correlated with times of maximum solar heating of the Martian surface. |
| | There are channels on Mars as much as 1500 km long and 200 km wide that appear to have been cut by running water. Under present atmospheric conditions on Mars (low pressure), water cannot exist as a free liquid on the surface (it must be gas or solid). Thus, evidence for water erosion suggests that the Martian atmosphere may have been more dense in the past. The following two images show portions of the Martian surface where the erosion patterns have regions that are very similar to those found for erosion by surface water on the Earth |
| | Enormous Shield Volcanoes |
| | In the previous section we saw an image of Mars with 3 large volcanoes on the left limb of the image. The volcanoes on Mars are now extinct, but they indicate a preceding period of significant Martian volcanism. Such volcanoes are called shield volcanoes, because they look like shields. The largest volcano on Mars is not one of the three shown previously. It is called Olympus Mons, and is illustrated below |
| | êîíåöôîðìûíà÷àëîôîðìûOlympus Mons is 600 km across its base and about 25 km above the surrounding plain. A perspective model is shown below. |
| | For reference, the largest shield volcano on the Earth is Mauna Loa, which is only about 200 km across its base, and the 25 km height of Olympus Mons is about 2 1/2 times the height of Mount Everest. This raises the interesting issue of why Mars in its past developed a few very large shield volcanoes, while on the Earth the more normal pattern is for volcanoes to develop in strings of smaller volcanoes. As we shall see, the answer is thought to lie in plate tectonics. |
| | êîíåöôîðìûíà÷àëîôîðìûMars has polar caps that wax and wane with the Martian seasons. The following image and animation show the North Polar Cap. |
| | êîíåöôîðìûíà÷àëîôîðìûThese polar caps appear to be partially composed of frozen carbon dioxide ("dry ice") and partially composed of frozen water. |
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