RED+-+FitzgibbonA


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The southern hemisphere of Mars is predominantly ancient cratered highlands somewhat similar to the [|Moon]. In contrast, most of the northern hemisphere consists of plains which are much younger, lower in elevation and have a much more complex history. An abrupt elevation change of several kilometers seems to occur at the boundary. The reasons for this global dichotomy and abrupt boundary are unknown (some speculate that they are due to a very large impact shortly after Mars' accretion). [|Mars Global Surveyor] has produced a nice [|3D map]of Mars that clearly shows these features. The interior of Mars is known only by inference from data about the surface and the bulk statistics of the planet. The most likely scenario is a dense core about 1700 km in radius, a molten rocky mantle somewhat denser than the Earth's and a thin crust. Data from Mars Global Surveyor indicates that Mars' crust is about 80 km thick in the southern hemisphere but only about 35 km thick in the north. Mars' relatively low density compared to the other [|terrestrial]planets indicates that its core probably contains a relatively large fraction of sulfur in addition to iron (iron and iron sulfide). Like [|Mercury] and the Moon, Mars appears to lack active [|plate tectonics] at present; there is no evidence of recent horizontal motion of the surface such as the folded mountains so common on [|Earth]. With no lateral plate motion, hot-spots under the crust stay in a fixed position relative to the surface. This, along with the lower surface gravity, may account for the Tharis bulge and its enormous volcanoes. There is no evidence of current volcanic activity. However, data from Mars Global Surveyor indicates that Mars very likely did have tectonic activity sometime in the past. Valley Network There is very clear evidence of **erosion** in many places on Mars including large floods and small river systems. At some time in the past there was clearly some sort of fluid on the surface. Liquid water is the obvious fluid but [|other possibilities] exist. There may have been large lakes or even oceans; the evidence for which was strenghtened by some very nice [|images of layered terrain] taken by Mars Global Surveyor and the [|mineralology results] from MER Opportunity. Most of these point to wet episodes that occurred only briefly and very long ago; the age of the erosion channels is estimated at about nearly 4 billion years. However, images from Mars Express released in early 2005 show what appears to be a [|frozen sea]that was liquid very recently (maybe 5 million years ago). Confirmation of this interpretation would be a very big deal indeed! (Valles Marineris was NOT created by running water. It was formed by the stretching and cracking of the crust associated with the creation of the Tharsis bulge.) Early in its history, Mars was much more like Earth. As with Earth almost all of its carbon dioxide was used up to form carbonate rocks. But lacking the Earth's [|plate tectonics], Mars is unable to recycle any of this carbon dioxide back into its atmosphere and so cannot sustain a significant [|greenhouse effect]. The surface of Mars is therefore much colder than the Earth would be at that distance from the Sun. Mars has a very thin atmosphere composed mostly of the tiny amount of remaining carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%). The average pressure on the surface of Mars is only about 7 [|millibars] (less than 1% of Earth's), but it varies greatly with altitude from almost 9 millibars in the deepest basins to about 1 millibar at the top of Olympus Mons. But it is thick enough to support very strong winds and [|vast dust storms] that on occasion engulf the entire planet for months. Mars' thin atmosphere produces a [|greenhouse effect] but it is only enough to raise the surface temperature by 5 degrees (K); much less than what we see on [|Venus] and [|Earth]. South Polar Cap Early telescopic observations revealed that Mars has permanent ice caps at both poles; they're visible even with a small telescope. We now know that they're composed of water ice and solid carbon dioxide ("dry ice"). The ice caps exhibit a layered structure with alternating layers of ice with varying concentrations of dark dust. In the northern summer the carbon dioxide completely [|sublimes], leaving a residual layer of water ice. ESA's [|Mars Express has shown] that a similar layer of water ice exists below the southern cap as well. The mechanism responsible for the layering is unknown but may be due to climatic changes related to long-term changes in the inclination of Mars' equator to the plane of its orbit. There may also be water ice hidden below the surface at lower latitudes. The seasonal changes in the extent of the polar caps changes the global atmospheric pressure by about 25% (as measured at the Viking lander sites). Recent observations with the [|Hubble Space Telescope] have revealed that the conditions during the Viking missions may not have been typical. Mars' atmosphere now seems to be both colder and dryer than measured by the Viking landers ([|more details]from STScI). The[|Viking]landers performed experiments to determine the existence of life on Mars. The results were somewhat ambiguous but most scientists now believe that they show no evidence for life on Mars (there is still some controversy, however). Optimists point out that only two tiny samples were measured and not from the most favorable locations. More experiments will be done by future missions to Mars. A small number of [|meteorites](the SNC meteorites) are believed to have originated on Mars. On 1996 Aug 6, David McKay et al announced what they thought might be evidence of ancient Martian microorganisms in the meteorite ALH84001. Though there is still some controversy, the majority of the scientific community has not accepted this conclusion. If there is or was life on Mars, we still haven't found it. Large, but not global, weak magnetic fields exist in various regions of Mars. This unexpected finding was made by [|Mars Global Surveyor]just days after it entered Mars orbit. They are probably remnants of an earlier global field that has since disappeared. This may have important implications for the structure of Mars' interior and for the past history of its atmosphere and hence for the possibility of ancient life. When it is in the nighttime sky, Mars is easily [|visible] with the unaided eye. Mars is a difficult but rewarding target for an [|amateur] telescope though only for the three or four months each martian year when it is closest to Earth. Its apparent size and brightness varies greatly according to its relative position to the Earth. There are several [|Web sites] that show the current position of Mars (and the other planets) in the sky. More detailed and customized charts can be created with a [|planetarium program].