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Fossa, Sulci, and Other Terms for Planetary Landforms

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The IAU has designated categories of names from which to choose for each planetary body, and in some cases, for each type of feature on a given planetary body. On Mercury, craters are named for famous deceased artists of various stripes, while rupes are named for scientific expeditions. On Venus, craters larger than 12.4 miles (20 km) are named for famous women, and those smaller than 12.4 miles (20 km) are given common female first names. Colles are named for sea goddesses, dorsa are named for sky goddesses, fossae are named for goddesses of war, and fluctus are named for miscellaneous goddesses.

The gas giant planets do not have features permanent enough to merit a nomenclature of features, but some of their solid moons do. Io's features are named after characters from Dante's Inferno. Europa's features are named after characters from Celtic myth. Guidelines can become even more explicit: Features on the moon Mimas are named after people and places from Malory's Le Morte d'Arthur legends, Baines translation. A number of asteroids also have naming guidelines. Features on 253 Mathilde, for example, are named after the coalfields and basins of Earth.

Susan Sakimoto, a scientist at NASA's Goddard Space Flight Center, is comparing the shapes of shield volcanoes from Mars with those from the Snake River Plain in Idaho.Amazingly, Idaho has less accurate topographical data than parts of Mars has, and Sakimoto and her team have had to take handheld GPS units and remap the topography of the volcanoes they are examining in Idaho. They find both Mars and Idaho have three main types of shield volcanoes: low-profile shields, dome-shaped shields like cowboy hats, and steep-summit shields with jagged peaks in their centers.The topography of the shield volcanoes in Idaho is created by the viscosity of the magma that is erupting, that is, the steepest volcanic peaks are formed by highly viscous lava that hardens before it flows. Sakimoto and her team suspect that the same mechanism may be at work on Mars.

Beyond the major shield and patera volcanoes, Mars has ample evidence for widespread, small-scale volcanic eruptions. Elemental maps of the surface from Mars Odyssey indicate a high potassium spot near the North Pole, and several high iron spots north of the dichotomy boundary. These may be volcanic centers. Many individual volcanic flows have been found on the planet through careful examination of photographs. One flow has been found that is 300 miles (480 km) long, with a width from three to 30 miles (5 to 50 km) and a height of 100 to 200 feet (30 to 100 m).

Both the Mars rovers (Opportunity and Spirit, shown in the upper color insert on page C-5) have found abundant volcanic rocks at their sites. Each has measured the compositions of rocks using its instruments, and together they have greatly increased the information on rock compositions on Mars. Remote sensing can have difficulty inferring true compositions of surface rocks because, beyond the difficulty of deciphering mineralogy from one spectra, the rocks have been altered by weathering and covered with dust, obscuring their true internal compositions.The rovers carried tools that could grind through the outer coating on rocks, allowing other instruments to measure the internal composition. A rock dubbed Clovis is shown in the image below being ground by the rock abrasion tool to a depth of 0.35 inches (8.9 mm).The hole is 1.8 inches (4.5 cm) in

The Mars Rover Spirit ground through the weathered rind of the rock dubbed Clovis to create a hole 1.8 inches (4.5 cm) in diameter where fresh rock can be examined. (NASA/JPL/Cornell/USGS)

diameter. Clovis is one of the softest rocks encountered in Mars because it contains mineral alterations that extend deeply from its surface, likely caused by acidic water as evidenced by its high levels of sulfur, chlorine, and bromine.

One small rock was struck by the Opportunity rover on landing, when the rover was still protected by a mass of large airbags.This 14-inch (35-cm) rock was named Bounce. After grinding the rock was available to the Thermal Emission Spectrometer, an instrument that measures the infrared radiation emitted by the rock from incident sunlight.The spectrum that Bounce gave off was the sum of the spectra of all its constituent minerals and so could be used in two ways: The spectrum could be compared to the spectra of rocks on Earth, whose bulk compositions are known, and the spectra of known minerals could be added up to see if they result in the spectrum of Bounce. Bounce's spectrum closely matched the spectra of Mars meteorites, though it is slightly enriched in silica. Bounce is therefore a basaltic rock similar to some of those that were knocked off Mars by impacts and traveled to Earth.

The Mars Global Surveyor orbiter also carried a Thermal Emission Spectrometer (TES), which detects infrared emissions from the planet at wavelengths from six to 50 micrometers while in orbit. The TES team, led by Phil Christensen of Arizona State University, identified two large regions on Mars that have distinctive spectral properties, though each is interpreted to be a dark igneous rock composition. In the southern highlands the dark igneous rocks appear to be basalts, similar to the magmas erupted from Hawaii on the Earth. In the northern lowlands the dark igneous rocks appear to be a type of lava higher in silica, known as andesite. On the Earth andesites are produced at subduction zones as the products of melting the mantle in combination with water. If these lavas really are andesites, then they may indicate something about the location of water and the role of plate tectonics on the early planet, or it may indicate that andesites can be formed in other ways unlike those on Earth.

Scientists had mixed reactions to the possibility of andesite on Mars. One question raised is how uniquely the spectra of Surface Type 2 matches andesite. Michael Wyatt and Harry Y. McSween, scientists at the University of Tennessee, and Timothy Grove from the Massachusetts Institute of Technology have taken another look at the Thermal Emission Spectrometer spectra by using a larger collection of aqueous alteration (weathering) products in the spectral mixing calculations.They show that weathered basalt also matches the spectral properties of Surface Type 2. Wyatt and McSween also note that Type 2 regions are generally confined to a large, low region that is the site of a purported ancient Martian ocean. They suggest that basalts like those in Surface Type 1 were altered in the ancient Martian sea. Independent data are needed to test the andesite versus altered basalt hypotheses.

Mars's extensive volcanism that has continued throughout its history is a problem for geoscientists. On Earth volcanism is driven largely by plate movements: Volcanoes form at convergent plate boundaries, boundaries where an oceanic plate is pressing against (converging on) a continental plate, and eventually sliding beneath it to form a subduction zone.Volcanoes on Earth also form at divergent plate boundaries, the midocean ridges along which new oceanic crust is formed and flows away. On Mars there is no plate tectonic movement, and if ever there was plate movement, it was a brief interval early in the planet's history. Mars's mantle must be converting under its thick solid lithosphere and allowing mantle material to melt in upwelling mantle plumes. No one has yet devised a convincing model that explains continuous volcanic activity over large portions of the planet's surface for the length of the planet's history, while sumiltaneously building one single immense volcanic edifice (Tharsis).

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