Paper 2000 Question: 1 (a) Al-Beruni

For many years very little was known about the planet, but in 1978 astronomers discovered a relatively large moon orbiting Pluto at a distance of only about 19,600 km (about 12,180 mi) and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another as seen from Earth between 1985 and 1990, enabling astronomers to determine their sizes accurately. Charon is about 1,200 km (750 mi) in diameter, making Pluto and Charon the planet-satellite pair closest in size to one another in the solar system. Scientists often call Pluto and Charon a double planet.

Every 248 years Pluto’s elliptical orbit brings it within the orbit of Neptune. Pluto last traded places with Neptune as the most distant planet in 1979 and crossed back outside Neptune’s orbit in 1999. No possibility of collision exists, however, because Pluto's orbit is inclined more than 17.2° to the plane of the ecliptic (the plane in which Earth and most of the other planets orbit the Sun) and is oriented such that it never actually crosses Neptune's path.

Pluto has a pinkish color. In 1988, astronomers discovered that Pluto has a thin atmosphere consisting of nitrogen with traces of carbon monoxide and methane. Atmospheric pressure on the planet's surface is about 100,000 times less than Earth's atmospheric pressure at sea level. Pluto’s atmosphere is believed to freeze out as a snow on the planet’s surface for most of each Plutonian orbit. During the decades when Pluto is closest to the Sun, however, the snows sublimate (evaporate) and create the atmosphere that has been observed. In 1994 the Hubble Space Telescope imaged 85 percent of Pluto's surface, revealing polar caps and bright and dark areas of startling contrast. Astronomers believe that the bright areas are likely to be shifting fields of clean ice and that the dark areas are fields of dirty ice colored by interaction with sunlight. These images show that extensive ice caps form on Pluto's poles, especially when the planet is farthest from the Sun.

Microsoft ® Encarta ® 2006. © 1993-2005 Microsoft Corporation. All rights reserved.

About 200 asteroids have diameters of more than 97 km (60 mi), and thousands of smaller ones exist. The total mass of all asteroids in the solar system is much less than the mass of the Moon. The larger bodies are roughly spherical, but elongated and irregular shapes are common for those with diameters of less than 160 km (100 mi). Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Certain asteroids may be binary, or have satellites of their own.

Few scientists now believe that asteroids are the remnants of a former planet. It is more likely that asteroids occupy a place in the solar system where a sizable planet could have formed but was prevented from doing so by the disruptive gravitational influences of the nearby giant planet Jupiter. Originally perhaps only a few dozen asteroids existed, which were subsequently fragmented by mutual collisions to produce the population now present. Scientists believe that asteroids move out of the asteroid belt because heat from the Sun warms them unevenly. This causes the asteroids to drift slowly away from their original orbits. 

The so-called Trojan asteroids lie in two clouds, one moving 60° ahead of Jupiter in its orbit and the other 60° behind. In 1977 the asteroid 2060 Chiron was discovered in an orbit between that of Saturn and Uranus. Asteroids that intersect the orbit of Mars are called Amors; asteroids that intersect the orbit of Earth are known as Apollos; and asteroids that have orbits smaller than Earth’s orbit are called Atens. One of the largest inner asteroids is 443 Eros, an elongated body measuring 13 by 33 km (8 by 21 mi). The peculiar Apollo asteroid 3200 Phaethon, about 5 km (about 3 mi) wide, approaches the Sun more closely, at 20.9 million km (13.9 million mi), than any other known asteroid. It is also associated with the yearly return of the Geminid stream of meteors (see Geminids).

Several Earth-approaching asteroids are relatively easy targets for space missions. In 1991 the United States Galileo space probe, on its way to Jupiter, took the first close-up pictures of an asteroid. The images showed that the small, lopsided body, 951 Gaspra, is pockmarked with craters, and revealed evidence of a blanket of loose, fragmental material, or regolith, covering the asteroid’s surface. Galileo also visited an asteroid named 243 Ida and found that Ida has its own moon, a smaller asteroid subsequently named Dactyl. (Dactyl’s official designation is 243 Ida I, because it is a satellite of Ida.)

In 1996 the National Aeronautics and Space Administration (NASA) launched the Near-Earth Asteroid Rendezvous (NEAR) spacecraft. NEAR was later renamed NEAR Shoemaker in honor of American scientist Eugene M. Shoemaker. NEAR Shoemaker’s goal was to go into orbit around the asteroid Eros. On its way to Eros, the spacecraft visited the asteroid 253 Mathilde in June 1997. At 60 km (37 mi) in diameter, Mathilde is larger than either of the asteroids that Galileo visited. In February 2000, NEAR Shoemaker reached Eros, moved into orbit around the asteroid, and began making observations. The spacecraft orbited the asteroid for a year, gathering data to provide astronomers with a better idea of the origin, composition, and structure of large asteroids. After NEAR Shoemaker’s original mission ended, NASA decided to attempt a “controlled crash” on the surface of Eros. NEAR Shoemaker set down safely on Eros in February 2001—the first spacecraft ever to land on an asteroid.

In 1999 Deep Space 1, a probe NASA designed to test new space technologies, flew by the tiny asteroid 9969 Braille. Measurements taken by Deep Space 1 revealed that the composition of Braille is very similar to that of 4 Vesta, the third largest asteroid known. Scientists believe that Braille may be a broken piece of Vesta or that the two asteroids may have formed under similar conditions.

III SURFACE COMPOSITION
With the exception of a few that have been traced to the Moon and Mars, most of the meteorites recovered on Earth are thought to be asteroid fragments. Remote observations of asteroids by telescopic spectroscopy and radar support this hypothesis. They reveal that asteroids, like meteorites, can be classified into a few distinct types.

Three-quarters of the asteroids visible from Earth, including 1 Ceres, belong to the C type, which appear to be related to a class of stony meteorites known as carbonaceous chondrites. These meteorites are considered the oldest materials in the solar system, with a composition reflecting that of the primitive solar nebula. Extremely dark in color, probably because of their hydrocarbon content, they show evidence of having adsorbed water of hydration. Thus, unlike the Earth and the Moon, they have never either melted or been reheated since they first formed.

Asteroids of the S type, related to the stony iron meteorites, make up about 15 percent of the total population. Much rarer are the M-type objects, corresponding in composition to the meteorites known as “irons.” Consisting of an iron-nickel alloy, they may represent the cores of melted, differentiated planetary bodies whose outer layers were removed by impact cratering.

A very few asteroids, notably 4 Vesta, are probably related to the rarest meteorite class of all: the acho

drites. These asteroids appear to have an igneous surface composition like that of many lunar and terrestrial lava flows. Thus, astronomers are reasonably certain that Vesta was, at some time in its history, at least partly melted. Scientists are puzzled that some of the asteroids have been melted but others, such as 1 Ceres, have not. One possible explanation is that the early solar system contained certain concentrated, highly radioactive isotopes that might have generated enough heat to melt the asteroids.

IV ASTEROIDS AND EARTH
Astronomers have found more than 300 asteroids with orbits that approach Earth’s orbit. Some scientists project that several thousand of these near-Earth asteroids may exist and that as many as 1,500 could be large enough to cause a global catastrophe if they collided with Earth. Still, the chances of such a collision average out to only one collision about every 300,000 years. 

Many scientists believe that a collision with an asteroid or a comet may have been responsible for at least one mass extinction of life on Earth over the planet’s history. A giant crater on the Yucatán Peninsula in Mexico marks the spot where a comet or asteroid struck Earth at the end of the Cretaceous Period, about 65 million years ago. This is about the same time as the disappearance of the last of the dinosaurs. A collision with an asteroid large enough to cause the Yucatán crater would have sent so much dust and gas into the atmosphere that sunlight would have been dimmed for months or years. Reactions of gases from the impact with clouds in the atmosphere would have caused massive amounts of acid rain. The acid rain and the lack of sunlight would have killed off plant life and the animals in the food chain that were dependent on plants for survival. 

If the Tunguska object had exploded in a less remote area, the loss of human life and property could have been astounding. Military satellites—in orbit around Earth watching for explosions that could signal violations of weapons testing treaties—have detected dozens of smaller asteroid explosions in the atmosphere each year. In 1995 NASA, the Jet Propulsion Laboratory, and the U.S. Air Force began a project called Near-Earth Asteroid Tracking (NEAT). NEAT uses an observatory in Hawaii to search for asteroids with orbits that might pose a threat to Earth. By tracking these asteroids, scientists can calculate the asteroids’ precise orbits and project these orbits into the future to determine whether the asteroids will come close to Earth. 

Although chemical classification is not rigid, the various classes of chemical compounds that include a majority of minerals are as follows: (1) elements, such as gold, graphite, diamond, and sulfur, that occur in the native state, that is, in an uncombined form; (2) sulfides, which are minerals composed of various metals combined with sulfur. Many important ore minerals, such as galena and sphalerite, are in this class; (3) sulfo salts, minerals composed of lead, copper, or silver in combination with sulfur and one or more of the following: antimony, arsenic, and bismuth. Pyrargyrite, Ag3SbS3, belongs to this class; (4) oxides, minerals composed of a metal in combination with oxygen, such as hematite, Fe2O3. Mineral oxides that contain water, such as diaspore, Al2O3• H2O, or the hydroxyl (OH) group, such as bog iron ore, FeO(OH), also belong to this group; (5) halides, composed of metals in combination with chlorine, fluorine, bromine, or iodine; halite, NaCl, is the most common mineral of this class; (6) carbonates, minerals such as calcite, CaCO 3, containing a carbonate group; (7) phosphates, minerals such as apatite, Ca5(F,Cl)(PO4)3, that contain a phosphate group; (8) sulfates, minerals such as barite, BaSO4, containing a sulfate group; and (9) silicates, the largest class of minerals, containing various elements in combination with silicon and oxygen, often with complex chemical structure, and minerals composed solely of silicon and oxygen (silica). The silicates include the minerals comprising the feldspar, mica, pyroxene, quartz, and zeolite and amphibole families.

The study of minerals is an important aid in understanding rock formation. Laboratory synthesis of the high-pressure varieties of minerals is helping the understanding of igneous processes deep in the lithosphere (see Earth). Because all of the inorganic materials of commerce are minerals or derivatives of minerals, mineralogy has direct economic application. Important uses of minerals and examples in each category are gem minerals (diamond, garnet, opal, zircon); ornamental objects and structural material (agate, calcite, gypsum); abrasives (corundum, diamond, kaolin); lime, cement, and plaster (calcite, gypsum); refractories (asbestos, graphite, magnesite, mica); ceramics (feldspar, quartz); chemical minerals (halite, sulfur, borax); fertilizers (phosphates); natural pigments (hematite, limonite); optical and scientific apparatus (quartz, mica, tourmaline); and the ores of metals (cassiterite, chalcopyrite, chromite, cinnabar, ilmenite, molybdenite, galena, and sphalerite).

Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers. Many synthetic materials, such as plastics, fibers (; Rayon), adhesives, glass, and porcelain, are also to a large extent polymeric substances.

Branched polymers have side chains that are attached to the chain molecule itself. Branching can be caused by impurities or by the presence of monomers that have several reactive groups. Chain polymers composed of monomers with side groups that are part of the monomers, such as polystyrene or polypropylene, are not considered branched polymers.

In cross-linked polymers, two or more chains are joined together by side chains. With a small degree of cross-linking, a loose network is obtained that is essentially two dimensional. High degrees of cross-linking result in a tight three-dimensional structure. Cross-linking is usually caused by chemical reactions. An example of a two-dimensional cross-linked structure is vulcanized rubber, in which cross-links are formed by sulfur atoms. Thermosetting plastics are examples of highly cross-linked polymers; their structure is so rigid that when heated they decompose or burn rather than melt.

III SYNTHESIS
Two general methods exist for forming large molecules from small monomers: addition polymerization and condensation polymerization. In the chemical process called addition polymerization, monomers join together without the loss of atoms from the molecules. Some examples of addition polymers are polyethylene, polypropylene, polystyrene, polyvinyl acetate, and polytetrafluoroethylene (Teflon).

In condensation polymerization, monomers join together with the simultaneous elimination of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters, and certain polyurethanes.

Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers. Many synthetic materials, such as plastics, fibers (; Rayon), adhesives, glass, and porcelain, are also to a large extent polymeric substances.

Branched polymers have side chains that are attached to the chain molecule itself. Branching can be caused by impurities or by the presence of monomers that have several reactive groups. Chain polymers composed of monomers with side groups that are part of the monomers, such as polystyrene or polypropylene, are not considered branched polymers.

In condensation polymerization, monomers join together with the simultaneous elimination of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters, and certain polyurethanes.