:: wikimiki.org ::

Impact Crater

Impact crater

moon. NASA photo.]] An impact crater (impact basin or sometimes crater) is a circular depression on a surface, usually referring to a planet, moon, asteroid, or other celestial body, caused by a collision of a smaller body (meteorite) with the surface. In the center of craters on Earth a crater lake often accumulates, and a central island or peak (caused by rebounding crustal rock after the impact) is usually a prominent feature in the lake. Ancient craters whose relief has disappeared leaving only a "ghost" of a crater are known as palimpsests. Although it might be assumed that a major impact on the Earth would leave behind absolutely unmistakable evidence, in fact the gradual processes that change the surface of the Earth tend to cover the effects of impacts. Erosion by wind and water, deposits of wind-blown sand and water-carried sediment, and lava flows in due time tend to obscure or bury the craters left by impacts. Simple slumping of weak crustal material can also play a role, especially on outer solar system bodies such as Callisto which are covered in a crust of ice. However, some evidence remains, and over 150 major craters have been identified on the Earth. Studies of these craters have allowed geologists to find the remaining traces of other craters that have mostly been obliterated. Impact craters are found on nearly all solid surface planets and satellites. As the number of impact craters increases on a surface, the appearance of the surfaces changes; this can be used to establish the age of extraterrestrial terrain. After a period of time, however, an equilibrium is reached in which old craters are destroyed as quickly as new craters form.

History

geologists Daniel Barringer (1860-1929) was one of the first to identify a geological structure as an impact crater, the Barringer Meteorite Crater (or the "Meteor Crater") in Arizona, but at the time his ideas were not widely accepted, and when they were, there was no recognition of the fact that Earth impacts are common in geological terms. In the 1920s, the American geologist Walter H. Bucher studied a number of craters in the US. He concluded they had been created by some great explosive event, but believed they were the result of some massive volcanic eruption. However, in 1936, the geologists John D. Boon and Claude C. Albritton Jr. revisited Bucher's studies and concluded the craters he studied were probably formed by impacts. The issue remained more or less speculative until the 1960s. A number of researchers, most notably Eugene M. Shoemaker, conducted detailed studies of the craters that provided clear evidence that they had been created by impacts, identifying the shock-metamorphic effects uniquely associated with impacts, of which the most familiar is Shocked quartz. Shocked quartz Armed with the knowledge of shock-metamorphic features, Carlyle S. Beals and colleagues at the Dominion Observatory, (Victoria, British Columbia, Canada), and Wolf von Engelhardt of the University of Tübingen in Germany began a methodical search for "impact structures". By 1970, they had tentatively identified more than 50. Their work remained controversial, but the American Apollo Moon landings, which were in progress at the time, provided evidence of the rate of impact cratering on the Moon. Processes of erosion on the Moon are minimal and so craters persist almost indefinitely. Since the Earth could be expected to have roughly the same cratering rate as the Moon, it became clear that the Earth had suffered far more impacts than could be seen by counting evident craters. The age of known impact craters on the Earth ranges from about a thousand (e.g. the Haviland crater in Kansas) to almost two billion years, though few older than 200 million years have been found, as geological processes tend to obliterate older ones. They are also selectively found in the stable interior regions of continents. Few underwater craters have been discovered because of the difficulty of surveying the sea floor; the rapid rate of change of the ocean bottom; and the subduction of the ocean floor into the Earth's interior by processes of plate tectonics. Current estimates of the rate of cratering on the Earth suggest that from one to three craters with a width greater than 20 kilometers are created every million years. This indicates that there are far more relatively young craters on the planet than have been discovered so far.

Formation and structure

An object falling from open space hits the Earth with a minimum velocity of 11.6 km/s (7 mi/s). Since the energy from motion grows as the square of the velocity, this gives moving rock more energy per kilogram than ordinary chemical explosives. Massive objects can easily cause kiloton explosions that resemble nuclear explosions. Seismographs record about one multikiloton impact somewhere on the Earth each year, usually in mid-ocean. If the object weighs more than 1,000 tonnes, an atmosphere does not slow it down much, though smaller bodies can be substantially slowed by atmospheric drag, as they have a higher ratio of surface area to mass. In any case, the temperatures and pressures on the object are extremely high. These temperature and pressure extremes can destroy chondritic or carbonaceous chondritic bodies before they ever reach ground, but metallic iron-nickel meteorites have more structural integrity and can strike the surface of the Earth in a violent explosion. When the object hits, it compresses a column of air, water and rock into an extremely hot plasma. This plasma expands violently, and cools rapidly (i.e. it explodes). The plasma and other ejecta splashes at orbital or near-orbital speeds. It can be thrown off into space, or can travel several times around the planet before re-entering as secondary meteors. Airless planets usually preserve stains of the ejecta around impact craters as a pattern of "rays". It should be noted that other non-impact theories for crater-ray formation have been suggested in the scientific literature. plasma Very energetic chemistry occurs in the plasma. In an Earth impact, powerful acids can be formed from saltwater and air. The vaporized rock of the plasma condenses into characteristic cone-shaped droplets of glass called tektites, and these are widely distributed by the high speeds. Tektites are found in isolated strewnfields on Earth. Note: Several researchers reject the popular impact-origin theory of tektites based on comparisons to bonafide impactite glasses. Curiously, the largest and youngest (700,000 years ago) tektite strewnfield, known as the Australasian field, has no known crater associated with it; this fact strongly suggests that, at least in this case, the tektites are not linked to an impact. A giant "fresh" impact site, less than a million years old, should be visible on land or in the sea. No such Asian impact crater has ever been found.. Oceanic impacts can be considerably more damaging than those on land. Large objects will invariably penetrate or displace the water to impact the seabed, causing huge tsunamis over a large area. The impact at Chicxulub, Yucatán is believed to have produced tsunamis 50 to 100 metres (150-300 feet) high which deposited debris many miles inland. The result of an impact on land or at sea is a crater. There are two forms, "simple" and "complex". The Barringer crater in Arizona is a perfect example of a simple crater, a straightforward bowl in the ground. Simple craters are generally less than four kilometers across. Complex craters are larger, and have uplifted centers that are surrounded by a trough, plus broken rims. The uplifted center is due to the "rebound" of the earth after the impact. It is something like the ripple pattern created by a drop of water into a pool, frozen into the Earth when the melted rock cooled and solidified. Arizona In either case, the size of the crater depends on the size of the impactor and the material in the impact regions. Relatively soft materials yield smaller craters than brittle materials. The size of craters invariably changes over time; in the short term, craters shrink as a result of slumping, and over the longer term erosion and other geological processes quickly hide impact craters on the Earth. The Barringer Crater is one of the best-preserved on the planet, but it is only about 50,000 years old. There are almost no signs of the 65 million year-old Chicxulub crater on the Earth's surface, despite it being one of the largest known on the planet. Some volcanic features can resemble impact craters, and brecciated rocks are associated with other geological formations besides impact craters. Non-explosive volcanic craters can usually be distinguished from impact craters by their irregular shape and the association of volcanic flows and other volcanic materials. An exception is that impact craters on Venus often have associated flows of melted material. The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however. Impacts produce distinctive "shock-metamorphic" effects that allow impact sites to be distinctively identified. Such shock-metamorphic effects can include:
- A layer of shattered or "brecciated" rock under the floor of the crater. This layer is called a "breccia lens".
- Shatter cones, which are chevron-shaped impressions in rocks. Such cones are formed most easily in fine-grained rocks.
- High-temperature rock types, including laminated and welded blocks of sand, and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: it is reported in the scientific literature that some "shock" features, such as small shatter cones, which are often reported as being associated only with impact events, have been found in terrestrial volcanic ejecta.
- Microscopic pressure deformations of minerals. These include fracture patterns in crystals of quartz and feldspar, and formation of high-pressure materials such as diamond, derived from graphite and other carbon compounds, or stishovite and coesite, varieties of shocked quartz. Craters can also be created from underground nuclear explosions. One of the most crater-pocked sites on the planet is the Nevada Test Site, where a number of craters were purposely made during its years as a center for nuclear testing (see, for example, Operation Plowshare).

Crater categorization

In 1978, Chuck Wood and Leif Andersson of the Lunar & Planetary Lab devised a system of categorization of lunar impact craters. They used a sampling of craters that were relatively unmodified by subsequent impacts, then grouped the results into five broad categories. These successfully accounted for about 99% of all lunar impact craters. The LPC Crater Types were as follows:
- ALC — small, cup-shaped craters with a diameter of about 10 km or less, and no central floor. The archetype for this category is 'Albategnius C'.
- BIO — similar to an ALC, but with small, flat floors. Typical diameter is about 15 km. The lunar crater archetype is Biot.
- SOS — the interior floor is wide and flat, with no central peak. The inner walls are not terraced. The diameter is normally in the range of 15-25 km. The archetype is Sosigenes crater.
- TRI — these complex craters are large enough so that their inner walls have slumped to the floor. They can range in size from 15-50 km in diameter. The archetype crater is Triesnecker.
- TYC — these are larger than 50 km, with terraced inner walls and relatively flat floors. They frequently have large central peak formations. Tycho crater is the archetype for this class. Beyond a couple of hundred kilometers diameter, the central peak of the TYC class disappear and they are classed as basins.

Lists of craters

Notable impact craters on Earth

- Barringer Crater (US)
- Carolina bays (Eastern US)
- Chesapeake Bay impact crater (Eastern US)
- Chicxulub Crater (Mexico)
- Haughton impact crater (Canada)
- Lonar crater (India)
- Mahuika crater (New Zealand)
- Manicouagan Reservoir (Canada)
- Manson crater (US)
- Mistastin crater (Canada)
- Nördlinger Ries (Germany)
- Panther Mountain New York, (US)
- Rochechouart crater (France)
- Sudbury Basin (Canada)
- Silverpit crater (United Kingdom, located in the North Sea)
- Rio Cuarto craters (Argentina)
- The Siljan Ring (Sweden)
- Vredefort crater (Vredefort, South Africa)
- Weaubleau-Osceola impact structure (Central US)
- Kaali crater (Estonia) See the [http://www.unb.ca/passc/ImpactDatabase/essay.html Earth Impact Database,] a website concerned with over 160 identified impact craters on the Earth.

Some extraterrestrial craters

- Caloris Basin (Mercury)
- Hellas Basin (Mars)
- Mare Orientale (Moon)
- Petrarch crater (Mercury)
- South Pole-Aitken basin (Moon)
- Herschel crater (Mimas)

References

- Charles A. Wood and Leif Andersson, [http://adsabs.harvard.edu//full/seri/LPSC./0009//0003669.000.html New Morphometric Data for Fresh Lunar Craters], 1978, Procedings 9th Lunar and Planet. Sci. Conf.

External links

- [http://planetscapes.com/solar/eng/tercrate.htm Photographs of terrestrial impact craters.]
- [http://scsn.seis.sc.edu/Publications/GRLFinalDraft(web).pdf a study of a South Carolina crater]
- [http://www.unb.ca/passc/ImpactDatabase/CIDiameterSort.html The Geological Survey of Canada Crater database, 172 impact structures]
- [http://www.spacedaily.com/news/deepimpact-02k.html A recent news report about tektites]
- [http://www.ottawa.rasc.ca/astronomy/earth_craters/index.html Aerial Explorations of Terrestrial Meteorite Craters]
- [http://bbs.keyhole.com/ubb/download.php?Number=71111 Google Earth Placemarker based on the Geological Survey of Canada Crater database] Category:Craters
- Category:Planetary science Category:Depressions ko:크레이터 ja:クレーター

NASA

] The National Aeronautics and Space Administration (NASA), which was established in 1958, is the agency responsible for the public space program of the United States of America. It is also responsible for long-term civilian and military aerospace research.

Vision and mission

NASA's vision is "to improve life here, extend life to there, and to find life beyond." Its mission is "to understand and protect our home planet; to explore the Universe and search for life; and to inspire the next generation of explorers."

History

Space Race

:For additional background, please see the Space Race article Space Race launch of Redstone rocket and NASA's Mercury 3 capsule Freedom 7 with Alan Shepard Jr. on the United States' first human flight into sub-orbital space. (Atlas rockets were used to launch Mercury's orbital missions.)]] Following the Soviet space program's launch of the world's first man-made satellite (Sputnik 1) on October 4, 1957, the attention of the United States turned toward its own fledgling space efforts. The U.S. Congress, alarmed by the perceived threat to U.S. security and technological leadership, urged immediate and swift action; President Dwight D. Eisenhower and his advisers counseled more deliberate measures. Several months of debate produced agreement that a new federal agency was needed to conduct all nonmilitary activity in space. On July 29, 1958, President Eisenhower signed the National Aeronautics and Space Act of 1958 establishing the National Aeronautics and Space Administration (NASA). When it began operations on October 1, 1958, NASA consisted mainly of the four laboratories and some 8,000 employees of the government's 46-year-old research agency for aeronautics, the National Advisory Committee for Aeronautics (NACA), though the probably most important contribution actually had its roots in the German rocket program led by Wernher von Braun, who is today regarded as the father of the United States space program. NASA's early programs were research into human spaceflight, and were conducted under the pressure of the competition between the USA and the USSR (the Space Race) that existed during the Cold War. The Mercury program, initiated in 1958, started NASA down the path of human space exploration with missions designed to discover simply if man could survive in space. Representatives from the U.S. Army (M.L. Raines, LTC, USA), Navy (P.L. Havenstein, CDR, USN) and Air Force (K.G. Lindell, COL, USAF) were selected/requested to provide assistance to the NASA Space Task Group through coordination with the existing U.S. military research and defense contracting infrastructure, and technical assistance resulting from experimental aircraft (and the associated military test pilot pool) development in the 1950s. On May 5, 1961, astronaut Alan B. Shepard Jr. became the first American in space when he piloted Freedom 7 on a 15-minute suborbital flight. John Glenn became the first American to orbit the Earth on February 20, 1962 during the 5-hour flight of Friendship 7. Once the Mercury project proved that human spaceflight was possible, project Gemini was launched to conduct experiments and work out issues relating to a moon mission. The first Gemini flight with astronauts on board, Gemini III, was flown by Virgil "Gus" Grissom and John W. Young on March 23, 1965. Nine other missions followed, showing that long-duration human space flight was possible, proving that rendezvous and docking with another vehicle in space was possible, and gathering medical data on the effects of weightlessness on humans.

Apollo program

Following the success of the Mercury and Gemini programs, the Apollo program was launched to try to do interesting work in space and possibly put men around (but not on) the Moon. The direction of the Apollo program was radically altered following President John F. Kennedy's announcement on May 25, 1961 that the United States should commit itself to "landing a man on the Moon and returning him safely to the Earth" by 1970. Thus Apollo became a program to land men on the Moon. The Gemini program was started shortly thereafter to provide an interim spacecraft to prove techniques needed for the now much more complicated Apollo missions. Gemini program.]] After eight years of preliminary missions, including NASA's first loss of astronauts with the Apollo 1 launch pad fire, and the first spacecraft to orbit the Moon (Apollo 8) at the end of 1968, the Apollo program achieved its goals with Apollo 11 which landed Neil Armstrong and Buzz Aldrin on the moon's surface on July 20, 1969 and returned them to Earth safely on July 24. Armstrong's first words upon stepping out of the Eagle lander captured the momentousness of the occasion: "That's one small step for [a] man, one giant leap for mankind." Twelve men would set foot on the Moon by the end of the Apollo program in December 1972. NASA had won the moon race, and in some senses this left it without direction, or at the very least without the public attention and interest that was necessary to guarantee large budgets from Congress. After President Lyndon Johnson left office, NASA lost its main political supporter, and rocket scientist Wernher von Braun was moved to a position lobbying in Washington. Plans for ambitious follow-on projects to construct a space station, establish a lunar base and launch a human mission to Mars by 1990 were proposed but with the end to procurement of Saturn and Apollo hardware, there was no capability to support these. The near-disaster of Apollo 13, where an oxygen tank explosion nearly doomed all three astronauts, helped to recapture national attention and concern. Although missions up to Apollo 20 were planned, Apollo 17 was the last mission to fly under the Apollo banner. The program ended because of budget cuts (in part due to the Vietnam War) and the desire to develop a reusable space vehicle.

Other early missions

Although the vast majority of NASA's budget has been spent on human spaceflight, there have been many robotic missions instigated by the space agency. In 1962 the Mariner 2 mission was launched and became the first spacecraft to make a flyby of another planet – in this case Venus. The Ranger, Surveyor, and Lunar Orbiter missions were essential to assessing lunar conditions before attempting Apollo landings with humans on board. Later, the two Viking probes landed on the surface of Mars and sent color images back to Earth, but perhaps more impressive were the Pioneer and particularly Voyager missions that visited Jupiter, Saturn, Uranus and Neptune sending back scientific information and color images. Having lost the moon race, the Soviet Union had, along with the USA, changed its approach. On July 17, 1975 an Apollo craft (finding a new use after the cancelling of planned lunar flights) was docked to the Soviet Soyuz 19 spacecraft, in the Apollo-Soyuz Test Project. Although the Cold War would last many more years, this was a critical point in NASA's history and much of the international co-operation in space exploration that exists today has its genesis with this mission. America's first space station, Skylab, occupied NASA from the end of Apollo until the late 1970s.

Shuttle era

Skylab 1981 ]] The space shuttle became the major focus of NASA in the late 1970s and the 1980s. Planned to be a frequently launchable and mostly reusable vehicle, four space shuttles were built by 1985. The first to launch, Columbia did so on April 12, 1981. The shuttle was not all good news for NASA – flights were much more expensive than initially projected, and even after the 1986 Challenger disaster highlighted the risks of space flight, the public again lost interest as missions appeared to become mundane. Work began on Space Station Freedom as a focus for the manned space programme but within NASA there was argument that these projects came at the expense of more inspiring unmanned missions such as the Voyager probes. The Challenger disaster aside the late 1980s marked a low point for NASA. Nonetheless, the shuttle has been used to launch milestone projects like the Hubble Space Telescope (HST). The HST was created with a relatively small budget of $2 billion but has continued operation since 1990 and has delighted both scientists and the public. Some of the images it has returned have become near-legendary, such as the groundbreaking Hubble Deep Field images. The HST is a joint project between ESA and NASA, and its success has paved the way for greater collaboration between the agencies. In 1995 Russian-American interaction would again be achieved as the Shuttle-Mir missions began, and once more a Russian craft (this time a full-fledged space station) docked with an American vehicle. This cooperation continues to the present day, with Russia and America the two biggest partners in the largest space station ever built – the International Space Station (ISS). The strength of their cooperation on this project was even more evident when NASA began relying on Russian launch vehicles to service the ISS following the 2003 Columbia disaster, which grounded the shuttle fleet for well over two years. Costing over one hundred billion dollars, it has been difficult at times for NASA to justify the ISS. The population at large have historically been hard to impress with details of scientific experiments in space, preferring news of grand projects to exotic locations. Even now, the ISS cannot accommodate as many scientists as planned. During much of the 1990s, NASA was faced with shrinking annual budgets due to Congressional belt-tightening in Washington, DC. In response, NASA's ninth administrator, Daniel S. Goldin, pioneered the "faster, better, cheaper" approach that enabled NASA to cut costs while still delivering a wide variety of aerospace programs (Discovery Program). That method was criticized and re-evaluated following the twin losses of Mars Climate Orbiter and Mars Polar Lander in 1999.

NASA's future

Mars Polar Lander and the planned crew and heavy lift launch vehicles]] NASA's most publicly-inspiring mission of recent years has probably been the Mars Pathfinder mission of 1997. Newspapers around the world carried images of the lander dispatching its own rover, Sojourner, to explore the surface of Mars in a way never done before at any extra-terrestrial location. Less publicly acclaimed but performing science from 1997 to date (2005) has been the Mars Global Surveyor orbiter. Since 2001, the orbiting Mars Odyssey has been searching for evidence of past or present water and volcanic activity on the red planet. NASA expects to continue exploring the Red Planet with more spacecraft such as the Mars Reconnaissance Orbiter, which will reach Mars in 2006. The Space Shuttle Columbia disaster in 2003, which killed the crew of six American and one Israeli astronaut, and caused a 29-month hiatus in space shuttle flights, triggered a serious re-examination of NASA's priorities. The U.S. government, various scientists, and the public all considered the future of the space program. On January 14, 2004, ten days after the landing of Mars Exploration Rover Spirit, President George W. Bush announced a new plan for NASA's future, dubbed the Vision for Space Exploration. According to this plan, humankind will return to the moon by 2020, and set up outposts as a testbed and potential resource for future missions. The space shuttle will be retired in 2010 and the Crew Exploration Vehicle will replace it by 2014, capable of both docking with the ISS and leaving the Earth's orbit. The future of the ISS is somewhat uncertain – construction will be completed, but beyond that is less clear. Although the plan initially met with skepticism from Congress, in late 2004 Congress agreed to provide start-up funds for the first year's worth of the new space vision. Hoping to spur innovation from the private sector, NASA established a series of Centennial Challenges, technology prizes for non-government teams, in 2004. The Challenges include tasks that will be useful for implementing the Vision for Space Exploration, such as building more efficient astronaut gloves.

Criticisms

Some commentators, such as Mark Wade, note that NASA has suffered from a 'stop-start' approach to its human spaceflight programs. The Apollo spacecraft and Saturn family of launch vehicles were abandoned in 1970 after billions of dollars had been spent on their development. In 2004 the U.S. Government proposed eventually replacing the Shuttle with a Crew Exploration Vehicle that would allow the agency to again send astronauts to the Moon. Despite the reduction of its budget following project Apollo, NASA has maintained a top-heavy bureaucracy resulting in inflated costs and compromised hardware. Crew Exploration Vehicle on October 31, 1998.]] Currently, the ISS relies on the Shuttle fleet for all major construction shipments. The Shuttle fleet has lost two spacecraft and fourteen astronauts in two disasters in 1986 and 2003. While the 1986 loss was made up with a Shuttle built from replacement parts, NASA does not plan to build another shuttle to replace the second loss. (But see also CEV.) The ISS, which was intended to have a crew of seven as of 2005, now has a skeleton crew of two, causing many intended research projects to be delayed. Other nations that have invested heavily in the space station's construction, such as the members of the European Space Agency, are fearful that the ISS's fate will soon match the fate of Skylab. As of 2005, however, all of the European and Japanese contributions to the ISS are years behind development schedule themselves.

NASA spaceflight missions

Human spaceflight

- Mercury program
- Gemini program
- Apollo program
- Skylab
- Space Shuttle
- International Space Station (working together with ESA, Rosviakosmos and JAXA)
- Project Constellation

Robotic space missions

- Earth Observing
- Upper Atmosphere Research Satellite
- TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)
- Lunar missions
- Ranger
- Surveyor
- Lunar Orbiter
- Clementine
- Lunar Prospector
- Mercury missions
- Mariner 10
- MESSENGER
- Venus missions
- Mariner 2, 5 and 10
- Pioneer Venus
- Magellan
- Mars missions
- Mariner 4, 6, 7, 8 and 9
- Viking 1 and 2
- Mars Observer
- Mars Pathfinder
- Mars Climate Orbiter
- Mars Polar Lander
- Mars Global Surveyor
- 2001 Mars Odyssey
- Mars Exploration Rovers
- Mars Reconnaissance Orbiter
- Phoenix Lander (Planned for 2007)
- Mars Science Laboratory (Planned for 2009)
- Jupiter missions
- Pioneer 10
- Galileo
- Juno
- Saturn missions
- Cassini-Huygens together with ESA
- Multi-planet missions
- Pioneer 11 – Jupiter and Saturn
- Mariner 10 – Venus and Mercury
- Voyager 1 – Jupiter and Saturn
- Voyager 2 – Jupiter, Saturn, Uranus and Neptune
- New Horizons (Planned for 2006) – Jupiter, Pluto and Kuiper Belt
- Asteroidal/cometary missions
- NEAR Shoemaker
- Deep Space 1
- Stardust
- Deep Impact
- Dawn (Planned for 2006)
- Proposed or canceled planetary-asteroid missions
- JIMO (cancelled)
- CRAF (cancelled)
- NetLanders (cancelled)
- Pluto Kuiper Express (cancelled; New Horizons is replacement)
- Titan Explorer (proposed)
- Neptune Orbiter (proposed)
- Sun observing missions
- SOHO – ESA partnership
- Ulysses – ESA partnership
- Great Observatories for Space Astrophysics
- Hubble Space Telescope – ESA partnership
- Compton Gamma Ray Observatory
- Chandra X-ray Observatory
- Spitzer Space Telescope (formerly known as the Space Infrared Telescope Facility, SIRTF)
- Other observatories
- COBE
- FUSE
- Infrared Astronomical Satellite
- James Webb Space Telescope – ESA partnership
- WMAP

List of NASA administrators

# T. Keith Glennan (1958–1961) # James E. Webb (1961–1968) # Thomas O. Paine (1969–1970) # James C. Fletcher (1971–1977) # Robert A. Frosch (1977–1981) # James M. Beggs (1981–1985) # James C. Fletcher (1986–1989) # Richard H. Truly (1989–1992) # Daniel S. Goldin (1992–2001) # Sean O'Keefe (2001–2005) # Michael Griffin (2005–)

Field installations

In addition to headquarters in Washington, D.C., NASA has field installations at:
- Ames Research Center, Moffett Field, California
- Dryden Flight Research Center, Edwards, California
- John H. Glenn Research Center at Lewis Field, Cleveland, Ohio
- Goddard Space Flight Center, Greenbelt, Maryland
- Goddard Institute for Space Studies, New York, New York
- Independent Verification and Validation Facility, Fairmont, West Virginia
- Wallops Flight Facility, Wallops Island, Virginia
- Jet Propulsion Laboratory, near Pasadena, California
- Deep Space Network stations:
- Goldstone Deep Space Communications Complex, Barstow, California
- Madrid Deep Space Communication Complex, Madrid, Spain
- Canberra Deep Space Communications Complex, Canberra, Australian Capital Territory
- Lyndon B. Johnson Space Center, Houston, Texas
- White Sands Test Facility, Las Cruces, New Mexico
- John F. Kennedy Space Center, Florida
- Langley Research Center, Hampton, Virginia
- George C. Marshall Space Flight Center, Huntsville, Alabama
- Michoud Assembly Facility, New Orleans, Louisiana
- John C. Stennis Space Center, Bay St. Louis, Mississippi

Awards and decorations

NASA presently bestows a number of medals and decorations to astronauts and other NASA personnel. Some awards are authorized for wear on active duty military uniforms. Current NASA awards are as follows:
- Congressional Space Medal of Honor
- NASA Distinguished Public Service Medal
- NASA Distinguished Service Medal
- NASA Equal Employment Opportunity Medal
- NASA Exceptional Achievement Medal
- NASA Exceptional Administrative Achievement Medal
- NASA Exceptional Bravery Medal
- NASA Exceptional Engineering Achievement Medal
- NASA Exceptional Scientific Achievement Medal
- NASA Exceptional Service Medal
- NASA Exceptional Technological Achievement Medal
- NASA Outstanding Leadership Medal
- NASA Public Service Medal
- NASA Space Flight Medal

Related legislation

- 1958 – National Aeronautics and Space Administration PL 85-568 (passed on July 29)
- 1961 – Apollo mission funding PL 87-98 A
- 1970 – National Aeronautics and Space Administration Research and Development Act PL 91-119
- 1984 – National Aeronautics and Space Administration Authorization Act PL 98-361
- 1988 – National Aeronautics and Space Administration Authorization Act PL 100-685
- NASA Budget 1958–2005 in 1996 Constant Year Dollars

External links

General

- [http://www.nasa.gov NASA Home Page]
- [http://www.nasawatch.com NASA Watch]
-

Further research

- [http://history.nasa.gov/series95.html NASA History Series Publications]
- [http://history.nasa.gov/SP-4012/cover.html NASA Historical Data Books (SP-4012)]
- [http://www.hq.nasa.gov/office/pao/History/hhrhist.pdf Research in NASA History: A Guide to the NASA History Program (large PDF – over 1,012 kb)]
- [http://ntrs.nasa.gov/ NTRS: NASA Technical Reports Server]
- [http://www.eventscope.org Eventscope] Category:Independent Agencies of the United States Government ko:미국항공우주국 ja:アメリカ航空宇宙局 simple:NASA th:องค์การนาซา

Depression (geology)

Depression in geology is a landform sunken or depressed below the surrounding area. Depressions may be formed by various mechanisms, and may be referred to by a variety of technical terms.
- A blowout is a depression created by wind erosion typically in either a desert sand or post-glacial loess environment.
- A graben is a down dropped and typically linear depression or basin created by rifting in a region under tensional tectonic forces.
- An impact crater is a depression created by an impact such as a meteorite crater.
- A depression may be an area of subsidence caused by the collapse of an underlying structure. Examples include sinkholes in karst topography, calderas or maars in volcanic areas, or kettles in glaciated terraines.
- A depression may be a region of tectonic downwarping typically associated with subduction zones and fore-arc and back-arc basins associated with island arcs. These typically result in sedimentary basins as they fill with sediment from adjacent island arc or continental volcanism and uplift.
- A depression may result from the weight of overlying material such as an ice sheet during continental glaciation which is subsequently removed resulting in a basin which slowly rebounds.
- A depression may be a pothole - either a simple roadway depression or a fluvial erosional depression in a river streambed, or area affected by coastal water currents. One of many impressive depressions is the Great Rift Valley of East Africa. Perhaps even more impressive is the Atlantic Ocean basin.
- Category:Landforms

Natural satellite

The common noun moon (not capitalized) is used to mean any natural satellite of the other planets. There are at least 140 moons within Earth's solar system, and presumably many others orbiting the planets of other stars. The large gas giants have extensive systems of moons, including half a dozen comparable in size to Earth's moon. Mercury and Venus have no moons at all, Earth has one large moon ("The Moon"), Mars has two tiny moons, and Pluto has three, including a large companion called Charon (Pluto and Charon are sometimes considered a double planet).

Origin

Most moons are assumed to have been formed out of the same collapsing region of protoplanetary disk that gave rise to its primary. However, there are many exceptions and variations to this standard model of moon formation that are known or theorized. Several moons are thought to be captured asteroids; others may be fragments of larger moons shattered by impacts, or (in the case of Earth's Moon) a portion of the planet itself blasted into orbit by a large impact. As most moons are known only through a few observations via probes or telescopes, most theories about their origins are still uncertain.

Orbital characteristics

Most moons in the solar system are tidally locked to their primaries, meaning that one side of the moon is always turned toward the planet. Exceptions are Saturn's moon Hyperion, which rotates chaotically due to a variety of external influences, and the outermost moons of the gas giants, which are too far away to become 'locked' (an example is Saturn's moon Phoebe). It is not possible for a moon to have moons of its own: the tidal effects of their primaries would make such a system unstable. However, several moons have small companions in the Lagrangian points of their orbits (e.g., Saturn's moons Tethys and Dione). The recent discovery of 243 Ida's moon Dactyl confirms that some asteroids also have moons. Some, like 90 Antiope, are double asteroids with two equal-sized components. The asteroid 87 Sylvia has two moons. See asteroid moon for further information.

Moons of the Solar system

The largest moons in the solar system (those bigger than about 3000 km across) are Earth's Moon, Jupiter's Galilean moons Io, Europa, Ganymede, and Callisto, Saturn's moon Titan, and Neptune's captured moon Triton. For smaller moons see the articles on the appropriate planet. The following is a comparative table classifying the moons of the solar system by diameter. The column on the right includes some notable planets, asteroids and Kuiper belt objects for comparison.

Diameter(km)	Earth	Mars	Jupiter	Saturn	Uranus	Neptune	Pluto	Other objects
5000-6000			Ganymede	Titan
4000-5000			Callisto					Mercury
3000-4000	Luna		Io Europa
2000-3000						Triton		Pluto
1000-2000				Rhea Iapetus Dione Tethys	Titania Oberon Umbriel Ariel		Charon	90377 Sedna 90482 Orcus 50000 Quaoar 20000 Varuna 28978 Ixion
100-1000			Himalia Amalthea	Enceladus Mimas Hyperion Phoebe Janus Epimetheus Prometheus	Miranda Sycorax Puck Portia	Proteus Nereid Larissa Galatea Despina	S/2005 P 1² S/2005 P 2²	1 Ceres 2 Pallas 4 Vesta 10 Hygiea 511 Davida 704 Interamnia 3 Juno (and many others)
50-100			Thebe Elara Pasiphaë	Pandora	Caliban Juliet Belinda Cressida Rosalind Desdemona Bianca	Thalassa Naiad S/2002 N 4		(Too many to list)
10-50		Phobos Deimos	Carme Metis Sinope Lysithea Ananke Leda Adrastea	Siarnaq Atlas Helene Albiorix Telesto Pan Paaliaq Calypso Ymir Kiviuq Tarvos Ijiraq	Ophelia Cordelia Setebos Prospero Stephano Perdita S/2001 U 2 S/2001 U 3 Margaret Trinculo Mab Cupid	S/2002 N 1 S/2002 N 2 S/2002 N 3 Psamathe		(Too many to list)
less than 10	Cruithne¹		At least 47, see Jupiter's natural satellites for a listing.	Erriapo Narvi Skathi Mundilfari Suttungr Thrymr Pallene Polydeuces Methone S/2004 S 3 Daphnis				(Too many to list)

¹) Cruithne is not a real moon; it is mainly placed here for comparison's sake.
²) Diameters of the new Plutonian satellites are still very poorly known, but they are estimated to lie between 64 and 200 km.
In addition to the moons of the various planets there are also over 30 known asteroid moons, asteroids that orbit other asteroids.

External links

Jupiter's moons

- [http://www.ifa.hawaii.edu/~sheppard/satellites/jupsatdata.html Data on Jupiter's satellites]
- [http://www.ifa.hawaii.edu/faculty/jewitt/jmoons/jmoons.html Jupiter's new moons (discovered in 2000)]
- [http://www.ifa.hawaii.edu/~sheppard/satellites/jup.html Jupiter's new moons (discovered in 2002)]
- [http://www.ifa.hawaii.edu/~sheppard/satellites/jup2003.html Jupiter's new moons (discovered in 2003)]

Saturn's moons

- [http://www.news.cornell.edu/releases/Oct00/Saturn.moons.deb.html Saturn's new moons (discovered in 2000)]
- [http://www.ifa.hawaii.edu/~sheppard/satellites/sat2003.html Saturn's new moon (discovered in 2003)]

Neptune's moons

- [http://sse.jpl.nasa.gov/whatsnew/pr/030113A.html Neptune's new moons (discovered in 2003)]

All moons

- [http://www.planetary.org/learn/solarsystem/moons.html Moons of the Solar System (The Planetary Society)]
- [http://www.ifa.hawaii.edu/~sheppard/satellites Scott Sheppard's page]
- [http://ssd.jpl.nasa.gov JPL's Solar System Dynamics page]
- [http://www.space.com/scienceastronomy/planet_photo_040910.html Moon of an Object? First Photo of Satellite Beyond the Solar System]
- [http://planetarynames.wr.usgs.gov/append7.html USGS list of named moons] ----
- als:Satellit (Astronomie) ko:위성 ms:Satelit semulajadi ja:衛星 th:ดาวบริวาร

Meteorite

A meteorite is a small extraterrestrial body that impacts the Earth's surface. While in space these bodies are called meteoroids, and they are called meteors after entering Earth's atmosphere but before reaching the surface. These are small asteroids, approximately boulder-sized or less. When it enters the atmosphere, air drag and friction cause the body to heat up and emit light, thus forming a fireball or shooting star. More generally, a meteorite on a celestial body is a small body that has come from elsewhere in space.

Overview

shooting star (Full image)]] Most meteors disintegrate when entering the Earth's atmosphere, making impact events (Earth impacts) on the surface uncommon. About 500 baseball-sized rocks reach the surface each year. Large meteorites may strike the ground with considerable force, leaving behind an impact crater. The kind of crater will depend on the size, composition, degree of fragmentation, and incoming angle of the meteor. The force of collision may cause widespread destruction. Occasional damage to property, livestock, and even people has been recorded in historic times. In the case of comet fragments, which are largely composed of ice, a considerable concussion may occur, even though no fragment of the original meteoroid survives; the famed Tunguska event is thought to have resulted from such an incident. 79% of meteorites are chondrites - balls of mafic minerals with small grain size indicative of rapid cooling. In most chondrites small spherules, called chondrules, can be found. Chondrites are typically about 4.6 billion years old and are thought to represent material from the asteroid belt. It is unknown how they formed. Carbonaceous chondrites, thought to be unaltered solar nebula material, constitute about 5% of meteorites and contain small amounts of organic materials, including amino acids. Also, presolar grains are identified in carbonaceous chondrites. The isotope ratios of carbonaceous chondrites are similar to those of the Sun. Sun.]] Achondrites are similar to terrestrial mafic igneous rocks and sometimes are brecciated. Achondrites constitute about 8% of the incoming material and are thought to represent crustal material of some of the larger asteroids (mostly 4 Vesta) and occasionally Mars. About 6% of meteorites are iron meteorites with intergrowths of iron-nickel alloys, such as kamacite. Unlike chondrites, the crystals are large and appear to represent slow crystallization. Iron meteorites are thought to be the core material of one or more planets that subsequently broke up. Stony iron meteorites constitute the remaining 2%. They are a mixture of iron-nickel and silicate minerals. They are thought to have originated in the boundary zone above the core regions where iron meteorites originated. A small number of meteorites belong to additional groups or subgroups with unique chemical characteristics relative to other members of the larger groups, such as lunar meteorites or Martian meteorites. Tektites (from Greek tektos, molten), natural glass objects up to a few centimeters in size, were formed--according to most scientists--by the impact of large meteorites on Earth's surface, although a few researchers favor an origin from the Moon as volcanic ejecta. A classification of meteorite types can be found here. One theory stipulates that a large meteorite impact caused the mass extinction of the dinosaurs. It is also theorized that meteorites caused other mass extinction events as well throughout the history of the earth. mass extinction events]] The only reported fatality from meteorite impacts is an Egyptian dog who was killed in 1911, although this report is disputed. The meteorites that struck this area were identified in the 1980s as Martian in origin. The first known modern case of a human hit by a space rock [http://imca.repetti.net/metinfo/metstruck.html] occurred on November 30 1954 in Sylacauga, Alabama. There a 4 kg stone chondrite meteorite [http://internt.nhm.ac.uk/jdsml/research-curation/projects/metcat//detail.dsml?Key=S4530&index= ] crashed through a roof and hit Ann Hodges in her living room after it bounced off her radio. She was badly bruised. Several persons have since claimed [http://home.earthlink.net/~magellon/news1.html] to have been struck by 'meteorites' but no verifiable meteorites have resulted. Indigenous peoples often prized iron-nickel meteorites as an easy, if limited, source of iron metal.

Notable meteorites

- Heat Shield Rock
- Sayh al Uhaymir
- Willamette Meteorite (the largest meteorite ever found in the United States)
- Orgueil meteorite
- Canyon Diablo meteorite
- Sikhote-Alin Meteorite
- ALH84001

External links

- [http://www.aerolite.org/meteorite-photography.htm Aerolite.org: Meteorite photographs, articles on meteorite hunting]
- [http://www.meteorites.com.au/information.html Meteorites.com.au - Meteorite Information]
- [http://www.meteorite.fr/en/news/ Meteorite.fr - All about Meteorites]
- [http://www.nhm-wien.ac.at/NHM/Mineral/MetCollecte.htm Natural History Museum of Vienna]
- [http://www.meteorieten.com Heavenly Bodies - Meteorite information (E / NL)]
- [http://www.meteoriticalsociety.org/ Meteoritical Society]
- [http://flood.nhm.ac.uk/cgi-bin/earth/metcat/ The Natural History Museum's Meteorite Catalogue Database]
- [http://imca.repetti.net/metinfo/metstruck.html Meteorite hits]
- [http://www.jensenmeteorites.com/largestmeteorites.htm Largest meteorites]
- [http://ourworld.compuserve.com/homepages/dp5/dust2.htm Article with image of Hoba, world's largest meteorite]
- ko:운석 ja:隕石 th:อุกกาบาต

Crater Lake

__NOTOC__ :For the general term of a geological feature that goes by the same name, see crater lake. crater lake Crater Lake is a lake in the U.S. state of Oregon that is 5 by 6 miles (8 by 9.6 km) and 1958 ft (597 m) deep. It is Crater Lake National Park's most prominent feature and is famous for its deep blue color, water clarity and vertical driftwood, named Old Man of the Lake. The lake partly fills a nearly 4000 ft (1220 m) deep caldera that was formed around 6900 years ago by the collapse of the volcano Mount Mazama. Mount Mazama The lake's average depth is around 1500 ft (450 m). Its deepest point has been measured at 1932 ft (589 m) deep, though as with any lake its depth fluctuates with the climate, particularly rainfall [http://soundwaves.usgs.gov/2000/09/fieldwork.html]. This makes Crater Lake the deepest lake in the United States, the second deepest lake in the Western Hemisphere (Great Slave Lake is first) and the seventh deepest lake in the world (Lake Baikal is first). The caldera rim ranges in elevation from 7000 to 8000 ft (2130 to 2440 m). The Oregon state quarter, released in 2005, features an image of Crater Lake. 2005 2005

Geology

2005 For more detail, see Mount Mazama. The caldera was created in a massive volcanic eruption that lead to the subsidence of Mount Mazama around 4860 BC. Since that time, all eruptions on Mazama have been confined to the caldera. Lava eruptions later created a central platform, Wizard Island, Merriam Cone, and other, smaller volcanic features, including a rhyodacite dome that was eventually created atop the central platform. Sediments and landslide debris also covered the caldera floor. In time, the caldera cooled, allowing rain and snow to accumulate and eventually form a lake. Landslides from the caldera rim thereafter formed debris fans and turbidite sediments on the lakebed. Fumaroles and hot springs remained common and active during this time. In time, the slopes of the caldera rim more or less stabilized, streams restored a radial drainage pattern on the mountain, and dense forests revegetated the barren landscape. Some hydrothermal activity remains at the lake floor, suggesting that someday in the future Mazama may erupt again. [http://craterlake.wr.usgs.gov/geology.html]

References

- Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris, (Mountain Press Publishing Company, Missoula; 1988) ISBN 0-87842-220-X
- Geology of National Parks: Fifth Edition, Ann G. Harris, Esther Tuttle, Sherwood D., Tuttle (Iowa, Kendall/Hunt Publishing; 1997) ISBN 0-7872-5353-7

External links

- [http://www.crater.lake.national-park.com/ Crater Lake National Park information]
- [http://craterlake.wr.usgs.gov/ Crater Lake Data Clearinghouse] of the United States Geological Survey
- [http://www.pbase.com/ngruev/image/36607126/large Panoramic view of Crater Lake]
- [http://www.nationalparksgallery.com/parks/Crater-Lake-National-Park Crater Lake Pictures]
- [http://www.thetownmenu.com/klamathfalls/attractions/craterlakepano.html More Crater Lake photos]
- [http://maps.google.com/maps?ll=42.942383,-122.108499&spn=0.133038,0.187798&t=k&hl=en Satellite image of Crater Lake (Google Maps)]
- [http://www.pbase.com/image/33010525 Picture and description of the Old Man of the Lake]
- [http://video.google.com/videoplay?docid=-345603526721135094 video taken from from shore of Crater Lake] Category:Volcanic calderas Category:Volcanoes of Oregon Category:Lakes of Oregon Category:Cascade Range

Palimpsest

A palimpsest is a manuscript page, scroll, or book that has been written on, scraped off, and used again. The word palimpsest comes from two Greek roots (palin + psEn) meaning "scraped again." Romans wrote on wax-coated tablets that could be reused, and a passing use of the rather bookish term "palimpsest" by Cicero seems to refer to this practice. Cicero]]

Development of palimpsests

Because parchment and vellum, both prepared from animal hides, are more durable than paper or papyrus, most palimpsests known to modern scholars are parchment, which rose in popularity in western Europe after the 6th century A.D. Also, where papyrus was in common use, reuse of writing media was less common because papyrus was cheaper and more expendable than costly parchment. Some papyrus palimpsests still survive, and Romans referred to this custom of washing papyrus. The reed from which it was made did not grow in Italy. With the passing of time the faint remains of the former writing that had been washed from parchment or vellum, using milk and oat bran, would reappear enough so that scholars can make out the text (which they call the scriptio inferior, the "underwriting") and decipher it. In the later Middle Ages the surface of the vellum was usually scraped away with powdered pumice, irretrievably losing the writing. Therefore the most valuable palimpsests are those that were overwritten in the early Middle Ages.

Modern decipherment

Scholars of the 19th century used chemical means to read palimpsests that were sometimes very destructive, using tincture of gall or later, ammonium hydrosulfate. Modern methods using ultraviolet and photography are less damaging. Superposed photographs exposed in various light spectra, a technique called "multispectral filming," can bring up the contrast of faded ink against parchment that is too indistinct to be read by eye in normal light. Innovative digitized images have come to aid scholars in deciphering unreadable palimpsests.

The palimpsest as a form of destruction

Pagan manuscripts have often only survived as palimpsests. Much of the cultural heritage of Antiquity that is commonly said to have been preserved by the Church was actually transmitted inadvertently, through palimpsests. The primary cause of the purposeful destruction of vellum manuscripts was the dearth of material. In the case of Greek manuscripts, so great was the consumption of old codices for the sake of the material, that a synodal decree of the year 691 forbade the destruction of manuscripts of the Scriptures or the church fathers, imperfect or injured volumes excepted. The decline of the vellum trade with the introduction of paper exacerbated the scarcity, which was only to be made good by recourse to material already once used. Cultural considerations combined with such economic ones to motivate the creation of palimpsests. The demand for new texts might outstrip the availability of parchment in some centers, yet the existence of cleaned parchment that was never overwritten suggests that there was also a spiritual motivation, to sanctify pagan text by overlaying it with the word of God, somewhat as pagan sites were overlaid with Christian churches to hallow pagan ground. Or the pagan texts may have merely appeared irrelevant. Texts most susceptible to being overwritten included obsolete legal and liturgical ones, sometimes of intense interest to the historian. Early Latin translations of Scripture were rendered obsolete by Jerome's Vulgate. Texts might be in foreign languages or written in unfamiliar scripts that had become illegible in time. The codices themselves might be already damaged or incomplete. Heretical texts were dangerous to harbor: there were compelling political and religious reasons to destroy texts viewed as heresy, and to reuse the media was less wasteful than simply to burn the books. Vast destruction of the broad quartos of the early centuries of our era took place in the period which followed the fall of the Roman Empire, but palimpsests were also created as new texts were required during the Carolingian renaissance. The most valuable Latin palimpsests are found in the codices which were remade from the early large folios in the 7th to the 9th centuries. It has been noticed that no entire work is generally found in any instance in the original text of a palimpsest, but that portions of many works have been taken to make up a single volume. An exception is the Archimedes palimpsest (see below). On the whole, Early Medieval scribes were indiscriminate in supplying themselves with material from any old volumes that happened to be at hand.

Some famous palimpsests

- The Codex Ephraemi Rescriptus, Bibliothèque nationale de France, Paris: portions of the Old and New Testaments in Greek, attributed to the 5th century, are covered with works of Ephraem the Syrian in a hand of the 12th century
- Among the Syriac manuscripts obtained from the Nitrian desert in Egypt, British Museum, London: important Greek texts
- A volume containing a work of Severus of Antioch of the beginning of the 9th century is written on palimpsest leaves taken from 6th century manuscripts of the Iliad and the Gospel of St Luke, both of the 6th century, and the Euclid's Elements of the 7th or 8th century, British Museum
- A double palimpsest, in which a text of St John Chrysostom, in Syriac, of the 9th or 10th century, covers a Latin grammatical treatise in a cursive hand of the 6th century, which in its turn has displaced the Latin annals of the historian Granius Licinianus, of the 5th century, British Museum.
- The only known hyper-palimpsest: the Novgorod Codex, in which maybe hundreds of texts have left their traces on the wooden back wall of a wax tablet
- The Ambrosian Plautus, in rustic capitals, of the 4th or 5th century, re-written with portions of the Bible in the 9th century, Ambrosian Library
- Cicero, De republica in uncials, of the 4th century, covered by St Augustine on the Psalms, of the 7th century, Vatican Library
- Codex Theodosianus of Turin, of the 5th or 6th century
- the Fasti Consulares of Verona, of A.D. 486
- the Arian fragment of the Vatican, of the 5th century
- the letters of Cornelius Fronto
- the Archimedes Palimpsest, a work of the great Syracusan mathematician copied onto parchment in the 10th century and overwritten by a liturgical text in the 12th century
- Sinaitic Palimpsest
- the unique copy of a Greek grammatical text composed by Herodian for the emperor Marcus Aurelius in the 2nd century, preserved in the Osterreichische Nationalbibliothek, Vienna

Alternate usage

The word palimpsest also refers to a plaque which has been turned around and engraved on what was originally the back side. In planetary astronomy, ancient lunar craters whose relief has disappeared from subequent volcanic outpourings, leaving only a "ghost" of a rim are also known as palimpsests. Icy surfaces of natural satellites like Callisto and Ganymede preserve hints of their history in these rings, where the crater's relief has been effaced by creep of the icy surface ("viscous relaxation").

Uses in culture

- Gore Vidal titled his memoirs Palimpsest
- [http://palimpsest.org.uk/ Palimpsest] is an online arts discussion forum.

External links

- [http://www.opib.librari.beniculturali.it/english/progetti/rinascimanto_virtuale/rinascimento_storia.html OPIB Virtual Renaissance Network activities in digitizing European palimpsests: history and objectives of a cooperative venture] and [http://www.rrz.uni-hamburg.de/RV/areas.html a briefer view of the project]
- [http://web.archive.org/web/20041029014117/www.evellum.com/ductus/demo/engine/ductus/frames/bibliography/lowe_loew1972f.html Brief note on economic and cultural considerations in production of palimpsests] Category:Manuscripts

Erosion

Erosion is the displacement of solids (soil, mud, rock, and so forth) by the agents of wind, water, ice, movement in response to gravity, or living organisms (in the case of bioerosion). Although the processes may be simultaneous, erosion is to be distinguished from weathering, which is the decomposition of rock. Erosion is an important natural process, but in many places it is increased by human activities. Some of those activities include deforestation, overgrazing and road or trail building. Likewise, humans have sought to limit erosion by terrace-building and tree planting. A certain amount of erosion is natural and in fact healthy for the ecosystem. For example, gravels continually move downstream in watercourses. Too much erosion, however, can cause problems, clogging streams with gravel, filling reservoirs with sediment, reducing soil fertility and water quality.

Causes

water quality] What causes erosion to be severe in some areas and minor elsewhere? It is a combination of many factors, including the amount and intensity of precipitation, the texture of the soil, the steepness of the slope, and ground cover (from vegetation, rocks, etc.). The first three factors do not change much. In general, given the same kind of vegetative cover, you expect areas with high-intensity precipitation, sandy or silty soils, and steep slopes to be the most erosive. Soils with a lot of clay that receive less intense precipitation and are on gentle slopes tend to erode less. The factor that is most subject to change is the amount and type of ground cover. When fires burn an area or when vegetation is removed as part of timber operations, building a house or a road, the susceptibility of the soil to erosion is greatly increased. clay Roads are especially likely to cause increased rates of erosion because, in addition to removing ground cover, they can significantly change drainage patterns. A road that has a lot of rock and one that is "hydrologically invisible" (that gets the water off the road as quickly as possible, mimicking natural drainage patterns) has the best chance of not causing increased erosion. One of the most serious and long-running water erosion problems on the planet is in China, on the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow each year into the ocean. The sediment originates primarily from water erosion in the Loess Plateau region of northwest China. In materials science, erosion is the recession of surfaces by repeated localized mechanical trauma as, for example, by suspended abrasive particles within a moving fluid. Erosion can also occur from non-abrasive fluid mixtures. Cavitation is one example.

Erosion processes

Cavitation

Gravity Erosion

Mass-Wasting is the down-slope movement of rock and sediments, mainly due to the force of gravity. Mass-wasting is an important part of the erosional process, as it moves material from higher elevations to lower elevations where transporting agents like streams and glaciers can then pick up the material and move it to even lower elevations. Mass-wasting processes are occurring continuously on all slopes; some mass-wasting processes act very slowly, others occur very suddenly, often with disastrous results. Any perceptible down-slope movement of rock or sediment is often referred to in general terms as a landslide. However, landslides can be classified in a much more detailed way that reflects the mechanisms responsible for the movement and the velocity at which the movement occurs. Slumping happens on steep hillsides, occurring along distinct fracture zones, often within materials like clay, that, once released, may move quite rapidly downhill. They often will show a spoon-shaped depression within which the material has begun to slide downhill. In some cases the slump is caused by water beneath the slope weakening it. In many cases it is simply the result of poor engineering along highways where it is a regular occurrence. Surface creep is the slow movement of soil and rock debris by gravity which is usually not perceptible except through extended observation. However, the term can also describe the rolling of dislodged soil particles 0.5 to 1.0 mm in diameter by wind along the soil surface.

Water erosion

Splash erosion is the detachment and airborne movement of small soil particles caused by the impact of raindrops on soils. Sheet erosion is the result of heavy rain on bare soil where water flows as a sheet down any gradient carrying soil particles. Gully erosion results where water flows along a linear depression eroding a trench or gully. Valley or stream erosion occurs with continued water flow along a linear feature. The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside. In the earliest stage of stream erosion the erosive activity is dominantly vertical, the valleys have a typical V cross-section, and the stream gradient is relatively steep. When some base level is reached the erosive activity switches to lateral erosion which widens the valley floor and creates a narrow floodplain. The stream gradient becomes nearly flat and lateral deposition of sediments becomes important as the stream meanders across the valley floor. In all stages of stream erosion by far the most erosion occurs during times of flood when more and faster moving water is available to carry a larger sediment load.

Shoreline erosion

meander, England.]] Shoreline erosion, on both exposed and sheltered coasts, primarily occurs through the action of currents and waves, but sea level change can also play a role. Sediment is transported along the coast in the direction of the prevailing current (longshore drift). When the upcurrent amount of sediment is less than the amount being carried away, erosion occurs. When the upcurrent amount of sediment is greater, sand or gravel banks will tend to form. These banks may slowly migrate along the coast in the direction of the longshore drift, alternately protecting and exposing parts of the coastline.

Ice erosion

Ice erosion is caused by movement of ice, typically as glaciers. Glaciers can scrape down a slope and break up rock and then transport it, leaving moraines, drumlins, and glacial erratics in its wake typically at the terminus or during glacial retreat. Ice wedging is the weathering process where water trapped in tiny rock cracks freezes and expands, causing the breakup of the rock. This can lead to gravity erosion on steep slopes. The scree which form at the bottom of a steep mountainside is mostly formed from pieces of rock broken away by this means. It is a common engineering problem wherever rock cliffs are alongside roads and morning thaws can drop hazardous rock pieces onto the road.

Wind erosion

Wind erosion, also known as eolian erosion is the movement of rock and/or sediment by the wind. Windbreaks are often planted by farmers to reduce wind erosion. This includes the planting of trees, shrubs, or other vegetation, usually perpendicular or nearly so to the principal wind direction. The wind causes dust particles to be lifted and therefore moved to another region.

Tectonic effects of erosion

The removal by erosion of large amounts of rock from a particular region, and its deposition elsewhere, can result in a lightening of the load on the lower crust and mantle. This can cause tectonic or isostatic uplift in the region.

Figurative use

The concept of erosion is commonly employed in analogy to various forms of perceived—or real—homogenization, "leveling out", collusion, or even the decline of anything from morals to indigenous cultures. It is quite a usual trope of the English language to describe as erosion the gradual, organic mutation of something thought of as distinct, more complex, harder to pronounce, or more refined into something indistinct, less complex, easier to pronounce, or (disparagingly) less refined.

Reference

- World Bank 2001: China: Air, Land, and Water. Category:Geomorph

Impact crater

History

Formation and structure

Crater categorization

Lists of craters

Notable impact craters on Earth

Some extraterrestrial craters

References

See also

External links

NASA

Vision and mission

History

Space Race

Apollo program

Other early missions

Shuttle era

NASA's future

Criticisms

NASA spaceflight missions

Human spaceflight

Robotic space missions

List of NASA administrators

Field installations

Awards and decorations

Related legislation

See also

Other space agencies

External links

General

Further research

Depression (geology)

Natural satellite

Origin

Orbital characteristics

Moons of the Solar system

See also

External links

Jupiter's moons

Saturn's moons

Neptune's moons

All moons

Meteorite

Overview

Notable meteorites

See also

External links

Crater Lake

Geology

See also

References

External links

Palimpsest

Development of palimpsests

Modern decipherment

The palimpsest as a form of destruction

Some famous palimpsests

Alternate usage

Uses in culture

External links

Erosion

Causes

Erosion processes

Gravity Erosion

Water erosion

Shoreline erosion

Ice erosion

Wind erosion

Tectonic effects of erosion

Figurative use

See also

Reference