Atomic Energy

 Atomic Energy

 
UN nuclear age was born almost at the same time. Scary World War II, Hiroshima and Nagasaki atomic
bombings at the end, the house needs to resolve the nuclear issue brought. From his first address to the UN General Assembly established the Atomic Energy Commission issues raised by the discovery of nuclear energy to deal with. And in 1953 President Dwight D. Eisenhower of the United States from historical address, "Nuclear for peace", 1957 International Atomic Energy Agency (IAEA) led the establishment.

Today, nearly 16 per cent of the world's 439 nuclear power reactors generate electricity. In nine countries, more than 40 per cent of energy production comes from nuclear energy. IAEA, an international organization in the UN family, nuclear energy safe, secure and peaceful use helps develop and use nuclear technology for sustainable development is to ensure.

Nuclear Weapons (NPT) extension under the 1968 agreement, the IAEA conducts site inspections to ensure that nuclear material is used only for peaceful purposes is used. Before the 2003 Iraq War, Iraq banned its inspectors uncover and eliminate weapons programs and capabilities have played an important role. In 2005, the Agency Director General Mohammad ElBaradei, Nobel Peace Prize that was awarded "They are being used for military purposes and to prevent nuclear energy is nuclear energy for peaceful purposes, to ensure efforts are used in the safest way possible is for. "

The United Nations Conference on Disarmament, the sole multilateral negotiating forum on disarmament, Comprehensive Nuclear Test Ban Treaty, which was adopted in 1996, is produced. Office for Disarmament Affairs nuclear disarmament and non-proliferation promotes. On peaceful uses of outer space nuclear power sources in outer space committee on the use of production rules is 1992. Ionizing radiation effects of nuclear radiation threat level and report security and safety standards around the world to provide scientific basis for the United Nations Scientific Committee on the Effects.

Discovery Tropical Reefs' Surviving Environmental Stresses: Corals' Choice of Symbiotic Algae May Hold the Key

DiscoveryTropical Reefs' Surviving Environmental Stresses: Corals' Choice of Symbiotic Algae May Hold the Key
Corals that host fewer species of algae are less sensitive to disturbances

Coral known as Acropora on a fringing reef at NSF's Moorea Coral Reef LTER site. Credit and Larger Version

The following is part ten in a series on the National Science Foundation's Long-Term Ecological Research (LTER) Network. Visit parts one, two, three, four, five, six, seven, eight and nine in this series.
Symbiodinium, it's technically called, but more popularly it's known as zooxanthellae.
Either way, these microscopic algae that live within a coral's tissues hold the key to a tropical reef's ability to withstand environmental stresses.
The effects on tropical corals of global warming, ocean acidification, pollution, coastal development and overfishing may all come down to how choosy the corals are about their algae tenants.
Reef corals are the sum of an animal and the single-celled algae that live inside its tissues. The animal is called the host and the algae are called endosymbionts.
It's a mutually beneficial arrangement. The corals provide the algae with protection in sunlit, shallow seas. The algae produce large amounts of energy through photosynthesis, which the corals use to survive and to build their skeletons.
The stability of this symbiotic relationship is critical to corals' survival. When corals lose their algae, they bleach out and often die.
Researchers at the University of Hawaii and other institutions have found that the more flexible corals are about their algal residents, the more sensitive they are to environmental changes.
"It's exactly the opposite of what we expected," says Hollie Putnam of the University of Hawaii and lead author of a paper published this week in the journal Proceedings of the Royal Society B.
"The finding was surprising; we thought that corals exploited the ability to host a variety of Symbiodinium to adapt to climate change."
But more is not always better, say Putnam and co-authors Michael Stat of the University of Western Australia and the Australian Institute of Marine Science; Xavier Pochon of the Cawthron Institute in Nelson, New Zealand; and Ruth Gates of the University of Hawaii.
"The relationship of corals to the algae that live within them is fundamental to their biology," says David Garrison, program director in the National Science Foundation's (NSF) Division of Ocean Sciences, which funded the research.
"This study gives us an important new understanding of how corals are likely to respond to the stresses of environmental change."
The research was conducted at NSF's Moorea Coral Reef Long-Term Ecological Research (LTER) site, one of 26 such NSF LTER sites around the globe in ecosystems from deserts to freshwater lakes, and from forests to grasslands.
Putnam and colleagues took samples from 34 species of corals at the Moorea LTER site. By analyzing the DNA from the algae in the samples, they identified the specific species of Symbiodinium.
The findings reveal that some corals host a single Symbiodinium species. Others host many.
"We were able to link, for the first time, patterns in environmental performance of corals to the number and variety of endosymbionts they host," says Putnam.
The patterns show that corals termed generalists--those that are flexible in their choice of algae residents--are more environmentally sensitive.
In contrast, environmentally resistant corals--termed specifists--associate with only one or a few specific species of Symbiodinium.
Generalists such as Acropora and Pocillopora are some of the most environmentally sensitive corals.
Conversely, specifists such as Porites harbor few Symbiodinium species and are environmentally resistant.
"Coral reefs are economically and ecologically important, providing homes for a high diversity of organisms and are necessary for food supplies, recreation and tourism in many countries," says Gates.
"The better we understand how corals respond to stress, the more capable we will be of forecasting and managing future reef communities."
It's likely that the reefs of tomorrow, say Putnam and co-authors, will be shaped by the coral-Symbiodinium assemblages of today.

Discovery Gigantic Gravity "Lenses" Magnify Galaxies Far, Far Away

Discovery
Gigantic Gravity "Lenses" Magnify Galaxies Far, Far Away
Submillimeter observatories penetrate cold, dusty galaxies to see stars forming in the primeval past

This diagram illustrates a cosmic phenomenon known as gravitational lensing. Credit and Larger Version

November 19, 2010
A chance alignment of galaxies, recently observed by a space observatory, presents the perfect opportunity for studying star-forming galaxies billions of light-years away.
For decades, astronomers have used Einstein's concept of a "gravitational lens" to study the magnified images of distant objects.  A lensing effect occurs when a massive object, such as a galaxy, is located exactly between another object and an observer.
Because the galaxy in front is so massive, its strong gravitational pull bends the light coming from behind it, creating distorted images of the background galaxy.
Sometimes, the image is so warped that it appears as a ring or arc around the foreground galaxy. Other times the result is repeated images of the background galaxy, similar to what happens when looking in a funhouse mirror. But in all cases, the distorted images are magnified, making the background galaxies appear brighter and therefore easier to study.
The universe contains billions of galaxies, but getting the chance alignment that produces a gravity lens is something like finding a needle in the haystack. Scientists must sift through large volumes of data to discover only a few possibilities, and then study these further to verify that a lensing event has occurred.
"Since about 1978, astronomers probably have found about 200 gravitational lensing events, but that involved a very slow search through lots and lots of data," said Asantha Cooray, Professor of Physics and Astronomy at University of California in Irvine.  He and dozens of other scientists from around the world collaborated on a recent project to make the process faster.
Largest space mirror
Using data collected by the Herschel Astrophysical Terahertz Large Area Survey (Herschel‑ATLAS) the team of researchers have demonstrated a way to quickly and easily find strong gravitational lensing events with close to 100 percent efficiency.
Herschel is a European Space Agency (ESA) mission with significant participation from NASA. It was launched in May of 2009, and now orbits a distant point about 1.5 million kilometers (930,000 miles) from Earth on the opposite side of our planet from the sun.
Because Herschel's mirror is the largest astronomy mirror ever launched in space (3.5 meters, or about 11.5 feet across), it can take the sharpest pictures to date at the wavelengths it detects.
"Herschel-ATLAS is aimed at understanding how stars form inside galaxies and to shed some light on the past history of galaxy growth," said Cooray who is now the lead US (NASA) investigator for Herschel-ATLAS. Cooray is supported by a CAREER award from the National Science Foundation (NSF).
Cold, dusty star factories
"Astronomers believe most galaxies undergo a rapid phase of star-formation where hundreds to a thousand stars are born within a galaxy over a year, with such activity lasting over a total period of a few to several million years," Cooray said.
"This phase involves stars that are formed in clouds enshrouded by dust," he said. "Unfortunately existing ground and space-based observatories miss most of this picture since they operate at optical wavelengths and optical light cannot penetrate dust clouds to see the young stars inside."
Seeing the (submillimeter) light
Although the background galaxy behind a gravitational lens often can't be seen by visible light telescopes, these ancient objects still glow with radiant energy in what's called the submillimeter range-wavelengths of light from about 0.3 millimeters to 1 millimeter. The Herschel observatory was specifically designed to detect energy at submillimeter wavelengths, between the microwave and far-infrared regions of the electromagnetic spectrum.
In a recent article in the journal Science, Cooray and the rest of the team describe how they used data from the Herschel-ATLAS survey, along with instruments at several ground-based observatories all over the world, to search out and pin down these dim star factories.
Using maps made by Herschel, the researchers chose five gravitational lens "candidates." These were the brightest spots they could find in the submillimeter-wave images taken of just one small section of the universe.
"To demonstrate that these bright galaxies are indeed lensing alignments of a more distant faint galaxy magnified by an intervening massive galaxy, we needed to establish the distances to both the background galaxy seen in Herschel and the foreground galaxy seen in optical images along the same sky directions," Cooray said.
To calculate these distances, they tested each candidate for redshift.
A shift in the red direction
Redshift is caused by the expansion of space as the universe ages. The further away an object, the longer its wavelengths stretch. Because red light has longer wavelengths than other visible light, the phenomenon is called redshift.
To measure redshift of the galaxies, NSF contributed support for two critical instruments: The Z-Spec, located at the Caltech Submillimeter Observatory (CSO), on the summit of Mauna Kea in Hawaii, and the Zpectrometer, at the National Radio Astronomy Observatory's Green Bank Telescope (GBT) in West Virginia.
According to Andrew Harris, professor at the University of Maryland's Department of Astronomy, the two instruments were named after the letter ‘z," which in astrophysics is used as a symbol of redshift. Harris is the primary investigator for the Zpectrometer, at GBT.
"The goal of this project was to build a ‘redshift machine' that can find the redshifts of distant galaxies with very little additional information," Harris said. "Then we can use the information from the spectra to understand the galaxies' interstellar media."
Galactic fingerprints
Both the Z-Spec and the Zpectrometer measure the spectral lines of incoming radiation. Spectral lines are patterns of light or dark lines that show up at specific wavelengths for specific atoms or molecules. These lines can be used as a signature or fingerprint, to identify the types of gases found in the galaxy being measured.
The instruments measure the redshift by measuring the change in wavelength of these spectral lines caused by the expansion of the universe. Greater distance means greater redshift.
"Of course, to measure a change in wavelength, you must first know what the original wavelength was," said James Aguirre, professor at the University of Pennsylvania and Primary Investigator for the Z-Spec at CSO. "This is greatly helped if we can identify an emission line whose identity is known, and for which we know the emission wavelength at rest from laboratory measurements."
Because carbon monoxide (CO) is abundant in distant galaxies, scientists use it to as a "fingerprint" to calculate the redshift of a galaxy.
"You can think of the CO spectral lines as marking on a ruler, which stretches depending on the distance of the galaxy," Aguirre said. "Measuring the amount of stretch gives the redshift."
"The larger the redshift the farther away the galaxy," Cooray said. "The galaxies that we have found are at redshifts around 3, which roughly correspond to an age of the universe about 2 to 3 billion years--that's about 11 billion years into the past from today."
Lenses confirmed
Aguirre and the Z-Spec team at the Caltech Submillimeter Observatory observed three of the five lens candidates and detected redshifts which were higher than and inconsistent with the redshifts derived from the optical spectroscopy of the visible, foreground galaxy.
"Z-Spec has specifically shown these three Herschel-ATLAS galaxies are at high redshift," said Aguirre. "Armed with this fact, other researchers were able to confirm that the brightest galaxies in the survey are being gravitationally lensed."
Meanwhile, the Zpectrometer team at the Green Bank Telescope was able to independently confirm the distance of one of the five galaxies, and to measure the distance of another.
"We showed all five of the bright galaxies seen by Herschel are lensed, which is not true for example when we look at optical images of the sky," Cooray said. Without Herschel, these lens events would have been completely missed.
Understanding the stars
"Given that we found five lensed galaxies easily with only two percent of our final map covered, the expectation is that we will find hundreds of such events with Herschel," Cooray said.
And with these multiple lenses, the scientists hope to tunnel further into the past, in order to better understand the history of the early universe.
"Big galaxies in this early era are unexpected because of our current model of mass assembly in the universe," said Harris, "So finding and explaining the existence of very massive young galaxies provides tight constraints on the models that drive our understanding."

--	Holly B. Martin
--	Lisa Van Pay, (703) 292-8796 lvanpay@nsf.gov

Investigators Asantha Cooray
Andrew Harris
James Aguirre

THE DISCOVERY OF NEPTUNE

(NASA, Jet Propulsion Laboratory, "Voyager at Neptune: 1989," JPL 400-353, U.S. Government Printing Office, Washington, D.C., March 1989.)
Neptune was the first planet located through mathematical predictions rather than through systematic observations of the sky.
In the years following William Herschel's discovery of Uranus in 1781, astronomers noted that Uranus was not faithfully following its predicted path. Uranus seemed to accelerate in its orbit before 1822 and to slow after that. One possible explanation was that the gravity of an undiscovered planet was affecting the orbit of Uranus.
Two young mathematicians, each working independently and with no knowledge of the other, were intrigued by the mystery and set out to solve it.
In England, John Couch Adams began work on the problem in 1841 and pursued it sporadically. By the fall of 1845, he felt confident enough in his calculations to present them to the Astronomer Royal, Sir George Airy, at the Greenwich Observatory. It was, perhaps, Adams' youth and the fact that he was an unknown astronomer that caused the older man to give little attention to Adams' work at the time.
The next summer, however, French mathematician Urbain Jean Joseph Le Verrier published his own work on the topic. When Sir George noticed that Le Verrier's work closely matched that of young Adams, he directed Professor James Challis of Cambridge Observatory to begin a search of the heavens for this object. Challis was hindered, however, by the lack of up-to-date star maps of the area to be searched and, without these, it was difficult to quickly discern new bodies from known ones. His only course was to tediously scan and rescan the sky over a period of weeks, watching for planet-like motion. He missed recognizing Neptune several times.
In September 1846, Le Verrier, unable to interest French astronomers, sent his calculations to an assistant at the Berlin Observatory, Johann Gottfried Galle. Galle received the letter on September 23 and began a search for the object that night. Galle, too, might have missed the discovery had not a student, Heinrich Louis d'Arrest, provided him with the latest star map of the area. And there, within a degree of Le Verrier's predictions (and only a few degrees from Adams' predictions) was an unidentified disk. When, by the next night, the object had a new, position, the discovery could be claimed‹an eighth planet had been found.
An international brouhaha followed,with supporters of Adams contending with those of Le Verrier for recognition of their champion. In keeping with the established practice of naming planets for ancient Roman or Greek gods, however, the new planet was called Neptune after the Roman god of the sea.
Seventy-five years earlier or later, the problem would have been mathematically insoluble. At the time of the discovery, Neptune was in the one part of its orbit that allowed solution.
The orbit calculated by Adams and Le Verrier is not precisely Neptune's orbit. Differences between the actual and predicted orbits continued to be noted by astronomers. In 1915, American Percival Lowell predicted a ninth planet, based on the differences between calculated and observed orbits of Neptune and other planets. Motivated by Lowell's ideas, V. M. Slipher, the director of Lowell Observatory in Flagstaff, Arizona, hired astronomer Clyde Tombaugh to begin an exhaustive search for this ninth planet. In 1930, 84 years after Neptune's discovery, Tombaugh discovered the planet Pluto. Pluto is now known to be far too small to have caused the apparent differences between Neptune's predicted and observed orbits, however, and the source of these differences remains unresolved.

FACTS ABOUT NEPTUNE

(Bevan M. French and Stephen P. Maran, eds., "A Meeting with the Universe," NASA EP-177, U.S. Government Printing Office, 1981.)
Neptune is even further out, 4.5 billion kilometers (2.8 billion miles) from the Sun. Through the telescope it is a green, featureless world, about the size of Uranus. Two moons have been detected. Neptune has remained untouched by the activity of the Space Age, although clouds have been detected in its atmosphere. The planet remains an enigma, too far away to see well from Earth, almost too far away to reach.
Neptune, the most distant gas giant planet from the Sun, is so far away that it is only a tiny, blurred image in the 154-cm (60-inch) telescope at the Catalina Observatory. Three images show dark absorption bands, due to the presence of atmospheric methane (CH4), across the planet's equator. Bright regions at the poles are produced by a high haze of ice crystals. An image of Neptune shows the uniform, featureless appearance of the planet when seen in visible light. (The following is from the NASA publication "Voyager at Neptune: 1989," JPL 400-353, March 1989.)

Cloud patterns were detected in the atmosphere of Neptune by ground-based observations in 1978. Uranus and Neptune are often thought of as a pair, because of their great distance from the Sun and their similarities in size and color. But already scientists expect that Neptune will be vastly different from any of the other planets yet studied. Although Neptune is the fourth largest planet, it is invisible to the naked eye because it orbits in the outer regions of the solar system, 4 1/2 billion kilometers (nearly 3 billion miles) from the Sun. (In fact, Neptune is currently the farthest planet from the Sun‹since the early 1970s, Pluto has been closer to the Sun than Neptune has, and it will remain so until the end of this century.) At this distance, Neptune receives nearly 1,000 times less sunlight than Earth, and about two and one-half times less than Uranus, but its overall temperature is about the same as that of Uranus. Therefore, scientists believe that Neptune must have some internal heat of its own, as do Jupiter and Saturn. Neptune's seasons last more than 40 years. Its rotational axis is tilted about 30 degrees to the plane of its orbit around the Sun (Earth's axis tilts 23.5 degrees). At this phase in Neptune's sojourn around the Sun, it is summer in the southern hemisphere and there is continuous daylight at the south pole, while the north pole is cloaked in darkness. Both planets rotate at about the same rate‹Uranus' internal rotation rate is 17 hours 14 minutes, while Neptune's atmospheric rotation rate is between 17 and 18 hours. Rotation rates of planets can be measured in two ways: by tracking cloud features in the atmosphere or by monitoring the radio emissions generated by electrons spiraling into the planet's magnetic field. Radio emissions give the rotation rate of the bulk of the planet because the magnetic field is generated in the planet's interior. Measurements obtained by tracking cloud features include the additional effects of atmospheric winds. As Voyager 2 nears Neptune, the planetary radio astronomy experiment will determine the rotation rate of the planet's interior. With an equatorial diameter of about 49,400 kilometers (30,700 miles), Neptune is only slightly smaller than Uranus. But Neptune is denser, indicating that it must contain a larger quantity of heavier materials than does Uranus. Like Uranus, Neptune is believed to be composed primarily of rock and melted ice, mixed with hydrogen and helium. The combination of infrared and radio observations will provide a measurement of the relative amounts of helium and hydrogen in Neptune as compared with the amounts in the other gaseous outer planets and the Sun. Despite Neptune's remoteness, astronomers have been able to learn a few things about the planet's atmosphere. (Light emitted and reflected from an atmosphere contains information about the atmosphere's chemistry and composition.) At times, high-resolution images taken from Earth-based telescopes indicate the existence of thin atmospheric hazes over major portions of the planet. The haze, which comes and goes in a matter of days or weeks, may consist of methane ice crystals. If there are methane clouds on Neptune, they probably condense at a pressure of about 2 bars (twice the atmospheric pressure at sea level on Earth) and a temperature of about 85 kelvins (-305° F). Voyager 2's radio signals can probe to a pressure level of 3 to 5 bars, so there is a good chance of detecting the base of the methane clouds, which will indicate the amount of methane in Neptune's atmosphere. Although other cloud layers, including water-ice clouds, are expected deeper in the atmosphere, Voyager 2 will not be able to detect them. While the spacecraft is in Neptune's shadow, it will maneuver to precisely track the outer edge of the planet to enable Voyager's radio signal to probe Neptune's atmosphere. There is evidence that Neptune has a magnetic field, as do Mercury, Earth, Jupiter, Saturn, and Uranus. Voyager 2 is not likely to penetrate the planet's magnetosphere until the last day before the spacecraft's closest approach to the planet. RING ARCS Jupiter, Saturn, and Uranus are encircled by ring systems, but Neptune's rings may be a series of ring arcs. A classic technique in identifying ring systems is to monitor the brightness of a star as a planet's ring region passes in front of (occults) the star as seen by the observer. Rings may be deduced if the starlight blinks off and on in a regular pattern on both sides of the planet. However, the effects that may be due to ring material near Neptune have been seen in only about 7 percent of the occultation studies to date, and never has the same ring been seen on both sides of the planet. Pieces of rings, or ring arcs, could explain these results. Currently, scientists believe that there may be three narrow (8- to 20-kilometer or 5- to 12-mile) near-circular sets of arcs in or near Neptune's equatorial plane at distances of 17,000 to 42,000 kilometers (10,500 to 26,100 miles) from the planet's cloud tops. The size of particles comprising these rings could range from tiny dust particles to pebbles. Voyager 2's flight path carried the spacecraft close to the outermost set of possible ring arcs. As at Uranus, there is likely to be diffuse material that could fill much of the space within the ring arc region. Although such a diffuse sheet of material is not expected outside the area of the possible ring arcs, the flight path can be adjusted as late as 10 days before the closest approach to Neptune, should more-distant ring arcs be discovered. Several ring observations will be retargeted if individual ring arcs are located in images taken as the spacecraft approaches the planet. Retargeting to these ring arcs can take place as late as a day or two before closest approach. As the spacecraft passes behind the rings, changes in the radio signal will be analyzed to determine the sizes of the particles and the structure of the rings. (The following is from "The Solar System," NASA/ASEP, 1989, p. 11.) Neptune is named for the Roman god of the sea. Neptune's symbol is the fishing spear. This is the eighth planet from the Sun. Neptune is the smallest of the gas planets. Neptune circles the Sun every 164.1 Earth years. One day on Neptune is 16 hours and 7 minutes. The gravity on Neptune is 1.15 of Earth's gravity. The diameter of the Neptune is 30,780 miles. Neptune was discovered in 1846; this was the first time a planet was found by mathematical calculations. Neptune is a twin planet to Uranus. Neptune's bluish-green color is caused by methane gas (natural gas). Neptune's atmosphere is made up of hydrogen, helium, and methane. Neptune has three rings. Neptune has eight moons (Triton, Nereid and six others). Neptune's moon Triton is slowly spiralling in toward the planet. Triton is the only large satellite with retrograde orbital motion (east to west), and with the same face toward the planet (synchronous rotation). Neptune receives 900 times less sunlight than Earth, less than half of Uranus, but Neptune's temperature is the same as Uranus. Voyager 2 will fly-by the planet on August 25, 1989. Neptune's Great Dark Spot is reminiscent of Jupiter's hurricane-like storms and is large enough to contain the entire Earth. Neptune's moon Triton shows evidence of a remarkable geological history, and Voyager 2 images show active geyser-like eruptions spewing invisible nitrogen gas and dark dust particles several kilometers into space. Triton has a minimal atmosphere (1/70,000 of surface pressure on Earth). JPL SUMMARY ABOUT NEPTUNE (NASA, Jet Propulsion Laboratory, "Our Solar System at a Glance," NASA Information Summaries, PMS 010-A (JPL), June 1991.) Voyager 2 completed its 12-year tour of the solar system with an investigation of Neptune and the planet's moons. On August 25, 1989, the spacecraft swept to within 4,850 kilometers (3,010 miles) of Neptune and then flew on to the moon Triton. During the Neptune encounter, it became clear that the planet's atmosphere was more active than Uranus'. Voyager 2 observed the Great Dark Spot, a circular storm the size of Earth, in Neptune's atmosphere. Resembling Jupiter's Great Red Spot, the storm spins counterclockwise and moves westward at almost 1,200 kilometers (745 miles) per hour. Voyager 2 also noted a smaller dark spot and a fast-moving cloud dubbed the "Scooter," as well as high-altitude clouds over the main hydrogen and helium cloud deck. The highest wind speeds of any planet were observed, up to 2,400 kilometers (1,500 miles) per hour. Like the other giant planets, Neptune has a gaseous hydrogen and helium upper layer over a liquid interior. The planet's core contains a higher percentage of rock and metal than those of the other gas giants. Neptune's distinctive blue appearance, like Uranus' blue color, is due to atmospheric methane. Neptune's magnetic field is tilted relative to the planet's spin axis and is not centered at the core. This phenomenon is similar to Uranus' magnetic field and suggests that the fields of the two giants are being generated in an area above the cores, where the pressure is so great that liquid hydrogen assumes the electrical properties of a metal. Earth's magnetic field, on the other hand, is produced by its spinning metallic core and is only slightly tilted and offset relative to its center. Voyager 2 also shed light on the mystery of Neptune's rings. Observations from Earth indicated that there were arcs of material in orbit around the giant planet. It was not clear how Neptune could have arcs and how these could be kept from spreading out into even, unclumped rings. Voyager 2 detected these arcs, but they were in fact part of thin, complete rings. A number of small moons could explain the arcs, but such bodies were not spotted. Astronomers had identified the Neptunian moons Triton in 1846 and Nereid in 1949. Voyager 2 found six more. One of the new moons, Proteus, is actually larger than Nereid, but since Proteus orbits close to Neptune, it was lost in the planet's glare for observers on Earth. Triton circles Neptune in a retrograde orbit in under six days. Tidal forces on Triton are causing it to spiral slowly towards the planet. In 10 to 100 million years (a short time in astronomical terms), the moon will be so close that Neptunian gravity will tear it apart, forming a spectacular ring to accompany the planet's modest current rings. Triton's landscape is as strange and unexpected as those of Io and Miranda. The moon has more rock than its counterparts at Saturn and Uranus. Triton's mantle is probably composed of water-ice, but its crust is a thin veneer of nitrogen and methane. The moon shows two dramatically different types of terrain: the so-called "Icantaloupe" terrain and a receding ice cap. Dark streaks appear on the ice cap. These streaks are the fallout from geyser-like volcanic vents that shoot nitrogen gas and dark, fine-grained particles to heights of 2-8 kilometers (1-5 miles). Triton's thin atmosphere, only 1/70,000th as thick as Earth's, has winds that carry the dark particles and deposit them as streaks on the ice cap‹the coldest surface yet discovered in the solar system (-235 degrees Celsius, -391 degrees Fahrenheit). Triton might be more like Pluto than any other object spacecraft have so far visited.