by neufer » Wed Nov 10, 2010 4:17 am
Chris Peterson wrote:Garry Maxfield wrote:
Why do astronomers still write that comets are icebergs?
There has never been a probe that has ever found any traces of water on any comet.
For those who write these APOD's, please get your facts correct!
Get the facts right!
There is ample water detected spectroscopically in comet comas.
If it doesn't come from the nucleus, where do you suggest?
Other than that, comets have many ices besides water.
http://www.naoj.org/Pressrelease/2004/04/04/index.html wrote:
Discovery of Water Icy Grains in Comet LINEAR Approaching Earth, April 4, 2004
<<Near infrared spectroscopic observation of Comet LINEAR (C/2002T7) was carried out with the Subaru Telescope in September 2003. After analyzing the spectral data, we discovered water icy grains in the coma of Comet LINEAR.
Comet Hale-Bopp was the first example of the detection of water icy grains in a cometary coma, and this time is the second one.
Cometary nuclei are thought to be remnants of planetary sources (called planetecimals) existed in the early solar system. Since it is considered that the cometary nuclei were formed from icy dust (nonvolatile dust covered with ice mantle) in the interstellar medium or the solar nebula, research into such water icy grains in the cometary nuclei is very important to reveal the physical conditions of the early solar system.
It is known from past studies that cometary ices consist of water (H
2O) ice (more than 80 %), and the remainder is carbon monoxide (CO) and carbon dioxide (CO
2) (about 20 % or less). The CO and CO
2 ices are higher volatile materials than water ice, and they are easy to volatilize even in the low-temperature region far from the Sun. It is therefore possible that water icy grains can directly be observed from comets beyond the area of 3 Astronomical Units from the Sun where water icy grains do not volatilize, because the CO and CO
2 ices may be the power to release the water icy grains and dust from the cometary nucleus.
We observed Comet LINEAR approaching the Earth at about 3.5 AU from the Sun, and the brightness was only 1/100 of that of Comet Hale-Bopp at the same distance. Comet LINEAR is supposed to become brighter when the comet is getting closer to the Sun; however, it is difficult for the water icy grains to exist in the cometary coma since the water icy grains are heated by the Sun and easily vaporized. The successful detection of the water icy grains in Comet LINEAR was achieved by the high collecting power of the Subaru's 8.2m-primary mirror and the high spatial resolution to resolve the small region where the water icy grains existed.>>
Chris Peterson wrote:
An iceberg is still a reasonable thing to call an icy body.
I am unfamiliar with the "iceberg" description but perhaps "icy dirtball" is more apropos.
http://en.wikipedia.org/wiki/Comet_nucleus wrote:
<<Comets are often described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust. It has been suggested that comets should be referred to as "Icy dirtballs". Cometary nuclei are among the darkest objects known to exist in the solar system. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing. Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
On 4 July 2005 at 05:52 UTC,
Tempel 1 was deliberately targeted by one component of the
NASA Deep Impact probe, one day before perihelion. The impact was photographed by the other component of the probe, recording a bright spray from the impact site. It was also observed by earthbound and space telescopes, which observed a brightening of several magnitudes after the impact. Initial results were surprising as the material excavated by the impact contained more dust and less ice than had been expected. The only models of cometary structure astronomers could positively rule out were the very porous models which had comets as loose aggregates of material. In addition, the material was finer than expected; scientists compared it to talcum powder rather than sand. Other materials found while studying the impact included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact. Clays and carbonates usually require liquid water to form and sodium is rare in space. Observations also revealed that the comet was about 75% empty space, and one astronomer compared the outer layers of the comet to the same makeup of a snow bank. Astronomers have expressed interest in more missions to different comets to determine if they share similar compositions or if there are different materials found deeper within comets that were produced at the time of the solar system's formation. The probe's spectrometer instrument also discovered the presence of silicates, carbonates, smectite, metal sulfides (like fool's gold), amorphous carbon and polycyclic aromatic hydrocarbons.>>
http://en.wikipedia.org/wiki/Comet wrote:
<<Isaac Newton described comets as compact and durable solid bodies moving in oblique orbits, and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the sun. Newton suspected that comets were the origin of the life-supporting component of air. Newton also believed that the vapors given off by comets might replenish the planets' supplies of water (which was gradually being converted into soil by the growth and decay of plants), and the sun's supply of fuel.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and was never seen again after 1852. A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
Debate continues about how much ice is in a comet. In 2001, NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of Comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".
In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface, and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely cemented rubble pile.
More recent data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire." Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk, so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials." These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
Forthcoming space missions will add greater detail to our understanding of what comets are made of. The European Rosetta probe is presently en route to Comet Churyumov-Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.>>
[quote="Chris Peterson"][quote="Garry Maxfield"]
Why do astronomers still write that comets are icebergs?
There has never been a probe that has ever found any traces of water on any comet.
For those who write these APOD's, please get your facts correct!
Get the facts right![/quote]
There is ample water detected spectroscopically in comet comas.
If it doesn't come from the nucleus, where do you suggest?
Other than that, comets have many ices besides water.[/quote]
[quote=" http://www.naoj.org/Pressrelease/2004/04/04/index.html"]
Discovery of Water Icy Grains in Comet LINEAR Approaching Earth, April 4, 2004
[float=right][img3="The observed spectrum of comet LINEAR taken with the Subaru' CISCO (red points with error-bars), and modeled spectra (purple and blue solid lines). The purple line corresponds to the case of pure water icy grains. The red line corresponds to the case of water icy grains (5 μm in diameter) with dark materials. The model results are based on the optical constants of crystalline water ice, and the absence of 1.65 μm absorption feature in the observed spectrum indicates that the cometary water ice was in amorphous state at the observation. This means that the water ices formed under low-temperature condition (colder than -150 deg-C)."]http://www.naoj.org/Pressrelease/2004/04/04/Fig2.gif[/img3][/float]<<Near infrared spectroscopic observation of Comet LINEAR (C/2002T7) was carried out with the Subaru Telescope in September 2003. After analyzing the spectral data, we discovered water icy grains in the coma of Comet LINEAR.[b][color=#0000FF] Comet Hale-Bopp was the first example of the detection of water icy grains in a cometary coma, and this time is the second one.[/color][/b]
Cometary nuclei are thought to be remnants of planetary sources (called planetecimals) existed in the early solar system. Since it is considered that the cometary nuclei were formed from icy dust (nonvolatile dust covered with ice mantle) in the interstellar medium or the solar nebula, research into such water icy grains in the cometary nuclei is very important to reveal the physical conditions of the early solar system.
It is known from past studies that cometary ices consist of water (H[sub]2[/sub]O) ice (more than 80 %), and the remainder is carbon monoxide (CO) and carbon dioxide (CO[sub]2[/sub]) (about 20 % or less). The CO and CO[sub]2[/sub] ices are higher volatile materials than water ice, and they are easy to volatilize even in the low-temperature region far from the Sun. It is therefore possible that water icy grains can directly be observed from comets beyond the area of 3 Astronomical Units from the Sun where water icy grains do not volatilize, because the CO and CO[sub]2[/sub] ices may be the power to release the water icy grains and dust from the cometary nucleus.
We observed Comet LINEAR approaching the Earth at about 3.5 AU from the Sun, and the brightness was only 1/100 of that of Comet Hale-Bopp at the same distance. Comet LINEAR is supposed to become brighter when the comet is getting closer to the Sun; however, it is difficult for the water icy grains to exist in the cometary coma since the water icy grains are heated by the Sun and easily vaporized. The successful detection of the water icy grains in Comet LINEAR was achieved by the high collecting power of the Subaru's 8.2m-primary mirror and the high spatial resolution to resolve the small region where the water icy grains existed.>>[/quote]
[quote="Chris Peterson"]
An iceberg is still a reasonable thing to call an icy body.[/quote]
I am unfamiliar with the "iceberg" description but perhaps "icy dirtball" is more apropos.
[quote=" http://en.wikipedia.org/wiki/Comet_nucleus"]
[float=right][img3="[b]Infrared spectrum of Comet Tempel 1 impact.
[color=#008000] Vapor plume shown in green 0.6 seconds after impact.[/color]
[color=#0000FF]Blue spectrum is modelled with water & carbon dioxide
with no presence of organics between 3.3 & 3.6 µm.[/color]
[color=#FF0000]Red spectrum was taken 0.7 sec earlier than the green one.[/color][/b]"]http://deepimpact.umd.edu/gallery/jpg/spectrometer2-med.jpg[/img3][/float]<<Comets are often described as "dirty snowballs", though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath the crust. It has been suggested that comets should be referred to as "Icy dirtballs". Cometary nuclei are among the darkest objects known to exist in the solar system. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like tar or crude oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing. Roughly six percent of the near-earth asteroids are thought to be extinct nuclei of comets which no longer experience outgassing.
On 4 July 2005 at 05:52 UTC, [b][url=http://en.wikipedia.org/wiki/9P/Tempel]Tempel 1[/url][/b] was deliberately targeted by one component of the [b]NASA Deep Impact probe[/b], one day before perihelion. The impact was photographed by the other component of the probe, recording a bright spray from the impact site. It was also observed by earthbound and space telescopes, which observed a brightening of several magnitudes after the impact. Initial results were surprising as the material excavated by the impact contained more dust and less ice than had been expected. The only models of cometary structure astronomers could positively rule out were the very porous models which had comets as loose aggregates of material. In addition, the material was finer than expected; scientists compared it to talcum powder rather than sand. Other materials found while studying the impact included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact. Clays and carbonates usually require liquid water to form and sodium is rare in space. Observations also revealed that the comet was about 75% empty space, and one astronomer compared the outer layers of the comet to the same makeup of a snow bank. Astronomers have expressed interest in more missions to different comets to determine if they share similar compositions or if there are different materials found deeper within comets that were produced at the time of the solar system's formation. The probe's spectrometer instrument also discovered the presence of silicates, carbonates, smectite, metal sulfides (like fool's gold), amorphous carbon and polycyclic aromatic hydrocarbons.>>[/quote]
[quote=" http://en.wikipedia.org/wiki/Comet"]
<<Isaac Newton described comets as compact and durable solid bodies moving in oblique orbits, and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the sun. Newton suspected that comets were the origin of the life-supporting component of air. Newton also believed that the vapors given off by comets might replenish the planets' supplies of water (which was gradually being converted into soil by the growth and decay of plants), and the sun's supply of fuel.
As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.
However, another comet-related discovery overshadowed these ideas for nearly a century. Over the period 1864–1866 the Italian astronomer Giovanni Schiaparelli computed the orbit of the Perseid meteors, and based on orbital similarities, correctly hypothesized that the Perseids were fragments of Comet Swift-Tuttle. The link between comets and meteor showers was dramatically underscored when in 1872, a major meteor shower occurred from the orbit of Comet Biela, which had been observed to split into two pieces during its 1846 apparition, and was never seen again after 1852. A "gravel bank" model of comet structure arose, according to which comets consist of loose piles of small rocky objects, coated with an icy layer.
By the middle of the twentieth century, this model suffered from a number of shortcomings: in particular, it failed to explain how a body that contained only a little ice could continue to put on a brilliant display of evaporating vapor after several perihelion passages. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.
Debate continues about how much ice is in a comet. In 2001, NASA's Deep Space 1 team, working at NASA's Jet Propulsion Lab, obtained high-resolution images of the surface of Comet Borrelly. They announced that comet Borrelly exhibits distinct jets, yet has a hot, dry surface. The assumption that comets contain water and other ices led Dr. Laurence Soderblom of the U.S. Geological Survey to say, "The spectrum suggests that the surface is hot and dry. It is surprising that we saw no traces of water ice." However, he goes on to suggest that the ice is probably hidden below the crust as "either the surface has been dried out by solar heating and maturation or perhaps the very dark soot-like material that covers Borrelly's surface masks any trace of surface ice".
In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface, and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
The Stardust spacecraft, launched in February 1999, collected particles from the coma of Comet Wild 2 in January 2004, and returned the samples to Earth in a capsule in January 2006. Claudia Alexander, a program scientist for Rosetta from NASA's Jet Propulsion Laboratory who has modeled comets for years, reported to space.com about her astonishment at the number of jets, their appearance on the dark side of the comet as well as on the light side, their ability to lift large chunks of rock from the surface of the comet and the fact that comet Wild 2 is not a loosely cemented rubble pile.
More recent data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire." Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk, so comets also contain crystalline grains which were formed in the hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials." These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
Forthcoming space missions will add greater detail to our understanding of what comets are made of. The European Rosetta probe is presently en route to Comet Churyumov-Gerasimenko; in 2014 it will go into orbit around the comet and place a small lander on its surface.>>[/quote]