APOD: Northern Mercury (2012 Dec 01)

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APOD: Northern Mercury (2012 Dec 01)

Post by APOD Robot » Sat Dec 01, 2012 5:06 am

Image Northern Mercury

Explanation: Innermost planet Mercury would probably not be a good location for an interplanetary winter olympics. But new results based on data from the Mercury orbiting MESSENGER spacecraft indicate that it does have substantial water ice in permanently shadowed regions within craters near its north pole. The possibility of ice on Mercury has been entertained for years, inspired by the discovery of radar bright, hence highly reflective, regions near the north pole. Highlighted in yellow in this map based on projected MESSENGER images, radar bright regions are seen to correspond with floors and walls of north polar impact craters. Farther from the pole the regions are concentrated on the north facing crater walls. MESSENGER's neutron spectroscopy and thermal models for the craters indicate material in these regions has a hydrogen content consistent with nearly pure water ice and is trapped in an area with temperatures that remain below 100 kelvins (-280 deg.F, -173 deg.C). In circumstances similar to permanent shadows in craters of the Moon, debris from comet impacts is thought to be the source of ice on Mercury.

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by bystander » Sat Dec 01, 2012 5:52 am

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by ritwik » Sat Dec 01, 2012 10:32 am

what is "water ice" ?

is it normal ice formed from gradual cooling of liquid water :? how could liquid water exist at -173 deg celcius :|

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by sinanipek » Sat Dec 01, 2012 12:05 pm

I don't understand why small moons and planets are mostly colourless? I asked this questions about moon, and somebody said the moon is colourless because it has no oxygen in it. The colorurful compounds and minerals are mostly includes oxygen, he said. Is it a true and complete anwer? If so, why oxygen compounds are colourful? Is there a particular reason for this or is it just for our achestors has evolved in a planet mostly composed of oxygen? This is like, the spectrum we see is mostly includes frequencies the atmosphere cannot filter out? (I don't know if I use the right idioms to express myself, but...) If we evolve, say, in the moon or Mercury, would we "see" a colourful planet or just like a gray zone of colourlessness?

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by geckzilla » Sat Dec 01, 2012 1:36 pm

ritwik wrote:what is "water ice" ?

is it normal ice formed from gradual cooling of liquid water :? how could liquid water exist at -173 deg celcius :|
As opposed to, say, methane ice or any other compound capable of forming ice. It's an astronomy thing. Everyone else thinks ice is only ever frozen water. ...or diamonds, I guess. hehe (Edit: Or super pure meth!)
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Comets or Solar Wind?

Post by neufer » Sat Dec 01, 2012 1:49 pm

APOD Robot wrote:Image Northern Mercury

In circumstances similar to permanent shadows in craters of the Moon,
debris from comet impacts is thought to be the source of ice on Mercury.
Comets or Solar Wind :?:
http://asterisk.apod.com/viewtopic.php?p=188561#p185740
http://asterisk.apod.com/viewtopic.php?p=188561#p188561

Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere,
Nor any drop to drink.
Last edited by neufer on Sat Dec 01, 2012 2:35 pm, edited 1 time in total.
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by MarkD » Sat Dec 01, 2012 2:12 pm

This is just the latest example of water being found throughout the solar system. Perhaps Lou Franks Small Comet theory should be reexamined?

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by neufer » Sat Dec 01, 2012 2:22 pm

sinanipek wrote:
I don't understand why small moons and planets are mostly colourless? I asked this questions about moon, and somebody said the moon is colourless because it has no oxygen in it. The colorful compounds and minerals are mostly includes oxygen, he said. Is it a true and complete anwer?
Our own rocky planet is quite colorful thanks to lots of liquid water & vegetation.

Clouds make our sister planet Venus white.

Ice & no vegetation makes many moons white.

Io is yellow, red & black because of sulfur
Titan is orange because of hydrocarbons (in the atmosphere).
Mars is red because of iron(III) oxide / ferric oxide (i.e., rust).

Our own moon's surface is 14% black ferrous oxide (i.e., Iron(II) oxide).
And if it were more oxidized that might be red ferric oxide (i.e., Iron(III) oxide).
(so your friend is partially right).

However, our moon has lots of white oxides:
http://en.wikipedia.org/wiki/Moon#Surface_geology wrote:
Chemical composition of the lunar surface regolith (derived from crustal rocks)

Code: Select all

Compound 	     Formula 	Composition (wt %)
                            Maria   Highlands
--------------------------------------------------
silica    	     SiO2 	  45.4% 	45.5%
alumina 	       Al2O3 	 14.9% 	24.0%
lime    	       CaO 	   11.8% 	15.9%
iron(II) oxide 	FeO 	   14.1% 	 5.9%
magnesia 	      MgO 	    9.2% 	 7.5%
titanium dioxide  TiO2 	   3.9% 	 0.6%
sodium oxide 	  Na2O 	   0.6% 	 0.6%
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by Chris Peterson » Sat Dec 01, 2012 3:35 pm

ritwik wrote:what is "water ice" ?

is it normal ice formed from gradual cooling of liquid water :? how could liquid water exist at -173 deg celcius :|
"Ice" is just a term used to describe the solid phase of materials commonly found in other phases as well. So water ice is solid water. Given the temperatures and pressures found at the surface of Mercury, persistent liquid water isn't possible. Ice can accumulate directly from water molecules impacting a surface, however. There's no need for the water to pass through a liquid phase.
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by Chris Peterson » Sat Dec 01, 2012 3:47 pm

sinanipek wrote:I don't understand why small moons and planets are mostly colourless? I asked this questions about moon, and somebody said the moon is colourless because it has no oxygen in it. The colorurful compounds and minerals are mostly includes oxygen, he said. Is it a true and complete anwer?
It's largely true. Keep in mind, as well, that many small bodies are composed substantially of ices, or have icy surfaces. The types of ice found in our Solar System are fairly colorless. Silicates tend to be much simpler mineralogically when they exist in environments where they undergo few chemical reactions. The most colorful minerals on Earth tend to be oxides and hydrates.

That said, a lot of the color associated with the Earth is from water and vegetation. Take those away, and I don't think the Earth would look all that different from a number of other bodies in the Solar System.
If so, why oxygen compounds are colourful? Is there a particular reason for this or is it just for our achestors has evolved in a planet mostly composed of oxygen? This is like, the spectrum we see is mostly includes frequencies the atmosphere cannot filter out? (I don't know if I use the right idioms to express myself, but...) If we evolve, say, in the moon or Mercury, would we "see" a colourful planet or just like a gray zone of colourlessness?
Well, these compounds happen to interact quite strongly with wavelengths in the part of the spectrum we call visible light. Certainly, there are bodies that appear fairly colorless to us that show lots of detail at other wavelengths- IR, for example. Whether a species that evolved on another planet would see a different part of the spectrum would depend on what was physiologically possible, what the available energy sources looked like, and whatever evolutionary pressures might exist (e.g. whether there is any value in detecting different kinds of rocks, as opposed to detecting different kinds of plants).
Chris

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by sinanipek » Sat Dec 01, 2012 5:30 pm

Chris Peterson wrote:
sinanipek wrote:I don't understand why small moons and planets are mostly colourless? I asked this questions about moon, and somebody said the moon is colourless because it has no oxygen in it. The colorurful compounds and minerals are mostly includes oxygen, he said. Is it a true and complete anwer?
It's largely true. Keep in mind, as well, that many small bodies are composed substantially of ices, or have icy surfaces. The types of ice found in our Solar System are fairly colorless. Silicates tend to be much simpler mineralogically when they exist in environments where they undergo few chemical reactions. The most colorful minerals on Earth tend to be oxides and hydrates.

That said, a lot of the color associated with the Earth is from water and vegetation. Take those away, and I don't think the Earth would look all that different from a number of other bodies in the Solar System.
If so, why oxygen compounds are colourful? Is there a particular reason for this or is it just for our achestors has evolved in a planet mostly composed of oxygen? This is like, the spectrum we see is mostly includes frequencies the atmosphere cannot filter out? (I don't know if I use the right idioms to express myself, but...) If we evolve, say, in the moon or Mercury, would we "see" a colourful planet or just like a gray zone of colourlessness?
Well, these compounds happen to interact quite strongly with wavelengths in the part of the spectrum we call visible light. Certainly, there are bodies that appear fairly colorless to us that show lots of detail at other wavelengths- IR, for example. Whether a species that evolved on another planet would see a different part of the spectrum would depend on what was physiologically possible, what the available energy sources looked like, and whatever evolutionary pressures might exist (e.g. whether there is any value in detecting different kinds of rocks, as opposed to detecting different kinds of plants).
Thanks for the answer.

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Say "Wassily Kandinsky" 5 times fast

Post by neufer » Sat Dec 01, 2012 11:07 pm

http://sciencythoughts.blogspot.com/2012/08/names-for-nine-craters-in-north-polar.html wrote: Sciency Thoughts, 17 August 2012
Names for nine craters in the North Polar Region of Mercury.

<<The MESSENGER space probe has been in orbit about the planet Mercury since March 2011, since when it has beamed back over 100 000 images of the surface of the planet, helping scientists to construct detailed maps revealing many new geological features. This includes a number of impact craters in the North Polar Region that show signs of containing water ice, which was not expected on the surface of Mercury and is therefore of great scientific interest. Since these craters are likely to be the subject of considerable ongoing study, the NASA team behind the probe have named nine of the most prominent craters. Following current procedures for the naming of features on Mercury, all the craters are named after 'deceased artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field and have been recognized as art historically significant figures for more than 50 years'.

The first of these craters is named Egonu, after Uzo Egonu (1931-1996), a Nigerian/British painter considered one of the greatest African artists of the twentieth century.

The second crater is named Gaudi, after Antoni Gaudí (1852-1926), the Spanish architect noted for the flowing organic style of his work. Much of Gaudí's work can still be seen in the city of Barcelona, most notably the cathedral, probably his most famous work.

The third crater is named Kandinsky, after the Russian painter Wassily Kandinsky (1866-1944), noted for his abstract works.

The fourth crater is named Petronius, after Titus Petronius (c. AD 27-66), considered to be the author of the Satyricon, a Roman work of humerus [sic] fiction, concerning a retired gladiator who has trouble holding on to his young male lover.

The fifth crater is named Prokofiev, after Sergei Prokofiev (1891-1953), the noter Russian composer and pianist.

The sixth crater is named Tolkien, after J.R.R. Tolkien (1892-1973), the South African/British writer of the Lord of the Rings trilogy of fantasy novels, and related works.

The seventh crater is named Tryggvadóttir, after Nina Tryggvadóttir (1913-1968), the Icelandic abstract expressionist artist. Tryggvadóttir settled in the US in 1942, and married American artist Alfred L. Copley in 1949, but was refused permission to re-enter the US after a visit to Iceland later that year, on suspicion of being a Communist sympathizer.

The eighth crater is named Qiu Ying, after Shifu Qiu Ying (1494-1552), a Ming Dynasty realist painter, considered a master of Chinese gongbi painting.

The final crater in named Yoshikawa, after Eiji Yoshikawa (1892-1962), the Japanese historical novelist, noted for the reworking of classic tales in a more accessible style.>>
http://en.wikipedia.org/wiki/Wassily_Kandinsky wrote: "The 'Pioneer' [Kandinsky] did not just produce a body of work whose sensuous magnificence and rich inventiveness eclipse even the most remarkable of his contemporaries. Facing the hieroglyphs of the last canvases of the Parisian period (which are said to be the most difficult), they provide the Rosetta stone on which the meaning of these mysterious figures is inscribed".

"Kandinsky was fascinated by the expressive power of linear forms. Lyricism is the pathos of a force whose triumphant effort enters into action and encounters no obstacle. Because the straight line results from the initiative of a single, unopposed force, its domain is that of the lyric. When two forces are present and thus enter in conflict, as this is the case with the curve or the zigzag line, we are in domain of drama"..

"Kandinsky calls abstract the content that painting must express, that’s to say this invisible life that we are. In such a way that the Kandinskian equation, to which we have alluded to, can be written in reality as follows :

  • Interior = interiority = invisible = life = pathos = abstract".>>
Last edited by neufer on Sun Dec 02, 2012 1:41 pm, edited 1 time in total.
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by saturno2 » Sat Dec 01, 2012 11:19 pm

Ice of water in Mercury.
It is very important. The water is the first element for the life.
But.... -173 deg. C, perhaps isn´t very good.
Some micro-organism can to live in these temperature?
I don´t know :?

quigley

Re: APOD: Northern Mercury (2012 Dec 01)

Post by quigley » Sat Dec 01, 2012 11:45 pm

Would the planet Mercury experience "seasons" even though it has no axis tilt, but does have an irregular orbit? Does this affect the ice at the pole?

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by BMAONE23 » Sun Dec 02, 2012 12:57 am

quigley wrote:Would the planet Mercury experience "seasons" even though it has no axis tilt, but does have an irregular orbit? Does this affect the ice at the pole?
There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet's orbit. Since Mercury's axis is directly perpendicular to its motion (not tilted), it has no seasons.
http://www.astro-observer.com/solarsyst ... index.html
Due to the tenuous atmosphere, Mercury really has no weather to speak of other than wild fluctuations in temperature. In fact, Mercury has the largest diurnal temperature spread of any planet in our solar system. This is explained by a few factors. First, as stated above, Mercury rotates on its axis every 59 days relative to fixed stars such as our Sun. However a fictitious observer on Mercury would see that its actual length of day from sunrise to sunrise would take about about 176 Earth days to complete due to perspective.
http://www.crh.noaa.gov/fsd/?n=mercury

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by neufer » Sun Dec 02, 2012 1:11 am

quigley wrote:
Would the planet Mercury experience "seasons" even though it has no axis tilt, but does have an irregular orbit?
Seasons on Mercury are analogous to seasons
at the Earth's poles except that rather than:

an annual day/summer vs. night/winter
with 90º S being out of sync with 90º N

there is a bienniual day/summer vs. night/winter
with 90º E being out of sync with 90º W.
..................................................................
Those opposite longitudes (90º E & 90º W ?) which always
experience noon/midnight at aphelion (at 1 & 4) :arrow:
will experience relatively mild day/summer's.

Longitudes 90º from these (0º E & 180º W ?) which always
experience noon/midnight at perihelion will
experience relatively intense day/summer's.
quigley wrote:
Does this affect the ice at the pole?
The (average) secondary reflection off of polar crater tops will experience a factor of two (aphelion to perihelion) seasonal modulation; however, the asymmetry of the secondary reflection will usually exceed this effect.
Last edited by neufer on Sun Dec 02, 2012 3:48 pm, edited 3 times in total.
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by 1yioi87 » Sun Dec 02, 2012 2:41 am

When ever there is a discussion about the location of water, be that the earth, moon, or Mercury, it is usually said that the water came from comets. How did all that water come to be the main part of all the comets? Where was the water created in the first place?

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by BDanielMayfield » Sun Dec 02, 2012 4:14 am

1yioi87 wrote:When ever there is a discussion about the location of water, be that the earth, moon, or Mercury, it is usually said that the water came from comets. How did all that water come to be the main part of all the comets? Where was the water created in the first place?
The hydrogen in H2O was created when protons were formed in the big bang. The oxygen was formed in the cores of stars by the fusion of carbon and helium. After being blown into space in supernova explosions oxygen often combines with H2 (the most abundant gas in space) to form H2O. As this water cools it often freezes out on dust particles in space. These particles can coalesce into bodies that will become comets if they are ever heated by a star.

I hope that helped.

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by Ann » Sun Dec 02, 2012 5:25 am

BDanielMayfield wrote:
1yioi87 wrote:When ever there is a discussion about the location of water, be that the earth, moon, or Mercury, it is usually said that the water came from comets. How did all that water come to be the main part of all the comets? Where was the water created in the first place?
The hydrogen in H2O was created when protons were formed in the big bang. The oxygen was formed in the cores of stars by the fusion of carbon and helium. After being blown into space in supernova explosions oxygen often combines with H2 (the most abundant gas in space) to form H2O. As this water cools it often freezes out on dust particles in space. These particles can coalesce into bodies that will become comets if they are ever heated by a star.

I hope that helped.

Bruce
Like Bruce said, the hydrogen in H2O was created in the Big Bang, and the oxygen was formed in the cores of (massive) stars, and the oxygen was blown into space in titanic supernova explosions. Hydrogen and oxygen then combined (and keeps combining) to form H2O.

An interesting question, however, is why astronomers keep talking about water being delivered to the Earth by comets. Why should we assume that comets were born with large supplies of water, while the Earth was born dry?

The answer to that is the so-called the "snowline" in the early solar system. Inside that snowline, water ice generally melted and sublimated into gas. Therefore, the only solid "raw material" available to for planet construction inside the snowline was rock, mixed with some iron and nickel. The water there was gaseous and was generally not incorporated into the bodies that formed there, such as the proto-Earth.

Comets, however, formed beyond the snowline, so that all the H2O out there was solid. Water ice was just another solid that went into the formation of comets. And since there was fairly abundant water in the primordial disk that gave rise to the solar system, there was plenty of ice to incorporate into all sorts of solid bodies outside the snowline. This, by the way, is the reason why there is so much water ice in so many of the moons of Jupiter and Saturn. And "ice giants" Uranus and Neptune, as you can hear from their description "ice giants", hold enormous quantities of "inner ice".

But the Earth was born dry, because no ice was available as "building material". Yes, but after a while the inner solar system was bombarded by huge numbers of water ice-rich comets. Astronomers believe that enough of those comets (or ice-rich asteroids) crashed onto the Earth to deliver the water we have today.

And of course, most of the water ice that was delivered to the Earth was turned into that rare and fantastic commodity, liquid surface water.

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by 1yioi87 » Sun Dec 02, 2012 7:56 pm

Not being an Astronomer I find it hard to believe "that enough of those comets (or ice-rich asteroids) crashed onto the Earth, at just the right time,to deliver all the water we have today."

What was the temp of Earth during this period of a "huge numbers" of comets, and what would you consider a HUGE NUMBER? Is it just a coincidence that the temp was right, and the number of comets was right, at the same time so that the water didn't all boil off, as it apparently has on Mercury and other planets.

Also the Astronomers believe that the comets also rained down on Mercury and just happen to hit in the right spot to make an ice skating rink in the polar region. Are there HUGE NUMBERS of other places on Mercury that are identified as comet impact sites, enough so that if the temp was right, Mercury would have as much water as Earth has now. How many would that be.

Why isn't there still an occasional comet full of water coming down on earth or the moon or Mercury? When was the last one? Where is the "snowline" today?

"Comets, however, formed beyond the snowline, " How do comets form anyway and how long does it take? Do they just wander around out side the snowline like a big snowball rolling down hill and suck up all the H2O that is floating around out there somewhere? Then when they get big enough to make the trip worthwhile they swoop down on a piece of rock that is just the right temp and in just the right location and deposit their load of water. What are the odds of that. What causes them to depart from the birthing place when they are full grown and proceed to a intercept with Earth? If the collisions were just random then there must have been 10 to the 10 numbers of comets that missed the Earth at the same time, where did all of them end up?

As I said I am not an astronomer I am just interested. Seems like when the astronomers can't figure out why things are the way they are they make what they consider to be a probable story and they all agree to it. I don't know if or why Earth was created but there are some folks that believe in a greater force than that delivered by a comet impact. I am just saying --- it is hard for me to decide who is right. Am I wrong?

Thanks,
Cliff

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by geckzilla » Sun Dec 02, 2012 8:10 pm

Cliff, astronomy is just like any other science. There's a lot of evidence backing up models and theories. The history of the Universe and the Earth itself is always being refined and there are some things we may never know but one thing that there is certainly no evidence for is a "greater force" as you chose to call it. You can discuss evidence here all you want but if you intend to bait the discussion into debating philosophy beyond evidence, this isn't the place for it.
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by bystander » Sun Dec 02, 2012 8:12 pm

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Re: APOD: Northern Mercury (2012 Dec 01)

Post by neufer » Sun Dec 02, 2012 8:12 pm

1yioi87 wrote:
Not being an Astronomer I find it hard to believe "that enough of those comets (or ice-rich asteroids) crashed onto the Earth, at just the right time,to deliver all the water we have today."
http://apod.nasa.gov/apod/ap120524.html
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by Chris Peterson » Sun Dec 02, 2012 8:43 pm

1yioi87 wrote:Not being an Astronomer I find it hard to believe "that enough of those comets (or ice-rich asteroids) crashed onto the Earth, at just the right time,to deliver all the water we have today."
The idea that Earth's oceans came from comets is not highly regarded these days. That's largely because it appears that cometary ice has quite different isotope ratios than we find in water on the Earth. More popular now are models where much of the water was present from the formation of the Earth, with the possibility that more was delivered during early collisions with protoplanets. Nevertheless, the amount of water on Earth is quite small compared with the volume available in comets (especially much earlier in the evolution of the Solar System), so there's nothing inherently implausible about the theory on those grounds.
What was the temp of Earth during this period of a "huge numbers" of comets, and what would you consider a HUGE NUMBER? Is it just a coincidence that the temp was right, and the number of comets was right, at the same time so that the water didn't all boil off, as it apparently has on Mercury and other planets.
The temperature on Earth was probably about 200°C, but that was below the boiling point of water given the dense CO2 atmosphere that was probably present.
Also the Astronomers believe that the comets also rained down on Mercury and just happen to hit in the right spot to make an ice skating rink in the polar region.
Not at all. The shadowed polar region is simply the only place where water can survive. With high temperatures and no atmosphere, water impacting anywhere else returns to space.
Why isn't there still an occasional comet full of water coming down on earth or the moon or Mercury?
There is. Comet Shoemaker-Levy 9 hit Jupiter less than 20 years ago. We see comets hit the Sun regularly. The likelihood of a comet impact depends in part on the size of the target, which is why it makes sense that we have recently observed them in the cases of Jupiter and the Sun. Other impacts might well go unobserved, being very transient phenomena. On Earth, it is widely speculated that the Tunguska event of 1908 was produced by a small comet or comet fragment. Of course, the density of comets in the inner system is orders of magnitude lower today than it was during the early periods of bombardments in the first few hundred million years of our Solar System.
"Comets, however, formed beyond the snowline, " How do comets form anyway and how long does it take? Do they just wander around out side the snowline like a big snowball rolling down hill and suck up all the H2O that is floating around out there somewhere?
The mechanism by which comets form is not well established. Stellar system formation models suggest that combinations of density and temperature gradients result in regions of the protostellar disc that are suitable for the condensation of icy bodies.
Then when they get big enough to make the trip worthwhile they swoop down on a piece of rock that is just the right temp and in just the right location and deposit their load of water. What are the odds of that.
The odds for any one comet are millions to one. But there are trillions of comets, and they had hundreds of millions of years. So actually delivering the small amount of surface water present on the Earth isn't a statistical reach at all.
What causes them to depart from the birthing place when they are full grown and proceed to a intercept with Earth? If the collisions were just random then there must have been 10 to the 10 numbers of comets that missed the Earth at the same time, where did all of them end up?
Comets are perturbed out of their orbits by interactions with other Solar System bodies, or even interactions with nearby stars. Some are ejected from the Solar System completely, but most end up in the Sun.
As I said I am not an astronomer I am just interested. Seems like when the astronomers can't figure out why things are the way they are they make what they consider to be a probable story and they all agree to it.
Actually, for much of this, the theories have been narrowed down to just one or two that most astronomers consider probable. It isn't the job of scientists to create stories that all can agree upon, but to create theories that can be tested. It is exactly that way of thinking that has led to the loss of popularity in recent years of the theory that comets delivered the oceans- because tests suggest otherwise.
Chris

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neufer
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Re: APOD: Northern Mercury (2012 Dec 01)

Post by neufer » Sun Dec 02, 2012 10:40 pm

Chris Peterson wrote:
1yioi87 wrote:
Not being an Astronomer I find it hard to believe "that enough of those comets (or ice-rich asteroids) crashed onto the Earth, at just the right time,to deliver all the water we have today."
The idea that Earth's oceans came from comets is not highly regarded these days. That's largely because it appears that cometary ice has quite different isotope ratios than we find in water on the Earth. More popular now are models where much of the water was present from the formation of the Earth, with the possibility that more was delivered during early collisions with protoplanets. Nevertheless, the amount of water on Earth is quite small compared with the volume available in comets (especially much earlier in the evolution of the Solar System), so there's nothing inherently implausible about the theory on those grounds.
http://asterisk.apod.com/viewtopic.php?f=31&t=29083&p=179701#p179701 wrote:
Solar System Ice: Source of Earth’s Water
Carnegie Institution for Science | 2012 July 12
<<Scientists have long believed that comets and, or a type of very primitive meteorite called carbonaceous chondrites were the sources of early Earth's volatile elements—which include hydrogen, nitrogen, and carbon—and possibly organic material, too. Understanding where these volatiles came from is crucial for determining the origins of both water and life on the planet. New research led by Carnegie's Conel Alexander focuses on frozen water that was distributed throughout much of the early Solar System, but probably not in the materials that aggregated to initially form Earth.

The evidence for this ice is now preserved in objects like comets and water-bearing carbonaceous chondrites. The team’s findings contradict prevailing theories about the relationship between these two types of bodies and suggest that meteorites, and their parent asteroids, are the most-likely sources of the Earth's water. Their work is published July 12 by Science Express.

Looking at the ratio of hydrogen to its heavy isotope deuterium in frozen water, scientists can get an idea of the relative distance from the Sun at which objects containing the water were formed. Objects that formed farther out should generally have higher deuterium content in their ice than objects that formed closer to the Sun, and objects that formed in the same regions should have similar hydrogen isotopic compositions. Therefore, by comparing the deuterium content of water in carbonaceous chondrites to the deuterium content of comets, it is possible to tell if they formed in similar reaches of the Solar System.

It has been suggested that both comets and carbonaceous chondrites formed beyond the orbit of Jupiter, perhaps even at the edges of our Solar System, and then moved inward, eventually bringing their bounty of volatiles and organic material to Earth. If this were true, then the ice found in comets and the remnants of ice preserved in carbonaceous chondrites in the form of hydrated silicates, such as clays, would have similar isotopic compositions.

[Alexander’s team] analyzed samples from 85 carbonaceous chondrites, and were able to show that carbonaceous chondrites likely did not form in the same regions of the Solar System as comets because they have much lower deuterium content. The team suggests that carbonaceous chondrites formed instead in the asteroid belt that exists between the orbits of Mars and Jupiter. What's more, they propose that most of the volatile elements on Earth arrived from a variety of chondrites, not from comets.>>
Art Neuendorffer