
Rosetta finds comet's water vapour to be significantly different from Earth's - twowo
http://www.esa.int/Our_Activities/Space_Science/Rosetta/Rosetta_fuels_debate_on_origin_of_Earth_s_oceans
======
tokenadult
The two comments submitted previously as I type this explain why the article
title on HN is as it is. I was puzzled by what the title MEANT, so I looked
into the article, and what the article says farther down is "Previous
measurements of the deuterium/hydrogen (D/H) ratio in other comets have shown
a wide range of values. Of the 11 comets for which measurements have been
made, it is only the Jupiter-family Comet 103P/Hartley 2 that was found to
match the composition of Earth’s water, in observations made by ESA’s Herschel
mission in 2011." When I first read the headline, I wasn't sure if the claim
was that the water vapor had mixtures of other chemical molecules in it, or
what.

Okay, a different isotope ratio in water from a comet as contrasted with water
generally found on earth would indeed be a clue to how water might have
traveled from one orbiting body to another early in the development of the
solar system. This kind of isotype checking (for isotopes of other elements)
is one of the things done to confirm that rocks found on earth are
presumptively from other parts of the solar system.

~~~
emcrazyone
I'm surprised that scientists believe water was delivered. Why wouldn't water
be one of the by products of the big-bang and during Earth's creation?

~~~
tomr_stargazer
>>> Why wouldn't water be one of the by products of the big-bang

To address the first part of your question – The Big Bang (which happened 13.8
billion years ago) created ~only hydrogen and helium. So, no water existed in
the early Universe (a) because there were no oxygen atoms formed in the Big
Bang and (b) the early Universe was too hot to form water molecules even if
oxygen was present.

>>> during Earth's creation?

Whether water was present on the Earth from "day one" of its formation (~5
billion years ago, much later than the Big Bang) is still an open question,
but part of why it might be unlikely is that the Earth is close enough to the
Sun that it was too hot for water to stay as ice or liquid on Earth's surface
(before Earth's atmosphere formed). Water that forms in comets (further away
from the Sun) doesn't have that problem, as comets would be in colder parts of
the early Solar System.

~~~
flavien_bessede
So what happened between the Big Bang and Earth formation? For some reason I
always thought they were one and the same.

~~~
quantumet
Quite a lot; see for example:

[http://en.wikipedia.org/wiki/Graphical_timeline_of_the_unive...](http://en.wikipedia.org/wiki/Graphical_timeline_of_the_universe)

------
grecy
I didn't realize the standing theory is that Earth's water was delivered by
lots of meteor strikes.

Has there ever been an estimate made of the number of strikes needed to
deliver the volume of water currently on Earth? I have to think it's enormous.

~~~
pcrh
The amount of (surface) water on earth may not be as much as you would
intuitively think, given that it covers 70% of the surface area.

have a look at the image in this link:

[http://water.usgs.gov/edu/earthhowmuch.html](http://water.usgs.gov/edu/earthhowmuch.html)

~~~
dangayle
What about the water in the earth's core?

>>A reservoir of water three times the volume of all the oceans has been
discovered deep beneath the Earth's surface. The finding could help explain
where Earth's seas came from.

[http://www.newscientist.com/article/dn25723-massive-ocean-
di...](http://www.newscientist.com/article/dn25723-massive-ocean-discovered-
towards-earths-core.html#.VIjQPWbvgSo)

~~~
Balgair
My wife looks at carbon in the deep earth, but water is a very related
chemical in these cases. She says that the amount of water in the core is
indeterminable at this time, but it is likely a lot more than a mere 3x the
ocean volume (also, volume depends on pressure and temp, which are extreme in
the core)

Note: at extreme temps and pressures in the core, water as we know it is not a
good guidepost. It may be crystallized or associating strongly with other
exotic crystals and 'plastic'-y compounds at the Moho layer and other
transition zones deep down. Also, just counting O and H elements is likely to
not get you far there either. The story of how we got our oceans is FAR from
complete. I'd wager we are in the 2nd page of the prologue though, which, for
60 years of real work, is pretty good compared to most fields.

------
acqq
It doesn't appear to be a result for which the landing was necessary? Also it
seems it was something already achieved before ("measuring D/H"). The text
mentions that the D/H ratio was already measured in 11 comets, I believe by
just flying through their "tail" (1)?

1)
[http://adsabs.harvard.edu/full/1987A%26A...187..435E](http://adsabs.harvard.edu/full/1987A%26A...187..435E)

How was it done this time?

------
AYBABTME
They mention that water already on earth would have boiled off in its youth. I
don't understand this, wouldn't the vapor stay around the planet, then return
to liquid when the planet cooled?

Or else, what would have attracted the water away from the earth's atmosphere?
And if so, why wouldn't it have attracted away the azote and oxygen and all of
the atmosphere at the same time? Or was there not yet an atmosphere?

~~~
has2k1
Simulations do not like this scenario. The gravity is not enough to counter
the expansion due to the heat, so the water (vapour) leaks off far and wide
with little chance of coalescing in the inner solar system.

------
lnanek2
I think it is pretty obvious D/H would be higher for a comet that has been out
a long time and that doesn't really tell us much about if Earth's water came
from comets a long time ago since those crashed early. H is a lot lighter than
D, so it is a lot easier for a body to lose it, especially if the body has no
magnetic field like a comet or Venus (D/H is also very high on Venus).
Thinking the ratios would be the same seems like a mistake. Their graph should
really compare the values normalized for loss rates and time since solar
system formation.

------
wavesum
This one puzzles me:

"One of the leading hypotheses on Earth’s formation is that it was so hot when
it formed 4.6 billion years ago that any original water content should have
boiled off. "

So where would it actually have boil to? I thought gravity affected water also
in gas state. Just like the atmosphere doesn't "boil off", why would the
water?

~~~
dataduck
Actually, some fractions of the atmosphere _do_ "boil off":

[https://en.wikipedia.org/wiki/Atmospheric_escape](https://en.wikipedia.org/wiki/Atmospheric_escape)

Essentially, because the distribution of thermal energy has a fairly long
tail, it is possible for some lighter particles to reach escape velocity. The
hotter the atmosphere, the more (and heavier) particles escape.

------
smeyer
(For future readers, see dang's comment on how he changed the title from
"Rosetta first results: ocean water not from comets")[1])

I don't know why you changed the title to something claiming a stronger result
than their title: "ROSETTA FUELS DEBATE ON ORIGIN OF EARTH’S OCEANS". I'm not
a planetary guy, but I think while the evidence is moving that way, the claim
in your title is definitely premature.

Here's a distillation of their results (from their post) that seems far more
in line with their claims than your title:

>“Our finding also rules out the idea that Jupiter-family comets contain
solely Earth ocean-like water, and adds weight to models that place more
emphasis on asteroids as the main delivery mechanism for Earth’s oceans.”

[1]
[https://news.ycombinator.com/item?id=8731304](https://news.ycombinator.com/item?id=8731304)

~~~
aruggirello
I'm eager to see New Horizons arriving at Pluto, to see whatever water it
finds.

------
dang
The submitted title was "Rosetta first results: ocean water not from comets".
Since the article's own title is not very informative, we changed it to the
first sentence, which is. We also changed the url from
[http://blogs.esa.int/rosetta/2014/12/10/rosetta-fuels-
debate...](http://blogs.esa.int/rosetta/2014/12/10/rosetta-fuels-debate-on-
origin-of-earths-oceans/), which points to this.

~~~
bmm6o
It would be great if edits like this could be displayed at the top of the
thread rather than as the bottom-most comment.

~~~
dang
I understand why you'd say that, but the cure would be much worse than the
disease—the top subthreads would all be about the edits and editing practices.

~~~
bmm6o
It wouldn't have to be a replyable subthread, just a note somewhere are the
top of the page.

