Larry, and the Listoids,
All this talk of "compression" induced an involuntary
Google Storm on the compressibility of iron. Here's
what I found. At the Earth's core, the pressure is 330
to 360 gigapascals (3300000 to 3600000 atm). In a 1.4
Earth mass planet, the pressures would be still be
far from a terapascal (1000 gigapascals).
For Kepler 10b we only need 500 gigapascal results,
at most, but the only published data I could find
was for multi-terapascal pressures induced by shock
waves. Seems everyone wants to get up into the "teras"
in THEIR experiments!
http://iopscience.iop.org/0953-8984/21/45/452205/pdf/0953-8984_21_45_452205.pdf
At low pressure, the density of cubic lattice crystals of
iron is 7.875 and of FeNi, it's 7.884 to 7.860. At 100
gigapascals pressure, iron shifts to hexagonal close-
packed crystals with a density of 8.320 (an increase
of 5.561% in density) and FeNi to 8.330 (an increase of
5.98%). in the range of 400-500 gigapascals.
This doesn't even get us to 8.8. If you want to really
compress iron, try the inside of Jupiter at about 4-5
terapascals; the density will go up 15%! to about
9.0 for iron, which will stay in the hexagonal close-
packed phase up to... 10 or 15 terapascals? So, at
the most, we only get that 6% increase in iron
density inside Kepler 10b.
The freezing point of iron goes up with pressure, which
is why the Earth's core is solid though hot. There is an
eminently reasonable theory that our solid core "froze
out" of an originally liquid core. Some folks think it took
2 billion years to get a frozen core and other folks think
it didn't get there until a half billion years or so ago and
point to all the interesting changes in the planet 650
million years ago.
But Kepler 10b has had 12 billion years for its core to
cool down and "freeze." Despite its size, by now its core
could be solid and perhaps even in equilibrium with that
1600 C. surface temperature.
At any rate, it doesn't seem that simple squishing (I mean
gravitational compression) of an iron planet would get us
up to a density of 8.8. Yes, error bars, but the middle of the
error bars is the safest place to walk. Rock couldn't compress
enough, so we're left with the denser heavy metals to add
a little density.
A large solid core with a thin liquid iron layer acting as an
athenosphere at the base of an iron crust, topped with
dense alloy solids and then... oceans. I count 35 naturally
occurring elements denser than iron (up to densities of 22.6).
Some elements would easy mix with the iron (like its favorite
nickel at 8.92 density) but many would not. The one thing
I'm sure of? No volatiles...
Sometimes I feel embarrassed like I'm cooking up a planet
with liquid bromine oceans or something, but a planet in
this density range just has to be largely iron as nothing
else is as cosmically abundant. Still, those error bars run
from a 6.30 density planet to 11.3 density; that covers a
lot of ground. At a density of 6.30, it could just be a Super-
Mercury. At a density of 11.30, it would have to be odd and
compositionally unlikely.
Refining the measurement of the tidal radial velocity of
the star will sharpen that right up eventually, won't it?
Sterling K. Webb
------------------------------------------------------------------------------------
More Refs.
Other metals are more compressible than itron:
http://www.jetpletters.ac.ru/ps/1218/article_18419.pdf
Tantalum will go up to a density of 40!
Iron compressed without shock:
https://e-reports-ext.llnl.gov/pdf/333066.pdf
---------------------------------------------------------------------------------------
----- Original Message -----
From: <lebof...@lpl.arizona.edu>
To: "Sterling K. Webb" <sterling_k_w...@sbcglobal.net>
Cc: "Meteorites USA" <e...@meteoritesusa.com>; "Meteorite-list"
<meteorite-list@meteoritecentral.com>
Sent: Monday, January 10, 2011 10:54 PM
Subject: Re: [meteorite-list] NASA's Kepler Mission Discovers Its First
RockyPlanet
Hi Everyone:
An update. Geoff Marcy gave an invited talk this evening at the
meeting I
am at (American Astronomical Society). The density of the "new" planet
is
8.8 +/_ 2.5 g/cc (iron meteorites are 7-8). The large uncertainty (not
bad
given the size of the object) implies that the planet can be anywhere
from
a more compressed "Earth" (similar composition, but denser due to
greater
mass) to an object made up of 75% iron (closer to Mercury in
composition).
I find that interesting given that the star it orbits (and thus the
star
system) is iron poor relative to the Sun. There is something new every
day!
Larry
This is the top item on a list of Kepler "hits" waiting
to be verified by ground-based telescopes. The list is
roughly 700 "hits" long and we can expect a minimum
of 500 to be confirmed.
There are more hits in the data being teased out,
so we can expect a flood of planets to be slowly confirmed
and dribbled out. Planet-O-Rama!
Sterling K. Webb
--------------------------------------------------------------------------
----- Original Message -----
From: "Meteorites USA" <e...@meteoritesusa.com>
To: "Meteorite-list" <meteorite-list@meteoritecentral.com>
Sent: Monday, January 10, 2011 1:28 PM
Subject: [meteorite-list] NASA's Kepler Mission Discovers Its First
RockyPlanet
http://www.jpl.nasa.gov/news/news.cfm?release=2011-007&cid=release_2011-007&msource=11007&tr=y&auid=7605855
Not in the habitable zone, and 20 times closer to the Kepler 10 star
than Mercury is to our Sun, but it is 1.4 times the size of Earth
which is the smallest planet ever discovered outside our solar
system.
Way cool!
Regards,
Eric
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