[meteorite-list] 2003 EL61, IN PERSON

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon Sep 18 21:37:57 2006
Message-ID: <005301c6db8c$38d07dd0$f456e146_at_ATARIENGINE>

Hi,


    Here we've been wasting time talking about who found
2003 EL61 with not one word about the strangest planet in
the solar system (dwarf or not) itself! This is an utterly
fascinating place!

    First of all, there's its shape... Is it round? No, Is it irregular?
No. Is it squished? Well, sort of. Its dimensions are 1960 km
one way, 1518 km the other, and 996 km through the axis of
rotation. Hmm, can you picture that? Neither can I. So, here's
a picture of the shape of 2003 EL61:
http://hepwww.physics.yale.edu/quest/sedna/2003_el61.html

    Now, if you spin something fast enough (and EL61 spins
in under 4 hours per dizzy) and it's stretchy, you end up with
a shape like a squashed ball, or an oblate spheroid (or ellipsoid).
The Earth is so slightly squashed that it looks round, but Jupiter
appears squashed to the human eye.

    But 2003 EL61 is not a squashed ball, round and flattened.
No, it's much longer one way across than the other way across.
If it were made of ice or any substance that would move, even
very slowly, when force is applied to it, it couldn't maintain
this shape; it would even out over time. Likewise, if it was a
giant pile of rubble, it would adjust to the forces and be round
and flattened. And, there is an upper size limit to a rubble-loid,
where the energy needed to create rubble is so great it scatters
everything, so no rubble nor planet is left.

    Whatever 2003 EL61 is made of, it has to be stiff enough
to hold this shape as it whirls around every 3.9154 hours. That
creates a huge amount of force. 2003 EL61 is almost as big as
Pluto, the long way. If it was just round ("Why can't you be like
all the OTHER planets?"), it would be 1500 kilometers across,
bigger than Ceres, bigger than Charon. It has to be VERY stiff.

    We can calculate just how stiff it has to be to hold on its
elliptical midriff bulge while spinning, figure out its modulus of
rigidity and then look to see what materials are that stiff. The
answer is ROCK, rock of a high density. The estimates run from
a density of 2.6 to 3.4 gm/cm^3. For comparison, our Moon
has a density of about 3.3 gm/cm^3. Forget the "iceball" notion.
There can't be more than a smidge of volatiles in its composition
(like the Earth). The actual value is likely to be the highest or
a higher density, otherwise the planet would be right on the
borderline of being able to hold together and any of the ordinary
moderately big impacts you expect every billion years or so
would have shattered it.

    The currently favored explanation for the rapid rotation is
a giant impact. Likewise, the existence of two moons circling
2003 EL61 is attributed to a giant impact, like our Moon, like
Pluto and Charon; it's the moon-maker of choice these days...
But, the force of an impact great enough to spin 2003 EL 61
up to this speed is great enough to melt a rock body, and if
it had melted, the spin would have evened it out to a round but
flattened ball. Even if it hadn't melted, the rock would have
been soft enough to "creep" into a uniform oblate spheroid.

    The problem is, even though we can figure out how stiff
2003 EL61 has to be to hold onto its odd shape, that doesn't
explain how it got that shape in the first place...

    There are two ways out of this dilemma:

1.) Since resolution is poor at this distance, it could be that
2003 EL61 is a body that has been roughly chipped away by
multiple impacts into its present odd tri-axial shape, just as
Vesta seems to have been partially shaped by impacts (the
south pole crater). Is 2003 EL 61 a Super Vesta?
    But a "chipped" shape formed by multiple impacts into
a form so very extreme, with a ratio 4:3:2 for its axes, and a
"chipped" shape that size, 1000 to 2000 km, would likely be
shattered by multiple impacts strong enough to give it this
extreme shape, if it were only a "rockball."

    This leaves us with the other alternative:

2.) 2003 EL 61 IS a Super Vesta! That is, 2003 EL61 is a
fully differentiated planetary body, with a rocky mantle and
a iron-nickel core. Their densities are almost the same
(3.4 for 2003 EL61 vs. 3.4 for Vesta).
    As far as we know, the only way you can get an extreme
tri-axial shape is in bodies whose density is far so from
being uniform that the mass distribution distorts a two-axis
ellipsoid of revolution (ellipses having only two axes) into
that tri-axial shape -- in other words, it seems inevitable that
one would have to conclude EL61 is a differentiated body.

    Look at that picture at that URL above. Imagine it as a
composite of a round but flattened center section that is a
1000 km by 1500 km oblate spheroid with two "waves" on
opposite sides of the globe, "waves" that rise 250 kilometers
high from the oblate surface, like the (very much smaller!)
tidal bulges the Moon raises in the Earth's oceans.

    We pretty well have to assume that the spin-up impact and
the moon-forming impact are one and the same impact. These
Moon-forming impacts are relatively infrequent (Earth-Moon,
Pluto-Charon), else every planet would have a giant satellite
just like we do. Two huge impacts of that magnitude for
one small planet is asking a lot! So, one impact did it.

    Back when we thought the Earth's Moon was "spun off"
the early molten Earth and earnest mathematicians modeled
the process, their biggest problem, and the biggest objection
to the theory, was that the models always produced TWO
bulges ready to be spun off into satellites, and we only had
the one Moon, you see.

    So, it would appear that 2003 EL61's Big Whack caused
its molten core and soft lower mantle to merely distort into an
oblate spheroid, while its crust and asthenosphere became
completely fluid. It's possible the initial spin was even faster
than it is today and that the tidal interaction of its odd shape
with its Moon(s) has braked the rotation while moving the
moon(s) outward, just as with the Earth and its Moon.

    Moon One is 1/18 as bright as 2003 EL61. If we assume
that Moon One is as reflective as 2003 EL61 itself (a dubious
assumption, but what are you gonna do?), it would be about
350 km in diameter, mass about 1% of its primary (like the
Earth's Moon), and orbits at about 30-35 planetary radii (like
the Earth's Moon). Moon Two is smaller (about 1/4 as bright
as Moon One) at 170 km and closer (at 40,000 km instead of
Moon One's 50,000 km).

    Why do I say that its dubious that the Moons are as brightly
reflective as 2003 EL61? First, 2003 EL61 is a very bright body,
reflecting 70% of the light that falls on it, and it is indeed, as you
would suspect from this brightness, covered with water ice. BUT,
it's not old water ice, but new, freshly fallen crystalline ice,
otherwise known on our planet as snow. Apparently, it's like
Enceledus, the moon of Saturn, with water geysers which must
be driven by internal heat. If 2003 EL61 moons are water ice
bodies with internal heat like Enceledus, fine, but that's not
the type of satellite that would be formed by a planet-smashing
make-a-moon giant impact. The impact scenario on a rocky
body produces a rocky moon.

    If 2003 EL61's Moon One were as reflective as the Earth's Moon
(a typical rocky body), it would be 1100 kilometers across instead
of 350! If 2003 EL61's Moon One were as reflective as the typical
small TNO, it would be the size of Ceres. If 2003 EL61's Moon One
were just half as reflective as 2003 EL61, it would be bigger than
Vesta! (To illustrate the prejudice about outer system objects being
"tiny," if you were to put 2003 EL61's MOON in the asteroid zone,
it would be the fifth biggest asteroid, even at a "mere" 350 km!)
Water ice has been detected spectroscopically on Moon One, but
not the feature that indicates crystalline ice; its spectra is more like
Charon's. Charon is about 55% as reflective as 2003 EL61 which
would make Moon One about 680 km. My guess is that Moon
One will turn out to be at least 500 km in diameter.

    I'm having a lot of trouble reconciling this whole picture
of 2003 EL61 with the traditional image of a Kuiper Belt where
iceballs accrete by the gentle kisses of a trillion snowflakes...
In fact, to suggest that 2003 EL61 is a differentiated planetary
body like Vesta is enough to cause solar system formation
specialists to run screaming away or start gathering sticks and
branches for the traditional burning-at-the-stake. It's really
hard to explain, if true.

    It's bad enough to have all those battered iron cores out
on the asteroid zone when they should have stayed in the
terrestrial planet zone, but core-forming planetesimals all
the way out in Kuiper Belt?! Horrors! It's a mystery how
Enceledus generates water volcanoes; it's an even bigger
mystery how 2003 EL61 does it. Are the internal stresses
in 2003 EL61 so great they heat the whole body? Is the
surface in a continuous state of Richter 2.3? Anybody
want to live on a totally twitchy planet?

    I would give a great deal to see a nice sharp jpeg of 2003
EL61 from just a million kilometers away, but unfortunately
neither I nor anyone else on The List is likely to live long
enough for that to happen (nor for a 20-meter scope in orbit
nor anything else that would get us a good look...)

    And with ERIS and 2003 EL61 hogging all the limelight,
who pays any attention to the third big body announced
at the same time: 2005 FY9? Since I think its nickname of
"easterbunny" is ridiculous, I suggest a new one: the "shy
sister." The albedo of 2005 FY9 remains unknown, meaning
we have no way to estimate its size. Observations in the
infrared by the Spitzer, combined with similarities of spectrum
with Pluto yield a very conservative estimate of 1600 km,
or slightly bigger than 2003 EL61, which would make 2005
FY9 the largest known Kuiper belt object after Eris and Pluto.
But unlike Pluto's neutral hue, 2005 FY9 is much redder
than it or Eris and redder KBO's have lower reflectivities,
particularly big ones. If the "shy sister" has an albedo like
Quaoar's (10%), for example, the size of 2005 FY9 could
be a whopping 3000 km! That would make it the biggest of
them all.

    We just can't tell yet. The "shy sister" is currently the
second brightest Kuiper belt object after Pluto (despite its
great distance), with a magnitude of nearly 17. Back in 1930,
it was brighter still, and Clyde Tombaugh probably could
have found it but it was hiding out in the Milky Way! The
"shy sister" is so shy, she doesn't have a satellite (that we
can find). She's content to let everybody watch Eris and
2003 EL61 ("Elly"?); she's really very shy.


Sterling K. Webb
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Received on Mon 18 Sep 2006 09:37:51 PM PDT