[meteorite-list] Dwarf Planet 'Becoming A Comet' (2003 EL61)

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat, 3 Feb 2007 01:04:47 -0600
Message-ID: <008e01c74761$97577700$904ae146_at_ATARIENGINE>

Hi,

    This speculation that 2003 EL61 could become an inner
system Giant Comet is a very, very strange one. I find it
extremely puzzling. Brown, whom I admire greatly, is a finder,
not a dynamicist. He has a lot of dynamicist friends I don't
think much of, though.

    But, if 2003 EL61 became an inner system giant comet,
it would just be the capper on this strangest of all strange
worlds in the solar system! Here's its story. I posted some
of this about EL61 last year but if you don't want to read
about how odd it is, skip down to the heading GIANT
COMET, ok?

STRANGE WORLD

    First of all, there's its shape... 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).

    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 neither 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 EL61 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 semi-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.

ITS MOONS

    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 it's 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, EL61 is 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.

GIANT COMET?

    Now, we come to the Giant Comet Notion. Obviously,
2003 EL61's ice is a surface feature, a thin layer of volatiles
over what is essentially a rocky body. So, how much material
is there to be warmed by the Sun if EL61 got shuttled into the
inner solar system?

    Let's compare it to Comet Hale/Bopp, which more people
saw as a naked eye object than any recent comet (McNaught has
been sneaky). Hale/Bopp was 40 kilometers across and we don't
know how much of it was volatiles or how much of its surface
was volatilized by the Sun, but certainly not more than a small
percentage of the comet's bulk. If ALL of Hale/Bopp had been
volatilized, it would have been a hundred times (or more) brighter
and a thousand times more spectacular!

    If the water ice on the surface of 2003 EL61 were a mile deep,
it would have the volume of 136,460 Hale/Bopps! In fact, the
top one foot of 2003 EL61's icy surface contains 2.6 times the
volume of Comet Hale/Bopp!

    Since it seems likely that the freshness of the surface ice on
EL61 is because it is supplied by deep water geysers, there
would seem to be some depth of ice on EL61. If it were 5 miles
deep, the ice volume would be equivalent to the total volume of
682,300 Hale/Bopps. And if the layer were 20 miles deep, the
ice volume would be the equivalent to the total volume of
2,729,200 Hale/Bopps!

    Incidentally, my Ice Unit, 1.0 Hale/Bopp Unit, is exactly
268,082.57 cubic kilometers, or 2.6808257 x 10^14 cubic
meters of ice, weighing 2.6808257 x 10^17 kilograms!
This amount of ice, One Hale/Bopp Unit, is 7.5 times
the mass of ALL the interplanetary dust presently in the
solar system, that which causes the visible reflection
which we can see with our naked eye, the Zodiacal Light.

    The crucial question would be how deeply into the solar
system a perturbed 2003 EL61 would travel in its new,
perturbed orbit? If its perihelion were in Saturn Country,
it would simply become the Big Cheese of the Centaur
Group (of which there a 100 or so) and the Super Comet
might show traces of coma in a telescope. If its perihelion
were near to Jupiter (what an unstable orbit that would be!),
it would be both bright and visually comet-like.

    If its perihelion were any closer, there is another factor
to consider: Danger. Brown treats this (at least as quoted
in this press story) as an almost "touristy" event: "When it
becomes a comet, it will be the brightest we will ever see."

    But if its perihelion were INSIDE Jupiter's orbit, it would
pass through (and thus perturb) the Asteroid Belt! Hey now!
Wait a minute! Perturb the Asteroids? That doesn't sound
so great. No, definitely not a Great Idea. Lively, but not prudent.

    Anybody have a lot of enthusiasm for a Rain of Ice and
Iron like we haven't seem in Eons? No? I don't have much...
That's not "a good thing" (to quote Martha Stewart). Having
a planet-sized body traversing the Asteroid Belt can only
spell Big Trouble.

    Well, what if it didn't pass through the Asteroid Belt?
No way of knowing what the inclination of a perturbed
2003 EL61 would be, after all. What if its perihelion was
closer in than the Asteroids?

    It certainly would have a vaster and more extensive coma
then, wouldn't it? If it came in as far as Mars or even the Earth,
it would be incredibly bright, visible both day and night, with
a tail many millions of kilometers long, and a coma 10 or 20
thousand kilometers across, perhaps 100,000 kilometers
across if it got warm enough. That certainly qualifies as a
celestial "tourist event"!

    And if the perihelion were even closer to the Sun , the
volume of material pouring off the new "comet" would be
many millions of tons per second. The solar wind pushes
against dispersed material, dust, gas, ice; that's what forms
a comet's tail. But if the volume were great enough, the inner
solar system would fill up with, and become clouded with,
a fog of highly reflective particles, despite the best efforts of
the solar wind.

    The Sun's light would be scattered, diffused and dispersed.
The skies would be brighter at night and dimmer in the daytime.
Eventually, they would look the same both night and day as
the volume of cometary "fog" increased. There would be no
more night. The amount of solar energy reaching the inner
planets would be drastically reduced.

    The situation would be exactly like passing through a GMC
(Giant Molecular Complex) or interstellar cloud, only much worse.
This amount of volatiles released in the inner solar system would
"cocoon" the Sun in a gas and dust envelope.

    If the Earth's solar input dropped by 1%, the planet's mean
temperature would drop by less than 1 degree C. No problemo!
We'd get rid of that pesky Global Warming, you know.

    A mere 10% decrease would lower the planetary temperature
by 7 degrees C. That's not so pleasant. That's less than what
the global temperature was during the last Ice Age (6 degrees
cooler). We don't want the Ice Age back, do we? No, I don't
think so...

    A 20% decrease would lower the planetary temperature by 15
degrees C! You might be saying that a 15 degree drop doesn't
sound all that bad, but we're talking about the planetary mean
temperature. It hasn't been that low in the entire geological history
of the Earth, including the era when there was sea ice at the equator!
At this temperature, the entire planet would be covered by ice
from Pole to Pole, frozen solid!

    That's from a 20% decrease, but the gas production of a truly
planet-sized "comet" could easily produce a 30% to 50% reduction
in solar incidence, which would drop mean temperatures by up
to 50 degrees C. The equatorial temperature of the Earth would be
about what the temperature of Antarctica is now. Mars would fall
to the temperature range of Saturn (or Pluto). Even Venus, the solar
system's Hellball, could become too cool for comfort. Everything
from the Asteroids on out would be at the temperature the Kuiper
Belt is now.

    So, you see, it's not exactly a tourist bonanza nor a celestial
event to be anticipated with bouncy enthusiasm. No, this is not
good at all. This is worse than being bombarded with big asteroidal
impactors and other assorted cosmic disasters. It would not only
make our own planet uninhabitable, but also all the other worlds
where we might conceivably, with herculean effort, attempt to take
refuge, mucking up the entire solar system. (You got any relatives
at Alpha Centauri?) Actually, there's a name for this sort of event...

    I believe it's called The End of The World.

    So, I have to ask myself: how likely is this? Is this going to
happen? Ever?

> Brown has calculated that the object could be due [for] a close
> encounter with the planet Neptune. If so, Neptune's gravity could
> catapult it into the inner Solar System as a short-period comet.

    The likelihood of any "close encounter" repeating itself
is easy to calculate. Suppose two bodies with orbital periods
of 2 and 3 years respectively have a "close encounter." Two
years later, when the first body has returned, the slower body
is still a year away from the potential encounter point. Wait
another two years (four years total); the slower body is now
a year past the encounter point. Only after six years will the
encounter repeat. That is the product of the two orbital
periods ( 2 times 3 = 6 ) and is the time between encounters,
or indeed any specific configuration of the two orbits.

    2003 EL61's period of 284.5 years times Neptune's period
of 164.88 years is 46,900.36 years. That means that the two
had an encounter like what is proposed 46,900 years before, and
another such close encounter 46,900 years before that, and...
Well, 21+ such encounters every million years. Since the very
beginning of the solar system, they've had almost 96,000 such
encounters, and in exactly NONE of them has 2003 EL61 gone
off to visit the inner solar system, not even once.

    How likely is it that next close encounter will have that
result? Or even the next 100 close encounters? How has EL61
managed to hang onto its moons through all these "close"
encounters? Why, after the solar system has held together
for more than four billion years, should it decide to unravel
right now? Is the solar system just coming apart?

    On the other hand, there's this: we explain 2003 EL61's
extraordinary shape and fantastic spin to a Major Collision
with Something. We also assume it happened in the far distant
past, early in the history of the solar system (mostly because
it was more crowded then and also because we don't want
to think it could happen now). What if the body that "collided"
with 2003 EL61 was a moon of Neptune? What if it was "only"
half a billion years ago (or less) and that the collision altered
EL61's orbit to make a too-close encounter and a voyage to
the inner solar system possible?

    Now, there's a nasty thought...

    Which is why, instead of a news snippet with three sentences
of potential information, you ought to actually publish something
yourself, with, you know, real numbers and real calculations and
real information, Mike. To modify a great movie line: Show me
the numbers!

    Since EL61 has only been observed for 2-3 years of its 285
year orbit (1%) and the earliest prediscovery photo is 1955 (18%
of an orbit ago), just how accurate are those orbital determinations
and the resultant calculations? Inquiring minds want to know...

    My considered scientific opinion?

    'Tain't happenin', dude!


Sterling K. Webb
-------------------------------------------------------------------------------------------
----- Original Message -----
From: "Ron Baalke" <baalke at zagami.jpl.nasa.gov>
To: "Meteorite Mailing List" <meteorite-list at meteoritecentral.com>
Sent: Wednesday, January 17, 2007 11:32 AM
Subject: [meteorite-list] Dwarf Planet 'Becoming A Comet' (2003 EL61)

http://news.bbc.co.uk/2/hi/science/nature/6268799.stm

Dwarf planet 'becoming a comet'
By Paul Rincon
BBC News
January 17, 2007

An unusual dwarf planet discovered in the outer Solar System could be en
route to becoming the brightest comet ever known.

2003 EL61 is a large, dense, rugby-ball-shaped hunk of rock with a fast
rotation rate.

Professor Mike Brown has calculated that the object could be due a close
encounter with the planet Neptune.

If so, Neptune's gravity could catapult it into the inner Solar System
as a short-period comet.

"If you came back in two million years, EL61 could well be a comet,"
said Professor Brown, from the California Institute of Technology
(Caltech) in Pasadena.

"When it becomes a comet, it will be the brightest we will ever see."

Cosmic oddball

2003 EL61 is a large object; it is as big as Pluto along its longest
dimension. It is one of the largest of a swarm of icy objects that
inhabit a region of the outer Solar System known as the Kuiper Belt.

But it is extremely unusual: spinning on its axis every four hours, it
has developed an elongated shape.

2003 EL61 is apparently composed of rock with just a thin veneer of
water-ice covering its surface. Other Kuiper Belt Objects (KBOs) contain
much more water-ice.

Professor Brown's computer simulations show that the object is on a very
unstable orbit and set for a close encounter with Neptune.

The eighth planet's gravitational force could either sling the icy rock
ball into the inner Solar System as a comet, out into the distant Oort
Cloud region, or even into interstellar space.

Orbits of Kuiper Belt Objects tend to be very stable, but the region is
thought to be a reservoir for short-period comets.

Occasionally, some of these objects must get tossed inward to become the
fizzing lumps of ice and dust that criss-cross our cosmic neighbourhood.

Shedding surface

Mike Brown and his colleagues have come up with a scenario to explain
2003 EL61's physical characteristics and behaviour.

About 4.5 billion years ago, the object that became 2003 EL61 was a
ball, half composed of ice and half of rock - like Pluto - and about the
same size as Pluto.

Some time early in its history, it was smacked, edge on, by another
large KBO. This broke off much of 2003 EL61's icy mantle, which
coalesced to form several satellites.

As expected, the satellites seem to be composed of very pure water-ice.

Professor Brown suggested that some of 2003 EL61's mantle may already
have made it into the inner Solar System as cometary material.

The oblique impact also caused 2003 EL61 to spin rapidly. This rapid
rotation elongated 2003 EL61 into the rugby ball shape we see today.

"It's a bit like the story of Mercury," Professor Brown explained.

"Mercury got hit by a large object early in the Solar System. It left
mostly a big iron core, with a little bit of rock on the outside. This
is mostly a rock core with a little bit of ice on the outside."

Mike Brown outlined details of his work during a plenary lecture at the
recent American Astronomical Society meeting in Seattle.
Received on Sat 03 Feb 2007 02:04:47 AM PST