[meteorite-list] CORRECTIONS TO Dwarf Planet 'Becoming A Comet'

From: Rob McCafferty <rob_mccafferty_at_meteoritecentral.com>
Date: Sat, 3 Feb 2007 17:41:17 -0800 (PST)
Message-ID: <899687.69087.qm_at_web50902.mail.yahoo.com>

Fascinating reading.


I can't help but feel that the use of the word comet
is typical of scientists attempts to engage the
layman.

In this case, I accept the term 'becoming a comet' in
a context of trying to engage the common plebian [we
call them 'the Sun' Readers in the UK]
The average Sun reader thinks they know what a comet
is. If they are paying taxes for scientists to explain
stuff then they want it in terms they think they
understand. If the scientist try to get all superior
then it's not on. That'd suggest the scientist have
one up on the guys who pay their salaries and that's a
no-no.

I accept that 2003 EL61 is a fascinating object but a
comet it ain't. Again I refer to the non-typical
Halley as an example. The density of this comet is
.25g/cm^3. Much less than 2003EL61. With an
essentially rocky core, this is surely a different
class of object.

Fascinating discussion. [I continue this until we can
establish once and for all whether CI or CM are
cometary]

Rob McC


--- "Sterling K. Webb" <sterling_k_webb at sbcglobal.net>
wrote:

> Hi,
>
> Just to clear up a few things: EL61 is a big
> rock
> with a thin layer of "snow." But it's so big and
> there's
> so much "snow" on a surface that size, that it
> amounts
> to all those Hale/Bopps. Some of my arithmetic last
> night was wrong (note to self: put on glasses, use
> calculator, dummie!), but the correct figures don't
> change the picture for the better.
>
> EL61's surface area is hard to calculate, given
> its odd
> shape, but it's about 9-10 million km^2. How do you
> measure the surface of that tri-axial shape? It
> would be
> just under 8 million km^2 if it were an efficient
> sphere,
> or 13 million km^2 if it were a box. EL61's shape
> has
> more surface than a sphere of the same volume, but
> less than the box it would fit in. Theories of
> packing
> efficiencies say a "long egg" is about 83%
> efficient,
> which would make EL61 about 10 million km^2.
>
> A "Hale/Bopp" sized object is about 33,500 km^3,
> so every kilometer of depth of ice on EL61 is equal
> to 298.5 "Hale/Bopp" sized objects. Every ten feet
> of "snow" on EL61 is equal to one Hale/Bopp. And
> every 45 cm of depth is a layer of material
> equivalent
> to the entire mass of the interplanetary dust of the
> Zodaical Light.
>
> If the ice on EL61's surface was one kilometer
> deep,
> its total volume would be the equivalent volume of a
> 270-kilometer diameter "comet," a pure iceball. If
> the
> ice is ten kilometers deep, it would be the
> equivalent
> volume of a 580-kilometer diameter iceball. As huge
> as
> that volume of ice is, it's nothing compared to the
> total
> volume of 2003 EL61, which is 1,760,000,000 km^3.
>
> Another critical factor is that those volatiles
> are all
> spread out on a vast surface with the maximum
> ability
> to intercept the sun's rays, a surface of a body
> that is
> spinning so fast that every point crosses the
> "nightside"
> in 2 hours, guaranteeing full exposure of most of
> the
> surface and an average exposure of 50% everywhere.
>
> This is less mass than I calculated (too
> quickly) last
> night, but still more than enough to produce the
> results
> I described. I hate when you're off by more than an
> order of magnitude too big, but when you discover
> and
> correct it, the results are just as lousy and
> discouraging
> as before. Really annoying, and just as dangerous.
>
> Could the ice on EL61 just be very shallow, so
> that
> there's no big deal? The only source of crystalline
> ice
> (snow) on such a world is water geysers which must
> be driven by internal heat and pressure, like
> Enceledus,
> the moon of Saturn. This argues for some good depth
> of ice to reach or generate that heat and pressure.
>
> With an albedo of 0.70, the surface of 2003 EL61
> is a mixture of "new-fallen" crystalline snow
> (albedo
> 0.90) and "older" icy surfaces (albedo 0.67). This
> suggests about 20% of 2003 EL61's surface is "new
> fallen" snow. Crystalline ice (snow) has destinctive
> spectral characteristics that ice (old and solid)
> does
> not. EL61's got it; its moons do not.
>
> If 20% of EL61 is covered with "new" snow, that
> suggests a fair rate of "geological" activity. If we
> had
> an orbiter watching it, it would probably find an
> active
> geyser or two going at any one time...
>
> Brown is using the term "become a comet" to
> describe the appearance, not the character, of 2003
> EL61 if it entered the inner solar system. It
> confuses
> the listener, because he doesn't mean it IS a comet.
>
> What a comet "is," is in flux right now, with
> all
> the recent missions and recoveries going on. They
> do not appear to be the traditional "dirty
> snowball,"
> but much more asteroidal. Conversely, we keep
> finding asteroids that may be "comets." Ultimately,
> I think all the Small Bodies are similar with a
> range
> of volatiles that is not as wide as we thought.
> Comets
> are rockier; asteroids are wetter, than believed.
> The
> difference may be between "hot" and "cold"
> asteroids,
> rather than asteroids and comets.
>
>
> Sterling K. Webb
>
-------------------------------------------------------------------
> ----- Original Message -----
> From: "Rob McCafferty" <rob_mccafferty at yahoo.com>
> To: "Sterling K. Webb"
> <sterling_k_webb at sbcglobal.net>;
> <meteorite-list at meteoritecentral.com>
> Sent: Saturday, February 03, 2007 4:25 PM
> Subject: Re: [meteorite-list] Dwarf Planet 'Becoming
> A Comet' (2003 EL61)
>
>
> Apologies for taking selected bits. Hope it's not
> out
> of context.
>
> --- "Sterling K. Webb"
> <sterling_k_webb at sbcglobal.net>
> wrote:
>
> ''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''
>
> Curiously, Halley's comet has an abledo of less than
> 4%, less than that of coal or black velvet. While
> Halley is not necessarily typical of comets, it is
> agreed that comets are very dark objects.
> Nucleus[nuclei] sizes have been estimated by
> removing
> modelled coma brightnesses from Hubble images and
> for
> nearby comets radar measurements seem to confirm the
> low albedo.
>
> Cometary dust may begin as silicate grained
> materials
> mantled with organic matter. To this hundreds of
> 0.01micron ice particles may form from a protosolar
> nebula into .5micron grains. These cluster into
> loose
> agglomerates which end up being part of the coma of
> comets. The evidence for this theory is the
> particles
> swept up by high altitude research planes [18km up]
> believed to be cometary in nature. This being the
> case
> it explains the brightness of the coma and -might I
> suggest- the brightness of EL61. It need not be
> covered in ice, just covered in this cometary 'snow'
>
>
> ''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.''
>
> Comets are generally considered to be a thin layer
> of
> rocky material over a lot of volatites, the complete
> opposite. I could well be wrong on this. Virgin
> comets
> are unusually bright on their first perihelion
> passage. One theory is that the surface volatiles ar
> vapourised away leaving this outer layer of dark
> material. This would suggest that if EL61 is indeed,
> becoming a comet, this is it's first journey inward
> which seems most unlikely. Also, comets sublimating
>
=== message truncated ===



 
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Received on Sat 03 Feb 2007 08:41:17 PM PST