Hi Sterling-
Yeah, I guess you're right, we're doomed to disagree <g>. (Did you see the
cleverly packaged MIT T-shirts distributed to MIT freshman at last years
orientation? The ones that nobody noticed until too late had "Because not
everyone can go to Caltech" printed on the back?)
I take exception to your point that the Moon "obviously" has a uniform crust
early on. This isn't obvious at all, and nobody has a good explanation for
why the Moon does not now have a uniform crust. This feature does not fall
out of any models. Gravity does not obviously explain why the crust should
be thinner towards the Earth. If the crust thickness variation developed
early (and the maria are certainly old) this would explain the reason that
maria are only present on one side. Since it is likely that the Moon was not
yet tidally locked when the basins formed, I don't see the effects of the
Earth as having contributed in an obvious way to their formation.
Chris
*****************************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
----- Original Message -----
From: "Sterling K. Webb" <[EMAIL PROTECTED]>
To: "Chris Peterson" <[EMAIL PROTECTED]>;
<[email protected]>; "Dawn & Gerald Flaherty"
<[EMAIL PROTECTED]>
Sent: Monday, June 27, 2005 3:55 AM
Subject: Re: [meteorite-list] fire flies or flying fires
Hi,
There are a variety of questions involved.
First, there is to the problem of identifying what constitutes a
"basin" as
the earliest basins are relicts of the earliest impacts and are obscured
to a
greater or lesser degree by later large basin forming impacts. Most
selenologists think Procellarum is the most ancient relict basin overlaid
by the
later Imbrium, Serenitatis, Tranquilitatis, Humorium and Nubium basins.
If Procellarum is a basin, it would be the largest on the Moon at 3200
km
(near side), followed by South Polar basin at 2500 km (on the edge but
mostly
near side), followed by Imbrium at 1160 km (near side), Crisium at 1060 km
(near
side), Orientale at 930 km (on the edge, side), down through a long list
sorted
by size, until you arrive at the Keeler-Heavyside basin at 780 km (far
side).
The size of the basin is not linearly related to the size of the
impactor,
as the state of the Moon was changing throughout the period of
bombardment. The
crust, its distribution and other characteristics, the mantle, the core,
were
continually evolving over the basin-impact period.
Obviously, when the Moon was mostly molten and starting to
differentiate,
the crust was a new thin skin everywhere. Procellarum, if a basin, formed
within the Moon's first 150 million years, before (during?) mantle
crystallization and would not have required as large an impactor as later
basins
of lesser size. At this point the differential crust thickness and the
eccentric mass center lay (mostly?) in the Moon's future.
The gravitational effects that displaced the Moon's center of mass
toward
the Earth only take effect after cooling and differentiation has
progressed to a
sufficiently great degree and the crust differential only develops late or
after
that mass center shift is mostly complete, then it "freezes" in place.
Imbrium, the second largest basin, formed when the Moon was 600 million
years old (?), is thought by many to have been the target of the largest
impact
object of the Moon's history, well after the crust differential had
developed.
Indeed, Imbrium appears to be one of the youngest basins in the solar
system,
hence requiring a stupendous whack from an object large enough to be a
respectable little moon itself.
Another young (or later, whichever you prefer) giant basin is Caloris
on
Mercury, larger at 3700 km, whose impactor came close to breaking the
planet, as
the vast stretch of chaotic terrain on the opposition point on Mercury's
surface
shows. Planet breakers, not a happy concept.
However, the point of all this is that ancient basins are identifiable
even
when overlaid by one or several or many later basins (like Procellarum
is). The
far side shows no traces, however faint, of former gigantic relict basins
to
rival those of the near side or the poles.
There was far less magna upwelling on the far side to obliterate traces
of
relict basins. The thick crust protects relict basins from obliteration
because
it is harder to form new large basins on (or is the word "in"?). Yet, the
central far side is the most depleted in traces of ancient basins,
implying far
fewer impacts. Large ringed craters / small basins are pretty much the
same
thing and in uniform distribution. The Moon has a huge number of them and
every
smaller size of impact, but giant ancient basins? Near side is the spot
where
the action was.
We also have to distinguish between ringed basins formed in an already
stiffened crust and flood basins formed by large upwellings alone,
although
Procellarum is so old it's hard to tell, it doesn't appear to be a ringed
basin,
although it did form on older crust.
The WHOLE dating question is still problematic. The oldest fragment
recovered by Apollo were inclusions formed at 4450 mya (million years ago)
which
would set the Earth-Moon impact (the current theory) at an Earth age of
almost
nothing! But a single crystallized grain is thin evidence. The next
oldest
samples are mare basalt fragments in younger lunar breccias at 4230 mya,
implying a lunar magma had upwelled to cool into new crust to be
fragmented by
impact, but that's a big gap. No complete lunar rock as a whole older
than 4150
mya. We think. No samples at all from the far side, of course. Get me
10,000
more samples and we'll straighten this whole thing out! Or would we?
The famous ancient zircon from Australia, now living in the lap of
luxury in
Wisconsin, is 4404 (+/-4) mya, formed in a time of temperate conditions
and
liquid water, they say. But wait, Time Zero is about 4560 mya, the
earliest
planetesimals start forming at 4555 mya. The Earth and other terrestrial
bodies
are essential complete by 4500 mya.
But the Earth-Moon impact theory, with it's 2-3 Mars mass body
completely
remelting an already formed and much smaller complete Earth and adding a
whole
new crust on top of the old crust, with full differentiation taking place
AGAIN,
including a new major addition to the core, when did that happen? After
4500
mya (complete first Earth) but before 4450 mya (lunar crystal), right? Or
at
4500 mya, at the very last minute? Why is the happy zircon so happy? And
when
it find a time and place to be happy in at 4404 mya?
If the 4450 mya lunar crystal is a lucky inclusion (like the zircon?)
was
there a happy Earth that got whacked by another planetoid and the lucky
zircon
is a chance survivor? The pro-zircon forces are in favor of a totally
Cool
Early Earth (I think it's pretty cool, myself). The Earth-Moon Impact
crowd
insist that their Early Earth be served Molten Hot Twice. Am I the only
one to
see a problem here? NO Hadean Epoch? TWO Hadean Epochs?
The happy zircon is also a funny zircon. Even it's a tiny crystal,
other
probe spots on it are younger than the 4404 mya spot, even though it's one
crystal. Contacting zircons in the one little rock the crystal is
included in
are as young as 3300 mya. There's no "this one whole microscopic crystal
is
4404 mya" object. It has had a complicated life for a happy zircon.
It's clear the happy zircon is a triumph of micro-probe dating, but
what is
it proof of? And the Moon? We know how long it would take the lunar
mantle to
crystallize and we know how long again before the core solidified, but
WHEN does
that timeline start? The Earth-Moon Impact crowd say the Moon was
complete by
4440 mya but about how long it took to assemble, there is silence and a
few
mutters, "a couple of days" say some, "100 million years" say some. The
models
are dynamically incomplete. So sorry. Recent remodeling says it was
"more
likely" quick, but then we want it, need it, to be quick.
We are still quarreling about the Late Collisional Bombardment, whether
there was one or not, for heaven's sake.
Stitching together the happy zircon timeline, the Earth-Moon impact
timeline, the planetesimal and accretion timeline, and all the other
timelines
is not complete. We're still looking for the needle and thread. Oddities
abound. Every living thing on Earth has the same little snippet of 16S
ribosomal RNA gene (in various related mutated versions). What is it good
for?
Why, metabolizing sulfur for energy! We all get our "go" from sulfur,
right?
No. The critters that need it are the archaic sulfur eating bacteria in
deep
sea hot mineral vents, and some biologists suggest that its universality
arises
from an early extinction of ALL life but them, and that life had to start
all
over again. Earth-Moon impact, anyone? After the happy zircon?
This stitching is like making a crazy quilt. Drives me crazy, anyway.
I'm a lot more certain about the predominance of near side ancient
basins,
though.
We're doomed to disagree, Chris, you (CalTech) and me (MIT).
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