[meteorite-list] Early Mercury Impact Showered Earth

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Sat Apr 8 10:34:33 2006
Message-ID: <009601c65a20$7a25f8c0$5f7dd745_at_ATARIENGINE>

Hi, Rob, List,


    The impact theory of Mercury's origin is not new;
it was first put forward by Cameron and Benz in 1988,
about the same time as the impact origin of the Moon
was put forward by Hartman (altho he'd been thinking
about it for a while, I'm told).
    Interesting and obvious things can be terribly misleading.
For example, it happens that the density of our Moon and
the density of the Earth's mantle is the same, to about one
part in 400. It's "obvious" that the Moon was formed from
the mantle of the Earth (except for the fact that it wasn't)!
    The figures are nothing but pure coincidence; their
compositions are radically different. But the obviousness
of it made mantle origin the main theory of the Moon's origin
for more than a century.
    Having received my astronomy education at a time
we had a lot less information and a lot more daydreaming to
fill in the holes, I've had the pleasant experience of having
the errors of the "obvious" revealed to me time and again.
    Another example is Mercury's "day." Because of the
powerful influence of the Sun's gravity, everyone assumed
that Mercury would be tidally locked, with one face always
to the Sun, like the Moon is to the Earth, but no, it ain't
necessarily so. Mercury's day is 2/3rds of its year. It is
locked in a 3:2 resonance instead of a 1:1 resonance.
    One so-called "proof" of the Mercurian impact theory
is the fact that Mercury's orbit is both eccentric (e = 0.2)
and very seriously tilted (7.2 degrees) with respect to
the rest of the really flat solar system, implying that only
a Big Whack can explain that. Only Pluto is more tilted
(inclined) than that and that's because things are looser
4,000,000,000 miles away.
    However, there's a big inconvenient fact right in the
middle of that. The Sun itself is tilted with regard to the
solar system plane, by 7 degrees, almost the same amount
as Mercury is inclined. Is that a coincidence? It is a damned
odd fact. Now, you can do two things with an oddity:
obsess over it or ignore it.
    I've actually met full-grown degree-wielding astronomers
that were unaware of it, blithely assuming the solar system
to be in the equatorial plane of the Sun. This tilt makes no
sense at all. How can a rotating solar system that carries
almost all of the angular momentum in the system,
whatever your theory of its origin is, form at an angle?
OK, I'll ignore it if you will...
    When a planet has an anomalous axial tilt (like Venus
or Uranus), we just call in The Big Impact as an automatic
explanation, but you can't Whack a star. How much torque
would it take to tilt the Sun? Got lots of zeroes handy?
    It is such an inconvenient piece of reality... Perhaps
Mercury isn't inclined at all, and the rest of the solar
system is. Although I find it hard to imagine what could
twist an entire solar system... Maybe our solar system
(like many extra-solar ones), once had gas giants in fast
close orbits, and nebular gas drag spiraled them in until
the Sun ate them; they might have enough mass and
momentum to affect the Sun's tilt, if they were really big
gas giants.
    Mercury has all kinds of oddities: a magnetic field, which
requires a liquid core, the most likely cause of which is the
element sulfur dissolving in during core-mantle separation
and forming FeS. Otherwise it's too small to have stayed
liquid and magnetic this long. The Mercurian mantle would
have to be about 2% sulfur. OK, but where'd this sulfur
come from?
    Sulfur is one of the more volatile elements, which means
that it's definitely not what you'd expect to find in a planet
formed right on the Sun's doorstep. Out at Io, yes; but at
Mercury, no. (Unless it is a leftover satellite of one of those
gas giants the Sun ate...) Another oddity, as it's really hard
to explain the core's magnetism without it. You could count
this problem as one more of the problems with the notion
that planets formed from narrow zones in place.
    The "escaped satellite" set of explanations are not popular
right now; computers have shown that it's a lot harder to pry
a satellite loose from a planet than we used to think, at
least without a huge wreck of an encounter. The notion that
Pluto was an "escaped satellite" of Neptune was very popular
in the 1950's. Not any more. The idea that Triton, Neptune's
retrograde moon, was captured is still played with, but it now
appears to be possible only if Triton was part of a binary system
whose missing other component was not too massive.
    Venus losing a satellite would require a much more
energetic event and is harder to explain. Of course, everything
about Venus is hard to explain, so that's no surprise. Half a
century ago, our understanding of the terrestrial planets was
"quasimythological." Mars was old, mature, drying out into
red deserts, water-short, with possible canals and ancient
Martians. Venus was young, primitive, super wet and cloudy,
with jungles and possibly some exotic dinosaurs. Earth, of
course, was in the prime of life, the Goldilocks planet:
it was just right. The chief mover in busting up this view with
regard to Venus was Carl Sagan in a 1962 paper that
resurrected the spectroscopic work of Wildt in the 1930's
and 40's and explained what it meant: the hot dry massive
greenhouse atmosphere planet. So long to those dinosaurs.
    The orbit of Venus has peculiarities, too. Venus's "year"
is 224.7 Earth days. Venus's "day" is 243.01 Earth days. But
because Venus's axial rotation is backward measured against
the Sun and stars, the Venusian "solar day" is only 116.75
Earth days long. Of course, we could just as well not describe
Venus's rotation as "backward," but just consider that Venus
rotates "normally" but with its axis turned completely
upside-down, by 177.4 degrees! However you look at it,
Venus is the only body of any size in the solar system to
rotate "backward."
    If you regard "normal" rotation as required, as it is, by
most theories of solar system formation, then you have to
invoke another Big Whack to turn Venus upside-down! That
would have to be one heck of a whack, too. The energy
transfer would be so great it's hard to imagine the planet
could have survived it.
    So, there's another theory: that the solar tides on the
thick atmosphere have braked Venus down to a standstill
and are now spinning'er up in the backward direction. Myself,
I think the atmospheric torque is just not big enough to do
the job, and since what little we know about the surface
suggests that there are virtually no winds at all at the
surface (and you have to apply atmospheric torque to
the surface), I think it's hooey. The math is complex and
not entirely convincing.
    The position of Venus in the Earth's sky cycles in the
time it takes Venus to lap the Earth in its orbit, 593.92
Earth days, the synodic period. Oddly, that period is
almost 5 Venus days, to be exact, 5.0014 Venus days.
This means, annoyingly, that when you're trying to radar
map Venus from the Earth at the close approach when you
have the highest resolution, you're looking at almost exactly
the same patch of Venus you were looking at the last time!
Over and over again.
    The synodic period of Venus, 593.92 Earth days, is almost
exactly 8/5ths of an Earth year, so that every eight Earth years
the positions of Earth and Venus line up very closely with only
a tiny amount of drift in position from cycle to cycle. Every 152
Venus synodic cycles of 593.92 Earth days, the line-up returns
to its original precise positions, creating a long cycle of precise
repetitions of the positions of Venus and the Earth. This long
cycle takes 243.01 Earth years.
    Now, if the number 243.01 seems familiar, it's because it
"happens" to be the length of Venus's axial rotation in Earth
days, the sidereal period! The extreme regularity of this
cycle of Venusian positions with respect to the Earth creates
the long and precisely repeating cycle of Venus's transits
of the Sun, meaningless except that these mark the timing
of the Sun's passage across the nodes of the mutual plane
of Earth's and Venus's orbits.
    So, how many 243.01 Earth day sidereal periods of
Venus does the transit cycle take? Why, 365.24 of them,
which "happens" to be the number of days in the Earth year,
just as 243.01 is the number of Earth days in the Venus day.
    Personally, I find that just plain spooky. Officially, these
coincidences are just that: coincidences. The Earth and
Venus are not in an 8:5 resonance, officially, yet when you
either regress or progress the orbits, these regularities
do not go away. They drift in and out of greater or lesser
regularity for as far as the floating point calculations can
go, for millions of years, without any divergence. It is an
extremely stable configuration.
    There was a lot of argument in the 1960's about whether
this was "really" a resonance or not, and by the 70's, it was
branded an annoying coincidence. Personally, I think it's
too neat to be a coincidence, so I was cheered last year when
I ran across a AGU paper that calculated that the differences
between the atmospheric tidal torques and the solid tidal torques
generated by Venus's tiny eccentricity acted to push Venus
back and forth into a recurring perfect face-to-face lock with
the Earth by minutely altering the length of its "day"!
    I stand on "spooky" as the best description of the Universe.
    If Venus had received a big impact or an encounter that
stripped away a moon as big as Mercury, you might well
consider that some orbital irregularity would remain, even
after billions of years. Mercury still has an eccentric orbit,
presumably, from that early impact. However, it should be
pointed out that the eccentricity is also sustained by that
3:2 resonance which depends on the varying orbital speed
of Mercury, and forces the Sun's tides to re-enforce the
eccentricity.
    Our Moon has an orbital inclination of about 5 degrees
with respect to the Earth's equator that is hard to explain
without resorting to an impact origin, since both the Sun
and Jupiter tug rather strongly on this tilt, trying to
straighten it out (and indeed this tilt may have been
greater in the past). Without the Moon to tug on the
Earth's axis and keep us reasonably precessing in a
narrow range of shallow axial inclinations, the many
perturbations on the Earth would push the cycle
of axial precession through a wild ride on a short leash,
moving Greenland and Siberia to the sub-solar point and
putting Brazil in the position of Antarctica (relative to the
Sun). It would be climatic chaos.
    Yet, Venus maintains the most circular, non-eccentric
orbit in the solar system, only 0.006, and seems to continue
this delicate balance of some kind of complex resonance
with the Earth on a long-term basis. It's hard to imagine
major life-changing immense impacts would leave no
greater trace.
    Of course, we have to define how major "major" is.
These early impacts (Moon-forming or Mercury crust-
stripping) are low-velocity or grazing encounters with
planet sized bodies, as big as Mars or bigger. The
range of eccentricities and inclinations in the Asteroid
Belt requires the gravitational "stirring" of a body at
least as big as Mars passing through the Belt repeatedly.
Uranus was hit hard enough to lay it right over on its
side. All these events are "major." They leave marks.
    The impact of an asteroid as big as Vesta or even
Ceres on a planet like ours doesn't qualify as "major"
from this standpoint. That impact wouldn't strip crust
or form a big satellite or alter a solar orbit in the least.
All it would do is boil the oceans instantly, vaporize
the top few kilometers of crust, creating a 3500 degree
rock vapor "atmosphere," and melt the rest of the crust
down to its base at the mantle. Nothing "major."
    It interesting to note that such an incident would
convert the Earth into a very convincing "twin" of Venus
in one day flat. We would have, in a very short time,
a brand-new basalt crust that was all the same age
everywhere on the planet ("check"), a 100-bar CO2
atmosphere from the breakdown of the oceanic
carbonates ("check") with an equilibrium temperature
of about 350 degrees C. ("check"), a cloud-deck with
the same photochemistry as Venus ("check"), totally
suppressed tectonics (all the plates would have
been fused completely into one when the crust
melted -- "check"), and so forth.
    Venus, despite all the landers, mappers, probes,
and effort expended on it, remains an on-going quarrel.
We thought more data would explain things; it just gets
worse. Consensus about how Venus came to be what
it is, what forces evolved it, and how they work today,
is not in sight. It could be merely because Venus is
just that different, but is annoying that no one can agree
on the crustal mechanics after all that radar mapping,
that we can't explain the landforms, that there are major
atmospheric constituents we can't identify, that we can't
unravel even the noble gas abundances, and so on.
    In view of all these difficulties in explaining Venus,
wouldn't it be funny if it was just a case of a perfectly
"ordinary" terrestrial planet that took just such a hit
from a big asteroid as described above. That would
certainly explain it very economically.
    Nasty way for a world to die, though.


Sterling K. Webb
------------------------------------------------------------------------
----- Original Message -----
From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
To: <meteorite-list_at_meteoritecentral.com>
Sent: Wednesday, April 05, 2006 4:49 PM
Subject: Re: [meteorite-list] Early Mercury Impact Showered Earth


>
> Definitely a thought provoking article. There are one
> or two things which have nagged me about Mercury and I
> see no reason why this article cannot point in the
> direction of solving them.
>
>
>> "We think that Mercury was created from a larger
>> parent body that was
>> involved in a catastrophic collision
>
> a large
>> proto-planet collided with a giant asteroid about
>> 4.5 billion years ago,
>> in the early years of the solar system.
>>
>> "Mercury is an unusually dense planet, which
>> suggests that it contains
>> far more metal than would be expected for a planet
>> of its size,"
>
> Now I know I'm not the first to suggest this, ideed, I
> got the idea from a professor I studied under.
>
> Could Mercury be an ex-moon of Venus? A large object
> hitting Venus creating it in much the same way as we
> predict the moon formed?
>
> I've seen a graph of (ln)Spin Angular Momentum vs (ln)
> mass of the planets and they all fit on the line bar
> the Earth, Venus and Mercury. However, Earth/moon
> combined does fit the line, as does Mercury/Venus
> combined. Is this a coincidence?
>
> That the moon is drifting out from the earth due to
> tidal effects and will one day be lost...The
> Venus/Mercury mass ratio has greater parity than
> Earth/moon. Could it not be that the same process took
> place there and Venus simply lost mercury long ago?
>
> I have never once heard this suggested in the popular
> press and they say some pretty "far out" stuff.
>
> Is this a theory which is generally considered
> nonsense and if so, why?
>
> In anticipation of far more knowledgable people
> telling me the current state of play...
>
> R McC
>
> --- Ron Baalke <baalke_at_zagami.jpl.nasa.gov> wrote:
>
>>
>>
> http://www.spacedaily.com/reports/Early_Mercury_Impact_Showered_Earth.html
>>
>> Early Mercury Impact Showered Earth
>> SpaceDaily
>> April 5, 2006
>>
>> Leicester, England (SPX) - New computer simulations
>> of Mercury's formation
>> show some of the resulting ejected material ended up
>> on Earth and Venus. The
>> simulations, which track the material's path over
>> several million years, also
>> shed light on why Mercury is denser than expected.
>>
>> Scientists at University of Bern, Switzerland,
>> produced the simulations,
>> which depict the fate of material blasted out into
>> space when a large
>> proto-planet collided with a giant asteroid about
>> 4.5 billion years ago,
>> in the early years of the solar system.
>>
>> "Mercury is an unusually dense planet, which
>> suggests that it contains
>> far more metal than would be expected for a planet
>> of its size," said
>> team leader Jonti Horner, who presented the research
>> at a meeting of the
>> Royal Astronomical Society.
>>
>> "We think that Mercury was created from a larger
>> parent body that was
>> involved in a catastrophic collision, but until
>> these simulations we
>> were not sure why so little of the planet's outer
>> layers were
>> re-accreted following the impact."
>>
>> To solve the problem, the team ran two sets of
>> large-scale computer
>> simulations. The first examined the behavior of the
>> material in both the
>> proto-planet and the incoming asteroid. The
>> simulations were among the
>> most detailed to date, following a huge number of
>> particles and
>> realistically modeling the behavior of different
>> materials inside the
>> two bodies.
>>
>> At the end of the first simulations, a dense
>> Mercury-like body remained,
>> along with a large swathe of rapidly escaping
>> debris. The trajectories
>> of the ejected particles were then fed in to a
>> second set of simulations
>> that followed the motion of the debris for several
>> million years.
>>
>> A second simulation tracked the ejected particles
>> until they landed on a
>> planet, were thrown into interstellar space, or fell
>> into the Sun. The
>> results revealed how much material would have fallen
>> back onto Mercury
>> and allowed the researchers to investigate ways that
>> debris is cleared
>> within the solar system.
>>
>> The group found that the fate of the debris depended
>> on where Mercury
>> was hit, in terms of its orbital position and the
>> angle of the
>> collision. Prevailing gravitational theory suggested
>> a large fraction of
>> the debris eventually would fall back onto the
>> planet, but the
>> simulations showed it would take up to 4-million
>> years for 50 percent of
>> the ejecta to return to Mercury, enough time for
>> much of it to be
>> carried away by solar radiation.
>>
>> This explains why Mercury retained a much smaller
>> proportion than
>> expected of the material in its outer layers, Horner
>> explained. He said
>> the simulations also showed a small fraction of the
>> ejected material
>> made its way to Venus and Earth - a finding that
>> illustrates how easily
>> material can be transferred among the inner planets.
>>
>> Given the amount of material that would have been
>> ejected in such a
>> catastrophe, Horner said, Earth could contain as
>> much as 16 quadrillion
>> tons of proto-Mercury particles.
>>
>> Related Links
>> RAS 2006 <http://www.nam2006.le.ac.uk/index.shtml>
>> Royal Astronomical Society <http://www.ras.org.uk/>
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Received on Fri 07 Apr 2006 04:51:35 AM PDT