From: Amara Graps (amara@amara.com)
Date: Wed Jul 16 2003 - 04:53:14 MDT
> Spike <spike66@comcast.net> wrote:
>> Mike Lorrey:
>>
>> ...Secondly, our moon is at least as important as Jupiter at saving
>> our asses from being nothing but a gravel pit...
Not for the reasons that you state, Mike.
> I don't follow your reasoning here Mike. Why
> would the moon have much affect on the number
> of meteors that hit the earth? Is that what
> you meant? Jupiter mops up most of the interplanetary
> stuff that would otherwise hit the earth. The moon?
> Nah, almost negligible.
>>Not so. Look at the impact record, especially on the far side
>>of the moon. If the moon were not so significant, you'd see
>>little variation between the near side and far side impact
>>records. Instead, the far side looks like it's suffered
>>multiple cases of chicken pox, cow pox, small pox, and
>>leprosy.
You must consider the time dimension.
After the moon formed (in a molten state- one theory called the
large-impact hypothesis describes why this is so), it was subjected
to intense bombardment. This was during the first 1/2 billion years
of its life. The Moon's crust was shattered, and you can see large
basins like the Mare Imbrium basin formed during this time. Then
between 4.1 and 3.9 billion years ago, the cratering rate fell
sharply, to about 1000 times less than its early heavy bombardment
rate.
While the Moon cooled rapidly after its formation, some process
heated the subsurface material and part of it melted, which produced
lava. The deep basins were flooded by successive lava flows between
3.8 and 3.2 billion years ago.
The key to understanding why the maria are nearly free of craters
and the highlands heavily cratered is the time of events and the
thickness of the Moon's crust and topography of its surface. The
cratering was intense and early. Then the lava flows filled in those
early records and created the maria, but not uniformly over the
surface of the Moon.
Studies have shown that the Moon's crust is thinner on the side
towards the Earth. Gravity mapping, shows that on average, the lunar
crust is about 70 km thick, but it varies from a few tens of
kilometers beneath the mare basins to over 100 km in some highland
areas. Under some of the largest basins, the crust was weakened so
much that the mantle has bulged upward. Moreover, the intrusion of
the dense mantle material into the crust changes the local gravity
field.
In addition to the gravity mapping, Clementine's laser altimeter
showed for the first time a relief map of the Moon's topography. The
global map of the Moon's topography showed that the near side of the
moon appears relatively smooth with typical relief of only 5 to 6km,
whereas on the far side one sees a full 16-km range of relief. The
difference is caused by wide-spread filling of the near side
basins by mare basalts and a relative scarcity of maria on the far
side. Moreover, the wide range of relief on the far side is caused
mostly by the presence of the enormous South Pole-Aitken basin, 12km
deep.
The Moon's center of mass is offset from its geometric center by
about 2 km in the direction of the Earth, probably because the crust
is generally thicker on the far side. Spudis says in (2) that it
might explain why so few maria exist on the far side of the Moon.
Imagine a subsurface boundary akin to a global water table,
attracted towards the center of mass with equal gravitation force at
every point. Because of the 2-km offset, this equipotential surface
lies farther from the top of the crust on the far side. It is
possible, therefore, that basalt magmas rising from the interior
reached the surface easily on the near side, but encountered
difficulty on the far side.
Instead of deflecting Near Earth Objects gravitationally, as you
hypothesize, the Moon's effect on the Earth is something different.
Spudis points out that without the Moon's stabilizing effect on the
Earth's rotational axis, life on Earth would experience extreme
climate changes and also wouldn't have evolution-catalyzing tides in
the oceans.
Amara
References
(1) Michael Seeds, Horizons: Exploring the Universe, 2002,
Brooks/Cole. publishers
(2) Paul Spudis, The Moon, _The New Solar System_, editors: J. Kelly
Beatty, Carolyn Collins Petersen, and Andrew Chaikin, 1999, Sky
Publishing Corporation
(3) Papers on this topic in NASA ADS
http://adsabs.harvard.edu/cgi-bin/nph-abs_connect?db_key=AST&sim_query=YES&aut_xct=NO&aut_logic=OR&obj_logic=OR&author=&object=&start_mon=&start_year=&end_mon=&end_year=&ttl_logic=OR&title=&txt_logic=OR&text=Earth%27s+rotational+axis+Moon+stability+secular+evoluday=&start_entry_mon=&start_entry_year=&min_score=&jou_pick=ALL&ref_stems=&data_and=ALL&group_and=ALL&sort=SCORE&aut_syn=YES&ttl_syn=YES&txt_syn=YES&aut_wt=1.0&obj_wt=1.0&ttl_wt=0.3&txt_wt=3.0&aut_wgt=YES&obj_wgt=YES&ttl_wgt=YES&txt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1
(4) The Stability of Habitable Planetary Environments
"...Geologic evidence for low-latitude glaciation during the
Precambrian era suggests that the obliquity of early-Earth may have
been much higher than it is today. Earth's obliquity could have been
reduced to its present value as a consequence of
obliquity-oblateness feedback. In this process, obliquity-driven
changes to continental ice volume and oblateness may have caused a
secular downward drift in obliquity of ~30o between 600 Ma and 500
Ma. Such an event may account for the present non-zero inclination
of the lunar orbit."
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998PhDT........23W&db_key=AST&high=3f058f138002444
-- Amara Graps, PhD Istituto di Fisica delle Spazio Interplanetario (IFSI) Istituto Nazionale di Astrofisica (INAF), Roma, ITALIA Amara.Graps@ifsi.rm.cnr.it
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