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Lecture

Geography 2240A/B Lecture Notes - Carbonaceous Chondrite, Impact Crater, Chondrule


Department
Geography
Course Code
GEOG 2240A/B
Professor
Philip Egberts

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Chapter 14 Impacts from Space - The Basics
1.0 Introduction
I've chosen to begin this chapter with a schematic figure of impacts and growth of Earth
from those impacts. There was no more
violent period in Earth's history than
when it was being built by the accretion
of impacting objects, and there is very
little doubt that if human civilization
lasts long enough it will be put in
jeopardy - and possibly destroyed - by
an impact event.
One million years ago an asteroid
impacted at Zhamanshin in Kazakhstan,
making a crater 13.5 km in diameter
(Because impact was in a mountainous
area that has since been severely
eroded, photographs are
of very poor quality). It
(apparently) did not
produced quite enough
disruption to equal what
we refer to as a 'nuclear
winter' (Fig. 2), and it
certainly was not big
enough to cause global
mass extinctions;
however, we recognize
that an impact like that,
today, could easily end
our civilization - which
is very fragile - but
certainly not our species.
To emphasize our
vulnerability, note that in 1989 astronomers were startled to see an asteroid roughly half
the size of the one that caused the Zhamanshin crater moving away from Earth, having
come within 700,000 km. To put that in perspective, Moon is about 380,000 km from
Earth. The asteroid's approach had not been detected at all. If it had approached only
two hours later, its orbit and Earth's orbit would have intersected (i.e. impact would have
occurred).
As far as we know, the Zhamanshin asteroid was the largest to impact on land in the last
million years; considering that 3/4 of Earth is water, we can make no statement about
impacts for which we cannot study the evidence. According to impact projections, we

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should expect one Zhamanshin-size (1 km diameter object) impact with Earth every 1
million years; if the last one was about 1 million years ago, statistics say we're due now.
Of course, if you want to believe the rigor of such statistics (which is dangerous), maybe
the 1989 close-approach was it, and we dodged the bullet!
We need a few definitions. Look at the mini-glossary as Appendix I to this chapter.
2.0 Asteroids and the Asteroid Belt
Let's consider the
importance of what we
call the asteroid belt. It is
from the asteroid belt that
Earth receives most (but
not all) of its large space
debris. We know that the
asteroid belt, located
between Mars and Jupiter
(Fig. 3), consists of
material that was never incorporated into a planet (so it is not the debris of either an
exploded small planet or two planets that crashed together). While we speak of the
'asteroid belt', upon closer
examination we see the
asteroids are actually
concentrated in a number
of distinct belts, separated
by clear gaps (Fig. 4). In
a search to explain that
arrangement, it was
discovered that the gap
distances are directly related to the orbit frequency of Jupiter.
Many of the asteroids have the same or certainly similar composition as the parent
ingredients from which the planets were assembled (I will say more about that in the next
chapter). We have collected many meteorites in this category of primitive material, and
we normally refer to them as chondrite meteorites (named for the tiny, 1 mm or so,
bead-like spherules they contain called chondrules). If you proceed into geochemistry
and petrology classes, you will use the composition of chondrites as standards against
which to judge the degree of differentiation or evolution from primitive composition (i.e.
chondrites) that particular rocks exhibit.
Many of the larger asteroids underwent roughly the same sort of differentiation history as
Earth and other large space objects. In other words, there was sufficient contained
radioactive material that the planetoids or asteroids melted and formed a core and mantle
a simple gravity-controlled process. With so many large bodies orbiting in relatively
close proximity it is inevitable that collisions take place - many more in the past than

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now. Head-on collisions at 16,000 km/hr certainly spray debris widely! Some of the
debris gets knocked into the gaps, from which it is whipped out in an eccentric orbit
(accelerated by the gravity of Jupiter), with a looping trajectory toward Sun - and thus
potential intersection with Earth's trajectory. From study of those fragments that do hit
Earth, we are commonly able to get a close look at the different parts of other
differentiated bodies and correlate our observations with what we know of Earth.
We are gradually getting around to the point where we have
enough information to consider impacts of space objects
with Earth, and we need to think just a bit more about those
asteroids which have been thrown out of the asteroid belt
gaps and into wide elliptical orbits. A large number of those
asteroids have, in the past, impacted the planets and their
satellites; we see the evidence clearly in the cratering on
Moon, for example (Fig. 5). By definition, Near Earth
Objects (NEOs) are those asteroids and comets whose
orbits intersects Earth’s orbit or comes very close to it (Fig.
6). Those NEOs that actually intersect Earth’s orbit are specifically referred to as
Apollos; those which intersect the
orbit of Mars rather than Earth
(but still come very close to
Earth) are referred to as Amors.
The largest impact feature we
have identified on Moon has a
diameter of 2200 km; an asteroid
measuring some 50-150 km in
diameter would have created that.
Earth has a gravitational force
that is 24 times that of Moon, thus Earth MUST have collected many more of the largest
objects than did Moon. If Earth were struck by any object (asteroid or comet) with a
diameter larger than 5 km diameter (the size of the nucleus of Halley's Comet) it has
been estimated that the atmosphere would be temporarily destroyed. Apparently this
happened several times in the very early history of Earth. Also, apparently, several times
prior to 3.8 billion years ago, objects also vaporized our oceans. The largest crater we see
preserved anywhere on land is Sudbury (Fig. 7), Ontario (at about 200 km diameter -
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