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Environmental perturbations caused by the impacts of asteroids and comets
REVIEWS OF GEOPHYSICS, 35 (1): 41--78 (1997)
Аннотация
We review the major impact-associated mechanisms proposed to cause
extinctions at the Cretaceous-Tertiary geological boundary. We then
discuss how the proposed extinction mechanisms may relate to the
environmental consequences of asteroid and comet impacts in general.
Our chief goal is to provide relatively simple prescriptions for
evaluating the importance of impacting objects over a range of energies
and compositions, but we also stress that there are many uncertainties.
We conclude that impacts with energies less than about 10 Mt are
a negligible hazard. For impacts with energies above 10 Mt and below
about 10(4) Mt (i.e., impact frequencies less than one in 6 x 10(4)
years, corresponding to comets and asteroids with diameters smaller
than about 400 m and 650 m, respectively), blast damage, Earthquakes,
and fires should be important on a scale of 10(4) or 10(5) km(2),
which corresponds to the area damaged in many natural disasters of
recent history. However, tsunami excited by marine impacts could
be more damaging, flooding a kilometer of coastal plain over entire
ocean basins. In the energy range of 10(4)-10(5) Mt (intervals up
to 3 x 10(5) years, corresponding to comets and asteroids with diameters
up to 850 m and 1.4 km, respectively) water vapor injections and
ozone loss become significant on the global scale. In our nominal
model, such an impact does not inject enough submicrometer dust into
the stratosphere to produce major adverse effects, but if a higher
fraction of pulverized rock than we think likely reaches the stratosphere,
stratospheric dust (causing global cooling) would also be important
in this energy range. Thus 10(5) Mt is a lower limit where damage
might occur beyond the experience of human history. The energy range
from 10(5) to 10(6) Mt (intervals up to 2 x 10(6) years, corresponding
to comets and asteroids up to 1.8 and 3 km diameter) is transitional
between regional and global effects. Stratospheric dust, sulfates
released from within impacting asteroids, and soot from extensive
wildfires sparked by thermal radiation from the impact can produce
climatologically significant global optical depths of the order of
10. Moreover, the ejecta plumes of these impacts may produce enough
NO from shock-heated air to destroy the ozone shield. Between 10(6)
and 10(7) Mt (intervals up to 1.5 x 10(7) years, corresponding to
comets and asteroids up to 4 and 6.5 km diameter), dust and sulfate
levels would be high enough to reduce light levels below those necessary
for photosynthesis. Ballistic ejecta reentering the atmosphere as
shooting stars would set fires over regions exceeding 10(7) km(2),
and the resulting smoke would reduce light levels even further. At
energies above 10(7) Mt, blast and Earthquake damage reach the regional
scale (10(6) km(2)). Tsunami cresting to 100 m and flooding 20 km
inland could sweep the coastal zones of one of the world's ocean
basins. Fires would be set globally. Light levels may drop so low
from the smoke, dust, and sulfate as to make vision impossible. At
energies approaching 10(9) Mt (>10(8) years) the ocean surface waters
may be acidified globally by sulfur from the interiors of comets
and asteroids. The Cretaceous-Tertiary impact in particular struck
evaporate substrates that very likely generated a dense, widespread
sulfate aerosol layer with consequent climatic effects. The combination
of all of these physical effects would surely represent a devastating
stress on the global biosphere.
тэги
- acid-rain;
- atmosphere;
- boundary;
- chicxulub
- crater;
- cretaceous-tertiary
- earths
- fall;
- imported
- k-t
- k/t
- meteor
- nuclear
- quartz;
- shocked
- shoemaker-levy-9
- tunguska
- winter;
