Impact Craters

• Impact Craters on Earth and Moon
• All craters on Moon are impact not volcanic
• Unique geological process are caused by impact
• Large objects will not survive impact
• Two types of impact craters:
  • Simple
  • Complex

Simple Impact Crater

• diameter: a few kms; about 10 x depth
• energy -> shock wave -> crater
• quickly filled with debris from walls

     

copyright Scientific American, 1990 April

Complex Impact Crater

• diameter: > 4 kms; up to 100 x depth.
• uplifted central structure, fractured rim.
• Like dropping a pebble into pool of water

copyright Scientific American, 1990 April

Difficult to Find Impact Craters

Detection of Impact Craters

• Pressure resulting from impact unique.
• Find shocked metamorphic rocks in craters
• Central uplift seen in complex craters.

Craters: Earth vs. Moon

• On moon all craters are impact (Apollo program needed to show this)
• On earth, nearly all craters are volcanic
• Larger mass and gravity of earth is key
  • Atmosphere retained - protective shield
  • Spread of ejecta is inhibited
  • geologically active - surface relatively young
  • Erosion, sedimentation - destruction of landforms.

Barrington Meteor Crater in Arizona - a simple impact crater

Old, Complex Impact Craters

The Earth’s Atmosphere - What it is and How it Got That Way

Introduction

• Earth’s surface temperature is unique in the solar system Þ H₂O is abundant liquid.
• Importance of atmosphere:
  • Moderates temperature
  • Major source of erosion
  • Oxygen in it makes life possible

• The first atmosphere > 2.5 Byr ago
  • Accumulated during formation?
  • Oxygen poor
  • Probably rich in hydrogen compounds: ammonia (NH₃), methane (CH₄)
  • Evidence from fossil blue-green algae, today found in oxygen poor environments.
  • Light gasses (H₂ , He) would have escaped via evaporation from top.
  • No water, no life?

Present Atmosphere:  originated from Volcanoes?

• Volcanic gas rich in H₂O, CO₂, N₂
• How do you reduce CO₂ content?
  • CO₂ is absorbed by oceans® limestones
  • Plants convert CO₂ to O₂ in last <2.5 Byr
• Solar UV radiation destroys (NH₃), and (CH₄). But UV radiation is bad for life
• Bombardment by comets yields H₂O(?)

The Appearance of Life

• Easy to get rid of oxygen - very reactive; supply must be continually replenished.
• About 2.5 Byr ago, plants created enough O₂ to make ozone O₃.
• Ozone blocks UV radiation and make life on land possible.
• Life creates the environment needed for its survival.
• Today’s atmosphere evolved slowly.

Structure of the Atmosphere

• Troposphere:
  • Lowest, densest level.
  • Absorbs re-radiated IR emission from the ground (colder at higher altitude).
  • Convective motions.
• Stratosphere:
  • Convective motions replaced by laminar flow.
  • Absorption of solar UV by ozone
• Mesosphere
  • Above ozone layer.  ineffective heating
  • Most important coolant is CO₂. 
• Ionosphere:
  • Partially ionized (by solar extreme UV radiation) gas.
  • High temperature because cooling ineffective.

This drawing is copyrighted 

Greenhouse Effect and Global Warming

Implications for the near Future

• Heat (IR radiation) is trapped in earth’s atmosphere and acts as a blanket.
  (the picture below is from a 1989 September article in Scientific American and is copyrighted by them)
• 

Energy Balance in Atmosphere

• Solar radiation is reflected and absorbed by the Earth:
  • Atmosphere + ground reflect ~30%.
  • Atmosphere absorbs 25%, ground 45%
• Energy conservation: Above atmosphere :
  • Energy absorbed=energy emitted (equilibrium)
• But: T above earth from satellites is 33 K cooler than actual mean for surface of earth

The Greenhouse Effect

• Heat (IR radiation) is trapped in earth’s atmosphere and acts as a blanket.
• Ground absorbs visible light, re-emits IR radiation in form of heat.
• Trace gases in atmosphere (CO₂, N₂, CH₄ , N₂O, CFC; also H₂O very efficient absorbers of IR but transmit visible.
• IR heat radiation is trapped; ground heats up until reach new, hotter equilibrium.
• This is the Greenhouse Effect.
• Example: car parked in sun, windows up.

Another illustration, copyright Scientific American May 1987

The Earth’s Carbon Cycle

• The capacity of the soil and oceans to remove carbon dioxide is limited.

A Cautionary Tale

• Small changes in temperature over past billion years had profound effect on life
  • Ice age can result from D T of < 10 C
  • Age of reptiles only 10-15 C warmer than now
  • For life, extremes in T can be fatal:

• • greater D T between equator and poles will drive more powerful storms and larger fluctuations.
  • animals that survive one extreme may not be able to survive other at same location.

Present Global Warming

• It is no longer a question "if" it exists, but "how much" is (will be) the change?
• Due to increase of greenhouse gases.
  • Average global T up ~ 1 deg. C in 100 years
  • Warming trend appears to be accelerating

  • strong correlation with amount of methane (CH₄^{) and CO}₂ ^{in atmosphere}

^{Rise in Atmospheric CO2}

• ^{The oscillations are due to seasonal variations}

^{Evidence for Global Warming}

• ^{Even coldest years of past decade are warmer than all but the hottest years of a century ago.}

^{History of Temperature Change}

• ^{Good correlation between increase in greenhouse gases and T from ice cores.}

• ^{Change in T greater than predicted from increase in greenhouse gases alone.}

• ^{Global warming has positive feed back:}

  • ^{Decay of organic matter is faster (releases heat)}

  • ^{Reduction in snow and ice cover}

  • ^{Death of forests (Blue Ridge, Amazon)}

^{CO2 and Global Temperature}

• ^{Data from ice core 2,200 meters long; age 0 to 160,000 years.}

• ^{Deuterium and 18O are good traces of temp. They agree}

• ^{CO2 is also good global thermometer as is methane (CH4)}

^{Effects of Global Warming}

• ^{Continued sharp rise in global temperature}

• ^{Continued increase in climate "extremes"}

• ^{Change in rainfall patterns}

• ^{Big rise in sea levels (Antarctic melting?)}

• ^{Social and Psychological effects}

^{Rising Sea Levels in 100 Years}

• ^{Although some of the local variations are due to unrelated effects, the overall global average is up by an amount ~ 4 to 5 inches in last century}

^{Predicted Effects of a CO2 Rise}

• ^{Good agreement for temperatures}

^{The Earth’s Magnetosphere}

• ^{Magnetic field generated by convective motion in molten core.}

  • ^{Deflection of solar wind around Earth.}

  • ^{Trapping of particles in "van Allen Belts"}

  • ^{Interaction between solar particles produces aurorae.}

  • ^{Magnetic field reverses polarity every million years or so (geological evidence).}

^{The Northern Lights or
Aurora Borealis}

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