GEO 330 Lecture Notes - Lecture 31: Valles Marineris, Valley Network, Dew Point
14 May 2018
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Weathering and impact erosion could make thick CO2atmosphere difficult to maintain
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Impact is energetic enough to accelerate vapor plume to escape velocity
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Mass of vapor plume must exceed mass of atmosphere above the point of impact
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Impacts can remove atmosphere if:
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Carr et al. (1999) calculated a 4bar atmosphere 4.4Gy ago would be reduced to a 700mbar
atmosphere by 4Gy ago
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Brain and Jakosky (1998) calculated that 50-90% of atmosphere could be lost by this
mechanism based on population of large Noachian craters
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Impacts can efficiently remove the atmosphere:
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If liquid water were present on the surface, chemical weathering could also act to remove
atmosphere
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5bar CO2atmosphere with a surface T of 273K would be removed in only 50My
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Schaeffer (1993), using a different model concluded that Pollack et al. greatly overestimated
CO2removal and that a thick atmosphere could last for much longer
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According to models of Pollack et al. (1987), removal of CO2by weathering might be very efficient
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Both Pollack and Schaeffer models predict removal of most CO2by the end of heavy
bombardment, unless CO2is replenished
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Removal by CO2and weathering may be interdependent processes
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Early high heat flow and rates of volcanism could have led quick burial and heating, breaking
down carbonates and returning CO2to atmosphere
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Heat from impact processes could also break down carbonates, but probably less efficient
than volcanism
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Hesperian volcanism at Syrtis Major may have interacted with subsurface carbonates to
release CO2much later
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CO2atmosphere could be retained by recycling of CO2from carbonates
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What happed , though, at the end of the Noachian when valley network formation rapidly
declined?
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Probably still had 0.5-5bar of CO2in the atmosphere at the time that temperatures fell below
freezing, causing rapid slow-down of valley formation
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Loss of CO2after era of valley formation may have occurred via sputtering
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Atoms are ionized in the upper atmosphere and accelerated by the solar magnetic field
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Process favor loss of light isotopes
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Jakosky and Jones (1997) estimate that 90% of exchangeable CO2reservoir lost by this
process since end of Noachian
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Ions then collide with other atoms at high velocities and eject them into space
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95-99% of atmosphere lost by combination of impact and sputtering processes since end of
Noachian
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If large amount of atmosphere was lost, we should not expect to see near-surface carbonates
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Post Noachian warm episodes cannot be attributed to a CO2-H2O greenhouse
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Major questions still exist regarding the nature of and retention of a thick Noachian greenhouse
atmosphere
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More prominent role of trace greenhouse gases
1.
Early sun was brighter than standard models predict
2.
CO2clouds are more efficient infrared scatterers than has been assumed
3.
Warm conditions were sporadic, resulting from large impacts or volcanic events
4.
Valley networks and other features apparently formed by liquid water really formed under
cold conditions
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Potential solutions include:
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Retention of a Dense CO2Atmosphere
Atmosphere and Climate Change
Wednesday, May 9, 2018
6:12 PM
GEO 330 Page 1
Document Summary
Weathering and impact erosion could make thick co2 atmosphere difficult to maintain. Impact is energetic enough to accelerate vapor plume to escape velocity. Mass of vapor plume must exceed mass of atmosphere above the point of impact. Carr et al. (1999) calculated a 4bar atmosphere 4. 4gy ago would be reduced to a 700mbar atmosphere by 4gy ago. Brain and jakosky (1998) calculated that 50-90% of atmosphere could be lost by this mechanism based on population of large noachian craters. If liquid water were present on the surface, chemical weathering could also act to remove atmosphere. According to models of pollack et al. (1987), removal of co2 by weathering might be very efficient. 5bar co2 atmosphere with a surface t of 273k would be removed in only 50my. Schaeffer (1993), using a different model concluded that pollack et al. greatly overestimated. Co2 removal and that a thick atmosphere could last for much longer.
