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Mars's lack of an atmosphere

  1. Apr 25, 2007 #1
    I hope someone can clear up some issues about Mars's atmosphere, specifically:
    -- Since most evidence indicates that Mars once had surface water, I assume that it must have had a considerably more substantial atmoshpere than it currently has.
    --- A magnetic field is necessary for the preservation of such an atmosphere, as is the case on earth.
    ---The fact that Mars no longer has a magnetic field implies that its field vanished at some point and that explains the dissaperance of the atmosphere in sufficient density to preserve surface water.

    This all seems reasonable to me but I'm guessing at most of it. Anyone out there care to comment on the validity of these points?
  2. jcsd
  3. Apr 26, 2007 #2


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    Why would a magnetic field be necessary to hold an atmosphere? Ours is held in place by gravity.
  4. Apr 26, 2007 #3
    I didn't say "hold" but rather "preserve". What I ment by that was that the magnetic field protects the atmoshphere from errosion by the particles in the "solar wind" which can impart sufficient kinetic energy to the molecules that they can exceed the escape velocity.
  5. Apr 26, 2007 #4


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    Are you suggesting that the solar wind has been responsible for "blowing away" all the air molecules of what used to be the Marsian atmosphere?

    I'm quite skeptical, but it doesn't appear to be too outrageous a claim.
  6. Apr 26, 2007 #5
    Yes, that is what I am suggesting. I have no idea how long that would take. However, it seems to me there is no other way to explain the previous presence of surface water (and therefore an atmoshpere) and the current absence of both. How Mars's magnetic field could have just "quit" is not at all clear to me. It may be related to how the earth's field periodically reverses direction. I hope our's doen't just quit during its next reversal, which is a bit overdue.
  7. Apr 26, 2007 #6
    I'm no scientist, so please take my post with a grain of salt
    To my knowledge air molecules are held in place by gravity, as stated before. but none of the inner planets have enough gravity to completely prevent dissipation of air molecules into the atmosphere.

    This happens on Earth, too. If you go back some hundred million years ago... you'll find creatures (Giant Dragonflys, Pterodactyls, etc.) that would be way too heavy to fly in our current atmosphere, but were able to do so back then because the atmosphere was once much thicker.

    As our planet does have a molten core, our atmosphere is replenished somewhat by volcanic eruptions. Mars, being smaller and (to my knowledge) geologically dead, withered and died.

    But, All the inner planets were geologically very active during and just after formation . The clumping together of material to make the planets produces alot of heat. This heat was enough to create an atmosphere on each of them in the beginning
  8. May 8, 2007 #7


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    Mars doesn't need to have had a very substantial atmosphere - it had surface water but that didn't necessarily stay long. Rather than stable long lasting oceans you could think of underground ice that melted, flowed to the surface in a flash flood and then evaporated fairly quickly.

    ps - the earths atmosphere wasn't thicker to allow giant insects although there have been periods when the oxygen content was higher (up to about 35%). The size limit for insects is the length that oxygen can diffuse into their bodies.
  9. Apr 18, 2008 #8
    Gravity dominates

    A planet's atmosphere is 'held in place' by gravity. However, first the planet needs to be close enough to the sun so that surface temperature is high enough to avoid liquifying any atmospheric gas. The four inner planets satisfy that condition. If the temperature conditions allow an atmosphere, then gravity comes into play. The planet needs to have sufficient mass to retain its atmospher. Earth and Venus satisfy that condition. Mars' gravity is strong enough to have had an atmosphere but not strong enough to prevent the atmosphere from drifting into space. However, Earth's gravity is not strong enough to prevent helium and hydrogen from drifting away from earth.

    A magnetic field has a secondary effect on retaining a planet's atmosphere. It diverts the solar wind around the planet. If it was not present, the solar wind could cause erosian of the atmosphere at very high altitutes.
  10. Apr 18, 2008 #9


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  11. Apr 18, 2008 #10


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    If Mars ever had a significant magnetic field it would have been produced by some circulating molten metals in the martian core. I'm pretty sure this kind of motion doesn't just stop, not even in the 4 billion years since the planet was formed. Look at the Earth.

    Although, perhaps something the Earth has (large moon?) keeps the core spinning, but I'm not too clear on this point. Would be interesting to look at martian rocks and see if any of them are magnetically aligned, suggesting the presence of a magnetic field.

    Anyways, Mars is pretty small so even if it had an atmosphere it was not stable, so perhaps it did just leak away due to lack of mass and protective magnetic field.
  12. Apr 18, 2008 #11

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    Mar's k2 Love number was assessed about 4 years ago by people at JPL. The high value (0.153) rules out a completely solid core. Mars has a remnant of a molten core.

    BTW, if you google for "Mars k2 Love number" beware that you will get a lot of hits that have nothing to do with Mars' k2 Lover number.
  13. May 2, 2008 #12
    The lack of a magnetosphere and extremely thin atmosphere in Mars and insufficient atmospheric pressure makes it impossible to retain water in its liquid form(water instead sublimates to a gaseous state)!!!!Your points are perfectly valid, bdrosd!
  14. May 11, 2008 #13
    There is another thing to consider about earlier atmosphere on Mars:

    Point 1: Mars has 24 degree tilt producing seasons similar to as earth : warmer summer and colder winter. Mars also has a very eccentirc orbit. At the closest point to the sun, Mars picks up so much extra warmth that it results in global dust storms. I guess that the eccentirc orbit of Mars causes nearly as much warming and cooling as the seasons produced by the planet's tilt.

    Point 2: Geographically, southern Mars is much higher elevation than northern Mars. Southern Mars (except Hellas basin) will never support liquid water - the thinness of the air causes ice to melt into vapor, never allowing water to exist as liquid. Northern Mars has much more air pressure, almost enough to support liquid water, but the rub is that Northern summer occurs when Mars is farthest from the sun, so there is not enough heat to bring water ice to melting point.

    As Mars orbit precesses, there must be some time in the past (anyone know how long ago?), when Mars northern summer occurred at the closest point of Mars orbit to the sun. This means northern ice would easily have melted in the northern summer heat. Even if all the water sublimated into the atmosphere, the atmospheric pressure would gradually increase until liquid water could exist in the northern plains. With the warmer northern summers, the north ice cap would diminish, further increasing the atmospheric density. And if the planet warmed globally as a result, the southern ice cap of carbon dioxide might diminish as well, which would also increase the atmospheric pressure.

    This presents the possibility that Mars has a very different climate when the northern summer is aligned with orbital perhelion - possibly to the extent of increasing the Martian atmospheric pressure (I'm hoping for 3x the current atmospheric pressure) and allowing some existance of liquid water. The current Martian climate, when the southern summer is aligned with perhelion, means that the north is permanently frozen, southern heating contributes little to the planetary climate, and the atmosphere ends up thinning out as the CO2 ice caps on the south pole gets thicker.

    Has anyone seen research along this line of thought? Any links would impress me.
  15. May 11, 2008 #14
    Just look at Mars. It seems like an ocean that dried up. I just think that Mars used to be like earth: full of water. They just evaporate over time. I guess the 2nd law of thermodynamics holds. Everything will eventually reach stable condition and obviously rocks are more stable than water. :P
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