Response to Ted Holden's question about Water on Mars

Ted Holden, who for a number of years was the principal proponent of Velikovskian and Saturnist style catastrophism on the USENET newsgroup, asked for a mechanism to explain the loss of water on Mars. The Martian landscape exhibits many features which give all the appearance of having been cut by copious quantities of running water. Where did the water go? I suggested impact erosion as a possible mechanism.

Ted Holden wrote:
>Our own Earth has gone through ice ages and warm periods before, and 
>nothing in any of that has caused our oceans to disappear.  Mars' oceans 
>have disappeared;  they're gone.  An ocean, of course, is simply bigger 
>than a polar ice-cap;  it's ludicrous to claim that the oceans of Mars 
>now reside either in rock or in the polar ice cap.  
>Somebody has to explain how Mars lost all its oceans;  again, nothing in 
>standard theories, to my knowledge at least, would account for that.

Impact erosion is an excellent way to account for the loss of both the
atmosphere and the water on Mars.  Actually, all we need to do is account
for the loss of atmosphere, since then the evaporation and loss of surface
water follows directly.

The escape velocity of Mars is about five kilometers per second.  It only
takes a modest sized impactor to accelerate atmospheric gases to escape
velocity on Mars.  Further, these hot gases can also erode the planet's
surface by accelerating rocks to escape velocity as well.  A number of such
rocks of probable Martian origin have been found in Antarctica.  The main
loss, though, is the atmospheric gases.

John Lewis and Hampton Watkins demonstrated back in 1982 that more than
90 percent of the Martian atmosphere could have been lost through impact
processes alone over geological time scales.  More recent work by Jay Melosh 
and Ann Vickery[1] suggest that the early atmosphere of Mars could have been 
dozens of times denser than at present.  The paper by Owen and Bar-Nun[2]
neatly summarizes the effects of impact erosion on Mars, among other topics, 
and provides a useful set of references on this topic.  Owen and Bar-Nun
suggest that the early atmosphere of Mars was dominated by CO2 and exhibited
a surface pressure of 7.5 bars.  Using Earth as a reference, the amount
of water corresponding to such an atmosphere would be equivalent to a 0.75
km global ocean.


(1)  Melosh, H. J. and Vickery, A. M.  "Impact erosion of the primordial
     atmosphere of Mars."  Nature 338, pp. 487-489.

(2)  Owen, Tobias and Bar-Nun, Akiva.  "Comets, Impacts, and Atmospheres."
     Icarus 116, pp. 215-226.

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Last modified by pib on July 6, 2003.