[Widdekind]

Early Mars may have had 20-30 times more water than it does today*.

* Michael A. Seeds. Horizons (8th Ed.), pg. 373 ; http://www.agu.org/sci_soc/prrl/prrl0019.html

CONCLUSION (?): Earth's Escape Velocity (~11 km / s) is about double that of Mars (~5 km / s). But, both planets orbit in the Sun's Habitable Zone, and hence they have comparable Surface Temperatures. Indeed, both planets are big enough, and cold enough, to keep the bulk of their water over Geologic Time Scales*.

* Seeds, ibid., pg. 370.

So, early Earth may have had around 10 times more water than it does today. Therefore, early Earth's Oceans may have once been about 40 km deep*. This could account for the fact, that Archaean Crusts were created underwater**, and even until about 2.0 billion years ago***. This suggests, that Sea Levels finally dropped down below the maximum height of Continental Crust (~9 km#) at that time.

* http://en.wikipedia.org/wiki/Ocean
** http://www.palaeos.com/Archean/Archean.htm
*** http://www.geosciences-forum.com/ftopic587.html
# Arthur Holmes. Principals of Physical Geology, pg. 642.. Earth's Oceans average around 4 km deep, while the Tibetan Plateau rests around 5 km above Sea Level. But, probably Earth's Crust was thinner, hotter, and weaker at that early epoch. Cf. Michael A. Seeds. Horizons (8th Ed.), pg. 372 ; http://www.astronomyforum.net/plane...us-have-drier-crusts.html

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[Widdekind]

REVISION:

Early Mars had 20-30x more water than it does today*. But, Mars has steadily lost its Atmosphere over the eons, causing its water to boil away**. Mars' Atmosphere was probably stripped away by the Solar Wind, since Mars lacks a shielding Magnetic Field***. Likewise lacking a shielding Magnetic Field, Venus also steadily loses Atmosphere, and water, to the Solar Wind#.

* Michael A. Seeds. Horizons (8th Ed.), pg. 373 ; http://www.agu.org/sci_soc/prrl/prrl0019.html
** http://www.pbs.org/wgbh/nova/origins/mckay.html
*** http://www.pbs.org/wgbh/nova/transcripts/3518_mars.html
# http://www.esa.int/esaMI/Venus_Express/SEM0G373R8F_0.html

But, by producing a comparatively powerful global Magnetic Field (from at least 3.2 Ga*), Earth has seemingly managed to maintain much of its Atmosphere. However, Earth's Magnetic Field does change in magnitude, and even direction, and some Atmosphere is surely stripped during such periods**. Thus, it is perhaps perfectly plausible, that the Earth has also lost most of its primordial water, to the viscissitudes of Deep Time. Indeed, every other Terrestrial World (Mercury, Venus, Moon, Mars) in the Solar System seems to have already lost most of its water.

* http://www.nature.com/nature/journa...136/full/nature05667.html ; http://www.huliq.com/17529/earth-ha...-as-3-2-billion-years-ago
** Dalrymple, cited at: http://www.talkorigins.org/faqs/faq-age-of-earth.html

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[Widdekind]

Oceanic Crusts have taken 10-12 times as much water, as exists presently upon the planet's surface, down into the Mantle, at Subduction Zones*. That fact agrees exactly with this analysis.

* http://www.geosciences-forum.com/htopic676-.html