Thursday, January 19, 2012

Why is the Earth 70% Covered With Water?

This is a good time to get into Tectonics - the movements of the Earth's crust. There are some questions that are basic to both kids and adults, like why is the sky blue? And why is the Earth 70% covered with water?


It has been over thirty years since I took a class in historical geology, but I have always been fascinated with the concept of continental drift. My question is: Given the evidence for continental drift in the earth's early history, is it likely that continental drift is typical of the geological history of all planets? In the search for extrasolar, water-bearing planets, is it likely that some of them-- if they do indeed have oceans--might exhibit water/land mass configurations similar to Pangaea with one huge continent on one side of the planet and a huge ocean on the other? I have also thought it strange that the surface of the earth is roughly 70% water and 30% land, whereas the rest of the terrestrial planets are--what--90%+ land? Why is the surface of the only significant water-bearing planet predominately water whereas the other planets don't show even trace elements of liquid water? 
Thank you for considering my inquiry.
--Amado N.


Continental drift apparently "turned on" sometime in the early history of the Earth. It took awhile for the accretion process to segregate itself into a silica-rich crust, an iron-and-olivine-rich mantle, and an iron core. It took even more time for the proto-continents to form - to "float" from isostacy above the denser oceanic crust. It then took awhile for heavy radionuclides accumulating in the deep Earth to create enough heat, and to begin the convection process that now drives the crustal segments around in what we call continental drift.

I have just a modewrate understanding of the other "solid" planets in the solar system, mainly from tracking the scientific literature in magazines such as Nature and Discovery. However, as I recall Mars and the Moon are thought to not have continental drift - this implies either less radionuclides remaining in their interiors for whatever reason, or it implies that the cores are cooled and solid. I think this holds for Mercury and Titan also, but too little is known of each to be sure of this. Titan does have an atmosphere, even an ocean and landmasses, but the chemistry is radically different - it's primarily a weather analog, as the main liquid (gas) is methane. Io is a tiny moon that lies deep within Jupiter's gravity well, and the huge gravity gradient there drives monster tidal forces that will make this planet a font of volcanism forever - but that's a different process. It's like squeezing PlayDough forever will keep it warm and pliant.

I'm unaware of water - for certain - on any other solar system planet - the solid planets are 100% "land" as far as I have ever heard, though there is a deep suspicion from visual evidence that Europa and perhaps Enceladus have watery oceans beneath their strange crusts. In the case of Venus, the temperature (~900C) is too high for liquid water, and in the case of Mars the gravity is so weak that most volatiles like water have long since wafted off into space.

The reason we have 70% surface water coverage on Earth stems from several unique circumstances that we find with our planet. One is the fact that the Earth lies in a relatively narrow "Life Zone" of moderate temperature where water can have a triple-point: be gaseous, liquid and solid. Mars is so cold that if water were somehow still present in any significant amounts on the surface, it would be solid* due to the very cold temperatures, though there is growing evidence of water not only in ancient times, but perhaps still present beneath the crust today.

The other reason is that the Earth is so large that gravity and weathering together enforce a pretty flat surface. The tallest mountain on Earth is Mauna Loa (not Everest), and it is only 14 kilometers "tall" from where it sits on the Pacific Ocean floor. The radius of the Earth is about 6,370 kilometers - so our tallest mountain is only slightly more than 0.002 of this: two tenths of a percent. Our planet is a very smooth blue marble! If there were 100-kilometer-deep trenches on Earth instead of what we now have (the deepest trench, in the Marianas, is just 12 km deep), then I suspect that the relatively small percentage of water on this planet would all pool in a relatively few of those sorts of structural lows. In other words, we would have far less surface covered by water if the Earth's surface was proportionally as rugged as Mars' surface is. To help you better understand the powerful effect of the greater gravity and weathering on Earth, Olympus Mons, the best-known volcano on Mars, stands 25 kilometers above the surrounding plains!

There is still a relatively incomplete understanding of why we have rocky planets vs. gas giant planets in our Solar System - or for that matter, what the cores of those gas giants may consist of. Jupiter has a powerful magnetic field, suggesting a conductive core - iron? Mercury probably almost certainly has no water or volatiles because of the intense heat and ferocious solar wind that it is subject to.

* For decades now NASA and astrogeology academics have speculated that there is water beneath the
surface of Mars. The mantra for a long time has been "Follow the Water." I was once a co-principal investigator for a NASA-USGS proposal to prove this. We proposed a balloon-carried electromagnetic "snake" designed to traverse the planet under its day-night thermal transition and thin-air storm regime, all the while sounding the surface for conductive material. In the case of a conductor on Mars, the likely cause would be water or sulfides - if widespread enough, a conductive anomaly would have to be water. Water will be absolutely crucial to ever establishing a human presence on that planet... assuming that the vast fuel, and cosmic and solar radiation problems can ever be dealt with.


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