Day: September 13, 2008

You might ask “Ocean? Surely it’s still there where it’s always been?”

For the last 500 million years that has been more or less true, though sea levels have varied substantially, but in general things have been as always. But before that? A news bite from PhysOrg suggests evidence that a lot of seawater has ended up in the mantle. Part of the ocean has drained away. This would have had dramatic effects on the available land-levels and the potential for Life to benefit from shallow water – much of the deep ocean is desert, feed only by what is produced in the continental shallows. It may be no coincidence that the first macroscopic life, seaweed/macroalgae, appeared some billion years ago.

Looking back in Net-Time there’s also this curious research by some Japanese geoscientists…

Leaking Earth could run Dry

…a BBC news-bite from Sept 8, 1999. But still pretty much on the money. Shigenori Maruyama and colleagues estimated that sea-levels had dropped by 600 metres in the last 0.75 Gyr and the oceans would be gone in another billion. More or less in line with the new research that suggests half Earth’s water has drained into the mantle since the ocean formed. While that might sound like a lot it’s a depth of 5.3 kilometres of water, when averaged over the Earth’s surface. Just 0.25% of Earth’s total volume and the mantle’s total volume is 83%. Thus a drop of water in a bucket of lava…

Addendum
Eldridge Moores, a Professor Emeritus of Geoscience, has suggested for some years that sea-levels have declined over geological time, though due to a different process. By his reckoning the oceanic crust was thicker and thus isostasy meant the average ocean depth was shallower – meaning all that water covered the continents too. Only mountain peaks poked above the waves. Then, roughly as Rhodinia began forming, the thicker crust gave way to thinner oceanic crust and that super-continent of the day rose from the waves over the next few hundred million years.

When Earth Dried Out

How many times have I heard people fretting over the Solar Wind blowing a planet’s atmosphere away when talking about the terraforming of Mars or the Moon. People… not going to happen in a hurry. The Solar Wind is on average 5 million particles (mostly protons, just like the Sun) per cubic metre travelling at 400 km/s at Earth’s distance from the Sun – thus the flux (number passing through per square metre) is 400,000 m/s*5,000,000 /m^3 = 2E+12 protons per square metre per second. Their collective energy flux is their kinetic energy times their number flux – i.e. 1/2*(400,000)^2*(2E+12)*m(p)… where m(p) is the proton mass (1.673E-27 kg)… so we’re talking 2.68E-4 J/s. Less than 0.3 milliWatts per square metre. The sunlight is 1365 W/sq.m some 5.1 million times stronger, but qualitatively the two are quite different in how that energy is distributed. A proton slamming into an oxygen atom at 400 km/s is quite a bump. If all the momentum went from proton to atom, the atom would fly away at 25 km/s. But ions and atoms tend to collide elastically and bounce off each other. To conserve momentum and energy, the proton reverses direction and slows a bit, while the atom is flung away at high speed. But when trillions of atoms and ions are involved it’s not just one interaction – many could occur and eventually the ion might share its momentum with quite a few oxygens before escaping to space. So there’s three outcomes – one interaction between ion and atom, an even share (on average) of ion momentum, and finally an even share of ion energy. Surprisingly the first is the least efficient – an upper Venusian atmosphere some 6,200 km in radius loses just 6.4 kg/s. The second case loses 2.5 atoms (assuming 10 km/s escape speed at altitude) per ion – thus 16 kg/s. The third case sends 100 atoms into the void per ion – 640 kg/s. Sounds respectable but Venus has 4.6E+20 kg of gas to space… meaning 2.3 trillion years, 0.911 trillion years and 0.023 trillion years respectively to lose Venus’s air via the solar wind. A long time.

So why is Mars described as losing all its air in the early days? Back when the Sun was young its Wind was up to 1,000 times stronger. Mars, being smaller than Venus, would lose its air very rapidly in those days (Venus would still take billions of years in scenario 1 &2), but not now the Sun is better behaved. There are complications to this picture too – magnetic fields and ionization of the impacted atoms – but the basic picture is pretty straightforward. Solar Wind erosion is SLOW…