How Does Earth's Water Compare to Potential Ocean and Desert Planets?

[lpetrich]

Let's first look at our homeworld, to use as a reference.

The Earth's surface water has a mass about 0.00023 times the Earth's total mass, with the planetary ocean having 96.5% of it. The ocean has a mean depth of 3.8 km, and they cover 71% of the Earth's surface, giving a planetwide average of 2.7 km. I use singular, because what are usually called oceans are one continuous body of water, sometimes called the World Ocean.

Turning to the Earth's interior, estimates vary widely. Water distribution on Earth - Wikipedia from https://www.sciencenews.org/article/quest-trace-origin-earth%E2%80%99s-water-%E2%80%98-complete-mess%E2%80%99: 1.5 to 11 oceans. https://www.coursehero.com/file/p4khek5/Estimates-of-the-mass-of-water-inside-Earth-range-from-1-O-to-50-O-where-1-O/: 1 to 50 oceans, noting Drake & Campins (2006) estimating 10 oceans and Marty (2012) estimating 4 to 12 oceans.

If the Earth had much less surface water, then most of it would be in the crust as groundwater. About 1.69% of the Earth's surface water is groundwater, so if the Earth had 1% of its present amount of surface water, then the Earth would look waterless: a desert planet.

If the Earth had much more surface water, then all its land area would be drowned, making it an ocean planet. One can estimate how much water would be needed by considering the height of the highest possible mountain. From the surrounding terrain, the highest mountains on our planet are Mauna Loa and Mauna Kea at about 10.3 km. Mt. Everest is the highest in gravitational potential, but it rests on some already-elevated terrain. That makes it about 12.6 km above the average ocean floor. That means that 4 to 5 times the water would be enough.

Why might the Earth have almost enough water to cover its land area, but not quite?

One can try to work it out from theory, but the theory of planetary formation already has had some nasty curveballs thrown at it by observations. Curveballs like warm and hot Jovian planets. But one can look for Earth-sized habitable-zone planets, and indeed, it has been possible to find some such planets.

[1704.04290] Updated Masses for the TRAPPIST-1 Planets -- though those masses have big error bars, they are enough to show that at least five of those planets are likely ocean planets: b, e, f, g, h. Planet d straddles the all-rock line, meaning that it also may be an ocean planet, and planet c is between the rock and iron lines, as Venus and the Earth are. It would be difficult to rule out a few-hundred-km-deep ocean on c, however.

So the Earth's amount of water might be a rarity.

Anything on other exoplanets?

 

[Buzz Bloom]

Hi lpetrich:

I found your post quite interesting. I was wondering if you had given any thought about the implications of your analysis to the Drake Equation.

https://en.wikipedia.org/wiki/Drake_equation​

It seems to me that in particular the value of the n_e and f_l terms (items iii and iv in the Wikipedia article) would be influenced by the fraction of the planet that would be liquid water. Do you have any thoughts about this?

Regards,
Buzz

 

[lpetrich]

Of the Drake-equation factors, f_l refers to the fraction of suitable planets where organisms emerge, so it would not be affected. However, n_e will be affected.

As to evidence of life on other planets, that has been seriously discussed. The most observable evidence is biomarker gases like oxygen and ozone and methane, especially together. [astro-ph/0609398] Spectral Evolution of an Earth-Like Planet, New Biomarkers Honed to Help Search for Life on Earth-like Exoplanets - Scientific American, Exoplanet Exploration: Planets Beyond our Solar System: Detecting biomarkers on faraway exoplanets.

As to the origin of life, there appears to be only one example on our planet that has present-day survivors. But research has gotten surprisingly far on the earliest phases of the evolution of our planet's biota, like the Last Universal Common Ancestor (LUCA). Our last common ancestor inhaled hydrogen from underwater volcanoes | Science | AAAS, The physiology and habitat of the last universal common ancestor : Nature Microbiology.

But that's another story.

 

[lpetrich]

Ocean planets have the interesting property that if their oceans are deep enough, the water at the bottom may be frozen as high-pressure phases of ice. Water has a delightfully complicated phase diagram with numerous solid phases: Water phase diagram, Ice - Wikipedia. Familiar ice is Ice Ih, other phases are higher Roman numerals.

Here is a table of some melting points. The pressure will be in Earth-ocean-kilometer equivalents: 100 bar or 10⁷ pascal.

• 0 C, 273 K -- 62 km -- liquid 1.19 g/cm^3, Ice VI 1.31 g/cm^3
• 30 C, 303 K -- 105 km -- liquid, Ice VI 1.31 g/cm^3
• 82 C, 355 K -- 222 km -- liquid 1.35 g/cm^3, Ice VI 1.31 g/cm^3, Ice VII 1.57 g/cm^3
• 125 C, 398 K -- 280 km -- liquid, Ice VII, 1.69 g/cm^3 -- highest organism-growth temperature
• 190 C, 463 K -- 400 km -- liquid, Ice VII, 1.85 g/cm^3 -- dry-DNA stability limit
• 400 C, 673 K -- 1700 km -- liquid, Ice VII 2.11 g/cm^3
• 700 C, 973 K -- 10,500 km -- liquid, Ice VII 3.19 g/cm^3
• 1000 C, 1273 K -- 42,000 km -- liquid, Ice VII, Ice X, both ices 4.5 g/cm^3

So it will take a volcano to melt the ice if it is thick enough.