Seventy-one percent may seem like average, but when it comes to Earth’s surface water percentage, that number appears to be ideal for advanced civilization. A scientific agency like NASA recognizes that life cannot exist without liquid water—thus, their long-held astrobiology mantra is to “follow the water.”1 That pursuit leads astrobiology researchers to look for evidence of life or life’s remains on astronomical bodies where there is at least a possibility of surface liquid water.
However, NASA’s slogan is not helpful since water is the third most abundant molecule in the universe, right after the two different forms of molecular hydrogen, H2 and H3. The universe is “soaking wet.”
To add to the difficulty, two Harvard University astronomers, Manasvi Lingam and Abraham Loeb, have published a paper wherein they explain how the location and quantity of liquid water on a planet’s surface seriously constrain the possibility of life.2
Problem of Too Much Water Coverage
Where astronomers are able to measure the water content of Earth-like extrasolar planets, the vast majority possess water content far exceeding that of Earth, while the remainder are bone dry. For the majority, the water fraction by weight ranges from 8 percent to 50 percent.3 Earth’s water fraction is only 0.045–0.251 percent, of which just 0.02 percent is surface water.4
Any planet with a water fraction one percent or greater, where at least some of the water is liquid, will exhibit these features: (1) they will have surfaces with deep oceans and no landmasses, or (2) they will have deep subterranean oceans and be completely covered in ice. Such worlds will lack continental landmass weathering. The lack of such weathering will limit the availability of phosphates to the tiny amount generated by submarine weathering. This tiny amount might permit the existence of a small biomass of prokaryote microbes but not the existence of animals.5
Any planet with a water fraction five percent or greater will possess an ocean that is at least 100 kilometers deep. An ocean deeper than about a hundred kilometers will permit no mineral weathering at all. At the bottom of such oceans, pressures will be extreme enough to produce tetragonal (crystal-forming) ice. Tetragonal ice has a density greater than that of liquid water.6 Thus, an ice layer will form at the ocean bottom that will create a permanent barrier between the liquid water and the minerals of the planet’s interior (see figure 1). The oceans of such worlds will lack the nutrient density to support life. They will also be acidic.7
Figure 1: Water World Cross Section
Planets with a deep surface or subterranean ocean will possess a tetragonal ice layer at the ocean bottom that will permanently separate its liquid water from its mineral interior. Such an ocean will lack the nutrient density to support life. Diagram credit: Hugh Ross
Problem of Too Little Water Coverage
Planets overwhelmingly dominated by surface landmasses will face a precipitation problem. The predominant source of precipitation onto land comes from the evaporation of ocean water and landmass precipitation is proportional to the surface area of a planet’s oceans.
Another critical factor for balanced precipitation is that the larger the percentage of a planet’s surface area that is covered by land, the more uneven is its landmass precipitation distribution. Where oceans cover less than 10 percent of a planet’s surface area, very little precipitation falls on the landmasses. Most of the landmass area receives no precipitation at all. The thin strips of land that do receive precipitation are able to sustain only a tiny fraction of the net primary biological productivity that would be typical of a present-day continent or island on Earth today.
Fine-Tuned Water Coverage
For any kind of life to be possible a planet must not be 100 percent or 0 percent covered with water. Temporary microbial life can exist on a planet that is 5–25 percent covered or 80–95 percent covered.
However, the long-term existence of plants and animals requires a planet that efficiently recycles nutrients. This necessity mandates that there must be a rough balance between surface oceans and surface landmasses. For global high-technology human civilization to be possible, a planet that is almost exactly the size of Earth is required. Today, on Earth the oceans cover 71 percent of the surface area and landmasses cover the remaining 29 percent. Less landmass coverage means less space to accommodate a large population of humans, their animals, their farms, and their technology. More landmass coverage means less precipitation falling on the landmasses and less even distribution of that precipitation—with consequences for food crop production.
Since the origin of life 3.8 billion years ago, Earth’s landmass coverage has been steadily increasing. Evidently, humans appeared on Earth at the optimal time for them to launch and sustain global civilization.
So far, astronomers have only found worlds beyond Earth that are either 100 percent or 0 percent (surface) covered with water. That we are 71 percent covered with water and 29 percent covered with landmasses appears to be no accident, but rather a testimony of purposeful design.
- NASA Fact Sheet, “Follow the Water: Finding a Perfect Match for Life,” April 16, 2007, https://www.nasa.gov/vision/earth/everydaylife/jamestown-water-fs.html.
- Manasvi Lingam and Abraham Loeb, “Dependence of Biological Activity on the Surface Water Fraction of Planets,” Astronomical Journal 157, no. 1 (January 3, 2019): id. 25, doi:10.3847/1538-3881/aaf420.
- Sheng Jin and Christoph Mordasini, “Compositional Imprints in Density-Distance-Time: A Rocky Composition for Close-In Low-Mass Exoplanets from the Location of the Valley of Evaporation,” Astrophysical Journal 853, no. 2 (February 1, 2018): id. 163, doi:10.3847/1538-4357/aa9f1e; Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds,” Astrophysical Journal 834, no. 1 (December 27, 2016): id. 17, doi:10.3847/1538-4357/834/1/17; Leslie A. Rogers, “MOST 1.6 Earth-Radius Planets Are Not Rocky,” Astrophysical Journal 801, no. 1 (March 2, 2015): id. 41, doi:10.1088/0004-637X/801/1/41; C. T. Unterborn, N. R. Hinkel, and S. J. Desch, “Updated Compositional Models of the TRAPPIST-1 Planets,” Research Notes of the American Astronomical Society 2, no. 3 (July 3, 2018): id. 116, doi:10.3847/2515-5172/aacf43; David Charbonneau et al., “A Super-Earth Transiting a Nearby Low-Mass Star,” Nature 462 (December 17, 2009): 891–94, doi:10.1038/nature08679; Linda T. Elkins-Tanton and Sara Seager, “Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets,” Astrophysical Journal 685, no. 2 (October 1, 2008): 1237–46, doi:10.1086/591433; Geoffrey Marcy, “Water World Larger Than Earth,” Nature 462 (December 17, 2009): 853–54, doi:10.1038/462853a.
- Richard C. Greenwood et al., “Oxygen Isotope Evidence for Accretion of Earth’s Water before a High-Energy Moon-Forming Giant Impact,” Science Advances 4, no. 3 (March 28, 2018; corrected update July 13, 2018): eaao5928, doi:10.1126/sciadv.aao5928.
- Jochen J. Brocks et al., “The Rise of Algae in Cryogenian Oceans and the Emergence of Animals,” Nature 548 (August 31, 2017): 578–81, doi:10.1038/nature23457; Christopher T. Reinhard et al., “Evolution of the Global Phosphorus Cycle,” Nature 541 (January 19, 2017): 386–89, doi:10.1038/nature20772.
- A. Levi, D. Sasselov, and M. Podolak, “Structure and Dynamics of Cold Water Super-Earths: The Case of Occluded CH4 and Its Outgassing,” Astrophysical Journal 792, no. 2 (August 25, 2014): id. 125, doi:10.1088/0004-637X/792/2/125.
- Hugh Ross, “Waterworld Planets Are Acidic, Primordial Earth Was Not,” Today’s New Reason to Believe (blog), Reasons to Believe, May 14, 2018, https://www.reasons.org/todays-new-reason-to-believe/read/todays-new-reason-to-believe/2018/05/14/waterworld-planets-are-acidic-primordial-earth-was-not.