Perhaps the most memorable lines in The Rime of the Ancient Mariner are “Water, water, everywhere, nor any drop to drink.” 1 Seawater won’t help sailors in peril, but a new scientific discovery adds yet more evidence to how remarkably designed planet Earth is to allow us to have water to drink and to use to sustain our civilization.
Water is the third most abundant molecule in the universe—right after H 2 and H 3. In a certain respect, the universe is soaking wet.
What makes Earth unique is not that it possesses liquid water on its surface but that it possesses so little liquid water. Compared to other known rocky planets where astronomers can measure the planets’ water content, Earth contains about 500 times less water. 2
Two distinct events caused Earth to lose nearly all its primordial water. The first was the birth of the solar system in a giant dense cluster of stars. In the solar system’s birth cluster, Earth was blasted by aluminum-26 and other short-lived radiometric isotopes radiated by nearby supernova eruption events. Intense heat released from the radioactive decay of these radioisotopes drove off most of Earth’s initial atmosphere and water ocean. 3 The second event was the collision between the primordial Earth and another solar system planet, Theia, that resulted in the formation of the Moon. 4
Characteristics of the Moon-Forming Colliders
For the past several years, planetary astronomers who specialize in Moon-formation research have been debating the relative sources of Earth’s present supply of surface water. Some have claimed that the collision between primordial Earth and Theia was so violent that Earth lost 100 percent of its primordial atmosphere and surface water (see figure 1). In these Moon-formation models, Earth’s present water came predominantly from a late veneer (late accretion of asteroidal or cometary material to terrestrial planets) of remaining planetesimals, comets, and asteroids that peaked about 50–100 million years after the Moon-forming event. 5 Other astronomers have pointed to measurements indicating that the total mass of late veneer objects in the solar system was only about 0.1 percent of Earth’s mass. 6
Figure 1: Artistic Rendition of the Collision between Theia and the Proto-Earth. Image credit: NASA/JPL-Caltech.
Now, a team of eight planetary astronomers and geophysicists led by British planetary astronomer Richard Greenwood has performed the most detailed and precise oxygen isotope measurements to date on lunar and Earth rocks. 7 Greenwood’s team has analyzed these measurements and drawn several tentative conclusions about the Moon-forming event and the source of Earth’s water. Their results potentially could yield a lot more evidence for the fine-tuned designs of both Earth and the Moon that are critical for Earth’s capacity to support advanced life.
The team’s oxygen-17 measurements show “no significant difference between lunar rocks and terrestrial olivine,” . . . “but a statistically significant difference of 3 to 4 ppm [parts per million] when basaltic samples are included in the terrestrial data set.” 8 This difference, the team demonstrates, is inconsistent with Theia (the planet that impacted the proto-Earth to form the Moon) being of aubritic composition (meteoritic material that is predominantly igneous, chemically reducing, iron-poor, and magnesium-rich). It is consistent, though, with Theia and the proto-Earth being much closer in their oxygen isotopic compositions.
The team also showed that the oxygen-17 difference is consistent with a high-energy impact between Theia and the proto-Earth. An independent team, comprised of Oxford University geoscientists Jon Wade and Bernard Wood, used the present chemical compositions of the Moon and Earth to constrain the composition and mass of Theia. 9 The second team concluded that Theia was much less oxidized than either the Moon or Earth and it approximately matched the composition of Mercury. They also determined that Theia’s mass = 10–20 percent of Earth’s present mass. (For comparison, Mars = 10.7 percent of Earth’s present mass.)
Source of Earth’s Present Water
Geoscientists have yet to accurately determine Earth’s present total water content (interior plus exterior). Estimates range from a low of 2 to a high of 12 global ocean units. (1 global ocean unit = total mass of Earth’s hydrosphere, which = 1.38 x 10 21 kilograms.) As a percentage of Earth’s total mass, these estimates translate to 0.046–0.277 percent. (Earth is water-poor, as mentioned earlier.)
Greenwood’s team showed that if the late veneer consisted entirely of carbonaceous chondritic meteoroids (the most ancient of known meteoroids with a water content = 3–22 percent), similar to the Allende meteoroid (see figure 2), then the late veneer “could have furnished all, or most, of Earth’s water.” 10 However, a lower oxygen-17 difference between lunar rocks and terrestrial basalts than what Greenwood’s team measured would be expected. On the other hand, the team showed that if the late veneer consisted entirely of enstatite chondritic meteoroids (the driest objects known in the solar system with a water content of just 0.01 percent), like the Abee meteoroid (see figure 3), then the late veneer would have delivered very little water to Earth. In this case, though, the expected oxygen-17 difference between lunar rocks and terrestrial basalts would be 6–15 times greater than what they had measured.
Figure 2: Section of the Allende Meteoroid, a Carbonaceous Chondrite.
Image credit: Shiny Things, Creative Commons Attribution.
Figure 3: Section of the Abee Meteoroid, an Enstatite Chondrite.
Image credit: Captmondo, Royal Ontario Museum, Creative Commons Attribution.
The best fit with the team’s measurements would be a late veneer consisting approximately of 80 percent enstatite chondritic meteoroids and 20 percent carbonaceous chondritic meteoroids. Such a fit implies that the late veneer provided, at most, just 0.7 of a global ocean unit of water or, at best, about 35 percent of Earth’s total water budget. Thus, Greenwood’s team concluded that a large fraction of Earth’s water survived the Moon-forming collision event between Theia and the proto-Earth.
Greenwood’s team of astronomical researchers’ conclusions represent a start in the right direction. They considered only three different mixtures of just two relatively rare kinds of meteoroids. Other mixtures and especially other kinds of meteoroids need to be considered. Also, it is possible that at the time of the Moon-forming collision event (4.46 billion years ago) that one or more or all kinds of meteoroids were more hydrated than they are today. Lastly, the Greenwood team’s oxygen-17 measurements need to be confirmed by more extensive and more accurate measurements.
What the Greenwood group did not comment on is that the survival of so much water (possibly 35 percent) through the Moon-forming collision event will require even more exquisite fine-tuning of the characteristics of Theia, the proto-Earth, and the collision/merger event. Such fine-tuning is guaranteed to generate even more “ philosophical disquiet“ 11 among Moon-formation modelers seeking strictly naturalistic explanations. For Christian theists, however, it will be cause for rejoicing and celebration.
- Samuel Taylor Coleridge (1772–1834), The Rime of the Ancient Mariner, Part II, https://www.poets.org/poetsorg/poem/rime-ancient-mariner.
- 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 (October 2008): 1237–46, doi:10.1086/591433; Geoffrey Marcy, “Extrasolar Planets: Water World Larger than Earth,” Nature 462 (December 17, 2009): 853–4, doi:10.1038/462853a.
- J. D. Gilmour and C. A. Middleton, “Anthropic Selection of a Solar System with a High 26Al/ 27Al Ratio: Implications and a Possible Mechanism,” Icarus 201 (June 2009): 821–23, doi:10.1016/j.icarus.2009.03.013.
- For a recent, thorough review of the latest research on the Moon-forming event, see chapter 5 in Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016), 48–60, https://shop.reasons.org/product/283/improbable-planet.
- Hilke E. Schlichting, Paul H. Warren, and Qing-Zhu Yin, “The Last Stages of Terrestrial Planet Formation: Dynamical Friction and the Late Veneer,” Astrophysical Journal 752 (June 10, 2012): id. 8, doi:10.1088/0004-637X/752/1/8; Francis Albaréde, “Volatile Accretion History of the Terrestrial Planets and Dynamic Implications,” Nature 461 (October 29, 2009): 1227–33, doi:10.1038/nature08477; William F. Bottke et al., “Stochastic Late Accretion to Earth, the Moon, and Mars,” Science 330 (December 10, 2010): 1527–30, doi:10.1126/science.1196874.
- R. Brasser et al., “Late Veneer and Late Accretion to the Terrestrial Planets,” Earth and Planetary Science Letters 455 (December 1, 2016): 85–93, doi:10.1016/j.epsl.2016.o9.013.
- 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 (March 28, 2018; corrected update July 13, 2018): eaao5928, doi:10.1126/sciadv.aa05928.
- Greenwood et al.,”Oxygen Isotope Evidence,” 2.
- Jon Wade and Bernard J. Wood, “The Oxidation State and Mass of the Moon-Forming Impactor,” Earth and Planetary Science Letters 442 (May 15, 2016): 186–93, doi:10.1016/j.epsl.2016.02.053.
- Greenwood et al., “Oxygen Isotope Evidence,” 5.
- Tim Elliott and Sarah T. Stewart, “A Chip Off the Old Block,” in “Planetary Science: Shadows Cast on Moon’s Origin,” Nature 504 (December 5, 2013): 90, doi:10.1038/504090a; Hugh Ross, “Increasing Lunar Coincidences Lead to ‘Philosophical Disquiet,'” Today’s New Reason to Believe (blog), Reasons to Believe (March 3, 2014), http://www.reasons.org/explore/blogs/todays-new-reason-to-believe/read/tnrtb/2014/03/03/increasing-lunar-coincidences-lead-to-philosophical-disquiet.
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