NASA’s astrobiology program leans heavily on the assumption that any location where liquid water can persist is a potential place for life to emerge and evolve. Consequently, those interested in the question of life beyond the Earth have typically limited their searches to watery places. Usually those were planets orbiting within their particular “continuously habitable zone” (CHZ), defined as the distance from the host star where H2O could remain in the liquid state for long periods of time. The CHZ has inner and outer radii with temperatures between 0 and 100°C, the freezing and boiling points for H2O. If a planet stays within the CHZ throughout its orbit, it is deemed “habitable” whether or not it has inhabitants.
Later astrobiologists realized that other locations with liquid water exist. Subsurface oceans of water are suspected on icy moons like Europa at Jupiter, Enceladus at Saturn, Triton at Neptune, and possibly a few others. Because in situ investigation of those places are unlikely till far in the future, we will restrict our discussion to the orbital CHZs. One caveat about habitable zones is that they can migrate. Some types of host stars become hotter or cooler over time. The CHZ, correspondingly, will move outward or inward.
Faint Young Sun
Our own sun is thought to have been 20 percent cooler in its early history. As Earth could not have migrated inward to adjust, this creates a “faint young sun paradox” that astrobiologists must address in their models of life’s history on Earth. If a faint sun resulted in Earth orbiting outside the CHZ for a time, it could have become a giant “Snowball Earth” that could only melt back to normal with difficulty. A Snowball Earth could be a dead end; the high albedo of water ice would reflect more solar warmth back out to space. Some doubt it could ever recover. It’s best, therefore, to avoid snowball scenarios in models of Earth history.
A deeper dive into requirements for habitability shows that it is too simplistic to assume that being “in the zone” (CHZ) qualifies a planet for habitability. The right atmosphere, crustal composition, inclination, obliquity, rotation period, and other factors bear strongly on the question. Books such as The Privileged Planet, Rare Earth, and A Fortunate Universe have added to the list of requirements, including factors like stellar class, the avoidance of tidal locking, and presence of a stabilizing large moon. Most recently, Denton’s The Miracle of Man and the earlier books in his Privileged Species series have focused attention on essential chemical elements for life — over a dozen of them — that must be available near the surface of putative habitable planets. His book The Wonder of Water (see the video below) explains H2O’s many properties that benefit life.
And yet one property of water — its ion content — has been largely neglected by astrobiologists. Table salt (NaCl) is the most common ionic compound in sea water. Its ease of dissolving in water sets up electrical properties between its positive sodium (Na+) and negative chlorine (Cl–) ions. As a paper discussed below says, “Salt affects seawater density and ocean dynamics via direct mass effects and through its influence on charge density and ionic interactions with polar water molecules.” One effect of salinity is lowering the freezing point of water; this is the reason for salting roads in winter.
Sea water on Earth presently contains about 35g/kg of NaCl. Has this value remained constant throughout the history of the Earth? And does the concentration of salt in a planet’s oceans have any effect on its habitability? Surprisingly, the relationship between salinity and habitability has received scant attention till now. News from Purdue University announced that “salt may be the key to life on Earth and beyond.”
The composition of the atmosphere, especially the abundance of greenhouse gases, influences Earth’s climate. Researchers at Purdue University, led by Stephanie Olson, assistant professor of earth, atmospheric, and planetary sciences, have recently found that the presence of salt in seawater can also have a major impact on the habitability of Earth and other planets. [Emphasis added.]
The Purdue team modeled the effects of salinity and found that increases or decreases in ocean salt concentration have profound effects on habitability. Their paper, by Olson et al., “The Effect of Ocean Salinity on Climate and Its Implications for Earth’s Habitability,” was published open access in Geophysical Research Letters.
The influence of atmospheric composition on the climates of present-day and early Earth has been studied extensively, but the role of ocean composition has received less attention.
A major finding in the paper is that high salinity warms the climate by affecting ocean currents. This may answer, the authors believe, the faint young sun paradox: i.e., how our planet avoided the Snowball Earth scenario when the solar luminosity (solar energy per unit area, in watts per square meter) was 20 percent lower, according to theories of stellar evolution for G2 main sequence stars like our sun.
We find that saltier oceans yield warmer climates in large part due to changes in ocean dynamics. Increasing ocean salinity from 20 to 50 g/kg results in a 71% reduction in sea ice cover in our present-day Earth scenario. This same salinity change also halves the pCO2 threshold at which Snowball glaciation occurs in our Archean scenarios. In combination with higher levels of greenhouse gases such as CO2 and CH4, a saltier ocean may allow for a warm Archean Earth with only seasonal ice at the poles despite receiving ∼20% less energy from the Sun.
Too much salt, on the other hand, can be hostile to life. Watch plant roots bend to avoid salt in a news item from the University of Copenhagen. The Purdue authors did not consider the effects on organisms with 50g/kg NaCl (their highest model value). Some organisms are remarkably salt-tolerant now, but evolutionists do not think they began that way. The Dead Sea, with over 340 g/kg, is dead for a reason. Rising salinity in California’s Salton Sea has killed most of the fish that once attracted anglers to its shores (Desert Sun). On Mars, the pervasive concentration of perchlorate salts worries some astrobiologists about the possibility of life there.
Other consequences of changes in salinity not discussed by the paper in detail include interactions with other ions and elements critical for life. Tinkering with salt is likely to cause unintended consequences.
The paper’s conclusions rest on assumptions that are difficult to test and are somewhat dubious. For instance, modeling high salt concentration initially to keep the planet from freezing under a cooler sun could appear like special pleading; how do they know salt concentrations did not start initially low instead, increasing as water eroded the continents? Do they have an experimental basis for presuming higher salinity in the past? They cite a couple of papers, but note that
Archean salinity remains poorly constrained. Our goal is thus not to offer a definitive view of a single moment in Earth’s history; instead, our goal is simply to explore the response of the climate system to changing ocean salinity and to assess the potential significance of these effects in the context of reduced solar luminosity on early Earth.
More important for a design view of the Earth is the relation between salinity and habitability. Is the value of 35g/Kg NaCl a “Goldilocks” value? Has the salinity value remained stable while life was present, but fluctuated, increased monotonically, or decreased prior to life’s appearance? If both questions yield affirmative answers, there might be evidence of fine timing to consider, a possible homeostasis in salt geology as well as salt biology. Notice the delicate balance that results from changes in salinity, according to the authors:
Present-day seawater with a salinity of 35 g/kg freezes (and is most dense) at −1.9°C, and saltier oceans freeze at progressively lower temperatures. In combination, these three density effects may profoundly affect the density structure of the ocean, its circulation, and ocean heat transport to high latitudes with consequences for sea ice formation. Even small differences in sea ice formation may yield significant climate differences through interaction with the positive ice-albedo feedback.
Then the authors point out that salinity is a dynamic value. It thus becomes crucial to understand the sources and sinks of salt.
Sodium (Na+) and chlorine (Cl−) are the primary ions contributing to ocean salinity today. The residence times of Na+ and Cl− ions in the ocean are 80 and 98 Myr, respectively, much shorter than the age of the Earth.
The authors point out that salinity also affects the concentration of atmospheric CO2. This becomes another complication not previously considered in climate models. Notice the word “coincidence” in this eye-opening statement:
The salinity evolution of Earth’s ocean is not yet well constrained, but constant salinity through time would be a notable coincidence or imply some currently unknown feedback. Climate models that implicitly assume present-day salinity may thus yield misleading views of Earth’s climate history.
The paper raises interesting new questions more than it provides definitive answers:
It is thus unclear whether accounting for changes to sea salt aerosol in our model would have a large effect on climate and whether these effects would amplify or offset warming with increasing salinity in our model scenarios. The relationships between ocean salinity, atmospheric water vapor, cloud nucleation, precipitation patterns, and surface temperature on short and long timescales remain an exciting opportunity for future work.
A Critical Role
That’s enough quotation to point out the criticality of salt to habitability. Those interested in the details can follow the authors’ arguments in the paper. Suffice it to say that a planet designer would have had to regulate an additional factor — salt — to make it livable. Liquid water alone is not enough to maintain a CHZ. One cannot tinker recklessly with salt concentration without knocking a planet out of the Goldilocks zone. If the models require beginning with a cooler sun, was it a lucky coincidence to start with higher salinity to keep the Earth warm, then decrease it steadily as the sun brightened?
The Purdue research adds two factors to the list of requirements for habitability that Denton, Gonzalez, Richards and others have compiled: (1) fine tuning of salt concentrations for a stable climate, and (2) fine timing of salt dynamics under a changing solar constant. Maybe there is something new under the sun after all: the salt of the Earth.