Why Life Is Beginning to Seem Very Special Indeed
"Life should not exist. This much we know from chemistry. In contrast to the ubiquity of life on Earth, the lifelessness of other planets makes far better chemical sense."
So writes Professor James Tour, one of the world's foremost synthetic organic chemists, based at Rice University in Texas. Intimately acquainted with the latest research in prebiotic chemistry, Tour has expressed severe skepticism that a plausible naturalistic mechanism for the origin of life will be found any time soon. But he goes even further:
We synthetic chemists should state the obvious. The appearance of life on Earth is a mystery. We are nowhere near solving this problem. The proposals offered thus far to explain life's origin make no scientific sense. Beyond our planet, all the others that have been probed are lifeless, a result in accord with our chemical expectations. The laws of physics and chemistry's Periodic Table are universal, suggesting that life based upon amino acids, nucleotides, saccharides and lipids is an anomaly. Life should not exist anywhere in our Universe. Life should not even exist on the surface of the Earth.1
Dr. Tour's views have surfaced at a time when astronomers have been peering into the depths of space, searching for intelligent signals from hypothetical alien civilizations. Yet although they have been listening for more than half a century, ET has not chimed in. The quest to detect life beyond the Earth is admittedly in its infancy, but the negative results thus far produced have caused more than a few scientists to question the underlying assumptions made by the early pioneers in the quest to find extra-terrestrial life: Frank Drake and Carl Sagan.
Despite what the general media report, there are a number of serious problems with the standard origin-of-life models, for which their proponents have failed to provide good answers.
The Problem of Homochirality
For example, life on Earth requires a source of homochiral molecules, that is, molecules that are capable of rotating the plane of polarized light either to the left (L) or to the right (D). Specifically, life invariably requires L amino acids and D sugars. But so far, chemists have been unable to identify a plausible natural mechanism by which these left- and right-handed biomolecules can be generated at the high level of purity necessary for the first cells to form. Indeed, such molecules can only be synthesised under highly constrained laboratory conditions, using purified (read "brought in") reagents, which have little or no relevance to the environment of the early Earth. And while meteorites have been found that contain small amounts of amino acids, they invariably are shown to contain equal amounts of L and D isomers (technically known as a racemic mixture).
In short, no conceivable naturalistic scenario could result in the generation of the large, stable ensembles of homochiral ribose and homochiral amino acids that all naturalistic origin-of-life models require, affirming why no such natural sources have ever been found.2 I recently asked Dr. Tour directly if the problem of homochirality had been solved, and he firmly responded, "No; it is far from solved."
The Phosphorus Conundrum
The element phosphorus is vital for the proper functioning of living cells, being a constituent of both RNA and DNA, as well as of adenosine triphosphate (ATP), the universal energy currency of all known life forms. But recent work conducted by Cardiff University astronomers suggests that phosphorus could be scarce in many parts of the universe. "Phosphorus is one of just six major chemical elements on which Earth organisms depend," says Dr. Jane Greaves,
and it is crucial to the compound ATP, which cells use to store and transfer energy. Astronomers have just started to pay attention to the cosmic origins of phosphorus and found quite a few surprises. In particular, phosphorus is created in supernovae—the explosions of massive stars—but the amounts seen so far don't match our computer models. I wondered what the implications were for life on other planets if unpredictable amounts of phosphorus are spat out into space and later used in the construction of new planets.3
The Cardiff team used the UK's William Herschel telescope, situated on La Palma in the Canary Islands, to measure the levels of phosphorus and iron in the Crab Nebula, a well-known supernova remnant. They compared those figures to measurements taken earlier from another supernova remnant known as Cassiopeia A (Cas A). Their preliminary results proved very surprising. While the measurements of Cas A showed relatively high levels of phosphorus, those from the Crab Nebula showed far lower levels. "The two explosions seem to differ from each other, perhaps because Cas A results from the explosion of a rare type of super-massive star," said Dr. Phil Cigan, another member of the Cardiff team. "If phosphorus is sourced from supernovae," added Greaves,
and then travels across space in meteoritic rocks, I'm wondering if a young planet could find itself lacking in reactive phosphorus because of where it was born? That is, it started off near the wrong kind of supernova? In that case, life might really struggle to get started out of phosphorus-poor chemistry on another world otherwise similar to our own.4
Re-evaluating the Drake Equation
Ever since the American astronomer Frank Drake introduced his famous eponymous equation in the early 1960s, astronomers have produced widely varying estimates of the number of extant extra-terrestrial civilizations present in the Milky Way Galaxy. Until fairly recently, the estimates varied from 10,000 to a few million. Countering these estimates, some scientists have re-examined the so-called Fermi Paradox, posed by the distinguished Italian physicist Enrico Fermi in the form of a question: If the universe is so large, with innumerable habitable planets, then why have we not detected any sign of ET?
A team of scientists and philosophers based at the Institute of Humanity in Oxford University has taken a new look at the reasoning behind the Drake equation, and found that its optimistic expectations are linked to models like the Drake equation itself. The problem, as these researchers point out, is that all such models "implicitly assume certainty regarding highly uncertain parameters." Indeed, following an analysis, they concluded that "extant scientific knowledge corresponds to uncertainties that span multiple orders of magnitude." When these uncertainties are introduced, the outcome is strikingly different: "When the models are re-cast to represent realistic distributions of uncertainty, we find a substantial ex ante probability of there being no other intelligent life in our observable universe, and thus that there should be little surprise when we fail to detect any signs of it." This result, they assert, "dissolves the Fermi paradox, and in doing so removes any need to invoke speculative mechanisms by which civilizations would inevitably fail to have observable effects upon the universe."5
Questioning the Mediocrity Principle
Over the past few decades, astronomers have discovered thousands of exo-planets orbiting nearby stars, so that now there is little doubt that the number of planets in the observable universe likely exceeds the number of stars. Exo-planet hunters have discovered that many of these planets orbit their stars within the so-called habitable zone—that narrow annulus around a star that allows for the stable existence of water on a planet's surface. Nevertheless, as geologist Peter Ward and astronomer Donald Brownlee argued in their highly influential book, Rare Earth; Why Complex Life Is Uncommon in the Universe,6 many of the features of planet Earth that have made it suitably equipped to allow both microbial and complex life to flourish on it over billions of years are likely very rare in the rest of the universe.
For instance, the vast majority of potentially habitable exo-planets orbit low-mass red dwarf stars, which make up 75 percent of all the stars in the galaxy.7 These stars are much more active than sun-like stars, thus exhibiting higher rates of flaring than does the Sun. Many such stars also generate strong stellar winds that could strip away the atmospheres of their planets.8 And many planets are located so close to their parent stars that they have become tidally locked, meaning that they do not rotate on an axis but constantly present the same face to their stars as they move in their orbits. Yet another issue pertains to the potential of gravitational perturbations of a habitable planet by its neighbouring planets. Even small changes to the orbital characteristics of a planet could extirpate any developing life that might exist upon it. All these conditions raise many problems for the development of any hypothetical life forms on the surface of these planets over long periods of time.
NASA's Hubble Space Telescope is currently being utilized in a special program called HAZMAT—Habitable Zones and M Dwarf Activity Across Time. And the early results from the program do not look encouraging. Preliminary data on just a dozen young red dwarf stars show that the frequency of flaring is much higher in them than in stars like the Sun; they typically emit flares with energies that are between 100 and 1,000 times higher than those of their elder counterparts. The most energetic red dwarf flares, dubbed Hazflares, are far more energetic than the most energetic flares ever to come from the Sun. "With the Sun, we have a hundred years of good observations," says Parke Loyd, a member of the scientific team involved in the project.
And in that time, we've seen one, maybe two, flares that have an energy approaching that of the Hazflare. In a little less than a day's worth of Hubble observations of these young stars, we caught the Hazflare, which means that we're looking at superflares happening every day or even a few times a day.9
So-called super-earths—worlds larger than the Earth but smaller than Neptune—have recently been identified as possible candidate worlds for the development of life, but there is as yet no scientific consensus on whether they can maintain or even allow plate tectonic activity to occur in their crusts. Without plate tectonics, there will be far less efficient nutrient re-cycling, which would greatly hinder the flourishing of hypothetical life forms.
In March 2019, a team of astronomers based at the Australian National University dealt yet another blow to the prospects of finding viable exo-planetary biosystems. Modelling the magnetic fields of a large number of exo-planets, the astronomers concluded that planets with a strong magnetic field, like Earth, are likely to be very rare. "Magnetic fields appear to play an essential role in making planets habitable, so I wanted to find out how Earth's magnetic field compared to those of other potentially habitable planets," says Sarah Macintyre, the lead author of the paper.10 "We find most detected exo-planets have very weak magnetic fields, so this is an important factor when searching for potentially habitable planets," she added.
Life on Mars?
Scarcely a year goes by without the question arising of whether or not Mars has microbial life. This issue was brought into sharp focus in June 2018, when NASA scientists announced the discovery by the rover Curiosity of organic matter in the soil of an ancient lakebed.11 But "organic matter" means different things to different people. Simply put, matter that is carbon-rich is not necessarily derived from biogenic sources.
More broadly though, if evidence of either extant or past life on Mars is uncovered, it might well also be discovered that such life originated on Earth. Indeed, it is estimated that over the 4-billion-year history of life on Earth, so much terrestrial soil has found its way to Mars that the Red Planet can boast an average of 2 kilograms of terrestrial soil per square kilometer of its surface (or about 11.3 pounds per square mile).12 It is certainly possible that some microbial life was delivered there along with the soil—in fact, the discovery of either extant microbial life or microfossils on Mars might well be anticipated. If that happens, astrobiologists will need to consider the possibility that it came from Earth before claiming that any such life originated there.
Questioning Biosignatures on Exo-planets
Oxygenic photosynthesis by plants is the mechanism that produces the vast majority of the molecular oxygen in the terrestrial atmosphere. So for several decades, astrobiologists have speculated that the detection of oxygen in the atmosphere of an exo-planet would provide good evidence that life must exist there.13 While the detection of substantial levels of this gas would certainly be suggestive of the presence of plant life as we know it, it pays to remember that there are established abiotic mechanisms (mechanisms derived from non-living sources) that also can generate substantial molecular oxygen.
A group headed by Chinese astronomer Feng Tian of Tsinghua University published two interesting papers in 2009 that show that stars having less than 50 percent of the mass of the Sun (i.e., the majority of stars) emit copious quantities of hard UV rays and soft X rays throughout their long nuclear burning phases of up to 10 billion years.14 They also showed that when a lifeless exo-planet possessing carbon dioxide in its atmosphere is irradiated, the rays can break down the CO2 into carbon atoms and molecular oxygen. Over time, the carbon atoms, being less massive, escape into space, leaving the molecular oxygen behind. Tian's calculations show that this molecular oxygen can reach concentrations of a few percent and so might be confused with a genuine biosignature.
When a team of chemists from Johns Hopkins University simulated the atmospheres of exo-planets beyond the solar system, they found that they could create simple organic molecules and oxygen under various scenarios without the mediation of life.15 "Our experiments produced oxygen and organic molecules that could serve as the building blocks of life in the lab, proving that the presence of both doesn't definitively indicate life," says Chao He, assistant research scientist in the Johns Hopkins Department of Earth and Planetary Sciences. "Researchers need to more carefully consider how these molecules are produced." Up-and-coming missions, such as the highly anticipated ones utilizing the James Webb Space Telescope, would need to take results like these into account before jumping to any firm conclusions about the habitability of a candidate planet.
In a recent development, a team of planetary scientists led by Li Zeng at Harvard University estimated that as many as 35 percent of exo-planets may have impenetrable water oceans hundreds of kilometers deep.16 But while NASA has long adopted the mantra, "follow the water," the same scientists caution that these planets are very unlikely to be habitable. Their fathomless ocean worlds would generate pressures millions of times greater than those found on Earth, resulting in exotic, rock-like ice formations many kilometers deep (such as ice VII) covering their floors. Such conditions would prevent any nutrient recycling from occurring, thus rendering these planets sterile.
Call for Caution
Investigating whether extra-terrestrial life exists or not is a profoundly important and interesting scientific endeavor, but at this point, there are good grounds for remaining skeptical about whether it actually exists. Given the arguments raised in this article, it is entirely reasonable to think that life might be extraordinarily rare in the universe, perhaps even unique to Earth. Only time will tell.
1. James Tour, An Open Letter to My Colleagues (August 2017): http://inference-review.com/article/an-open-letter-to-my-colleagues.
2. Hugh Ross and Fazale Rana, Origins of Life (RTB Press, 2014).
3. "Paucity of phosphorus hints at precarious path for extraterrestrial life" (Apr. 4, 2018): eurekalert.org/pub_releases/2018-04/ras-pop040318.php.
5. Anders Sandberg et al., "Dissolving the Fermi Paradox" (June 8, 2018): https://arxiv.org/pdf/1806.02404.pdf.
6. Peter D. Ward and Donald Brownlee, Rare Earth; Why Complex Life Is Uncommon in the Universe (Copernicus Books, 2000).
7. "Superflares from young red dwarf stars imperil planets," NASA News (Oct. 22, 2018): https://exoplanets.nasa.gov/news/1527/superflares-from-young-red-dwarf-stars-imperil-planets.
8. O. Cohen et al., "Magnetospheric Structure and Atmospheric Joule Heating of Habitable Planets Orbiting M-Dwarf Stars," Astrophysical Journal 790 (July 2014): doi:10.1088/0004-637X/790/1/57.
9. Ibid., note 7.
10. "Strong planetary magnetic fields like Earth's may protect oceans from stellar storms," Royal Astronomical Society (Mar. 14, 2019): https://m.phys.org/news/2019-03-strong-planetary-magnetic-fields-earth.html.
11. Jennifer L. Eigenbrode et al., "Organic Matter Preserved in 3-Billion-Year-Old Mudstones at Gale Crater, Mars," Science 360 (June 8, 2018): https://doi:10.1126/science.aas9185.
12. Ibid., note 2.
13. Carl Sagan et al., "A Search for Life on Earth from the Galileo Spacecraft," Nature 365 (Oct. 21, 1993): nature.com/articles/365715a0.
14. Feng Tian, "Thermal Escape from Super Earth Atmospheres in the Habitable Zones of M Stars," Astrophysical Journal 703 (Sept. 2, 2009): https://dspace.mit.edu/bitstream/handle/1721.1/96200/Tian-2009-THERMAL%20ESCAPE%20FROM.pdf;sequence=1; Feng Tian et al., "Thermal Escape of Carbon from the Early Martian Atmosphere," Geophysical Research Letters 26 (Jan. 31, 2009): https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008GL036513.
15. Chao He et al., "Gas Phase Chemistry of Cool Exoplanet Atmospheres: Insight from Laboratory Simulations," ACS Earth Space Chemistry (Nov. 26, 2018): https://pubs.acs.org/doi/10.1021/acsearthspacechem.8b00133.
16. Li Zeng et al., "Growth model interpretation of planet size distribution," PNAS (Apr. 29, 12019): pnas.org/content/early/2019/04/23/1812905116.
holds a Ph.D in biochemistry and a BSc in physics & astronomy. His latest book, Chronicling the Golden Age of Astronomy (Springer, 2018), explores four centuries of visual astronomy.This article originally appeared in Salvo, Issue #50, Fall 2019 Copyright © 2019 Salvo | www.salvomag.com https://salvomag.com/article/salvo50/origin-stories