by Ted S. Frost, U of W, class of ‘54 & ‘04
Having time to pursue one’s real interests instead of schlepping around making a living is one of the benefits of retirement. In my case, that’s biology. For the past three years I’ve been attending biology orientated college courses. My previous college attendance was in the early 1950’s, so it’s been a culture shock. Not only in terms of the students of today (exposed belly buttons, pierced body parts, appearing young enough to be in grade school and, irrespective of that, their extreme brightness), but more importantly the enormous, incredible, inconceivable increase in knowledge that has accumulated since I was in school. The exponential expansion of facts, data, hypotheses, technological devices, and information boggles my Rip Van Winkle brain. It is, indeed, a great time to be alive if one is turned on by such things.
The University of Washington’s football team may have fallen on hard times, but its Earth & Space Sciences Department has assembled an outstanding faculty. People of international reputation such as professors David Catling, Roger Buick, Peter Ward, etc. This past year, I’ve had the privilege of attended two upper division classes and a graduate class presided over by these gentlemen, to whit: Astrobiology 437, Geobiology 313, and Astrobiology 502. Ever hear tell of those subjects? I hadn’t, because they’re of recent origin and still emerging. Geobiology is the study of the effect life has had on Earth and, conversely, the effect Earth has had on the development and evolution of life. Astrobiology is the study of the origin, distribution, and destiny of life in the universe. Since pursuit of these disciplines is associated with the Earth, the Moon, and the stars, I thought all you astronomy buffs might be interested in a few of the tidbits I’ve learned this year.
Every knowledgeable person now knows that Earth is a very special place with very special attributes. It has to be, since intelligent life as we know it requires special environmental attributes in order to exist. Those who’ve read Peter Ward and Donald Brownlee’s book Rare Earth or have perused the Drake Equation know what I’m talking about. What I didn’t realize, though, is just how special, special is. How extraordinary the circumstances were that made this the place it is.
Take the Moon, for example. Like the girl next door, we frequently see the Moon but take it for granted. But we shouldn’t. The Moon should be appreciated not only for its romantic appearance, but more importantly because it is a big factor why Earth is such an agreeable place for life to exist and evolve. We should be particularly appreciative because it turns out its creation was an anomaly with several strange attributes.
First off, we are the only terrestrial planet in our solar system to have one. Well, that’s not exactly true. Mars has two itty-bitty moons, Phobos and Deimos, but they are peewees - about 20 kilometers in diameter compared to our 3,476 diameter Moon -and are thought to be irregularly shaped asteroids that happen to have been captured by Mars rather than “real” moons. Other than Earth, the outer Jovian planets are the only ones with moons. In addition, ours is a single moon whereas the other moon systems are multiple. Another oddity is the Moon’s humungous size relative to us – a little over one quarter Earth’s diameter. Other moons in our solar system are tiny relative to the planets they orbit around. And our Moon is less dense than Earth (3.34 g/cm3 vs. 5.52 g/cm3), whereas other moons are denser than their associated planets. Finally, we know from being there that the Moon’s geochemical composition is the same as Earth. We are cut from the same cloth, as it were. Moons of other planets are compositionally different animals. (Note: With regard all the foregoing, let the record show I’m considering Pluto and Charon to be Kuiper Belt objects rather than a planet and a moon.)
How did all this come about? Why is it even there? Current thinking abetted by computer simulations has it that our Moon is the result of a highly unusual and stupendous collision involving Earth and a Mars-sized planet occurring some four billion years ago. It was an oblique collision rather than a glancing or retrograde type of blow that resulted in Earth and Planet “X” exchanging body fluids. They merged, in other words, with tossed-out lighter debris from the collision coalescing into our Moon. So, rather than Earth and the Moon being mere cold accretions of solar nebular matter, they are largely the result of a violent celestial train wreck.
That is all very interesting, but why was this chance event critically important to our being here? Consider the following laundry list of consequences:
First, we wound up with a bigger proportion of heavy materials than ordinary accretion of solar matter would have given us. Earth is the densest of all the planets. Second, kinetic energy (½ x m x v2) from the collision caused a big melt down of the merged material. This allowed the materials to differentiate into density layers rather than stay in a homogeneous or semi-homogeneous condition. Heavy metals like iron and nickel sank to the core, basalts to the mantle, and lighter granite stuff to the crust. Third, residual kinetic energy plus radioactive decay has kept part of the Earth’s core in a liquid state as well as maintain a temperature gradient in the mantle. Fourth, the collision knocked the Earth cockeyed. We have about a 23° obliquity (that’s “tilt” to
us laypersons). Fifth, Moon and Earth are locked in a gravitational embrace so Earth doesn’t wobble. It pretty much maintains its slant. Sixth, the gravitational attraction between the two causes the tides to ebb and flow. And seventh, it is important what the collision did not do. Luckily, it did not knock us out of our mildly elliptical Goldilocks orbital distance from the Sun. We stayed far enough away from the Sun to avoid water evaporation and run-away greenhouse like Venus, yet close enough to avoid Mars-like freezing.
So, here we are today. Sitting fat, dumb and happy as multicellular eukaryotes of Subphylum Vertebrata miraculously possessing the cognitive powers to analyze and contemplate it all. What benefits do we enjoy from the fortuitous birth of the Moon?
Well, for starters, the stupendous kinetic energy (probably about 6x1031 Joules, or 1016 times a one megaton nuclear bomb) of such a collision would have resulted in Earth possessing considerable residual heat. Enough to liquefy its mantle and facilitate fractionalization of its materials and possibly even partially offset the faint early Sun paradox. Concentrated differentiation of Earth’s materials plus the high temperature gradient of its mantle and core set the stage for plate tectonics. The concentrated differentiation of Earth’s materials plus the high temperature gradient of its mantle and core together with liquid water are essential for plate tectonics, something Mars and Venus don’t have. With plate tectonics we have upwelling of magma in ocean ridges and the so-called “black smokers” which are thought to be the environmental womb for creation of life where energy, chemicals, and protection necessary for creation of life came together. With plate tectonics, we have the subduction of heavier ocean crust under lighter continental crust. We have substantial volcanism. We have weathering and erosion of crust materials into the oceans and the carbon-silicate cycle. The interplay of all these things leads to stirring the Earth’s geobiochemical pot which aids weather stabilization and temperature control and the recycling of elements essential to life.
The liquid portion of Earth’s core gives us a strong magnetic field which protects us wimpy land animals from the deleterious effects of radiation from space. The gravitational embrace of Earth and Moon locks the Earth into a stable position. If it weren’t for this, Earth’s tilt would vary in a topsy-turvy manner causing chaotic climatic flip flops, making it hard for complicated life forms to adjust. Yet, Earth’s permanent 23° ± tilt allows rotisserie-like distribution of the Sun’s warmth throughout Earth and, at the same time, seasonal weather changes which stimulate evolutionary diversity by moderately varying environmental situations. As does the Moon-caused ocean tides which create a mixing of essential nutrients as well as a favorable air-water ocean beach habitat. A very recent hypothesis theorizes that the impact creating the Moon left Earth with an enrichment of heavy metals in the form of workable ore deposits. Hence, the raw materials of our technological civilization and a way to escape the Stone Age.
It all adds up to the Moon’s existence giving us a stable and benign platform for the creation and continued existence of life, yet, not so stable and benign as to not give evolution something to work against. So, the next time you look up at Old-Man-In-The-Moon, how about giving him a tip of your hat, or curtsy, as the case may be, and say: “Thank you, Mr. Moon, for being there.”
But all good things must come to an end, so enjoy the Moon while it lasts. Friction from the tidal bulge the Earth-Moon system creates is causing Earth’s rotation to slow down. Due to the Law of Conservation of Angular Momentum, Earth’s slower rotation is causing the Moon’s orbital radius to increase. As a result, Mr. Moon is receding from us at the rate of 3 cm per year. But not to worry. By the time its recession has a serious effect on us, we’ll be biomarkers in the fossil record and whatever succeeds us will have plenty of other things to worry about.