"SCIENCE, in its most fundamental definition, is a fruitful mode of inquiry, not a list of enticing conclusions. The conclusions are the consequence, not the essence."
-- Stephen Jay Gould, The Flamingo's Smile
The underlying theme of this lecture is the evolution of scientific methodology for addressing questions about the Earth around us, and the nature of processes both routine and catastrophic in our environment. We'll proceed from the ridiculous to the sublime, asking questions that have puzzled humankind from the earliest moments of leisurely circumspection. Let's begin with looking at a rock.....
A chuck of rocky material, comprised of bluish Fluorite crystals is our starting point. From the earliest exploitation of the materials in the environment, humans have explored the physical properties of rocks and other natural substances. Initially this was prompted by the practical necessity of tool and weapon making. As civilization advanced, the consideration of the fundamental essence of our surroundings also deepened. By the time of the Greek civilization, continual scraping for subsistence had subsided, and leisure time allowed intellectual pursuit of fundamental questions: what is a rock, why do they differ, and why are rocky substances different from human tissue, plants, or other living organisms?
The Greek intellectuals began to organize the mode of human inquiry, bringing in the notion of experimentation and hypothesis testing of a formal type. Practical observation led them to adopt the notion that solid, inanimate material was comprised of atoms, tiny building blocks of all materials. For rocks, these atoms are organized in regular structures, yielding crystals, that in turn comprise distinct mineral forms. Regular ordering of the atoms in crystal lattices is controlled by fundamental properties of the atoms, and their ability to interact with one another. While the Greeks lacked any analytic tools capable of probing the details of atomic structure, their systematic thought processes led them in the correct direction. This birth of Scientific Method was short-lived, and unfortunately disciplined human inquiry lapsed into conflict with theological dogma over the next 1500 years, and little more was learned for a long time about the nature of a rock.
Similar consideration of materials that grow and change, such as an insect, led to a recognition that tiny atoms also comprise the materials of animate objects, but the atoms are organized differently, into molecules that are organized structures, but do not have the lattice structures characterizing rocky materials. The Greeks also made the association that life forms were deriving their subsistence from the environment around them, consuming other life forms, some of which seemed to derive their very essence from the rocks and soils in which they grew. This led to the appreciation that human organisms are very much a product of their environment, indeed evolutionary theory now sees us as having evolved in a manner so as to optimize our utilization of our environment (for example our eyes are tuned to be most sensitive to the peak in the spectrum of solar radiation). As inquiry broadened into a more profound consideration of what are human organisms, there was some tension between the recognition that we were in some way a product in harmony with our environment and the philosophical perception that we are so special that divine explanations must be invoked. Again, the advance of scientific approaches to the nature of living organisms was impeded for many centuries by theological dogma.
With the flourishing of knowledge that followed the upheavals of the Renaissance and the onset of the Industrial Age, scientific method was reinvigorated and greatly expanded as an intellectual mode of inquiry. The ability to experiment and measure advanced rapidly with a proliferation of discoveries and technical innovations. Fields of science emerged, such as chemistry and physics, followed by applied fields such as geology. Scientists developed many new ways to determine compositions of minerals and molecules, adding depth to the fundamental understanding of matter. Chemistry and physics systematically identified the distinctive atomic materials, charting out the Periodic Table of the Elements. Tools such as mass spectrometers, which are able to take tiny samples of materials and break them apart to measure the relative abundance of their constituent atoms. Such machines are now widely used, and you can find many of them on campus, including in the laboratories of the Earth and Marine Sciences Building, where we continue to ask questions such as what is a rock and how did it come to be? Another critical tool was spectroscopy, the analysis of radiation emitted by an object when it is heated. The light given off is generated by the excitation states of the constituent atoms in the material, which imparts a distinctive 'color' to the light. By using methods of chemistry and physics, mass spectrometers, and spectroscopic sensors, humans began to count and measure the materials in our environment...
Let's up the scale a bit, and now ask a question germane to this class: What is the Earth and what is it made of? This was an issue also probed by the early Greeks, who came to the recognition that the Earth is a rocky planet with fluid envelopes in the oceans and atmospheres. They also recognized that descending deep into mines led to hotter conditions, and combined with the outpourings of magma during volcanic eruptions, it was decided that the planet was hotter in its interior. Even after the ups and downs of the Dark Ages and the Scientific revolution, geologists have been confronted with the question even up to today. Given that the deepest drill hole ever achieved is a scant 10 km deep, out of the 6371 km radius of the Earth, and that even the deepest rocks brought up to the surface in volcanic eruptions appear to have originated no deeper than a few hundred kilometers, how can we answer this question?
In part, the answer comes from looking outside the Earth rather than into its unreachable depths. This is one of the most important attributes of scientific method, the sometimes unexpected directions to solution of a problem that emerge from rethinking the problem logically. The approach is as follows: to understand the Earth, we must understand the system in which it evolved. Initially, this led us to consider the Earth as part of the Solar System, and then the Solar System as part of the Milky Way Galaxy, and the Milky Way as part of the Universe. Each expanding perspective probes more deeply into the question of what is the Earth, and ultimately what are we humans, which are products of the Earth itself.
The Greeks again were on the right track, viewing the Earth as one of the planets orbiting the Sun. The Heliocentric notion of a solar system was born with Pythagoras (580-500B.C.), but languished through the Dark Ages as theological perspectives placed the Earth in the center of the Cosmos. Copernicus (1473-1543 A.D.) and Kepler (1571-1630 A.D.) drew upon new tools such as the telescope and systematic measurements using careful scientific method to establish the correctness of the Heliocentric solar system and the laws of planetary motion. The recognition that the Earth was but one of a system of diverse planetary bodies orbiting a star of vastly greater mass led to a profound enlightenment. Given that stars themselves evolve (a notion made clear by observations of supernova explosions), one secret to understanding what the Earth is made of is to address how it formed when the entire solar system did. Assuming that the Sun and its planets formed together (as strongly suggested by the common rotational plane of the planets - the plane of the ecliptic), it is very reasonable to think that the Earth must have a distribution of rocky materials very similar to that of the Sun, as the primary mass of the solar system resides in the star. To make progress, we must ask: What is the Sun made of?
The composition of the Sun is not readily subject to direct measurement, so once again it is scientific method that leads to a non-obvious solution of this question. A critical development was provided by an eyeglass maker, Frauenhofer, in 1814. He developed the first glass prisms, which refracted white light from the Sun, separating the different wavelengths of radiation. (If we think of each color of light as a propagating light wave, the wavelength is the spatial distance between crests of the wave. For a surfer this would be length between peaks in a set of waves. The period of the wave is the time it takes between the passage of one wave crest and the next past a fixed point. The wavelength, L, is equal to the speed of the wave, c, times the period, T. L=cT. In this case the speed is the speed of light, which is the same for all light in a vacuum.)
Using a prism to separate 'white' sunlight into its separate colors (as happens naturally by air moisture to produce a rainbow), one finds colors ranging from violet to blue to green to yellow to orange to red, but amidst the colors there are dark lines. The colors represent emissions, and the dark lines represent absorptions of certain wavelengths of light. By doing experiments and theory, scientists established what wavelengths of light are emitted or absorbed by different materials under very hot conditions. From comparing the observed spectrum of the sun, resolved in great detail in the range 3900 to 6000 angstroms, we are able to state with good accuracy the relative abundance of elements in the Sun (!).
Now, even if we accept the notion that the composition of the Sun, inferred by the unexpected method of looking at its light radiation, tells us the primary composition of the entire solar system (since most of the mass of the solar system is in the sun), clearly there are important differences between the Earth and Sun. For example, the Sun's spectrum indicates a great relative abundance of hydrogen (H) and helium (He) relative to, say, Silicon (Si). Silicon is a major component of the (silicate) rocks of the Earth, so there would have to be vast amounts of H and He somewhere in the Earth if our planet had the exact same composition as the Sun, but this cannot be reconciled with measurements of the atmosphere, the ocean, or the mean density of the Earth. Clearly, the Earth has fewer of the light, volatile gasses that the Sun has retained This makes sense because the Earth's gravity is so much less than that of the Sun that very light gasses easily escape into space, leaving only a modest amount of nitrogen (N) and oxygen (O) on the planet. But, this difference raises the question: if the Earth's size modifies its composition relative to the Sun, what other differences are there? So, we need another estimate of the bulk composition of the solar system, preferably provided by a sample more similar to the rocky Earth.
Well, such samples fall to Earth daily, in the form of meteorites. Such objects are believed to share some common origins with the Earth, forming as small chunks of cool debris from the cloud of gas and dust that the solar system emerged from. But meteorites come in vastly different types, some being pure metal, some extensively melted rock, some rock-metal combinations that have little evidence of melting, and a special class, called Carbonaceous Chondrites, being meteorites that have experienced very little heating, remixing, or melting, and appear to have formed from the most primitive soup of the solar nebula, just as did the Sun. While still almost devoid of very light gasses like, H, He, O, N, these meteorites have been carefully studied, as they may represent what the ingredient were that went into the Earth 4.5 billion years ago when it formed. Prior melting and mixing of the Earth has undoubtedly redistributed the components internally, but the bulk composition of the entire planet may be very similar to a Carbonaceous Chondrite.
Strong support for this hypothesis comes from comparing the bulk composition of the Sun, inferred from spectroscopy, with that of the bulk composition of the meteorite inferred from mass spectrometry. The correlation is astounding for all materials other than the volatile gasses H, He, O, C, and N. The inference that the same relative abundance of the more refractory heavier elements exists in the Earth as exists in the Sun and the primitive meteorites gives us our best answer as to what is the Earth made of.
So, we can say now, based on this form of Scientific inference, coupled with other direct observations that we will hear about later, that the Earth is:
A Layered Planet:
Atmosphere has Nitrogen, Oxygen, Neon
Oceans have Water (H2O), Salt, trace elements
Crust (from 6-70 km thick)
Mantle (from below the crust to 2890 km deep)
Core (down to the center of the Earth).
The compositions are:
Crust + Mantle 69% Total Mass Compound Weight % SiO2 48% MgO 34% FeO 7.9% Al2O3 5.2% CaO 4.2% Core Compound Weight % Fe (iron) 89 Ni (nickel) 6 S,O,Si 5
While the process of inference is such that these numbers have some uncertainty, most scientists debate values at the level of 1%, not the bulk numbers at all. Strange, isn't it, that we can come to strong conclusions about material deep in the Earth that we have never directly seen or touched, largely as a result of looking up at the Sun and catching small rocks that tumble from the sky?
Now, having some idea of the bulk composition of the Sun, the Earth, and the Solar System, one can follow up with the obvious question: Is the composition of the Earth unusual relative to the whole Universe? This probes to the heart of the uniqueness of this planet and of the processes leading to its existence. Spectroscopy again supplies much of the answer, as we can look at the radiation from distance stars, even distance galaxies, and infer gross chemical compositions from the absorption/radiation spectra. This leads to an estimate of the Cosmic Abundances of the Elements:
#Atoms/atom of Silicon (Si) H 27,200 Hydrogen He 2,180 Helium C 12.1 Carbon O 20.1 Oxygen Ne 3.8 Neon N 2.5 Nitrogen Mg 1.1 Magnesium Si 1.0 Silicon Fe 0.0008 Iron
The preponderance of the lightest elements, H and He, not only in our Sun, but in the entire visible universe, begs for an explanation. How did there come to be so much of these simple atoms, and what controls the relative distribution of all atoms that were available to build up the Earth? What process could produce so much H and He, and so little Fe, yet lead to a huge concentration of iron in the Earth's core? These questions drive us to perhaps the biggest scale we can consider: what is the Universe?
The answer has come in stages, with two key advances enabling progress: The first was the ability to estimate distances to very distance objects, and the second was to assess how those objects are moving relative to the Earth. The first issue, measuring distances, exploits the changing position of the observers on the spinning Earth, or the changing position of the Earth itself, relative to a planet or star that serves as a reference point. By looking at the angle (the parallax) subtended by two observing positions relative to a known reference point while imaging a very distant object, one can estimate distance. The details can be found in any introductory astronomy book. From such measurements, we know that the Milky Way Galaxy in which our solar system resides is about 100,000 light years across, and that it is 2,000,000 light years to the next nearest galaxy, the Andromeda Nebula. (A light year is the distance traveled at the speed of light in one year. Given that the speed of light is about 300,000 km/s, this is a long way).
In addition to measuring the distance to an object, we must determine whether the object is moving toward or away from the Earth. This is essential for looking either forward or backward in time to understand how the Universe is evolving, a clear key to understanding what the Universe is. The important notion here is the concept of the Doppler Shift.
Assume some distant object is X km away from an observer, and the object radiates a particular light wave with a wavelength, Ltrue = c T, where c is the speed of light, and T is the period of the wave. At the speed of light, a particular peak of a wave reaches the observer at time to = X/c. If the object is stationary, the next peak of the wave will come in one period, T, later, where T is the time between peaks, and the observer will measure the true period of the waves. Now, however, assume that the object is moving relative to the observer at velocity V. In the time between radiating one wave peak and the next, the object will move a distance dX = VT. The observer will now see the second peak arrive at a time after it is radiated of t' = (X + dX)/c = (X + VT)/c. This is simply because the wave travels a different distance than it did for the earlier peak. The observer measures the wavelength of the wave arriving at his censor by observing the time difference between peak (the apparent period) multiplied by the velocity of light: Lmeasured = c(T + t'-to) = cT + VT. This will differ from the true wavelength if the object is moving. The ratio of observed to true wavelength is:
Lobserved/Ltrue = 1 + V/c.
If V is positive (the object is moving away from the observer), the observed wavelength is longer than the true one, which shifts the observation toward the red (long wavelength) end of the spectrum. If the object is moving toward the observer the wavelength will be shorter, toward the blue end of the spectrum. The Doppler shift is the explanation for the differences in sound detected by your ears when a noise-emitting object (say, an airplane or a train) is moving toward you (negative V; results in louder, higher frequency sound) versus away from you (positive V; results in quieter, lower frequency sound). There is nothing mysterious about the Doppler shift for light (well, other than the constancy of the velocity of light of all wavelengths), so the astronomical observations are highly robust.
Armed with the ability to measure distances, and the knowledge that certain fundamental radiation excitations of basic gaseous materials in the universe have predictable wavelengths, Edwin Hubbell in 1929 found a remarkable thing: Looking at the distant galaxies in the Universe, the light from them is shifted toward the red end of the spectrum. In every case! Also, the further away the galaxy is, the more red shifted the spectrum is, with the amount of shift increasing proportional to distance! He estimated distances for distant galaxies based on the brightness of their light, and plotted the degree of red shift versus distance, finding strong proportionality. This provided strong evidence that the relatively nearby galaxies are expanding outward and Hubble postulated that this holds for the entire Universe. This has been repeatedly confirmed and refined by observation after observation (further and further objects have been measured, and their degree of red-shift is close to that predicted by extrapolating Hubble's observations) to the point where there is scientific consensus that this expansion is indeed taking place. It also supports the Cosmological Principle, which is the notion that the Universe should look the same to observers in all galaxies if the scale is big enough and motions are less than the speed of light. The expansion of the Universe is often likened to how any point on a balloon moves relative to all other points on the balloon when it is inflated. Space itself expands with time rather than galaxies flinging further out into a pre-existing volume, just as the surface area of the balloon expands. All other positions move away from a reference point, with increasing velocity at larger distances, and the same behavior is found for any reference point on the balloon.
The refinded measurements of distant objects made since the 1950's indicate that the rate at which the most distant objects are moving away from Earth is actually somewhat higher than a simple extrapolation of the rates from more nearby (younger) objects seen today. This means that older (more distant) objects in the early days of the Universe expanded more quickly; the expansion rate is slowing down slightly with time due to the pull of gravity of all the mass in the universe, which resists the expansion. Recent observations tend to support the notion that the decrease in rate implies that eventually the Universe will contract, with gravity pulling all matter back together in the distant future; a Big Implosion. This suggests that the curvature of space is bounded (like that on the surface of a sphere) rather than being an open, hyperbolic (saddle-like) curvature that would lead to expansion for infinite time.
The discovery of an expanding Universe has profound implications for what the Universe is, but the most dramatic is to consider what happened long ago, effectively to take the measured velocities and run all the galaxies back through space and time, computing their relative positions in the past. The systematic convergence (letting the air out of the balloon), brings all points together again on a time scale of 13-15 billion years ago. This leads to the notion that all material in the Universe was in one place, say 14 billion years ago (the current best estimate), and has been spreading outward ever since, creating space as it goes. It was projected outward by the BIG BANG, and you and me and everything around us, every atom in the universe, or its precursory forms of atomic material was there! It was quite a moment, I'm sure.
So, our expedition from speculating about rocks and toenails has brought us up to the most dramatic catastrophe of all, the massive explosion that produced the Universe. Not all details of the expansion of the Universe are sewed up; current measurements by the Hubbell telescope suggest that the rate of expansion is accelerating. This may mean their is unknown force or mass in the universe that we have not included in our theories. Recent ideas suggest that the Universe may have expanded and collapsed multiple times in the past, with it at least looking as though collapse is the ultimate fate of this universe. But, it looks very likely that the Big Bang actually took place, and we'll now consider why the Universe and the Earth end up with the particular composition that they have.
![[return icon]](../dinoheadt.jpg)
Return to the Earth Sciences 80A Lectures Page