Of oceans and crystals

kw: geology, earth, crystal chemistry

In a number of recent discussions some basic questions of Earth science have arisen. Putting together a number of ideas, I realized that on the grossest scale, the Earth consists of three oceans and five crystals.

From outermost inward:

• The first ocean is the atmosphere. This is of geophysical and biological interest, but air is not a "rock". However, atmospheric processes have a lot to do with the movement of mineral materials on and in the upper parts of the crust.
• The second ocean is "the Ocean", primarily the saltwater that covers 71% of the surface to an average depth of 4 km. Low-salinity and fresh waters make up a fraction of a percent, and ice constitutes more than half of that. When I lived in South Dakota, we thought of snow as a special kind of sand, and just drove on it. The interaction of the water ocean with oceanic crust produces continental crust.
• The first crystal is the continental crust. To a first approximation, it is composed of feldspar with impurities. It averages about 20 km thick, ranging up to 40 km or so beneath large mountain ranges. Why do I call it a crystal? Most of the "rock forming" minerals are actually a framework of oxygen ions held together with metal ions, chiefly silicon, aluminum, magnesium and iron. Due to the activity of water and living things on shallow minerals, there is also a considerable component of calcium oxide, AKA limestone. But this is also a framework of oxygen held together by calcium ions. This immense oxygen has a comparatively low density, around 2.6. It was distilled from the minerals in the next crystal by tectonic activity coupled with water's dissolving power.
• The second crystal is the oceanic crust. It is primarily a mixture of olivine and pyroxene, which are also oxygen frameworks held together by the same metallic ions, but in a denser configuration, with a density of around 3.2 to 3.4. The thickness of oceanic crust is about 8 km, but quite variable. It is formed by expulsion of mantle materials into the depths of the water ocean by tectonic activity.
• The next two crystals constitute the mantle. The upper mantle grades from primarily pyroxene to enstatite, as the pressure and temperature increase downward. The density of enstatite approaches 4. 
• The lower mantle is primarily magnesium silicate with the structure of Perovskite, and its density reaches 6 in the very deep mantle. This mineral is a close approximation to a close-packed structure. Think of cannon balls stacked in a 3-sided pyramid. This is "cubic closest packing" (CCP); the pyramid is a corner cut off a cube. Each ball is touching 12 others. In high-pressure Perovskite, each oxygen is touching eight other oxygens and four magnesium (or iron) ions, which at this pressure are similar in size to oxygen. Together they form a CCP network with the tiny silicon ions fitting into the spaces between.
• Next we have the third ocean, the outer core. This is generally considered a liquid, particularly because it will not transmit S-type seismic waves, only P-waves. However, it is a stiff liquid, and a chunk of it suddenly brought to the lab would sit there (burning a hole in even an asbestos bench) and flow even more slowly than cold molasses – although that might change very rapidly once the huge pressure is released. Nonetheless, its fluid motions are apparently what create the Earth's magnetic field. It is composed of iron and nickel, with up to 10% sulfur. Although it is less than a tenth of Earth's volume, with a density more than 10, it is more than a quarter of the mass of the planet.
• Finally, the fifth crystal is the inner core. Also composed of iron and nickel, it probably has much less of a sulfur constituent. It is crystalline, and measurements of its seismic anisotropy indicate it may indeed be a single huge (2,400 km diameter) metallic crystal. It is not known if its rotation rate exactly matches that of the rest of the Earth. Its density is about 15, and its temperature may exceed 6,000K, or 11,000°F. Subtracting the amount of heat we think is generated within the earth by radioactive elements (U, Th and K, mainly), it is thought that this central crystal is growing at a rate of a few cm per century.

To sum up simply: an ocean of air, an ocean of salty water, four crystals consisting of ever-denser packings of oxygen held by metal ions, an ocean of nickel-iron, and a metallic crystal.

Opposite Hemispheres

kw: geography, photographs, earth

It has been said that Earth is a water planet. If your first view of the planet was from the perspective on the left, that would be quite justified. This screen capture from Google Earth is centered above 17°S, 150°W, to the southwest of Tahiti. The other image is centered above the antipode, 17°N, 30°E, in central Sudan. Here, "Earth" is more fitting. A view a little more to the East would show an even higher proportion of land to water surface.

These virtual views are from a simulated altitude of about 10,000 miles, or 16,000 km, roughly halfway to geosynchronous orbit. From much farther away, you'd be able to see most of North America in the "water" view, and most of South America and Antarctica in the "land" view.

Interestingly, the name for our planetary home in every language I know of is related to or derived from the word for dirt ("earth" is the much older word in English, though), even in languages of Polynesia and other island societies. The ground you stand on is your principal frame of reference! If we ever decipher dolphin languages, I suspect they will call their environment by a word derived from "water".

Warming from the inside

kw: earth, history, geology, radioactivity

It didn't require much sideways thinking, after yesterday's post about uranium and radioactive decay, to recall the history of these elements in the Earth. The age of the planet is 4.5 billion years (Gy), and even these long-lived isotopes must have changed over time. One question that comes up sometimes in Freshman Earth Science courses is, "How much does radioactivity affect the Earth's temperature?" A more thoughtful student may also ask whether it was much greater in the past.

The short answer to the first question is, "Not much." In spite of a century of intensive Geological exploration, the total heat flow from Earth's interior is known only within a factor of about 2, as between 30 and 60 Terawatts (3-6x10¹³W). A commonly accepted figure it 40 TW. About 75% of this is thought to be radiogenic heat.

This diagram, representing a view toward the upper end (60TW), has radiogenic heating near 52TW, and gives the breakdown by the four isotopes that contribute significantly. The source of the image is this Jrank article. In spite of this diagram, the article's author, David Rothery, considers the total heat flux from Earth's interior to be closer to 40 TW.

Let us first compare that to solar influx. The Solar Constant (which is variable in a narrow range) is about 1,350 W/m², or 1.35x10⁹ W/km². The Earth's nominal radius is 6,370 km, so it intercepts 1.275 km² of sunlight, a total of 1.72x10¹⁷ W. One-third of this is reflected outright by clouds and ice, leaving about 1.1x10¹⁷ to reach the ground and heat the surface. Divide this by 40 TW, and we see it is 2,800 times the internal heat flow.

Now, whether radiogenic heating is 52 TW, as shown in this diagram, or closer to 30 TW (0.75x40 TW), it doesn't contribute much heat compared to the Sun. How about in the past? The diagram shows that when Earth first came together, radioactive heating from these four isotopes was 8x what it is today. There were other short-lived isotopes that no doubt added significantly to this, but they didn't last long and we have no evidence how abundant they were 4.5 billion years ago (Ga).

Now, the Sun was 40% fainter then, but we don't know how much of its radiation reached the surface. At least during the Hadean period, between 4 and 4.5 Ga, when the entire planet was molten, there were not likely any water clouds in the atmosphere, but we don't know what the atmosphere was like. Still, the radiogenic heat probably never supplied more than about 1/400th of Solar heat. It has never been much of a factor in the planet's temperature.

However, today it supplies about 3/4 of the energy that drives plate tectonics. The other quarter is remnant primordial heat and the heat of crystallization as the outer liquid core slowly freezes onto the solid inner core. Two billion years ago, radiogenic heat was more than twice what it is today, and I expect that plate tectonics ran at a brisker clip. There was also more volcanism than today.

Can we predict the future? I don't have a good handle on how much internal heating is required to keep plate tectonics going. Without it, the biosphere and atmosphere would change a lot, and the continents would erode down to just below sea level, leaving an ocean planet. If the critical value is half of today's amount (this is a wild guess), we can predict that such a level will be reached in about two more billion years. That's the time we have left to figure out how to live on an ocean planet, or how to leave the planet altogether. Given humanity's tendency to procrastinate, it probably isn't enough time!

That was the day that was - 6 megahits

kw: book reviews, nonfiction, sun, moon, earth

I just Googled "that was the day that was" and got 5.9 million hits (Yes, I used quotes. Without them, it's 1.1 Billion(!) hits). Raconteur Michael Sims spends a day with us in Apollo's Fire: A Day on Earth in Nature and Imagination, and like all semi-fictional days, it is one packed day. Authors aplenty have written "day in the life of" novels and memoirs. This book is a day in the life of...a Day.

It is only recently, historically speaking, that people determined that the Sun is large and comparatively far away. How large? Its diameter is more than 100 times the Earth's, so its volume is greater than one million Earth volumes. How far? Whether you think in miles (93 million) or km (149 million), it is FAR: nearly 12,000 times Earth's diameter, or 3,700 times around, and around, and around...

But for thousands, perhaps hundreds of thousands, of years, since we had a thinker to think with, the Sun was thought of as a powerful but familiar Presence, perhaps the size of the next big town down the road, hung on a big bowl of a sky held up by the distant hills or mountains.

By turns didactic, lyrical, bare-bones factual, yet always engaging and witty, Sims takes us, not hour by hour, but event by event, through one day: pre-dawn darkness, dawn, morning, midday, afternoon and "brillig", sunset, twilight, and night. Here both science and mythology have a place.

Every culture bases its religion or mythology on daily events and the dominant players, the Sun and Moon. It could hardly be otherwise. Suppose that one side of Earth faced the sun perpetually, as the Moon's visible hemisphere always faces Earth. There would be no notion of "reliable as the sunrise". The Moon, which circles Earth monthly, would be the primary mobile item in a fixed Heaven, and would likely be the dominant deity; the Sun is "just there all the time, and does nothing". Subtler events like the slow orbits of Mars and Saturn take a people with plenty of leisure to puzzle out.

Many in the West know the Biblical formulation, "The Sun to rule the day, and the Moon to rule the night." But have you noticed the Moon in the daytime? This image from a spacecraft on its way to the outer planets captures an amazing view: Not only is the Moon at First Quarter, so is the Earth.

Those in Earth's nighttime see the half-moon high in the sky at sunset, rapidly descending to set about midnight. But consider those on the late afternoon portion of Earth. The Moon is (barring overcast skies) visible as a white half-cookie pasted on a blue sky, about halfway around the sky from the Sun's position. About half the time, the Moon is a daylight object, and near New Moon, it hardly appears after dark.

And that blue color? It is the blue sky seen from above. The air scatters light; even very pure air containing no dust will scatter light. Blue scatters more than red, because the short wavelength of blue light is more closely matched to the size of the molecules that make up the air. When you look at a mountain range such as the Rockies, seen from forty or fifty miles away, the blue, hazy effect is the blueness of the air between. Air is visible. Even after dark, if the moon is up, the sky appears grayish, not black. If our color vision were more sensitive, we would see that moonlit skies are also blue. Maybe lemurs, with their huge eyes see blue nighttime skies. But they are dark to us.

Darkness. What is the reason? The existence of a dark sky is visible proof that the Universe is finite, either in time, in extent, or probably both. Sims takes us rapidly through the wranglings of Galileo, Kepler, Olbers, and finally Slipher, as the puzzle of the dark sky was gradually solved...well, as solved as it can be at present. We still don't know for sure whether the expansion following the Big Bang is slowing down or, as many now contend, speeding up (My take: slowing down. The apparent anomaly in supernova brightness at very great distances is related to elemental evolution. The stars billions of years ago, having very low "metals" abundance, got bigger, burned brighter, and blew up more energetically than is the rule now. Type Ia supernovae in the early universe were genuinely brighter.).

By the end of the book, I felt like I'd been sitting with a comfortable, knowledgeable companion through a long, pleasant day.