From the April 2006 issue

Bob Berman’s strange universe: Center of the action

April 2006: When ancient Greeks figured out Earth's a sphere, a new issue arose. Every ball has an "inside." Well, what's inside Earth?
By | Published: April 1, 2006 | Last updated on May 18, 2023
Bob Berman
Until the 20th century, astronomers were strictly outsiders. They ignored star and planet interiors. They had to. Telescopes showed only the tops of things. Scientists knew that a ball’s “inside” constitutes virtually all its mass, and the limitation was frustrating. Basic questions went unanswered. For example, what lies below the dazzling solar surface? How can the Sun keep shining when even the highest-quality Sun-size lump of coal would burn itself out in 2,000 years? Observers suspected that fascinating, critical processes — and maybe life itself — might not hang out where it’s convenient for us to view — like on surfaces. Of course, this frustration began long before the telescope.
Deep caves give the impression that Earth’s interior is chilly. Anyone who has a basement knows a thermometer reads a constant 55° Fahrenheit (13° Celsius) all year just a few feet down. Because of this simple observation, the Ancients suspected Earth has a cool interior.

But volcanoes and hot springs provide counterexamples. These days, deep mines provide some answers. After the cool rocks of the first few dozen feet below ground, temperatures start to climb by 1° F for every 100–200 feet (30–60 meters) of depth. Today’s deepest working mine, a gold mine in South Africa at 12,500 feet (3,800m), roasts at 131° F (55° C). Experimental borings drill even deeper, to nearly 40,000 feet (12,200m). That’s still just 1/500 of the way to Earth’s center.

Just last year, researchers confirmed Earth’s inner core — a solid ball the size of Pluto (about 1,500 miles [2,400 kilometers] wide) — rotates faster than the rest of the world, as if there’s a planet within our planet. Could there be an empty chamber there where a person would float weightless? Gravity is indeed zero at Earth’s center, but the location suffers the pressure of 4,000 miles (6,400 km) of metal and rock crushing down from all directions.

The martian core froze up, destroying its magnetic field. That’s one reason land is so cheap there.
Earth’s liquid-metal core, swishing around that inner solid-iron core, is electrically conductive and creates our magnetic field. This field shields us from solar and cosmic radiation. Some of the bad stuff leaks through anyway, yielding a steady rate of biological mutations that alters Earth’s animals over time. By comparison, look next door: The martian core froze up, destroying its magnetic field. That’s one reason land is so cheap there.

Earth’s center bubbles at a fierce 8,500° F (4,700° C), thanks mostly to the decay of radioactive materials. Standing barefoot, you feel a bit of this heat percolating from deep below. But we get 5,000 times more warmth from the Sun above than from the ground below. On other worlds, the story is reversed. Jupiter and Saturn get twice as much heat from their own interiors as they do from the Sun. Because life needs energy, the prospects for creatures living “down below” on various worlds is now taken seriously. After all, life exists hundreds of feet below Earth’s surface in places forever deprived of even secondary solar energy. Seemingly lifeless worlds might teem with underground biology, and the standard sci-fi image of aliens in surface cities may be as wrong as it is unimaginative.

Nucleosynthesis in stars, too, occurs out of sight. The solar surface is merely where we view the energy-release. It isn’t remotely hot enough for fusion. That emerging inside-stuff is mostly photons of friendly visible light. But deep within, the power starts out as brutal gamma rays, whose photons fortunately stretch like taffy during countless encounters along their million-year journey to the Sun’s surface.

It’s worse elsewhere. Most of Orion’s stars have higher core pressures that generate far more ultraviolet energy. Extraterrestrials on nearby planets would need sunscreen by the barrel.

Even within the same star, things change. The Sun’s mutating magnetic field forms at a region called the tachocline — 7/10 of the way from the Sun’s center to its surface — and guides solar-particle emissions. This boundary between the radiative core, which rotates like a solid ball, and its convective zone, the outer one-third with regions of differential rotation, is a hot new area of study.

The European Space Agency’s Solar and Heliospheric Observatory — commonly known as SOHO — rests nearly a million miles sunward of Earth and investigates solar acoustical patterns, among other things. SOHO studies the Sun’s interior by monitoring its low-frequency oscillations.

Backyard starwatchers lack such sophisticated instruments. As we gaze into the April sky, those dots of light barely hint at their true stories — each a tale that begins at the middle.