Billions of years ago, the globe spun twice as fast as today; a complete rotation took just 6 hours. 

Then Earth’s rotation slowed, and that’s why you’re here listening to this episode. Let me explain.

The atmosphere of early Earth was made up of methane, CO2 and sulfur gases. But no oxygen.

Eventually, as noted on a prior EarthDate, cyanobacteria, blue-green algae, began to produce oxygen through photosynthesis.

At first, the amount of oxygen they released was so small that it was absorbed by iron in seawater, and no oxygen entered the atmosphere.

Scientists researching this phenomenon found a similar low-oxygen, high-sulfur environment in sinkholes at the bottom of Lake Michigan, where modern blue-green algae grows.

There, and in a lab mimicking that environment, they tested the effects of day duration on oxygen production.

Turns out blue-green algae is dormant in the morning. In a short day, it was nearly dark again by the time it started producing oxygen. 

And that small amount was reabsorbed by the algae before it could enter the water.

As days lengthened, the algae had enough time to produce enough oxygen to escape.

Over millions of years, oxygenated water first gave rise to aerobic sea life. 

Then an oxygen-rich atmosphere allowed land creatures to develop and thrive, which eventually led to…

you, and me, and a radio show called EarthDate.

Greenhouse gases trap heat in the atmosphere and keep Earth warm. Without them it would be a frozen iceball – as it once was before.

There are many greenhouse gases. Water vapor is by far the largest by volume, and does most of the warming.

Carbon dioxide is the weakest greenhouse gas in warming potential, but the second most abundant.

Carbon is always moving between Earth and sky in what’s called the carbon cycle. 

But since humans have industrialized, we’re transferring carbon that was held in fossil fuels into the atmosphere, where it lasts about a century.

We measure the warming impact of other gases in CO2 equivalent. 

Methane, for instance, is 80 times more potent than CO2, but lasts in the atmosphere only a few decades.

It’s naturally produced by decaying plants in swamps, lakebeds, and forest floors. And by human sources like natural gas leaks and livestock.

Nitrous oxides also occur naturally, emitted from soils beneath wild plants and oceans. And through our agriculture, by using nitrogen-based fertilizers. 

These are 300 times more potent than CO2, though produced in much smaller quantities.

Fluorinated gases are manmade, and emitted only from human sources, like leaking refrigerant. 

The volume is tiny, but they’re thousands of times more potent than CO2, and endure for thousands of years.

Only by understanding the mix and potency of greenhouse gases can we understand how to best manage them.

Five centuries ago, the last emperor of the Aztecs drank a full gallon each day of a coveted beverage, rich in vitamins and caffeine. 

The Aztec word for it: xocolatl Chocolate.

Just like today’s chocolate drinks, his was made from the ground seeds of the cacao fruit. 

The Olmecs and Mayans before him domesticated the cacao, an understory tree from Ecuador.

There and in Mexico, it thrived – under very specific conditions: 

Steady warm temperatures. Abundant rainfall. Well drained volcanic soil, high in minerals.

When the cacao tree blooms, its flowers must be pollinated within 48 hours, by ants or small flies. As few as 20% of pollinated flowers produce fruit. 

And those fruits hold only a handful of seeds, which must be fermented to mellow their bitter taste before they’re fit for consumption.

Mexican civilizations mixed their cacao with water, chilis, and spices. European invaders added sugar and milk to make chocolate bars.

They took the cacao to their island colonies and to Africa, where growing conditions mimicked Mesoamerica.

Today, rising heat, changing rainfall, deforestation, and overproduction jeopardize the global cacao trade.

But choco-holics don’t despair. Scientists are working with cacao farmers to develop new hybrids and sustainable farming practices to keep this sweet treat with us for centuries to come. 

We often think of rivers as mostly straight or gently bending waterways. 

If they are, it’s because we’ve made them that way, guiding them with levees or dredging them for navigation.

When viewed from the air, natural rivers, especially across nearly flat landscapes, bend dramatically, sometimes folding back on themselves. 

Rivers meander, constantly changing their paths.

Rivers have several sources: lakes, springs, drainage plains, and melting snow or glaciers. 

When they originate in steep areas, gravity moves the water fast.

Here, rivers are mostly straight, as they have the power to cut their own v-shaped channels into the land.

As the slope becomes gentler, so does the river. It loses some of its cutting power and begins to bend around high points.

When the land flattens, the river begins to erode sideways, rather than down. 

That’s because water on the outside of a bend moves faster than on the inside, cutting into the bank.

Over time, that pushes the bend farther and farther out. Dozens of wide bends, one after another, can give the river a shape like bunched ribbon.

Over the last 50 years, scientists have turned to math, physics, and satellite photos to study the natural paths of rivers and better predict where and how their bends will form -- and when they might meander into farmland or development.

That’s often when we give the river a little human guidance to conform to our desired landscape.

In 1984, an engineer walked into a nuclear power plant under construction and set off radiation alarms. 

But there was no nuclear material yet on site. Where was the radiation coming from?

Testing revealed the man was carrying it, on his clothes. 

Where did he become so contaminated? Turns out, at home.

Scientists visited his house and found levels of radon gas 600 times higher than normal – the highest level ever measured inside a building.

They immediately moved the man and his family out. Then began testing ways to mitigate the gas.

Radon starts as uranium, which decays to become other elements and eventually radon -- the only element in that chain that occurs naturally as a gas.

It’s odorless, colorless…and common, in small quantities, in air, water, and soil -- especially above bedrock that contains trace amounts of uranium.

But there, it can leach into basements where it concentrates and becomes a health hazard. Radon is the leading cause of lung cancer in non-smokers – and is even more dangerous to smokers.

A quick Google search can tell you if your area is prone to indoor radon accumulation. 

If so, buy a do-it-yourself testing kit. If you find high levels, hire a professional. 

They may recommend sealing all cracks in basement floors and walls, or ventilating under the foundation.

With some simple precautions, you’ll be much safer.

Around the world, a small group of secluded libraries contain some of the rarest and most fragile objects on Earth. 

They’re staffed by scientists, equipped with redundant power supplies… and kept very, very cold.

That’s because the ‘books’ in these libraries are ice cores. 

They’re collected in some of Earth’s harshest environments, like Antarctica and Greenland, at great cost, then transported frozen to these libraries.

Each core is one of a kind. And once it’s been read – small samples melted to reveal its secrets – it can never be replaced.

But those secrets make all this worthwhile. Because the glaciers and ice sheets the cores came from have captured, over endless seasons, the history of our planet.

Layers of ash reveal the timing, and geological makeup, of ancient volcanic eruptions. 

Pollen samples reveal global plant populations, as they’ve changed over millennia. 

Black carbon deposits tell of forest fires. 

Trapped isotopes of oxygen and other gases paint a picture of past atmospheres.

The top layers, representing the most recent few thousand years, record humans’ influences on Earth: traces of agriculture, industry or war that carried on winds and settled on the ice.

These ice cores, and the libraries that preserve them, are simply irreplaceable, providing a nearly 3-million-year window into Earth’s past. 

Now that’s some cool science.

Humans have lost most of our body hair, but the beard has stubbornly remained – though anthropologists can find no function for it.

Perhaps that’s why men have been grooming it, or removing it, for thousands of years.

Early cave paintings show men with smooth faces. 

Archeologists have found flint ‘razors’ they may have used to shave. And shell ‘tweezers’ they may have used to pluck off their beards. Ouch!

The Egyptians were first to engage in elaborate shaving rituals. 

Facial hair was considered unclean, and men used copper razors or caustic chemicals to burn off their beards. Ouch again!

Centuries later, Alexander the Great’s soldiers wore beards -- until he ordered them gone. 

Greeks reversed that tradition. For them, beards represented maturity and wisdom. 

The Romans reversed things again, and civilized men shaved or went to barber shops.

Through the Middle Ages, beards came and went with the styles of influential leaders. 

Christian priests remained clean-shaven, to separate themselves from rabbis and Muslim clerics, who often had beards.

In the 20th century, in the U.S. and Europe, shaving became synonymous with uprightness and business success. 

In the 21st century, things changed again, as beards reflect individual style and even environmental sustainability – avoiding the waste of disposable shaving products.

If you’re wondering ‘to beard or not to beard,’ there’s no right answer. Wait a while and styles may change anyway!

Geothermal energy, the heat of the earth, can be found under our feet everywhere on Earth.

Historically, we’ve only been able to access it in a few places, commonly in hot springs. 

Much less commonly, in far fewer places, we find more intense heat, so close to the surface that it can be used to heat cities or generate electricity.

That intense heat could be developed anywhere… if we could go deep enough, cheaply enough. And that’s always been the challenge: 

How to reach it, how to bring it back to the surface… and do so economically. New technology pioneered by the oil industry may be able to help.

Over the last two decades, in pursuit of oil and gas trapped in shale, drillers have perfected longer and ever more precise vertical and horizontal wellbores, at economies of scale.

Simultaneously, they’ve perfected fracturing techniques.

These may finally allow very deep, precision wells into hot rock formations, where water could be circulated to bring up the heat. 

This could be done by fracturing the rock, or by running fluid through pipes.

Instead of water, some companies are testing supercritical CO2, which behaves like a fluid and could transfer heat more efficiently. 

Costs for traditional drilling will remain a challenge. So other companies are experimenting with a high-powered laser to blast a deep hole.

There are many obstacles yet to overcome. But the promise of widely available geothermal energy is inspiring innovative solutions. 

Volcanoes often form where tectonic plates collide or pull apart. But sometimes they’ll form in the middle of plates, due to geologic hotspots.

A hotspot is a plume of hot mantle that rises from within Earth, and heats, expands and deforms the crust above it, to create mountains, volcanoes or volcanic islands.

The classic example is the chain of Hawaiian Islands, which formed as the Pacific plate traveled across a hotspot.

Scientists always thought hotspots were stationary, while tectonic plates moved over them. But new research from beneath the Indian Ocean has challenged that idea.

There, an undersea mountain range runs north-south for 3000 miles, along the 90-degree East longitudinal line.

This long ridge was formed by a hotspot—but scientists studying the age and geologic makeup of each mountain on the chain realized they grew faster than could be possible by the plate alone moving.

The hotspot must also have moved, possibly by hundreds of miles.

This simple discovery will cause us to rethink hotspots and volcano formation, all over the world.

It may turn out that other well-studied hotspots, like those beneath Iceland and Yellowstone, are moving too. Which could help us better understand the geothermal and volcanic activity there.

And that’s how science works. It’s always on the move—sort of like hotspots.

You may remember that opposite electrical charges attract.

If a positive charge builds up, it seeks to connect to a negative charge, to reach equilibrium.

This is what creates lightning. As explained in a prior episode, it’s generated by the static electricity of water vapor and ice molecules rubbing against each other. Billions and billions of molecules can produce a whopping charge.

Positive and negative charges build within the cloud, but they’re separated by air, which is a poor conductor. But once the charges are large enough, they overcome that resistance and connect as lightning—within the cloud or to Earth.

Sometimes, though, a powerful charge builds in the top of the cloud, and rather than striking down, it seeks equilibrium by going … up!

The charge leaves the top of the cloud and explodes into the atmosphere in what’s called a Transient Luminous Event, or TLE.

There are several types, called Sprites, Blue Jets and Elves. That’s ELVES—Expanding Light emission from Very Energetic Sources. They take the shapes of columns, glowing rings or giant ‘jellyfish.’ But they last just a fraction of a second.

Eyewitnesses saw TLEs 300 years ago. But it wasn’t until the advent of super high-speed video that scientists could capture their remarkable images to study them.

Another reminder that there are still many things to discover, right here on—or just above—Earth.