Jim Baen's Universe

Quarks to Quasars:The Science of Science Fiction

Columns

Meteoroids, Meteors and Meteorites

Written by Ben Bova

The headline screamed: “OMG! We Come From Space!”

Right under it was an image of a really hideous alien from a sci-fi flick.

The news story accompanying the headline and picture was much more subdued. It told of a report by researchers from Imperial College, in the United Kingdom, that they had found the chemical uracil in a meteorite.

Uracil is one of the key chemicals in the RNA and DNA molecules. It’s a fundamental part of every living cell on Earth.

Did it originate in outer space and ride to Earth inside a meteoroid? Maybe.

How did life on Earth begin? Was it somehow cooked up out of non-living chemicals here on Earth, or were the basic chemical ingredients for life carried to our planet by asteroids and comets that crashed here? If Earth was seeded from outer space with the chemicals of life, where in blazes did those chemicals come from?

As Yul Brynner said in The King and I, “Is a puzzlement.”

Let’s begin at the beginning.

According to current knowledge, the universe began roughly 14 billion years ago in a spectacular explosion that’s been dubbed the Big Bang. All the matter and energy in the universe came into existence in one titanic blast. “Let there be light!”

Where did all that stuff come from? That’s a question for cosmologists to ponder, and maybe the subject of another column.

For now, looking back some 14 billion years, we have a newly-hatched universe that consists mostly of hydrogen atoms, with a little helium and a trace of lithium. The three lightest chemical elements. Ferocious as it was, the Big Bang couldn’t create elements heavier than lithium.

But stars could. The earliest stars coalesced out of clouds of those light elements and began cooking heavier elements in their cores. Stars are nuclear fusion reactors. They fuse the nuclei of hydrogen atoms to produce helium nuclei. And energy. We call it starlight. Closer to home, we call it sunshine.

As stars age and run low on hydrogen, they begin to fuse helium into carbon, oxygen and neon. They keep producing constantly heavier elements until they finally run out of energy resources and collapse.

Very massive stars blow up in cataclysmic supernova explosions. Our middling-sized Sun won’t explode: it will just puff away its outer layers and quietly collapse into a white dwarf state. Of course, even such a mild end will boil away Earth’s atmosphere and oceans. But that won’t happen for a few billion years, so don’t panic. Yet.

What’s happening is that the stars are blowing into space those heavy elements that they’ve cooked up in their interiors. Those heavier atoms become the building blocks for new stars.

The universe is a big recycling project. Most of the atoms in your body were created inside stars. We are stardust, quite literally.

So now it’s about five billion years ago and our solar system is coalescing out of a cloud of interstellar gas and dust that contains elements such as carbon, oxygen, nitrogen, potassium, calcium, etc. At the center of the cloud a star is born, our Sun. In the maelstrom of gas and dust swirling around the newborn star, planets are forming.

Not just planets. Moons, comets and asteroids are being built too. Comets are basically chunks of ice, with a smattering of sooty dust thrown in. “Dirty snowballs,” they’ve been called. Actually they’re more like dirty icebergs.

Asteroids are chunks of rock that didn’t get sucked into a planet or moon. They’re like the crumbs left over after a banquet.

Today, the planets move in widely-spaced orbits, calm and sedate. It’s a news event when an asteroid crashes into Earth, although you can see plenty of asteroids flash through the sky on any clear night. Meteors are often called “falling stars.” Actually, they’re falling asteroids, most of them no bigger than a dust mote.

In the early days, however, the solar system was more like a shooting gallery. There were chunks of rock almost the size of planets whizzing around. Earth was hit again and again by gigantic asteroids and comets.

While billions of years of rain and weathering have erased most of the impact craters on Earth, you can see the evidence of this early meteoric bombardment on the battered face of the Moon. In fact, the Moon itself was created when a planetesimal the size of Mars smashed into Earth in a grazing blow that tore off a layer of rock that eventually coalesced to form our Moon.

This pounding from asteroids and comets may have seeded Earth with water and chemicals (such as uracil) that led to the development of life. But the pounding could also have obliterated the very earliest living things even as they were forming out of those chemicals. No telling how many times life arose in some spot on Earth, only to be smashed out of existence by a meteoric impact.

Nature is persistent, but lazy. Our solar system formed out of the elements that happened to be available in interstellar space: the debris of aged stars that spewed out their atoms in their death throes. The simple forces of gravity and chemistry produced our planet Earth, together with the rest of the solar system. Life on Earth was likewise produced by those forces, working with the materials that were available. The processes that led to life on Earth used the most abundant ingredients on hand and the easiest chemical pathways for putting those ingredients together.

The chemicals precursors of life exist in deep space. Radio astronomers have detected water, ammonia, and hundreds of organic chemicals in interstellar clouds, including formaldehyde, hydrocyanic acid, and the more complex polycyclic aromatic hydrocarbons (PAHs). Those chemical were undoubtedly present in the interstellar cloud that gave birth to our solar system.

We are talking now of prebiotic chemistry, chemical reactions that can build up organic molecules that become the building blocks for life.

Comets are particularly interesting to researchers studying prebiotic chemistry. Laboratory simulations have shown that prebiotic chemistry can take place in comets’ ice.

How can chemistry take place in ice? If you sprinkled ice cubes with carbon, nitrogen, etc. and kept them in a freezer, no chemical reactions would occur. The ice is a solid, crystalline lattice; its atoms cannot move around and recombine into more complex compounds.

But the ice in comets has a different structure. (Shades of Kurt Vonnegut and the “ice-nine” of Cat’s Cradle.) The ice in comets is amorphous ice, a form that’s been known to chemists since 1935. Amorphous ice is not a crystalline lattice; its internal structure is more like that of glass, which is structurally a fluid that happens to remain solid at normal room temperature.

When amorphous ice is exposed to ultraviolet radiation (as a comet is in space) it can flow like a liquid even though it’s only a few degrees above absolute zero. Atoms can begin to mix and mingle in amorphous ice. The chemistry of life can begin in comets.

Experiments with amorphous ice in laboratory chambers have produced ethers, alcohols, PAHs and even amino acids – the building blocks of proteins. And astronomers have seen organic chemicals in the tails of passing comets. Comets could bear the seeds of life; they could have brought them to Earth, together with abundant water.

As the recent discovery of uracil in a meteorite shows, asteroids can bring organic material to Earth, too. Meteorites are asteroids that have fallen to Earth. Since the 1960s it’s been recognized that some meteorites – particularly the carbon-rich chondrites – contain water and organic chemical in them. The water isn’t liquid: it’s locked chemically to the rock. But it’s water!

The Swedish Nobel laureate Svante Arrhenius proposed in 1908 that life originated elsewhere in the universe and was wafted as spores through the vast interstellar distances to arrive eventually here on Earth. This panspermia hypothesis envisions life pervading the universe, but it does not tell us how life began.

The discoveries of organic chemicals in meteorites and the studies of prebiotic chemistry in amorphous ice show that the first steps toward living organisms may indeed have happened in space and been brought to Earth by infalling comets and asteroids.

It seems likely that, yes, we do indeed come from outer space. Or at least some of the chemicals that served as the starting point for the eventual birth of living organisms on Earth were originally created in space and carried here on comets and asteroids.

While most of those chemicals were destroyed in the fiery impact when those asteroids and comets smacked into our planet, here and there, now and then, some survived. And became the starting point for the development of life on our world. Prebiotic chemistry built up constantly more complex organic molecules until, at some point, molecules that could reproduce themselves out of simpler chemicals arose. Life began.

Life on Earth consists of long-chain carbon-based molecules that can make more of themselves out of simpler chemicals. Once such molecules arose, biology began. Darwinian evolution took over and those earliest molecules developed, over eons of time, into bacteria and amebas and sponges and fish, flowers, trees, whales, you and me.

It didn’t take a miracle to produce life: just the right chemicals in the right conditions. Life needs a building-block atom, a medium in which chemical reactions can take place, and an input of energy. For us, living on the surface of a rocky but water-rich planet, the building block atom is carbon, the mixing medium is water, and the energy is sunlight.

There are other possibilities. Right here on Earth there are microscopic extremophiles that don’t need sunlight; living deep underground, their energy source is the planet’s internal heat.

We know that Mars was once warm enough to have had large bodies of water on its surface. Life probably developed on Mars. We may find traces of extinct Martian organisms in those frozen red sands. Or perhaps even living organisms that exist underground. There are oceans of water beneath the ice mantles of at least three of Jupiter’s major moons. Life could be found there. And what of giant Jupiter itself? There’s undoubtedly an ocean beneath that planet’s swirling cloud deck, an ocean ten times wider than Earth. What creatures might live in those waters?

From all that we know today, life is a normal part of the universe, most likely as commonplace as stars and rocks. Future explorations of the solar system will bear this out, I am convinced.

But what about intelligent life? Is intelligence a normal outcome of life’s development? Or is it a rare, perhaps even strange, adaptation that we shouldn’t expect to find on other worlds?

We’ll look into that next time.