Al Stahler : Three, two, one
A week from today – if schedule holds – a rocket will lift off from the east coast.
A hundred miles above Earth – traveling some thirteen thousand miles an hour – the rocket’s engine will cut out.
Thirteen thousand miles an hour is not fast enough to keep something in space. The rocket will fall back, into the Atlantic. But not before releasing a second, smaller rocket from its nose. Igniting its engine, the second-stage rocket will carry its passenger another fifty miles upward; and then, a hundred-fifty miles above the Earth, its engine, too, will cut out. The rocket will then be traveling over seventeen thousand miles an hour – fast enough to remain in orbit … but only briefly.
The second stage will re-ignite its engine, and continue firing, until its velocity reaches well over twenty-five thousand miles an hour.
At twenty-five thousand-plus miles an hour, the rocket will not – cannot – fall back to Earth. Nor can it remain in orbit. At twenty-five thousand-plus, the rocket and its passenger will escape from Earth forever.
The rocket engine will cut out, and the second stage will release its passenger: A robot geologist, which will cruise, in its shell, for half-a-year, to planet Mars.
Travel back, four-and-a-half billion years, to a time when there is no Earth, no Mars … not even a sun. There is, rather, a humongous cloud of gas and dust, spinning slowly as it drifts through our galaxy. The ball of dust and gas now-and-then approaches a star … then drifts off. But just once, the ball of gas-and-dust approaches a supergiant star, just as the star ends its life – the star explodes.
Like a hammer-blow, the explosion sends a shockwave through the ball of gas and dust. Motes of dust and gas smash together, and stick, forming larger motes … which themselves collide, creating larger clumps yet. Some clumps grow large enough to draw others to them with gravity.
The center of the cloud grows dense; its gravity grows strong. The center draws the surrounding gas and dust inward; the cloud collapses.
Caught out without gloves on a cold winter’s day, we rub our hands to keep them warm. Within the cloud, clumps of matter slam together, and likewise grew warm. The temperature at the center of the cloud shoots up, to millions of degrees, hot enough to ignite nuclear fusion. Atomic nuclei glue themselves together, releasing enormous energy.
The center of the cloud becomes the core of a star: Our sun is born.
Not all the dust and gas falls into the new-born sun. Around the young star, dust and gas coalesce into smaller bodies: Icy comets … rocky asteroids … planets. Among the planets: Earth … and Mars.
The planets are warmed by collisions, and by the intense radioactivity of the early solar system. The planets grow so hot, they melt.
Melting allows the young Earth to sort itself out. Heavy stuff, like iron, sinks down toward the center of the planet. Lighter stuff, like rock (compared to iron, rock is light) floats upward.
The molten Earth cools … but heat can escape only slowly from its core. Heat escapes so slowly, much of the iron in Earth’s core is still hot … and liquid.
The outer parts of the Earth cool quickly. Rock on the surface solidifies. Water vapor – steam – condenses to form cloud droplets. Droplets merge into raindrops. Rain falls.
Rainwater collects; flows downhill in rivers; rivers fill the first oceans.
The night sky is filled with stars, some older than our sun, some younger. Comparing the stars, we see that, as stars grow older, they grow hotter. Young stars are born cool.
Orbiting a cooler sun, the young Earth should have cooled to the point where its rivers and oceans froze solid. And yet, studying atoms in the oldest minerals found on our planet – sand-size crystals of the gemstone zircon – we find evidence that, early on, Earth was warm enough for rivers to flow, oceans to slosh.
Measuring the sun’s energy output today – and given our distance from the sun – we can calculate what temperature we should expect to find on Earth today. Earth’s average surface temperature, today, should be zero degrees Fahrenheit – well below freezing. Our rivers and oceans should be frozen solid.
But Earth’s average temperature, today, is well above zero – life on Earth is possible – because our atmosphere is rich in greenhouse gases – carbon dioxide, methane, ammonia, water vapor – gases which absorb heat coming off the surface of Earth, then send it back out. Heat sent back down to the surface raises Earth’s average temperature to a much cozier 59 degrees F.
How did the early Earth keep its surface warm enough for rivers to flow, oceans to slosh? It’s tempting to suggest a mix of greenhouse gases … but no one, so far, has been able to put together a realistic mix of gases that could do the job. We are left with a mystery: Evidence for liquid water on a planet whose sun should not have been strong enough to keep the water from freezing. This is the faint young sun (FYS) paradox.
Mars has what look very much like ancient – very ancient – river valleys. But, farther from the sun than Earth, Mars, billions of years ago, should have been way colder – way more frozen – than Earth … yet another FYS paradox.
Our robot geologist – aka “Perseverance” – will look for atoms in Mars rocks, which – like atoms in zircons – record the planet’s ancient history. Christina Viviano (Johns Hopkins Applied Physics Laboratory) hopes such atoms can tell us “… to what extent liquid water was actually present and stable at the surface” of Mars.
As Earth and Mars circle the sun, they are sometimes close to one another, sometimes quite distant. The next close encounter is fast approaching, giving us two weeks to get our robot geologist off the planet, and heading to Mars.
Al Stahler enjoys sharing science and nature with friends and neighbors on KVMR-FM, and can be reached at firstname.lastname@example.org.
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