NASA’s Parker solar probe blasted off from Earth August 12. If all goes as planned, it will go where no probe has gone before—so close to our host star that it enters the sun’s atmosphere. But first up for Parker: a close flyby of Venus Wednesday. It will use the planet’s gravity to slingshot itself closer to the sun.
The historic flyby is a fitting moment to reflect on how fortunate we are to orbit a star such as the sun, and from a planet with an atmosphere like Earth’s, and not like that of Venus. It’s thanks to this double good fortune that advanced creatures like ourselves can exist here, and do astronomy.
Even as late the 1950s scientists imagined that intelligent life might exist on Venus. Not anymore. We now know that the planet’s surface temperature is hot enough to melt lead—over 800 degrees Fahrenheit—due in part to the planet’s thick, CO2 atmosphere.
So Venus is deadly to life. But even if intelligent life had somehow emerged and thrived there, what then? Carl Sagan once asked readers to imagine such an alien race. Then he posed a question:
“Would it then invent science? The development of science on Earth was spurred fundamentally by observations of the regularities of the stars and planets. But Venus is completely cloud-covered … nothing of the astronomical universe would be visible if you looked up into the night sky of Venus. Even the Sun would be invisible in the daytime; its light would be scattered and diffused over the whole sky—just as scuba divers see only a uniform enveloping radiance beneath the sea.”
Light and Air
It is remarkable enough that the sun and most stars beam out radiation mainly in the tiny Goldilocks region of the electro-magnetic spectrum. The two types of electromagnetic radiation useful to life—visible light and infrared—occupy two exceedingly tiny regions in the immensity of the electromagnetic spectrum, and these just happen to be the same regions in which the sun and indeed the vast majority of stars emit nearly all of their radiation. To grasp how small this life-giving slice of the spectrum is, imagine just a few playing cards in a stack of cards stretching from here to beyond the Andromeda Galaxy. Andromeda is more than 25 million light years away! But our good fortune stretches further still.
Before this life-giving slice of the spectrum can enable life on a planet’s surface, at least two further preconditions must be satisfied. One, the atmosphere must allow the sun’s life-giving light to penetrate right down to the ground to permit life-essential photosynthesis. And two, a portion of the sun’s infrared radiation must be absorbed by and held in the atmosphere. This heat radiation warms the Earth above the freezing point of water and animates the atoms of life for chemistry.
Happily, our atmosphere—unique in our solar system—obliges us in this critical double task.
So Earth’s atmosphere is just right for photosynthesis and just right to warm it into the ambient temperature range, enabling “light eating” aerobes like ourselves to thrive on the planet’s surface.
Keeping Out the Riff Raff
And it’s not just that our atmosphere lets through the right light. It also strongly absorbs radiation from the dangerous regions of the electromagnetic spectrum on either side of the visual and near infrared regions.
The only other electromagnetic radiation not absorbed is in the radio region and the far microwave region. Very little strong or ionizing radiation in the UV, X-ray, and gamma ray regions—that is, radiation less than 0.3 microns—penetrates to the Earth’s surface. Further, our atmosphere absorbs radiant energy of wavelengths longer than fifteen microns (in the far infrared and near microwave regions). Very little of it reaches the ground. This is also almost certainly protective because microwave radiation has many reported damaging effects on living systems even at very low radiation fluxes.
What if our atmosphere absorbed a slightly different region of the electro-magnetic spectrum? For example, imagine it shifted ever so slightly to the right from its current position, so that the atmosphere absorbed all the visual light and all the infrared and let through instead the adjacent far ultraviolet. Then not only would photosynthesis be impossible, but the world would have suffered a runaway greenhouse effect. It would be a hot hell-house like Venus because our air would absorb all the sun’s infrared radiance.
In such a scenario, no carbon-based life could survive on the Earth’s surface, and certainly no air-breathing aerobes like ourselves!
Conversely, if we imagine it shifted to the left, all the light and infrared would have been absorbed by the atmosphere, again causing a runaway greenhouse effect.
Miracles of Fortuity
That the slice of the electromagnetic radiation emitted by the sun and the slice allowed through the atmosphere should both be largely restricted to the same tiny useful regions is an extraordinary example of a special fitness in nature for our type of aerobic life on a planetary surface. The fit is truly stunning.
Notice too that our atmosphere not only allows for life. It also allows us to see and study the stars, a privilege crucial to the birth of science.
The existence of technological creatures like ourselves depends on these and a host of other coincidences in nature’s order, ones I explore in my ongoing Privileged Species series. Altogether these coincidences convey an overwhelming impression of design. How else can we describe these coincidences except as miracles of fortuity?
Michael Denton is a Senior Fellow with Discovery Institute’s Center for Science and Culture and holds a PhD from King’s Collee in London. He is the author of the new book Children of Light: The Astonishing Properties of Sunlight that Make Us Possible.