Welcome to Stuff to Blow Your Mind, a production of I Heart Radio.

Hi, my name is Joe McCormick and this is The Artifact, a short form series from Stuff to Blow Your Mind,

focusing on particular objects, ideas, and moments in time.

One of the most powerful images from the history of space exploration is notable not because it captures the immensity

of the great and far, but because it shows us that in the words of Carl Sagan, on the scale of worlds,

to say nothing of stars or galaxies, humans are inconsequential, a thin film of life on an obscure

and solitary lump of rock and metal.

This image is the famous Pale Blue Dot, a photograph of Earth, taken by the Voyager 1 space probe

on the 14th of February 1990, when Voyager was about 6.4 billion kilometers away, beyond the orbits of Neptune and Pluto,

rushing ever farther at more than 60,000 kilometers per hour.

In a maneuver suggested by Carl Sagan and executed by a team including Candy Hansen of NASA JPL and Carolyn Porco of the University of Arizona,

the Voyager probe turned its camera back toward the solar system that it was departing to capture final images of the local planets, including Earth.

That image of Earth, the Pale Blue Dot, is a humbling peak at our world from the vantage point of the universe beyond.

To call it a dot is almost generous, it's more like a waning glimmer of a dot, a speck that barely stands out from the noise

and could fall invisible under a passing shadow.

Many authors in the years since have written of the way this photograph highlights the fragility and cosmic insignificance of our species.

Every human who has ever lived and all the things that they fought for are contained within that dot, has Sagan writes, on a mode of dust suspended in a sunbeam.

But today I wanted to focus on one sometimes overlooked aspect of the dot.

Why is it Pale Blue? Sagan himself had an answer for this, the color comes in part from the oceans and in part from the sky.

When you look at water in a drinking glass it doesn't usually appear to have any color at all, but this is only because of the tiny volume of the glass, the deeper the water the bluer it gets.

According to Sagan, by the time you have a column of water at least 10 meters in depth, almost all of the red light that strikes it is absorbed, while blue frequencies of light are scattered and reflected for us to see.

Hence the reason, or at least one of the major reasons, that the oceans are blue.

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