Free printable SSAT practice test for Grade 10

Far below the reach of sunlight, in water only a few degrees above freezing, it is always snowing. The flakes are not ice. They are clumps of dead and dying matter: the remains of microscopic algae, the shed shells of tiny animals, waste pellets and grains of dust blown off distant deserts, all bound together by a sticky film of mucus that many sea creatures produce. Ocean scientists call this steady drift "marine snow," and although a single flake is hardly worth noticing, the snowfall as a whole is one of the planet's most important means of moving carbon.

The process begins at the sunlit surface. Phytoplankton, single-celled organisms that float near the top of the sea, absorb carbon dioxide and use sunlight to turn it into living tissue, much as land plants do. Phytoplankton multiply quickly and die quickly, and they are eaten by small grazing animals that in turn release compact waste pellets. Each of these fragments contains carbon that was, not long before, dissolved in the water and in contact with the air above it.

Most of that material never travels far. Bacteria break much of it down long before it leaves the upper ocean, releasing the carbon back into the water, where it can return to the atmosphere. But some particles collide, stick together and grow heavy enough to sink faster than they can be consumed. A clump may spend weeks falling, and along the way it serves as a moving meal. Animals of the deep, from small crustaceans to the vampire squid, which gathers drifting particles with long, sticky filaments, depend on this rain from above in a place where almost nothing grows.

Only a small fraction of the original carbon reaches the deep ocean or the seafloor. That fraction matters enormously, however. Carbon carried into the deep can remain isolated from the atmosphere for centuries, and some of it is buried in sediment for far longer. Scientists refer to this whole chain, from surface growth to deep storage, as the "biological pump." Without it, the amount of carbon dioxide in the air would be considerably higher than it is.

Researchers study marine snow with sediment traps, which are containers anchored at various depths to catch what falls, and increasingly with cameras and robotic floats that can count particles as they pass. Their measurements remain uncertain, partly because the ocean is vast and the snowfall varies with season and place. A spring bloom of phytoplankton can send down a heavy burst; a nutrient-poor stretch of open ocean may produce only a thin drizzle.

What the research makes clear is that the deep sea, often imagined as remote and unchanging, is closely tied to the surface world. The carbon in a flake drifting past a vampire squid may have been in the air above the waves only a season earlier.

On a clear night, the stars seem to hang in a single moment, as if the sky were a photograph taken just now. In fact, it is something stranger: a collage assembled from many different pasts. Every point of light you see left its source long ago, and each has been traveling for a different length of time.

The reason is that light, though astonishingly fast, is not instantaneous. In empty space it covers about 186,000 miles every second. That speed is so great that for much of history people assumed light arrived the instant it was given off. One of the first serious challenges to that idea came in the 1670s, when the Danish astronomer Ole Romer noticed that the eclipses of Jupiter's moon Io came earlier or later than predicted, depending on whether Earth was moving toward Jupiter or away from it. The most reasonable explanation was that light needed extra time to cross the extra distance. Light, in other words, had a finite speed, and that speed could be measured.

Once that was established, distance and time became two ways of describing the same thing. Sunlight takes a little more than eight minutes to reach Earth, so we always see the Sun as it was about eight minutes earlier. For greater distances, astronomers use the light-year, a unit that sounds like a measure of time but is actually a measure of length: the distance light travels in one year, nearly six trillion miles.

Finding out how far away a particular star is proved far harder. The method that finally worked is called parallax. Hold a finger at arm's length and look at it first with one eye and then with the other; the finger appears to jump against the background. Astronomers do the same thing on an enormous scale, observing a nearby star from opposite sides of Earth's orbit, six months apart, and measuring how much it seems to shift against far more distant stars. The shift is tiny, much smaller than the unaided eye can detect, and it was not measured successfully until 1838, when Friedrich Bessel reported the distance to a star called 61 Cygni.

The results were humbling. The nearest star beyond the Sun, Proxima Centauri, is a little more than four light-years away, so its light is over four years old when it arrives. The Andromeda Galaxy, often described as the most distant object visible without a telescope, is about 2.5 million light-years away. The faint smudge it makes on a dark autumn night is light that set out before modern humans existed.

This delay is not merely a curiosity; it is one of astronomy's most powerful tools. Because light from remote objects is old, telescopes that gather extremely faint light are, in effect, looking into the past. By studying galaxies at greater and greater distances, astronomers can observe the universe at earlier stages of its history, much as a geologist reads older rock in deeper layers. The night sky, it turns out, is less a picture than an archive.