After a morning rain or a dewy night you’ve probably noticed blades of grass bejeweled with silvery water droplets. You’re more likely to spot them if you procrastinate on mowing the lawn — longer blades lean over and hold water beads like peas in a pod. Exactly this sight caught my attention earlier this week, so I zoomed in for a closer look with the camera.
The larger drops were oval-shaped, deformed either by the pull of Earth’s gravity or coaxed to spread by the texture of the grass. But the tiniest droplets formed nearly perfect spheres, each a tiny lens that brought into focused a fisheye view of the background scene within its curved confines. You might wonder why tiny water drops mold themselves into spheres and vaguely recall something about surface tension from high school chemistry. You are correct!
Each water molecule is made of one oxygen atom and two hydrogen atoms; joined together they make good old H2O. Surface tension is caused by molecules that attract one another or cohere. Water molecules enjoy each other’s close company because at the atomic level the hydrogens feel an attraction to oxygen and vice versa. And that’s because hydrogen in a water molecule has a slightly negative charge, while the oxygen is positively charged. Opposite charges attract. This also seems to happen in human relationships but on a decidedly different level.
Water has a high surface tension, the most of any liquid other than mercury. You may have seen how water striders exploit this property, darting about on the surface of a pond without ever breaking through and getting soaked.
Surface tension is especially strong on the surface of a water droplet (or water surface of a pond) because the molecules on the outside have no neighboring atoms above (or outside the droplet), so all their attractive force goes to tugging on their nearest neighbors, forming a sort of web of interlocking molecules.
The attraction is strong enough for water to literally pull itself into a tiny ball or drop. That’s what you see when you stare into a wet lawn. Isn’t it fascinating to witness hidden atomic forces revealed at the macro level? Almost spine-tingly if you know what I mean.
A sphere is one nature’s preferred shapes. Think of the stars, planets and even a few asteroids. They’re spheres, too. The Earth, sun and moon appear so perfectly round, at least within our perception, that it’s almost miraculous. In truth, it’s just nature taking the easiest way out, one of its favorite tactics.
Surface tension can makes spheres but only on a tiny scale before gravity and other effects deform them. Big spheres like planets form differently — through self-gravity. Bodies like moons and planets are spherical because their own gravity literally crushes them into spheres. Smaller moons and most asteroids and comets are irregular in shape because their gravity is too weak to fully mold them.
Not every planet or moon is made of rock. Bodies made of ice, which is more easily crushable than solid rock, can earn their sphericity certificate (say that 10 times fast) when they reach about 250 miles (400 km) across. That’s the size of Saturn’s moon Mimas which has an icy interior. The minimum for a rocky moon is about 375 miles (600 km).
There are other ways irregular-shaped bodies can get rounded out in other ways such as interaction with other bodies. But get enough matter together and it will eventually ball up as all the atoms in it pull toward a common center of gravity. So maybe you’re wondering why Jupiter, the biggest, most massive planet in the solar system isn’t spherical. If you’ve ever seen it in a telescope it looks oval, like someone flattened the top and bottom of the planet.
Jupiter’s not a solid rocky body like the Earth but made of a lot of gas, plus it spins really fast: once every 9.9 hours. This causes the planet to bulge outward along its equator, making it look out of round.
Spheres. We love ’em. They’re so much fun to pick up and throw, drink as dew and raindrops and marvel at through our telescopes.