Why is Jupiter so bright?


Last week, I wrote that Jupiter is the third-brightest object in the nighttime sky after the moon and Venus. However, such matters are often more complex than simple pronouncements would suggest.

Jupiter’s brilliance is something of a puzzle. The planet is 390 million miles away right now.

Mars, visible as a dim, red point in the east just before morning twilight, is 232 million miles away, about half Jupiter’s distance, yet Jupiter shines almost 48 times brighter. What gives?

Part of it is sheer size. Mars is a scant 4,200 miles wide. Jupiter has a diameter of almost 89,000 miles. Jupiter may be farther away, but it’s also a much bigger mirror to reflect the light from the sun back into space.

Distance is a second factor. The inverse square law partially determines the brightness of an object. Here’s how an astronomer might put it: The intensity of the radiation, whether reflected or emitted, is inversely proportional to the square of the distance.

In other words, put an object like Jupiter at a single unit away (the length of the unit doesn’t matter), and it has a given brightness. Double that distance, and the object is four times fainter. Triple the distance, and it is nine times fainter.

That reduction in brightness adds up significantly as distance increases. At 100 times farther away, the object is 10,000 times fainter. At 1,000 times farther away, the object is a whopping one million times dimmer.

Another factor is how shiny the mirror is in the first place. At Jupiter’s distance, the planet would be exceedingly faint if it weren’t bright to start with.

Astronomers call a planet’s relative ability to reflect light its albedo, the Latin term for “whiteness.”

Albedo can range from a value of zero (no reflection at all) to a value of 1 (100% reflection). In practice, no object in the universe absorbs all the light that hits it, i.e., has an albedo of zero, not even a black hole. However, some surfaces come very close.

On Earth, forests, oceans, cities, and deserts all have different albedos. Forests have albedos of between 0.08 and 0.15; deserts have an albedo of about 0.30. Because snow and ice reflect a significant portion of the sun’s light back into space, their albedos sit between 0.6 and 0.9.

That’s a pretty wide variation, of course. Much depends on the smoothness of the surface. Rough and uneven surfaces reflect less light into space. Furthermore, the angle at which the sunlight hits the surface affects albedo.

Technically, water can have the highest albedo (almost 1.0) if it is A) perfectly still (no waves at all or a flat sheet of ice) and B) the sunlight strikes it at a shallow angle. It is exceedingly rare for both conditions to happen simultaneously on Earth.

However, Saturn’s icy moon Enceladus has an albedo of .99.

Our planet gives a good accounting of itself in the albedo department. Its giant oceans and clouds of water vapor contribute to its relatively high albedo of .39, 39% reflectivity.

Earth’s moon looks bright because of its proximity to us, only about a quarter of a million miles from Earth. However, nary a drop of liquid water or vapor clouds adds to its reflectivity. Thus, its albedo is a pathetic .16, about the same as an asphalt road on Earth.

By contrast, Jupiter is extraordinarily reflective because it is covered with clouds.

Suspended crystals of methane and ammonia reflect the sun’s light very efficiently, giving Jupiter has a respectable albedo of .51. Just over half of the light it receives from the sun bounces back into space.

Even though Jupiter is, on average, 474 million miles from the sun, a hefty chunk of the planet’s light gets beamed back in our direction. But Jupiter is a dim bulb compared to glorious Venus.

Venus, which is low in the southeast during morning twilight, is tiny, only the size of Earth at about 8,000 miles wide. It shines even brighter than Jupiter partly because of its proximity but mostly because its coating of white sulfuric-acid clouds is highly reflective.

Its albedo is a stunningly beautiful .76. Yes, albedo can be beautiful. Go out and look if you don’t believe me.

Tom Burns is the former director of the Perkins Observatory in Delaware.

No posts to display