Jupiter’s time to shine

By Tom Burns - Stargazing

As I said last week, Jupiter, Mars, and Saturn are now easily visible in our pre-dawn, twilight sky. Look to the southeast for bright Jupiter. Down and to the left are Saturn and Mars.

Mars is never much to look at in telescopes or binoculars. If you want a view of Saturn’s famous rings, I’d wait a month or so for Saturn to rise a bit higher in the sky to avoid the curse of atmospheric turbulence.

In other words, it’s Jupiter time, fellow and sister stargazers. Whatever optical aid you can muster, now is the time to go out and check out the solar system’s biggest planet. Look in the hour before sunrise to the southeast for the brightest thing you’ll see over there.

Jupiter is the planet that most optical instruments were born to observe. Binoculars show its four brightest moons lined up around the planet. Even the simplest telescope reveals its weird cloud bands lined up around it like horizontal zebra stripes.

Jupiter’s stunningly visibility rises partly from its enormity. At 89,000 mileswide, Jupiter could comfortably contain almost 1,500 Earths. Even at half a billion miles away, any surface detail is a relatively easy target. The real limit is closer to the planet you live on. During some nights, Earth’s atmosphere is turbulent, which reduces the surface of Jupiter into a bubbling, boiling mess.

Oddly, that “surface” is no surface at all. Jupiter is essentially a ball of liquid hydrogen and helium covered with clouds divided into alternating bright “zones” and darker brown and yellow “belts.”

Larger telescopes show details within the clouds — hook-shaped festoons and light and dark ovals that are giant storm systems the size of our planet. You also might see the pale orange-red oval in the planet’s southern hemisphere. That so-called Great “Red” Spot is the biggest storm of all at about 2.5 times the diameter of Earth.

The color of the Red Spot changes slowly over time. Over the years, I’ve seen it fluctuate from white, to pink, to vivid orange-red. The exact causes of the changes are still unknown, but the reason is probably less astronomical and more meteorological. As the Red Spot churns around like a gigantic typhoon, it churns up varying amounts of more colorful elements hidden below the surface of the atmosphere.

Also, notice that the planet is not truly spherical. It bulges out distinctly along its equator in the direction of the cloud bands.

Much of what you see is a direct result of the planet’s rapid rotation. Jupiter may be the largest planet, but it has the shortest day, which, oddly, makes it the best planet to observe with a telescope.

Tiny Earth rotates once every 24 hours. Jupiter spins once on its axis once every 10 hours. At 40 degrees latitude, we central Ohioans are rotating at about 680 miles per hour.

Jupiter is 12 times the diameter of Earth. At 40 degrees latitude, its outer region is zipping around at 27,500 miles per hour.

Spin a giant, liquid planet around that fast and it will bulge out at the sides and its cloud bands will be stretched out all the way around. At its equator, Jupiter takes an average of 9 hours, 50 minutes to rotate once. Near Jupiter’s poles, it takes 9 hours, 55 minutes. The planet’s weird “differential” rotation is one of the factors that stretch its clouds into zebra-stripes extending all the way around the planet.

Add rising and falling heat, and you have yourself a recipe for atmospheric chaos. Meteorologists have a difficult time predicting the weather on Earth because of the large number of variables involved. However, Earth is a calm and simple place compared to turbulent Jupiter.

A few thousand miles below the cloud tops, Jupiter’s hydrogen is so cold and so pressurized by Jupiter’s intense gravity that the planet becomes a giant, unending sea of liquid hydrogen. Below that, the hydrogen acts much like a metal, although it still remains liquid because no matter how cold or dense it gets, hydrogen can never become a solid.

Because it is in metallic form, hydrogen can do what most metals do. One quality of a metal is that it can conduct electricity. All that metallic hydrogen spinning and sloshing around gives Jupiter the mother of all planetary magnetic fields.

Orbiting the whole swirling, seething mass is a thin, nearly invisible ring system and at least 69 moons, the four largest of which, called the Galilean moons, are visible in binoculars or a small telescope.

Much has been made of the fact that a spacecraft could never land on Jupiter because it has no solid surface.

But that’s not even the half of it. In our mind’s eye, let’s dive down into Jupiter’s atmosphere.

At first, we would marvel at how clear the atmosphere is. It is 90% hydrogen and 10% helium. Mere traces of ammonia, methane, and sulfur do not block the view.

It wouldn’t be long before our spacecraft would be crushed like a soft drink can by the planet’s enormous atmospheric pressure, even in the upper regions of its atmosphere.

But let’s imagine that we had somehow built a spacecraft that could handle the burden. We would notice that the atmosphere got denser and denser as we descended, like a dense fog on bone-crushing steroids.

As we got further down, the gas would slowly change into a liquid. We would note that Jupiter doesn’t even have a liquid surface. The change from a gas to a solid would be subtle and imperceptible. The atmosphere would get denser and denser until you were immersed in liquid. You’d never notice the transition.

To observe Jupiter at its best, you should wait for a month or so to allow Jupiter to rise higher in the morning sky. Personally, my self-imposed isolation causes me to get up at 5 a.m. to take a look.

If you feel the same way, start with binoculars. You won’t see the planet as anything other than a bright point of light, but at least two or three of its moons will be lined up near the planet.

In a small telescope, look for the cloud bands, at least one of which will extend around the planet.

The larger your telescope, the more you’ll see. Keep looking. Because of the planet’s rapid rotation, hour-by-hour and day-by-day, new surface features are constantly rotating into view or swirling into existence on this ever-changing planet.


By Tom Burns


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

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