Late summer in Mars’ south polar region is a lot colder than ours on Earth. During winter, carbon dioxide freezes out and precipitates across the polar region. When summer arrives, much of that ‘dry ice’ vaporizes or sublimates into gas form and returns to the atmosphere. In the planet’s south polar region, it stays cold enough even during the summer for some of the dry ice to stick around. In this stunning image from Mars, we see a portion of the permanent southern polar cap. Flat-floored brown pits are filled with an estimated 10 feet of dry ice. Wouldn’t you just love to strap on some boots and go for a ski?
I watched a movie last night and so didn’t step out to walk the dog until after 11. Not only was I greeted by a warm, fragrant breeze, but Jupiter stared me straight in the face. You may have experienced a recent Jupiter encounter and possibly wondered what that hugely bright object was in the eastern sky.
The biggest of the eight planets is the brightest object outside the moon visible in the night sky right now. It rises around 9:30 p.m. but doesn’t clear the local scenery until after 10:30. Just above the planet is the “crooked index finger” of the constellation Aries the Ram, which appears temporarily fainter than normal because of moonlight. Jupiter is brilliant for a couple reasons: it’s huge and covered in clouds like the planet Venus. Clouds reflect light well and Jupiter’s 88,000 mile girth — 11 times Earth’s diameter — provides lots of clouds to reflect sunlight.
The planet is presently 399 million miles from us. Light traveling from Jupiter at 186,000 miles per second takes 36 minutes to reach your eyeballs, so a radio transmission between Earth and an orbiting spacecraft would take an hour and 12 minutes to make the round trip. As we explore the outer solar system, we learn great patience and an appreciation for distance.
Jupiter has more moons than any planet in the solar system with 63 at last count. Most are very small, probably captured stray asteroids, but four are hefty globes similar in size to our own moon. Each is visible in ordinary binoculars as a tiny point of light snugged up very close to the planet. What gives them away when viewed this way is that they typically lie in a straight line on either side of the planet. That’s because they all orbit in the same plane about the planet’s equator. Since each moon also lies at a different distance from Jupiter, they orbit the planet with different periods. Io, the closest, completes its orbit in just 1.8 days while Callisto, the farthest, takes 16.7 days.
When you combine the four different periods, you’re guaranteed a unique display of moons every single night through your binoculars. Occasionally one moon or another might be behind the planet or too close to Jupiter’s glare to see, so the number of moons visible on a particular night can vary from one (or none!) to four. No wonder Galileo saw Jupiter as a solar system in miniature.
To find out which moons are which for any night you’re looking, click over to Sky and Telescope’s Jupiter’s Moons utility. It shows you a little diagram with the moons’ locations at the current time as well as a list of moon eclipses and transits for telescopic observers. If you want to know what to expect in advance, you’ll need to convert the time shown, which is in UT or Universal time, to your time zone. For instance, 11 p.m. Central time September 9 equates to 4 a.m. UT September 10 or 5 hours ahead. UT is 4 ahead of Eastern time, 6 ahead of Mountain and 7 ahead of Pacific. Have fun