Like a lot of parents, we read Goodnight Moon by Margaret Wise Brown to our kids to get them ready for bed at night. The calming words and repetition soothed child and adult alike at the end of the day.
Maybe a sequel titled “Goodmorning Moon” will be written someday about waking up to the smiling crescent in the east and getting ready for the day. Tomorrow morning we’ll see exactly that, a very thin moon, low in the eastern sky at dawn. Its delicate arc will surely make you stop and realize how much beauty nature puts on the plate for enjoyment and study every day.
Venus seekers can use the moon to make one last attempt to find the planet, now nestled very low in the east just a degree or two above the horizon 40 minutes before sunrise.
While we’re on the topic of planets, NASA’s Cassini spacecraft will make a close flyby of Saturn’s moon Titan tomorrow September 22nd. At 3,201 miles (5,150 km) across, Titan is the solar system’s second largest moon, only 79 miles smaller than Jupiter’s Ganymede. It’s also unique in having a very thick atmosphere – 1.5 times thicker than Earth’s – a feature usually found only on planets.
It’s still not known how Titan managed to hold onto all its air, which consists of primarily nitrogen mingled with methane and various other hydrocarbons that react in sunlight to create an orange smog that gives the moon its distinctive color. Several other moons such as Ganymede, Rhea and even our own moon have atmospheres, but they’re exceedingly thin compared to Titan’s.
It’s thought that Titan maintains and replenishes its atmosphere through outgassing from its interior. The bitter cold temperatures at Saturn’s nearly billion mile distance from the sun along with Titan’s considerable gravitational pull undoubtedly help preserve and hang on to its air. Comet impacts may also contribute to the moon’s stockpile of ices and organic compounds.
Along with an atmosphere come clouds, though of methane rather than the water vapor variety found on Earth. Temperatures at the surface hover just 90 degrees above absolute zero (-290º F, -179º C), chill enough for methane clouds to form and supply at least some of the precipitation to lakes of liquid ethane, methane and propane below.
This will be Cassini’s 9th flyby of Titan this year. During a flyby, the craft zips by the moon at high speed while keeping its instruments precisely pointed at the target using either its reaction wheels or thrusters, which spin the spacecraft to track the moon as it passes by. Thrusters are also used to keep Cassini from tumbling when it experiences drag while passing through Titan’s upper atmosphere during close flybys.
Descent through Titan’s atmosphere made by the Huygens probe on January 14, 2005
On Monday, Cassini will be traveling at 13,000 mph (21,000 km/hr) and come within 870 miles of Titan’s surface as it photographs seas and lakes – including Ligeia Mars shown above – around the north pole. Another instrument will observe Titan’s southern hemisphere atmosphere in ultraviolet light by observing the dimming of Alkaid, the star at the end of the Big Dipper’s handle as its light passes through the moon’s varied atmospheric layers.