Comet CG Evaporates

Where do comet tails come from? There are no obvious places on the nuclei of comets from which the jets that create comet tails emanate. One of the best images of emerging jets is shown in the featured picture, taken in 2015 by ESA’s robotic Rosetta spacecraft that orbited Comet 67P/Churyumov-Gerasimenko (Comet CG) from 2014 to 2016. The picture shows plumes of gas and dust escaping numerous places from Comet CG’s nucleus as it neared the Sun and heated up. The comet has two prominent lobes, the larger one spanning about 4 kilometers, and a smaller 2.5-kilometer lobe connected by a narrow neck. Analyses indicate that evaporation must be taking place well inside the comet’s surface to create the jets of dust and ice that we see emitted through the surface. Comet CG (also known as Comet 67P) loses in jets about a meter of radius during each of its 6.44-year orbits around the Sun, a rate at which will completely destroy the comet in only thousands of years. In 2016, Rosetta’s mission ended with a controlled impact onto Comet CG’s surface. Outreach Astronomers: Future APOD writers sought.

Hills, Ridges, and Tracks on Mars

Sometimes, even rovers on Mars stop to admire the scenery. Just late last November the Curiosity rover on Mars paused to photograph its impressive surroundings. One thing to admire, straight ahead, was Central Butte, an unusual flat hill studied by Curiosity just a few days before this image was taken. To its right was distant Mount Sharp, the five-kilometer central peak of entire Gale crater, the interior of which Curiosity is exploring. Mount Sharp, covered in sulfates, appears quite bright in this colorized, red-filtered image. To the far left, shrouded in a very dark shadow, was the south slope of Vera Rubin ridge, an elevation explored previously by Curiosity. Between the ridge and butte were tracks left by Curiosity’s wheels as they rolled forward, out of the scene. In the image foreground is, of course, humanity’s current eyes on Mars: the complex robotic rover Curiosity itself. Later this year, if all goes well, NASA will have another rover — and more eyes — on Mars. Today you can help determine the name of this rover yourself, but tomorrow is the last day to cast your vote. Help Name the Mars 2020 Rover: Vote here!