On this night in 1610 the Pisan astronomer Galileo Galilei looked up at a bright Jupiter at opposition through his handmade telescope and noted three little “stars” next to it, piquing his natural scientific curiosity. Further observations over the next few nights showed that the planet wasn’t moving relative to the little “stars” as it would have if they were indeed background stars. In fact the smaller objects (of which he soon saw four) were moving along with Jupiter, each in its own little path. He realized that the little objects weren’t stars at all but rather moons orbiting the giant planet—and, most importantly, not the Earth. This revelation helped change our entire view of the Solar System… and caused no end of trouble for Galileo as the Church didn’t appreciate a restructuring of their conveniently Earth-centered Universe. But it also opened the door for later discoveries of many more moons around other planets.
There are a lot of moons in our solar system—175 major planet satellites, and three times that if you count every natural satellite of every known object (like asteroids)—but among them our own capital-M Moon is in many ways unique. At a full quarter the size of Earth, only Pluto has a moon so near in size to itself, and unlike the swarms of icy worlds orbiting the gas giants the Moon is oddly very similar in composition to Earth…so similar, in fact, that it’s been casting increased doubt on the accuracy of the best-accepted model of the Moon’s formation, namely the Giant Impact Hypothesis.
Suggested in 1975 by planetary scientists William K. Hartmann and Don Davis, the model claims that the Moon was created 4.5 billion years ago when a Mars-sized world that’s been named Theia impacted the newly-formed Earth, blasting a chunk of molten material out into orbit that solidified to form the Moon. The model is based on a lot of science and answers a lot of questions, but not all—including a key issue of why the Moon today appears compositionally identical to Earth and not a mixture of Earth and a completely different planet.
As advanced computer measurement and modeling capabilities have increased a new wave of researchers are tackling the conundrum of the Moon’s origins, and a few new scenarios are coming to light. While ancient impacts are still involved, the question is now how many? With what kind of world(s)? And what exactly happened after the event?
“In the past five years, a bombardment of studies has exposed a problem: The canonical giant-impact hypothesis rests on assumptions that do not match the evidence. If Theia hit Earth and later formed the moon, the moon should be made of Theia-type material. But the moon does not look like Theia—or like Mars, for that matter. Down to its atoms, it looks almost exactly like Earth.”
Read the full story by Rebecca Boyle in The Atlantic here: The Moon’s Origin Story Is in Crisis
Mars isn’t a planet well-known for its natural satellites but it actually does have two small moons. The larger, Phobos, is an irregularly-shaped, heavily grooved and cratered world only about 17 miles (27 km) across at its widest. It orbits Mars so closely that it completes 3 orbits every day, and isn’t even visible from some parts of the planet. But Phobos has an even smaller companion in orbit: Deimos, which at the most 7.5 miles across is half Phobos’ size. Deimos orbits Mars much further away as well, taking about 30 hours to complete one orbit.
Saturn’s “yin-yang” moon Iapetus (pronounced eye-AH-pe-tus) is seen in this image, a color composite made from raw images acquired by Cassini’s narrow-angle camera on March 11, 2017.
The color difference on Iapetus is due to a fine coating of dark material that falls onto its leading hemisphere, sent its way by the distant moon Phoebe traveling within the recently-discovered giant diffuse ring. This dark coating of dust causes that half of Iapetus’ surface to warm up ever-so-slightly-more than the other, making the underlying water ice evaporate and redeposit on the other side. This in turn reinforces the cycle…a positive feedback loop.
NASA’s OSIRIS-REx may be designed to study asteroids close up but recently it’s captured a view of something farther away and much, much larger: the giant planet Jupiter and three of its largest moons at a distance of over 400 million miles!
The image was taken on Feb. 12, 2017, when the spacecraft was 76 million miles (122 million km) away from Earth—near the Earth-Sun L4 point—and 418 million miles (673 million km) from Jupiter. It’s a combination of two images taken with the PolyCam instrument, OSIRIS-REx’s longest range camera, which will capture images of the asteroid Bennu from a distance of over a million miles.
Read the full article here: NASA’s OSIRIS-REx Takes Closer Image of Jupiter