Moonday: Amalthea

(Image from Wikipedia; full article here) At 83 km in diameter (~50 mi), Amalthea was the fifth discovered moon of Jupiter (the first four were the large Galilean satellites), and the last planetary satellite to be discovered by direct visual observation, as opposed to photographically. Given that the above Galileo images are among the highest quality available, it should be unsurprising not too much is known about this object. A couple of seemingly unrelated facts do constrain its history, though. First, Jupiter would loom in the sky, covering more than 45 degrees of arc; in other words, if you drew a horizon-to-horizon line, Jupiter would cover more than quarter of that. Second, recent observations strongly indicate that Amalthea is icy. Since the young planet was very hot, we can conclude Amalthea did not form where it is today. It either migrated from a more distant orbit, or was captured by Jupiter later in its history.

Moonday: Helene (Again)

I won't make a habit of using the same moon twice in a row, but this is too cool to pass up. NASA's image of the day is a high-resolution shot of Helene, captured by Cassini just two days ago. I'm struck by the evident streamers on the surface... I have some ideas what they might be, but I'll keep them to myself. Poorly informed guesses can cause more problems than they're worth. (Full size image here; definitely worthwhile)

Moonday: Helene

From The Planetary Society comes this image of Saturn and its moon, Helene. (Several other magnificent color pictures of Saturnian moons at the link.) You can tell at glance it's a fairly small moon; it's irregularly shaped and has enormous relief compared to its size. Larger bodies with more mass are more strongly effected by gravity and pull themselves into more spherical shapes. A quick check of Wikipedia gives its mean diameter as 17.6 km (11 miles). Though this moon was only discovered in 1980, it was not from a Voyager flyby, but from ground-based observations.

Moonday: Io

It has been my intention to draw attention primarily to moons that aren't as well known or "popular" in this series, but this image from yesterday's APOD takes the cake:(follow the link to the site and click the pic there for full-size awesomeness and detailed insets) Quoth APOD:

What's happening on Jupiter's moon Io? Two sulfurous eruptions are visible on Jupiter's volcanic moon Io in this color composite image from the robotic Galileo spacecraft that orbited Jupiter from 1995 to 2003. At the image top, over Io's limb, a bluish plume rises about 140 kilometers above the surface of a volcanic caldera known as Pillan Patera. In the image middle, near the night/day shadow line, the ring shaped Prometheus plume is seen rising about 75 kilometers above Io while casting a shadow below the volcanic vent. Named for the Greek god who gave mortals fire, the Prometheus plume is visible in every image ever made of the region dating back to the Voyager flybys of 1979 - presenting the possibility that this plume has been continuously active for at least 18 years. The above digitally sharpened image of Io was originally recorded in 1997 from a distance of about 600,000 kilometers. Recent analyses of Galileo data has uncovered evidence of a magma ocean beneath Io's surface.

Regarding that magma ocean, which I also mentioned last Moonday, Erik Klemetti had a good clarification last Wednesday on why "ocean" is probably not the best word to use to clearly communicate the nature of magma in Io's subsurface.

A couple of other loony images have come across the innertubz recently: the first is another APOD from May 12 of Enceladus and its cryovolcanism, which I had actually been intending to use for last week, until I got distracted by the Callisto/Callista differentiation. The second came from The Cassini Solstice Mission webpage this morning, and captures five of Saturn's moons in one frame!

Rhea (1,528 kilometers, or 949 miles across) is largest here and is closest to Cassini. Dione (1,123 kilometers, or 698 miles across) can be seen just above the rings near the center of the image. Tiny Prometheus (86 kilometers, or 53 miles across) is just barely visible in the rings to the right of Dione. Epimetheus (113 kilometers, or 70 miles across) is to the right of the rings, and Tethys (1,062 kilometers, or 660 miles across) is on the extreme right of the image.

I've linked the full-size image here; the only one that might be difficult to spot is Prometheus, which looks more like a tiny bump on the ring plane, rather that a moon, just to the right of Dione.

Not-A-Moonday: Callista

In the spirit of public education, I would like to caution readers against confusing the subject of today's Moonday, Callisto, with today's not-a-Moonday, Callista. While such confusion is perfectly understandable, there are some important differences:

• Callisto orbits a gas giant, Jupiter. Callista orbits a giant gasbag, Newt.
• Callisto is Jupiter's fourth satellite; Callista is Newt's third satellite.
• Callista made Newt convert to Catholicism; Callisto doesn't give a damn.
  
• While both are clearly ancient, Callista is not as heavily cratered as Callisto.
• Though it takes careful measurement to demonstrate this, Callista is slightly less spherical than Callisto.
• While neither shows any evidence of internal activity, Callista can appear to be active on the surface. The importance of this activity is unclear.
• There are many amazing and fascinating images of Callisto. Callista, not so much.
• Callisto's surface composition is overwhelmingly ice. Callista has significant amounts of clay, spackling compound, and pigmented proteinaceous material, in addition to ice.
• Callista gets upset when asked "Why so serious?" Callisto never gets upset.
• Finally, Callisto is often seen in the company of Jupiter's other satellites. Callista is never seen with Newt's other satellites.

(Click for full-size; Callisto is lower center; Europa is to the left and slightly below the great red spot over Jupiter's disk.)

So again, the confusion is perfectly understandable, even excusable. But these are the sorts of fine, nuanced observation that are necessary for making sense of a complicated, chaotic world.

Followup: oh yeah, of course there's that small matter of the terminal vowel. But I was focused more on the actual features, not so much on the fine details of spelling.

Followup II, Thurs. May 18: Peteykins at Princess Sparkle Pony beat me to this by nearly 2 months. I read that blog, so there is a strong possibility I unconsciously swiped this idea, forgetting I had seen it elsewhere. However, there is surprisingly little overlap between our points of distinction, so you should go read his post too.

Moonday: Callisto

Callisto is the outermost of the four Galilean moons of Jupiter (identified as such because they were discovered by Galileo Galilei, and important because they were the first objects discovered that clearly could not be orbiting the earth), and the only one not currently thought to possess a subsurface ocean. Io has been much in the news the last few days with new results supporting the hypothesis of a subsurface magma ocean, while Europa and Ganymede are thought to have water oceans under crusts of water ice. One reason to believe that Callisto does not possess such an ocean is its heavily-cratered and obviously very old surface, unmodified by tectonic processes, such as can be seen on Europa and Ganymede, or volcanism, such as can be seen on Io. A second reason is that Callisto is not subject to tidal forces as strongly as the inner three, so tidal kneading and heating are much weaker.

According to Wikipedia (which is also the source of the image), "It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass." Another way of putting this is that Callisto's density is about a third of Mercury's, a difference due to the latter's suspected massive iron core, and the former's large component of low-density water and ice.

Moonday: Dactyl

Dactyl is that itty-bitty dot to the right, the first known satellite orbiting an asteroid, 243 Ida, which is the larger body to the left. According to Wikipedia,

Ida's moon, Dactyl, was discovered by mission member Ann Harch in images returned from Galileo. It was named after the Dactyls, creatures which inhabited Mount Ida in Greek mythology. Dactyl, being only 1.4 kilometres (4,600 ft) in diameter, is about one-twentieth the size of Ida. Its orbit around Ida could not be determined with much accuracy. However, the constraints of possible orbits allowed a rough determination of Ida's density, which revealed that it is depleted of metallic minerals. Dactyl and Ida share many characteristics, suggesting a common origin.

Moonday: Phobos

Phobos is the inner and larger of Mars' two moons. It is named for "fear" (Greek, Phobos, the son of Ares, who in turn was the god of war). Like Mimas a couple weeks ago, this moon suffered a collision earlier in its history that was very large compared to the moon itself, and as you can see from the fractures radiating from the large crater on the right limb, the moon was almost destroyed by the blast. Image from The Big Foto, where there are a number of other stunning pictures of this moon and a couple of Deimos as well.

Moonday: Triton

From Wikipedia:

English: original NASA caption: Global color mosaic of Triton, taken in 1989 by Voyager 2 during its flyby of the Neptune system. Color was synthesized by combining high-resolution images taken through orange, violet, and ultraviolet filters; these images were displayed as red, green, and blue images and combined to create this color version. With a radius of 1,350 km (839 mi), about 22% smaller than Earth's moon, Triton is by far the largest satellite of Neptune. It is one of only three objects in the Solar System known to have a nitrogen-dominated atmosphere (the others are Earth and Saturn's giant moon, Titan). Triton has the coldest surface known anywhere in the Solar System (38 K, about -391 degrees Fahrenheit); it is so cold that most of Triton's nitrogen is condensed as frost, making it the only satellite in the Solar System known to have a surface made mainly of nitrogen ice. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying these pink ices are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes, some of which were found to be active during the Voyager 2 flyby. The bluish-green band visible in this image extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of "cryovolcanic" landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior.

I find cryogeology fascinating; it's not too hard for me to wrap my head around water ice as a rock-forming material, but nitrogen?

Moonday: Mimas

Phil Plait at Bad Astronomy posted this image last Wednesday, and I feel compelled to pass it along (full size here). That flat ding on the right of this moon is not an image artifact; it's the side view of an enormous crater.

Voyager 1 passed through the Saturn region in November of 1980, just three years after the first Star Wars film was released. Though Mimas had been discovered in 1789, the views sent back from that flyby immediately gave the moon it's nickname, the "Death Star moon." According to Wikipedia (which is also where the image below comes from), "The impact that made this crater must have nearly shattered Mimas: fractures can be seen on the opposite side of Mimas that may have been created by shock waves from the impact travelling through the moon's body." More information at the Wikipedia link and at the Bad Astronomy post.

Moonday: Miranda

I meant to kick this off last week, then got distracted by other shiny things. My intent is to post a weekly image from that under-represented group of solar system citizens, the moons. And I vow to keep doing this right up to the point that I don't anymore. This happens to be an auspicious day to start the series: tomorrow (which is already today in the Eurasian borderlands) is the 50th anniversary of Yuri Gagarin's orbital flight around the planet, and the dawn of manned spaceflight.

Here's my first selection: my favorite known moon, Miranda, which orbits Uranus. This was posted last week at The Astronomy Picture of the Day (a site which regularly features some out-of-this world geology- see today's pic, for example), with the title, "Verona Rupes: Tallest Known Cliff in the Solar System." The lighting angle and orientation are odd, but the cliff face is toward the lower right, and faces toward the bottom of the image. See that flat, bright surface? That is an estimated 20 kilometers of vertical. Miranda's gravity is much lower than earth's and it is thought that the 12-minute fall from the crest to the bottom might be survivable with the appropriate air bag at the bottom... presuming you could hit it. Click that last link to read more, and click the pic there for a larger field of view. Or click here to see a full-disk mosaic of Miranda. Or both, if you like.