Amalthea: The Reddest Potato In Orbit Around Jupiter

Everyone knows you shouldn’t eat green potatoes, but what about red potatoes (no, this doesn’t include potatoes with red skins)? I’m not sure on that count, but this red potato would likely taste like, well, rock and a whole lot of radiation courtesy of its parent planet, Jupiter. When I say this moon is red, it’s red, considered as the reddest object in our solar system. It is a moon that is not much more than a pile of icy rubble with a very low density. The most significant feature on its surface is a couple of craters, named Gaea and Pan, discovered and imaged by the Galileo space-craft in the 1990s. Pan is the larger of the two craters, with a diameter of 100km and a depth of up to 8km. In contrast, Gaea is a crater measuring around 80km across with a depth of up to 12km.

A grayscale image featuring three of Jupiter’s moons as imaged by the Galileo space-craft in January 2000. Amalthea is the biggest moon in the middle, showing one of its huge craters.

Amalthea is the biggest of the four moons that orbit within the moon Io’s orbit; the other three are Adrastea, Metis and Thebe. Of them all, Amalthea orbits the closest to Jupiter at an average distance of 181,400 kilometres away from the gas giant’s surface. It orbits Jupiter at an average speed of 95,362km/hr, taking just under half an Earth day to complete one revolution. Considering Jupiter’s enormous gravitational pull, it seems odd that this ruddy moon has not been torn apart to this day. It’s pretty simple: its dinky size means that tidal pull from Jupiter’s gravity has minimal influence on Amalthea, so it can carry on merrily orbiting the gas giant behemoth. Nevertheless, with time, it will eventually meet its demise when its orbit decays to the point that it will fall toward Jupiter and be torn apart forever.

Voyager 1 image of Amalthea, taken in 1979. Credit: NASA/JPL.

On a final note, you might think that, because it is so close to its parent planet, it would have been impossible for anyone to spot it before Voyager 1 flew past it in 1979. To the contrary! In fact, this modest little moon was discovered in 1892 by Edward Emerson Barnard, who spotted it through his 91 cm refractory telescope while he was working at the California-based Lick Observatory, which at the time, was only four years old (built in 1888.) It might be a tiny moon of little note in most mainstream astronomy textbooks and books in general, but it has been known since horse-and-carts were still in vogue.

To round off this post, enjoy a beautiful image of Amalthea casting its shadow on Jupiter as imaged by the JUNO space-craft currently (as of 2021) in orbit around the gas giant.

The shadow of Amalthea blots out a tiny patch of Jupiter’s vast red and white clouds. Credit: NASA/JPL-Caltech.

References:

“Amalthea Casts Shadow on Jupiter. (October 2017. Sci-news website.)

“Juno Completes its Eighth Flyby of Jupiter.” (September 2017. Sci-news website.)

“Amalthea” (Solar Views website)

Lick Observatory (Links to “About” Page)

In Depth: Amalthea. (NASA website)

By the Numbers: Amalthea (NASA website)

“SOLAR SYSTEM: Jupiter, Saturn, and Their Moons” (2005). Moore, P. In: Encyclopedia of Geology (pp. 282-289).

Europa and its Perpetual Glow-Up

Forget those cute little star stickers you used to have as a kid (or at least I did), because imagine an entire moon that glows in the night sky, and not because it is reflecting light from our favourite star in the universe. Rudolph’s glowing red nose pales next to this super-shiny moon orbiting a planet in our solar system, its light blue, but with little tinges of green and white. So, what exactly is this curious glowy moon that lights up some lucky planet’s night sky?

Do I suit this shade of blue? Credit: NASA-JPL Caltech

That moon is Europa, one of the so called Galilean moons of Jupiter, and its glow-up is, unfortunately, not from some benign cause; rather, it is all down to Jupiter’s radiation. Thanks to Europa’s proximity to Jupiter, this little icy moon “enjoys” a daily, relentless dose of high-energy Jovian radiation on its surface, the consequence being that its night-side would still glow even when facing away from Jupiter, as suggested by a series of lab experiments down by a science research team, with the paper released in November 2020.

Their lab experiment involved blasting Europan ice analogues (i.e. water ice with different compositions of salts) with high-energy electrons, and analysing the results’ various light emissions. They also related their findings back to earlier studies where they had discovered that with increased intensity of high-energy electron bombardment, the greater the spectral (i.e. light) intensity. The light emission was also noticeably stronger in ice containing epsomite, which is a very soft mineral (hardness of 2) with a chemical composition that includes magnesium and sulfate, as well as water. In contrast, samples with sodium chlorate or sodium chloride gave off the least light.

This study has several implications for future research and exploration of this frozen moon with its spectacular glow-up, especially more so with the potential launch of the Europa Clipper spacecraft maybe in the mid-late 2020s. For example, relating back to the amounts of light reflected depending on salt composition, scientists looking at the Europa Clipper’s imagery would be able to note darker patches as likely containing sodium chlorate and/or chloride, and brighter areas likely containing epsomite salts. These observations would be key to furthering whether Europa has the right conditions for life swimming somewhere deep under its icy crust. But, whether or not Europa’s glow could be detected by the space-craft’s cameras, is still yet to be determined or confirmed, but if it could, that could potentially be very breathtaking!

References:

https://www.nature.com/articles/s41550-020-01248-1 Laboratory Predictions for the night-side surface ice glow of Europa, Nature Astronomy, Nov 2020.

https://www.jpl.nasa.gov/news/news.php?feature=7779 Europa Glows: Radiation does a bright number on Jupiter’s Moon. Jet Propulsion Laboratory, NASA. Nov 2020.

http://www.physicsworld.com/a/jupiters-moon-europa-could-glow-in-the-dark Jupiter’s Moon, Europa, could glow in the dark, Physics World, Nov 2020.

Does Europa glow in the dark?

https://www.mindat.org/min-1393.html Epsomite, mindat.org

Adrastea: One of Io’s Little Friends

Jupiter’s volcanic moon, Io, has four little friends keeping it company in its orbit: Metis, Amalthea, Thebe, and our star of today’s post, Adrastea. It is a humble smol, the tiniest of the quartet in Io’s orbit, at just 8.2km in radius, give or take 2kms. Owing to its place in Io’s orbit, Adrastea snuggles up nice and close to Jupiter, at just 129,000km away from the big boy of our solar system. That is very, very close and personal to Jupiter. How much so? Below is a stunning photograph from the JUNO space craft taken only 7000km from Jupiter’s cloudtops, but the planet is so huge that the view wouldn’t look much different even from Adrastea’s surface as it orbits the worldly behemoth in just seven hours.

A close up image of Jupiter’s clouds, showing a sea of swirling white, orange, blue, brown, and even pink, taken by the JUNO space-craft from only 7000km away! CREDIT: Gerald Eichstädt and Sean Doran (CC BY-NC-SA) based on images provided Courtesy of NASA/JPL-Caltech/SwRI/MSSS

Its size gives it another advantage: despite its closeness to Jupiter, it is too small to be affected significantly by its gravity; unfortunately, it is not immune to orbital decay, and eventually, this plucky little moon will fall into Jupiter, though how long this may take is not known.

References

https://solarsystem.nasa.gov/moons/jupiter-moons/adrastea/in-depth/

https://www.nasa.gov/image-feature/jpl/jovian-close-encounter

Neptune officially has 14 Moons!

Once upon a time, Neptune had eight confirmed moons. Then it had thirteen, with one yet to be confirmed,. And that one yet to be confirmed? Well, it now has an actual name: Hippocamp. It was a SETI Institute scientist, Mark Showalter, who decided on the moon’s moniker, naming it for a mythological creature. It was Showalter, too, who had first discovered the moon’s possible presence while he had been developing a new way to find faint structures around Neptune, intrigued by the planet’s arc-like rings.

A dinky moon at only 17km to 34km in diameter (depending on the source you read), it is believed to have broken off from nearby moon, Proteus. Indeed, its orbit is very unusual, as it is very snug up with Proteus itself, just over 12000km away from aforementioned moon. According to the National Geographic article referenced below, it is highly unlikely that Proteus and Hippocamp just happened to form so close together, and the latter was probably born out of a comet impact on Proteus, an impact that might have formed the aforementioned moon’s crater, Pharos.

If you want to learn more about Neptune’s other moons, I have a post on the very topic if you are curious about learning more about this distant ice giant’s family of satellites.

References:

https://www.nationalgeographic.com/science/2019/02/meet-hippocamp-newest-known-moon-neptune-hubble-space/

https://www.nature.com/articles/s41586-019-0909-9

https://www.universetoday.com/141598/say-hello-to-hippocamp-the-new-moon-discovered-at-neptune-which-could-have-broken-off-from-the-larger-moon-proteus/

 

Solar System Moons: Dactyl

Planets aren’t the only objects to have moons, for asteroids have been discovered to have orbiting satellites as well. The first asteroid discovered to have its own moon was the asteroid 243 Ida, thanks to the Galileo spacecraft that flew past it in 1993 on its way to Jupiter. However, it wasn’t until February 1994 that Anne Harch, a member of the Galileo team, discovered Dactyl while studying images sent back to Earth from the space probe.

Dactyl isn’t big by any means, only around 1.5km in diameter, orbiting around 90km distant from its parent asteroid. Like Ida, it is primarily composed of silicate rock, and probably formed at around the same time as aforementioned asteroid. One way it may have formed is through an early impact on Ida that sent up debri into orbit that eventually became Dactyl.

Ida and its dinky little moon, Dactyl. Credit: NASA

References: 

http://hyperphysics.phy-astr.gsu.edu/hbase/Solar/Ida.html

https://solarsystem.nasa.gov/asteroids-comets-and-meteors/asteroids/243-ida/in-depth/

http://solarviews.com/eng/ida.htm

Solar System Moons: Phobos

Last time, we visited Mars’ moon, Deimos. Today, we turn our sights upon the red planet’s only other moon, Phobos. If Deimos is the quiet child slowly making their journey away from home to live an independent life, then Phobos is the sibling taking reckless joyrides down busy roads at 110km (68 miles) an hour, as well as visiting all the bungey jump spots (the higher the better), including the one in Queenstown, NZ. They are the one gripping life by the horns, passionately living by the old adage, “live fast and die young!”

While Deimos makes a stately thirty-hour journey around Mars, Phobos whizzes around a complete orbit in just four hours. With each orbit, it gets a teensy bit closer to Mars; in fact, it closes in by 1.8 metres (or 6 feet) to 2 metres (6.6 feet) every 100 years. It is estimated that in around 50 million years, Phobos will either be torn apart by Mars’ gravity to form a ring, or it will actually impact the planet’s surface.

Look at any photo of Phobos, and you will likely see its most eye-catching feature: its giant crater, Stickney. It’s a crater over 9km across scarring a moon just approximately 17km in diameter. Phobos is clearly a resilient moon, as it was not shattered by whatever created the huge crater in its past. The crater is named after Chloe Angeline Stickney Hall, the wife of Asaph Hall, who discovered Mars’ two satellites in 1877.

Phobos, with its crater, Stickney, clearly visible on the right. Credit: JPL NASA

Another feature you may notice about Phobos’ geomorphology are the grooves on its surface. A previous theory suggested they were a consequence of the great impact that created Stickney Crater. However, as the grooves did not necessarily originate at the crater, another–more likely–suggestion has been proposed that they are a result of stress from tidal pulling from Mars’ gravity. In essence, the grooves are early hints of structural failure, foreshadowing the moon’s ultimate fate.

Phobos’ dust-laden, atmosphere-less surface sees extreme temperature differences, depending if you are on the night or day side of the moon. On the side facing the sun, temperatures soar up to a balmy -4 degrees celsius (25F), and the night side sees lows of -112 degrees celsius (-170F). So make sure to wrap up warm and bring plenty of duvets if you’re going to camp overnight on Phobos.

References:

https://apod.nasa.gov/apod/ap180505.html

http://www.astronomy.com/news/2018/09/mars-moon-phobos-may-have-formed-from-a-giant-impact

https://www.nasa.gov/feature/goddard/phobos-is-falling-apart

https://solarsystem.nasa.gov/moons/mars-moons/phobos/in-depth/

https://www.space.com/20346-phobos-moon.html

Solar System Moons: Deimos

Mars has two moons, Phobos and Deimos, with the former being significantly bigger than the latter. Both are small and lumpy in shape, and many scientists believe they may have been asteroids captured by Mars’ gravity in the past. Because they have very little mass, their surface gravity is very low–as a matter of fact, if you landed on either moon, you would weigh less than a piece of paper does on Earth!

Deimos’ nearly circular orbit, in contrast to Phobos’ dizzying four-hour revolutions around Mars, is nearly in synch with aforementioned planet’s rotation. Mars takes about 25 hours to spin once on its axis, and Deimos takes 30 hours to complete one orbit around its parent world. How would this appear to an observer standing on Mars? Not that you would see much, as Deimos is so small, distant, and faint that it would appear not much brighter than a star. However, if you were keeping tabs on the moon’s path through the sky, it would appear to slowly move across the sky over a couple of days before it finally sets in the west.

This really is a dinky satellite whose diameter is 12.4 km across, with a smooth surface, few craters, and very low density, pegged at a mere 1.471g per cubic centimetre. The gravity on its surface is just 0.003m/s^2 (compared to Earth’s 9.81m/s^2) and the escape velocity is 20km/h; Earth’s is at least double that.

As mentioned before, Deimos does not have many craters to speak of, and, to date, only two of them are named, both after authors: Voltaire (French) and Swift (English). They are noted for their predictions in their stories that Mars had to have two moons, as that made mathematical sense. To people in their day, it appeared only logical, mathematically speaking: Mercury and Venus had zero, Earth had one, Jupiter (as far as they knew in their time) had four. So it made total sense that Mars had to have two.

What is Deimos’ final fate? Its end is the polar opposite to Phobos’, which will eventually crash into Mars’ surface, if it isn’t broken up to form a planetary ring first. Deimos, in stark contrast, is wandering away from Mars, until eventually it will escape the reaches of the planet’s gravity and wander off into space, alone and free.

References:

http://www.astronomycast.com/2016/11/ep-428-the-moons-of-mars/

https://www.space.com/20345-deimos-moon.html

https://en.wikipedia.org/wiki/Swift_(Deimian_crater)

https://en.wikipedia.org/wiki/Voltaire_(crater)

https://solarsystem.nasa.gov/moons/mars-moons/deimos/in-depth/

https://www.universetoday.com/14908/mars-moon-deimos1/

A-Z Quick Facts: Craters

 

Mimas, a moon of Saturn, as imaged by the Cassini space probe. Credit: NASA

 Everyone’s seen a crater before: you only have to look up at the moon on a clear night to see the hints of these impact events. If you have a telescope, or have access to one, they are even more spectacular close up, and with a good enough telescope, you can almost feel like you’re hovering right there over the moon’s surface. Earth has a few impact craters of its own from the past, though most of these have been eroded away thanks to geological processes over the eons. But unlike here on Earth, there is minimal erosional processes on many moons in the solar system, as well as the planet Mercury, which is why these bodies have so much cratering on their surfaces. There are very impressive craters in the solar system including Herschel, the famous crater on Saturn’s moon Mimas that makes it look like the Death Star from Star Wars. Herschel is a great example of a complex crater, while may of the other craters you see on Mimas’ surface are simple craters.

So what’s the difference between a simple crater and a complex crater? The number one difference is in their morphology. Unlike a complex crater, a simple crater is a bowl-shaped puncture in the planet or moon’s surface with smooth walls and floor. These tend to be small, on the order of a few kilometres in diameter, and lack a central peak. You can see a few craters like this on the surface of Mimas in the photograph above taken by the Cassini spacecraft.

​On the other hand, complex craters tend to have more complex features, as the name suggests. They often sport a central peak (author aside: when I was younger those central peaks always made me think of bellybuttons. I was a weird kid.) These craters also tend to have multiple terraces, several rims, and a flatter floor in comparison to simple craters. Bigger complex craters’ outside walls tend to collapse, forming the aforementioned terraces, unlike with much smaller simple craters. Their shape often erodes with time, losing their shape until eventually they become a faint scar on the planet/moon’s surface, overtaken by younger, more recent craters.

Craters can be useful in estimating the age of a planetary surface, especially on worlds and moons that experience minimal environmental erosion. Unlike our planet, many places do not have geologic activity like earthquakes or weather processes like wind to erode them away over the eons. The more craters on the surface, the older the surface of that world is. Callisto, a moon of Jupiter, is said to have the most ancient surface in our solar system, on the order of ~4 billion years old, and this is thanks in part to its many craters it sports on its crust.
​
There are a couple of great links below that goes into a lot more detail on the nature of craters, written in a very informative and approachable method that anyone can understand, which is very important in science communication.

Links: 
https://www.lpi.usra.edu/education/explore/shaping_the_planets/impact-cratering/
http://www.psi.edu/epo/explorecraters/background.htm

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Jupiter’s moon, Callisto. Credit: NASA