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Hi everyone! Sorry that the posts were thin on the ground last week. The head druid told me that I needed to honor the solstice by taking some time to reflect on the meaning of things [citation needed]. Anyway…since I didn’t blog last week, I failed to post these astonishing pictures of Jupiter’s giant moon Ganymede, which were photographed by NASA’s Juno spacecraft as it slaloms though the Jovian system.

Ganymede as imaged by NASA probe Juno

Although its lack of atmosphere and pockmarked plains of dust make it superficially resemble Earth’s moon, Gannymede is a very strange and unique heavenly object Of the 200 known moons in the solar system, it is the largest. Indeed it is 26% larger than the planet Mercury by volume (although it is only 45% as massive as the metalliferous first planet). Ganymede has a diameter of 5,268 km (3,273 mi), so each pixel in the full size image of the Jovian moon is equal to a kilometer (although you may want to check out the NASA image to really savor that scale–since WordPress has a noteworthy penchant for scrunching up my images in incomprehensible ways).

A photo of the dark side of Ganymede taken by Juno’s incredibly light sensitive navigational camera

Alone among moons in the solar system, Ganymede has a magnetic field, albeit a rather meager one compared to Earth or Jupiter. Scientists surmise that the magnetic field is created by convection within the liquid iron core of the moon–although answers are not forthcoming as to why it has a liquid iron core to begin with (these planetary cores seem to be the real determinant of what planets are like, but I feel like we know precious little about them). Thanks to its size (and maybe thanks also to its magnetosphere), Ganymede has a very thin oxygen atmosphere…but that just creates more question, since elemental Oxygen has a tendency to instantly bond to all sorts of other elements. The 20 percent or so of oxygen in Earth’s atmosphere did not become a mainstay until about 1.5 billion years ago when photosynthesizing bacteria finally became so prevalent that they overcame the constant loss of atmospheric oxygen thanks to oxidation. Hopefully Juno’s survey will help us solve atmospheric mysteries on Ganymede. Ganymede is also believed to have a vast subsurface ocean of icy water tucked away somewehere beneath its surface. Astronomers have reasonably speculated that this Ganymede underworld ocean may contain more water than all of Earth’s oceans combined!

This is the largest version of this interesting cross section which I could find

Ganymede is a Galilean moon–which means it was discovered by the great scientist, and is one of the first objects ever discovered to orbiting another planet (I still sometimes imagine the thrill Galileo must have felt when he realized what he was seeing). I wonder what surprises Juno will send back for us!


Based on what we are learning from the exoplanet surveys of the past decade, our galaxy is the home of an immense number of Jovian-size gas giant planets.  There are countless “hot Jupiters”–gas giants located close to their stars which whip around and around their orbits in ridiculously short “years”.  There are frigid slow gas giants and super massive ones—practically brown dwarves– which are larger than Jupiter.  There is an endless proliferation of Uranus and Neptune type giants. Imagine them all glittering in strange colors with weird shapes.  They are cloaked in alien clouds and covered in mysterious storms.  Who knows what lies beneath?


All of these billions of giant planets seem pretty hypothetical to me as I sit here at my cramped & cluttered desk on solid little Earth.  Yet they exist.  They are out there in numbers too vast to comprehend. However, right now, NASA is conducting the most comprehensive exploration yet of the gas giant we can access.  Juno’s mission is just getting underway in earnest, and the largest gas giant in our own backyard should reveal lots about all of the billions which are out of reach.


I am sad that I can neither understand nor convey the loftiness of this crazy ongoing mission. It is an astonishing undertaking—but we are so inundated by with murky political battles and vulgar popular drivel, that it is hard to see the utterly astonishing nature of this undertaking.


Maybe I can put it in perspective somewhat. Imagine back to the year 1609 AD when Henry Hudson was first seeing the river which was later named after him. Before him was an exquisite expanse of islands, bays, and sparkling river. The vast waterway flowed down from unknown mountains into a bay surrounded by lovely islands.  The whole expanse was filled with flocks of unknown birds and schools of fish. Beyond the thriving marshes, mysterious forests were filled with moving shadows.

Now multiply that a billion times: replace Henry Hudson with a tiny fragile robot and replace the Hudson River with luminous gas oceans large enough to entirely submerge scores of Earths.  That is what is happening right now.  As you sit reading this on a little glowing screen, we are making fundamental discoveries about a whole planet.

On August 27, 2016, Juno executed the first of 36 orbital flybys over Jupiter. The doughty spacecraft was only 4,200 kilometers (2,500 miles) above Jupiter’s atmosphere. It sent back the first detailed images of the north pole of Jupiter—and it is unlike the rest of the planet.


The North Pole of Jupiter as seen by Juno [NASA]

To quote Scott Bolton, one of the lead scientists of the Juno mission, “[The] first glimpse of Jupiter’s north pole…it looks like nothing we have seen or imagined before….It’s bluer in color up there than other parts of the planet, and there are a lot of storms. There is no sign of the latitudinal bands or zone and belts that we are used to — this image is hardly recognizable as Jupiter. We’re seeing signs that the clouds have shadows, possibly indicating that the clouds are at a higher altitude than other features.”

Jupiter’s clouds contain whole continent-like regions of air which are different than the rest of the planet’s storms and whirls.  We don’t yet know why or how, but we are finding out.  As we do so, we are peeling back a layer of mystery which surrounds all such worlds.


Solar Radiation Streaming over the North Pole of Jupiter


Happy (belated) Fourth of July! While everyone was out barbecuing and amusing themselves with colorful novelty explosions, there was big news in space exploration: NASA’s Juno probe, which launched from Earth five years ago, has finally reached the gas giant planet and entered orbit. The robot spacecraft, which is about the size of a basketball court, is now dancing nimbly amongst the system of moons and rings and radiation belts around the giant world.


The probe is a remarkable spacecraft.  It traveled 2.7 billion kilometers (1.7 billion miles) to reach the exact orbit which NASA planned for it.  The secret behind its astonishing precision (even when traveling at 165,000 mph) is the autonomy of its sophisticated navigational computer.  Mission controllers do not have to radio the probe from half-way across the solar system (which would take minutes—or longer. Instead the probe navigates itself. The ship computer is shielded beneath a titanium vault to keep radiation from frying its clever electronic brain.

earth-jupiterOh man!


Among the planets, Jupiter is a sort of greedy eldest child.  Scientists who study planetary formation believe that the gas giant formed first of all the planets and it took the lion’s share of available matter left over from the formation of the sun. Jupiter is more than twice as massive as all the other planets in our solar system put together: indeed, it is three hundred and eighteen times more massive than Earth.  Yet we know shockingly little about this bruiser. Very basic questions about Jupiter remain unanswered. For example we still do not know whether the planet has a rocky core beneath its vast colorful atmosphere.


As we learn more about exoplanets which orbit other stars, questions about the formation of solar systems have become more numerous.  Astronomers have been particularly perplexed by the number of “hot Jupiters,” giant gas planets which are extremely close to their stars.  Was Jupiter such a world at some point before moving to its current location, or is it a huge freak?  We simply do not know.  Scientists would also like to know more about the unimaginably vast cloudscapes of Jupiter.  What dynamics move these huge bands of pressurized gas?

As Jupiter formed, it was bombarded by strange radiation.  The depths of Jupiter’s storms must still feature giant lightning strikes. This sort of treatment can cause hydrocarbons and ammonia to form amino acids.  Maybe life has a Jovian origin.  Maybe Jupiter still has life floating around like aerial zooplankton.  Again, we just don’t know much about the giant world…

Did anybody see that amazing episode of "Cosmos"?

Did anybody see that amazing episode of “Cosmos”?

However, now that Juno has arrived we can start to answer some of these questions.  The probe will go through various start-up and test sequences until Oct. 19 when it moves to a 14-day orbit of the planet and really starts scrutinizing our giant neighbor.


Oh, one more thing—NASA has been getting better at PR to make space more accessible and “fun” for us laypeople following at home (as witnessed by the July 4th arrival).  Juno also has a crew of three Lego astronauts: Galileo, Jupiter, and Juno herself.  This leads me to write about Juno herself, for she is a terrifying figure among the gods.  More about her tomorrow!


Between giant planets and small stars exists a bizarre class of heavenly objects known as brown dwarfs. Brown dwarfs are not massive enough to fuse hydrogen elements together as do main sequence stars like the sun, however brown dwarfs larger than 13 Jovian masses are believed to fuse deuterium atoms and large brown dwarfs (65 Jovian masses and up) are believed to fuse lithium.  Since brown dwarfs can be very much like planets or like stars, there is a specific definition to describe the objects: a brown dwarf must have experienced some sort of nuclear fusion as a result of mass and temperature, however it cannot have fused all of its lithium (or it is considered a star or stellar fragment).  A stellar physicist reading this blog might object that medium and large stars have some lithium present in their outer atmosphere, or that a very young white dwarf could still have some unused lithium present, or even that an old heavy brown dwarf could have fused all of its lithium.  That physicist would be correct: she deserves some cookies and a pat on the head for poking holes in unnecessarily simple definitions.

Various Classifications of Brown Dwarfs

Various Classifications of Brown Dwarfs

Brown dwarfs were theorized to exist in the 1960s, but no astronomer managed to discover one until 1988 when a team of University of California astronomers who were studying white dwarfs found a bizarrely cool red spectral signature for a faint companion to the star GD 165.  Since then many brown dwarfs have been discovered and sorted into the major types M, L, T, and Y.  They occupy a strange ambiguous area at the bottom of the Hertzsprung-Russell diagram—objects which are luminous and massive in comparison to everything else but tiny and dim compared to real stars.


There are some planets which are known to orbit brown dwarfs and there also brown dwarfs known to orbit true stars.  It is beginning to seem that there a great many brown dwarfs out there: perhaps they are as numerous as true stars (or maybe they are even more common than that).  Since they are hard to detect, scientists do not have a very accurate assay of their frequency in the universe.  The question bears somewhat on our understanding of the universe–since a great deal of matter is  not accounted for.

An artist's conception of a brown dwarf seem from a closely orbiting planet

An artist’s conception of a brown dwarf seem from a closely orbiting planet

My mind keeps returning to the fact that some brown dwarfs have planetary systems.  Imagine these melancholic twilight ice worlds forever orbiting a dim glow which will never blaze into a true sun.   It is a melancholy picture, but not without a certain beauty.

A Brown Dwarf with Planet and Moon (painting by Lynette Cook from

A Brown Dwarf with Planet and Moon (painting by Lynette Cook from

Ferrebeekeeper has already posted about the aegis, the invulnerable shield of Jupiter/Zeus, which was fashioned by the king of the gods from the skin of his foster mother (and loaned to his favorite daughter.  However the concept of Jupiter’s shield has a larger significance.

Yesterday morning, an unknown object appears to have slammed into the planet Jupiter.  Oregon based astronomer Dan Petersen was watching the gas giant at 4:35 AM PST (September 10th, 2012) when a bright flash erupted from near the Jovian equator.  Another amateur astronomer, George Hall of Dallas, TX was filming the planet through his 12 inch telescope and recorded the flash (you can see the video here).

The September 10th, 2012 Flash on Jupiter (recorded by George Hall)

Thanks to the florid nature of science fiction entertainment, it is easy to imagine scaly green Guarillions testing out energy weapons against the huge planet, but the flash was almost certainly from a comet or asteroid striking the surface (we will know more as astronomers look at Jupiter this week).  Such impacts have proven to be much more common than imagined.

Jupiter has a mass of approximately 1.9 x 1027 kg (which is equivalent to 318 Earths).  The gas giant is 2.5 times more massive than all of the rest of the non-sun objects in the solar system added together. The sun itself comprises between 99.8% and 99.9% of the mass of the system (which should put some perspective on the precision required for our ongoing programs to scan the nearby galaxy for exoplanets).

Jupiter Relative to the Sun and the Earth (NASA Goddard Space Flight Center)

The huge mass of Jupiter (relative to other planets and moons) means that a great many asteroids, comets, meteors, and whatnot fall into its gravity well.  Were it not for Jupiter, these hazardous leftovers would otherwise fly all around the solar system willy-nilly knocking holes in things and creating unsafe conditions (just ask the poor dinosaurs about this).  The ancient myths of the Aegis provide a powerful metaphor for this protection. Jupiter does indeed provide a shield for the smaller planets:  If it did not suck up so many cosmic punches, who knows if life could even have survived?

(Lithograph by F. Heppenheimer)

An Artist’s conception of Jupiter seen from Amalthea

If you wanted to build a vacation home with a truly spectacular view, one of the possibilities you might consider is Jupiter’s moon Amalthea.  Discovered in 1892 by the American astronomer Edward Emerson Barnard (who also discovered Barnard’s star) Amalthea was the first Jovian moon discovered by someone other than Galileo Galilei.  Amalthea is the largest inner satellite of Jupiter and from its surface Jupiter would appear to take up 46.5 degrees of the sky (from the horizon to directly overhead is 90 degrees).  Amalthea is in synchronous rotation around Jupiter and so the planet would always appear in the same part of the sky (provided you were on the right part of the moon).  From Amalthea the sun would disappear behind the planet’s bulk for an hour and a half each revolution.

A digital rendering of Amalthea

The 66 known Jovian moons are largely named after the lovers and children of Jupiter/Zeus, however Amalthea is an exception: it is named for Jupiter’s step-mother the goat/nymph Amalthea who fed and cared for the young god as he quickly grew to adulthood and whose impervious skin was fashioned into the aegis of the king of the god.  The name Amalthea was used for the moon almost since it was discovered but was only formally adopted by the International Astronomical Union in 1975.

Amalthea Visited by the NASA spacecraft Galileo

Amalthea is a strange and mysterious moon which perplexes astronomers.  Its irregularly shape is somewhat like a potato and it is covered with deep craters and tall mountains.  The surface of the moon is deep red in color (in fact Amalthea is the reddest object in the solar system) however weird bright patches of green appearing on the mountain slopes–the nature of which is unknown.  The moon appears to be formed of ice and rubble, but if had formed where it now is during the early days of Jupiter, it would have melted.  The moon must have formed elsewhere and been captured by Jupiter—a recent paper speculated that it was originally a Trojan asteroid.  Since Amalthea is made of ice and heterogeneous rubble scientists are perplexed at why gravity has not rearranged its into a more spherical shape.  Since Amalthea is so close to Jupiter it’s orbit is decaying and it will one day fall into the gas giant (so you may want to get really good insurance on the vacation house I mentioned in the first paragraph).

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September 2021