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Exploring the Moons of Jupiter

lpetrich

lpetrich

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The European Space Agency's Jupiter Icy Moons Explorer is on its way -- ESA's Juice spacecraft.

ESA - ESA’s Juice lifts off on quest to discover secrets of Jupiter’s icy moons - launched atop an Ariane 5 rocket from the Kourou spaceport in French Guiana at 14:14 CEST on 14 April.

 Jupiter Icy Moons Explorer and ESA - Juice

The spacecraft will use a gravity-assist trajectory:
  • 2023 Apr - launch
  • 2024 Aug - Earth flyby 1
  • 2025 Aug - Venus flyby
  • 2026 Sep - Earth flyby 2
  • 2029 Jan - Earth flyby 3
  • 2031 Jul - Arrival at Jupiter
  • Some 35 flybys of Jupiter's moons Europa, Ganymede, and Callisto, those icy moons
  • 2034 Dec - Entry into orbit around Ganymede
  • 2035 Sep - End of mission: crash into Ganymede

The spacecraft will be powered by solar panels and it will contain several instruments:
  • Jovis, Amorum ac Natorum Undique Scrutator (JANUS) -- Latin: "comprehensive observation of Jupiter, his love affairs and descendants." -- optical camera
  • Moons and Jupiter Imaging Spectrometer (MAJIS) -- visual and near-infrared
  • UV Imaging Spectrograph (UVS)
  • Sub-millimeter Wave Instrument (SWI)
  • Ganymede Laser Altimeter (GALA)
  • Radar for Icy Moons Exploration (RIME)
  • JUICE-Magnetometer (J-MAG)
  • Particle Environment Package (PEP) -- for Jupiter's magnetosphere
  • Radio and Plasma Wave Investigation (RPWI)
  • Gravity and Geophysics of Jupiter and Galilean Moons (3GM) -- uses a radio transponder and an ultrastable oscillator
  • Planetary Radio Interferometer and Doppler Experiment (PRIDE)
Very comprehensive.
 
ESA - Juice sends first ‘selfies’ from space -- pictures of parts of itself, some of them with the Earth.
Juice has two monitoring cameras located on the ‘body’ of the spacecraft to record various deployments. The images provide 1024 x 1024 pixel snapshots. The images shown here are lightly processed with a preliminary colour adjustment.
Click to expand...

ESA - Juice’s first taste of science from space
With the critical deployment of the solar arrays and the medium gain antenna following launch, the next three months will focus on deploying and checking out the instruments.
Click to expand...


ESA - Operating in an extreme environment
The Jupiter Icy Moons Explorer (Juice) will have to cope with challenges like very high and low temperatures, a lack of sunlight at Jupiter, and high levels of radiation around Jupiter.

Juice has been specially designed to overcome all these challenges and many more. For example, shields have been built to protect the spacecraft’s sensitive electronics, large solar panels will enable it to collect lots of sunlight, Multi-Layer Insulation will keep it at a stable temperature, a large antenna will help it communicate with engineers on Earth, and a powerful onboard computer – Juice’s ‘brain’ – will help it to solve some problems independently, without needing to contact Earth at all.
Click to expand...
Solar-panel area: 85 square meters
 
Jupiter's four big moons were discovered by Galileo Galilei in 1610, with his telescope. On January 7, 1610, he noted that there were three stars near Jupiter. But when he next observed them, he noticed that they moved, and over the next two months, he observed them and discovered a fourth one.  Galilean moons

This discovery provoked a very curious response, from astrologer Francesco Sizzi in his book "Dianoia Astronomica". He argued that these moons could not possibly exist, because they have nothing to correspond to. Consider these sets of entities:
  • The seven traditional planets: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn
  • The seven openings in our heads: the two eyes, the two ears, the two nostrils, and the mouth
  • The seven metals: gold, silver, copper, iron, tin, lead, and quicksilver (non-astrological name for mercury)
  • The seven days of the week
No room for those four moons.
 
They definitely exist and are easy to see. Below is a photo I took with a 300mm lens on my DSLR with Venus and Jupiter, with three of the Galilean moons visible at the time.

F96EBB80-8F40-4427-86C7-F750901F1F88.jpeg
 
I myself have seen those moons with a small telescope.

How big are they?  List of gravitationally rounded objects of the Solar System Largest rocky and icy bodies sorted by equatorial radius:

Earth 6,378.1366 km, Venus 6,051.8 km, Mars 3,396.19 km, Ganymede (Jupiter) 2,634.1 km, Titan (Saturn) 2,576 km, Mercury 2,440.53 km, Callisto (Jupiter) 2,410.3 km, Io (Jupiter) 1,815 km, Moon (Earth) 1,737.1 km, Europa (Jupiter) 1,569 km, Triton (Neptune) 1,353.4 km, Pluto 1,188.3 km, Eris 1,163 km

By mass:

Earth 5.972*10^24 kg, Venus 4.8690*10^24 kg, Mars 6.4191*10^23, Mercury 3.302*10^23 kg, Ganymede (Jupiter) 1.4819*10^23 kg, Titan (Saturn) 1.3452*10^23 kg, Callisto (Jupiter) 1.0758*10^23 kg, Io (Jupiter) 8.94*10^22 kg, Moon (Earth) 7.3477*10^22 kg, Europa (Jupiter) 4.80*10^22 kg, Triton (Neptune) 2.14*10^22 kg, Eris 1.65*10^22 kg, Pluto 1.30*10^22 kg

By density, in order of distance:

Mercury 5.43 g/cc, Venus 5.24 g/cc, Earth 5.52 g/cc, Moon (Earth) 3.3464 g/cc, Mars 3.940 g/cc, Io (Jupiter) 3.528 g/cc, Europa (Jupiter) 3.01 g/cc, Ganymede (Jupiter) 1.936 g/cc, Callisto (Jupiter) 1.83 g/cc, Titan (Saturn) 1.88 g/cc, Triton (Neptune) 2.061 g/cc, Pluto 1.87 g/cc, Eris 2.43 g/cc

So of the four Galilean moons, Io is all rocky, Europa mostly rocky with a relatively thin layer of ice and a subsurface ocean, and Ganymede and Callisto have much thicker ice and possible subsurface oceans.
 
I was able to see three of them with binoculars. Very impressive and humbling, how four balls of ice and rock could help overturn millennia of human thinking.
 
Let's say you were observing from those moons. What would one see?

Jupiter's equatorial and polar angular diameters in degrees:

Io: 19.5,18.2 -- Eur 12.2, 11.4 -- Gan 7.7, 7.2 -- Cal 4.4, 4.1

One's fist at arm's length is about 10d across.

Here are the average angular sizes in arcminutes of the other big moons from each one of them:
  • Io: Eu 16.0, Gan 16.9, Cal 8.8
  • Eur: Io 18.7, Gan 16.9, Cal 8.8
  • Gan: Io 11.7, Eur 10.0, Cal 8.8
  • Cal: Io 6.7, Eur 5.7, Gan 9.6
One would be able to resolve them as disks, but not much more. The Moon's average angular diameter from the Earth is 31 arcmin and the Sun's is 32 arcmin.

From Jupiter's distance, the Sun's angular size is 6.1 arcmin, borderline resolvable without a telescope. The solar flux is about 1/27 of that at the Earth's distance, making the illumination roughly that on an overcast day or near sunrise or sunset.  Lux

Mercury, Venus, Earth, and Mars would go back and forth, alternately leading the Sun and trailing the Sun, as we see Venus doing. One would need a telescope to observe the Moon, and even then, one would not be able to see much detail on the Earth.

Maximum elongation, as it's called: Mer 4.3d, Ven 8.0d, Ear 11.1d, Mar 17.0d -- Venus from the Earth: 46d

The Moon from the Earth as seen from Jupiter: 1.7 arcmin, the Earth's angular diameter: 3.4 arcsec
 
The Galilean moons have some orbital resonances, and I decided to check them.

Io, Europa, Ganymede, Callisto, in days:
SSD = JPL Solar System Dynamics

The last one also has the mean orbital angular velocities of the moons in degrees/day: 203.49, 101.37, 50.32, 21.57

The first three of these moons are in a "Laplace resonance", having orbit periods with ratios of 1:2:4 --  Orbital resonance The mean longitudes (roughly their ecliptic longitudes) of those moons satisfy
L(Io) - 3*L(Eur) + 2*L(Gan) = 180d
to first approximation.

I collected these four sets of numbers to try to test this assertion. The first two sets seem derivative of the third one, and while the third one gets a good fit for the Io-Europa resonance, it does not do so for the Europa-Ganymede one or the Laplace interrelationship. But the fourth one got all the interrelationships right.


There are several other orbital resonances in the Solar System, like Jupiter-Saturn 2:5, (Saturn) Mimas-Tethys 1:2, (Saturn) Enceladus-Dione 1:2, (Saturn) Titan-Hyperion 3:4, Neptune-Pluto 2:3.

Several exoplanets are also in orbital resonances, and TRAPPIST-1's seven planets are in a chain of resonances with Laplace-like resonances for each triplet of neighbors.
 
Theory of the Four Great Satellites of Jupiter by R.A. Sampson in 1921 -- a pre-computer calculation of the perturbations of the motions of Jupiter's four big moons, perturbations caused by each other, Jupiter's equatorial bulge, and the Sun (I'm not sure about that, however). This calculation was in the form of trigonometric series for orbit parameters as a function of time.

Over the 19th and early 20th centuries, astronomers did lots of such calculations for planets and moons, often hiring assistants to do the calculations by hand or with adding machines. These assistants were called "computers" back then.

A common thing to calculate was big tables of results, so one could interpolate in them.

Nowadays, one can do numerical integration, something called "Cowell's method" in pre-computer days, and one can use computer algebra to find trig-series solutions.

With the discovery of exoplanets, some such calculations are now done for them also.

Why do such complicated calculations? Predicting positions to compare with observations, for starters. That's how Mercury's extra precession was discovered, by calculating what one would predict from the other planets. That was also good for finding masses of planets without moons, like Venus, and also planets not known to have moons, like Mars before 1877. Not surprisingly, it was good for finding masses of moons like Jupiter's and Saturn's big ones.

That is also good for exoplanet masses, like the masses of the seven TRAPPIST-1 planets.

A successor:
Theory of motion of Jupiter's Galilean satellites by J.H. Lieske in 1977

An interesting consequence is that Io's orbit eccentricity is forced by its resonance with Europa and Ganymede. That means that it alternately goes closer to and farther from Jupiter, and that planet's tides on the moon thus vary, kneading the moon. The other big moons are also kneaded by their planet's tides, though in lesser degree.
 
What spacecraft have visited Jupiter?

 Pioneer 10 -- launched on 1972 Mar 3 -- flew by Jupiter on 1973 Dec 3

 Pioneer 11 -- launched on 1973 Apr 6 -- flew by Jupiter on 1974 Dec 3 -- then flew by Saturn

 Voyager 1 -- launched on 1977 Sep 5 -- flew by Jupiter on 1979 Mar 5 -- then flew by Saturn

 Voyager 2 -- launched on 1977 Aug 20 -- flew by Jupiter on 1979 Jul 9 -- then flew by Saturn, Uranus, and Neptune

 Galileo (spacecraft) -- launched on 1989 Oct 18 -- flew by Venus and the Earth -- entered Jupiter orbit on 1995 Dec 7 -- was sent into Jupiter on 2003 Sep 21.

When it entered Jupiter orbit, it released a probe that entered Jupiter's atmosphere.

The spacecraft's high-gain antenna did not open, strongly limiting how much data the spacecraft could send back.
 
 Ulysses (spacecraft) - launched on 1990 Oct 6 -- flew by Jupiter on 1992 Feb 8 -- entered highly inclined heliocentric orbit for studying the Sun's emissions at high latitudes

 Cassini–Huygens -- launched on 1997 Oct 15 -- flew by Venus twice and then the Earth -- flew by Jupiter on 2000 Dec 30 -- then flew to Saturn and went into orbit there

 New Horizons -- launched on 2006 Jan 19 -- flew by Jupiter on 2007 Feb 28 -- then flew by Pluto and 486958 Arrokoth

 Juno (spacecraft) -- launched on 2011 Aug 5 -- flew by the Earth -- entered Jupiter orbit on 2016 Jul 5 -- still in operation

The recently-launched one:

 Jupiter Icy Moons Explorer (JUICE) -- launched on 2023 Apr 14

Of the flyby missions, three of them had some other primary destination: Ulysses, Cassini, and New Horizons.
 
NASA also has an upcoming Jupiter-moons mission, the  Europa Clipper -- Europa Clipper named after the mid-19th-cy. sailing ships:  Clipper

It is to be launched on a SpaceX Falcon Heavy rocket, though a Space Launch System rocket would have made possible a direct trip to Jupiter, without gravity assists. H
  • 2024 Oct - launch
  • 2025 Feb - Mars flyby
  • 2026 Dec - Earth flyby
  • 2030 Apr - Jupiter arrival
The spacecraft will stay in orbit around Jupiter instead of going into orbit around Europa, because that will keep down its radiation exposure. It will do some 44 flybys over 3.5 years, and it will use gravity assists from that moon, Ganymede, and Callisto to adjust its trajectory. In each flyby, it will travel close to a different part of the moon.

Like JUICE and Juno, the spacecraft will be powered by solar panels. It will have these instruments:
  • Europa Thermal Emission Imaging System (E-THEMIS) -- mid to far infrared
  • Mapping Imaging Spectrometer for Europa (MISE) -- near infrared
  • Europa Imaging System (EIS)
  • Europa Ultraviolet Spectrograph (Europa-UVS)
  • Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON)
  • Europa Clipper Magnetometer (ECM)
  • Plasma Instrument for Magnetic Sounding (PIMS) -- for Jupiter's magnetosphere and Europa's ionosphere
  • Mass Spectrometer for Planetary Exploration (MASPEX) -- for gases released from Europa
  • Surface Dust Analyzer (SUDA) -- for dust particles released from Europa
  • Gravity/Radio Science
 
JUICE and EC have a lot of overlap:
  • Ultraviolet, visible-light, and infrared imaging and spectroscopy
  • Radar and lidar
  • Space environment
  • Spacecraft tracking for navigation and gravity-field structure
For gravity fields, the obvious thing that one measures is an object's mass, but one can measure the departure from sphericity of an object's gravitational field.

This nonsphericity has been measured to varying degree for Mercury, Venus, the Earth, the Moon, Mars, Vesta, Ceres, Jupiter, Io, Europa, Ganymede, Callisto, Saturn, Titan, Uranus, Neptune.
 
To see how well one could do, I checked on pre-spacecraft measurements of the masses of the inner planets, notably this one
Title: On the System of the Planets Mercury, Venus, the Earth, and Mars
Authors: Leverrier, M.
Journal: Monthly Notices of the Royal Astronomical Society, Vol. 22, p.257
Bibliographic Code: 1862MNRAS..22..257L
Click to expand...
That was  Urbain Le Verrier
He gave his numbers as 1 / something:
Me: 3,000,000, Ve: 401,847, Ea: 354,936, Ma: 2,680,337

List of estimates for Mercury
Title: On the probable Mass and Density of Mercury
Authors: See, T. J. J.
Journal: Astronomische Nachrichten, volume 156, Issue 23, p.361
Bibliographic Code: 1901AN....156..361S
Click to expand...
From orbit-perturbation calculations, from 1 / 3,000,000 to 1 / 7,500,000.

 Standard gravitational parameter and Astrodynamic Parameters - some recent numbers are
Me: 6,023,657, Ve: 408,524, Ea: 332,946.049, Ma: 3,098,704

So ULV got a good mass for Venus, a somewhat good mass for Mars, and an awful mass for Mercury, all from how these planets pull on other planets.

He and John Couch Adams predicted the planet Neptune from discrepancies in the motion of Uranus, and he also proposed an intra-Mercurian planet to account for the extra precession of Mercury that he calculated. It was a legitimate hypothesis, so we should not beat up ULV about it, but no intra-Mercurian planets were ever found, and there is nothing there that's larger than about 20 kilometers. An alternate hypothesis was that the law of gravity is not quite correct, and that turned out to be correct in the form of general relativity.
 
Planetary Satellite Physical Parameters at JPL SSD

Using the  Standard gravitational parameter in km^3/s^2 -- the mass multiplied by the Newtonian gravitational constant. That can often be measured to much greater accuracy than that constant itself.

Io: 5959.91547(135), Europa: 3202.71210(181), Ganymede: 9887.83275(247), Callisto: 7179.28340(324)

Less than 1 part per million, mainly from radio tracking of the Galileo spacecraft. For reference:

Moon: 4904.8695(9), Mars: 42828.37(2), Earth: 398600.4418(8), Jupiter 126686534(9) , Sun: 1.32712440018(9) * 10^11

-

Galileo Gravity Results and the Internal Structure of Io | Science and p5262_0709.pdf in 1996

From tracking the Pioneer and Voyager spacecraft:

Io: 5961(10), Eur: 3200(10), Gan: 9887(3), Cal: 7181(3)

Very close to the Galileo-flyby results.

-

This was published after the Pioneers' flybys, but before the Voyagers' flybys.

Theory of motion of Jupiter's Galilean satellites
Title: Theory of motion of Jupiter's Galilean satellites
Authors: Lieske, J. H.
Journal: Astronomy and Astrophysics, vol. 56, no. 3, Apr. 1977, p. 333-352.
Bibliographic Code: 1977A&A....56..333L
Click to expand...

Quoted numbers: Io: 449.7, Eur: 252.9, Gan: 798.8, Cal: 450.4
These are in units of 10^(-7) Jupiter masses. Converted to km^3/s^2:
Io: 5697, Eur: 3204, Gan: 10120, Cal: 5706

-

I couldn't find what numbers RA Sampson had used in his work, but I've found this from 1895:

Adams's determination of the masses of Jupiter's satellites
Title: Adams's determination of the masses of Jupiter's satellites
Authors: Sampson, R. A.
Journal: The Observatory, Vol. 18, p. 293-295 (1895)
Bibliographic Code: 1895Obs....18..293S
Click to expand...
Unit: 10^(-7) Jupiter masses
Adams: Io: 283.113, Eur: 232.355, Gan: 812.453, Cal: 214.880
Damoiseau: Io: 168.770, Eur: 232.2696, Gan: 884.370, Cal: 424.751
Converted to km^3/s^2
Io: 3587, Eur: 2944, Gan: 10293, Cal: 2722
Io: 2138, Eur: 2943, Gan: 11204, Cal: 5381

The astronomers:
John Couch Adams
Marie-Charles Damoiseau
 
Collecting these results,
Io: 3587, Eur: 2944, Gan: 10293, Cal: 2722 -- Damoiseau
Io: 2138, Eur: 2943, Gan: 11204, Cal: 5381 -- Adams
Io: 5697, Eur: 3204, Gan: 10120, Cal: 5706 -- Lieske
Io: 5961, Eur: 3200, Gan: 9887, Cal: 7181 -- Pioneer, Voyager probes
Io: 5960, Eur: 3203, Gan: 9888, Cal: 7179 -- Galileo probe

Europa's and Ganymede's masses were fairly well-determined from their perturbations of the other moons' motions, but Io's and Callisto's weren't.
 
 Volcanism on Io - a remarkable discovery from the Voyager missions.

Before the flyby of Voyager 1 in 1979, Io was known to have some odd features, like extra mid-infrared emission and sodium-vapor spectral lines, but it was hard to say what those were.

Shortly before that flyby, three planetologists published a paper about their predictions of lots of volcanism from  Tidal heating

(Energy per unit mass dissipation rate) ~ (dissipation fraction) * (radius)^2 * (orbit eccentricity)^2 / (distance from primary)^(15/2)

Io should be heated much more than the other  Galilean moons -- Europa only 1/8.4 as much, Ganymede 1/7200 as much, and Callisto 1/13100 as much.
 
Then Voyager 1 sent back its closeup pictures. The moon had a multicolored surface with flow features and irregular-shaped depressions but with no evidence of impact craters -- its surface was young and reworked by volcanic activity.

Then technician Linda Morabito was working with some optical-navigation imagines, images deliberately overexposed to make stars visible. An odd feature was visible in one of the pictures: a sort of crescent-shaped something-or-other on the limb or line-of-sight edge of Io. I remember once watching a documentary on outer-planet discoveries and she showed off her use of a Modcomp early desktop computer to display the discovery picture.

What was that feature? It resembled no known camera artifact. If it was another moon, then that moon would have to be big enough to have been discovered long ago.

BTW, when Galileo announced his discovery of Jupiter's four big moons, one critic called them artifacts of the telescope. Galileo offered a big reward for a telescope that only makes such artifacts around Jupiter.

Back to that odd feature. Its location on Io was at one of those depressions, meaning that it was a plume from an active volcano.

Some other plumes were discovered in Voyager pictures, and it was evidence of eight volcanoes erupting at that time.
 
Compared to the Earth's Moon, the diameters of the four Galiliean moons are 1.05, 0.90, 1.52, 1.39.

Turning from Io to the other three Galilean moons, their surfaces are covered with ice, though rather impure ice.

Europa has relatively few impact craters, but instead has lots of colored streaks on its surface. Cracks in its surface ice? Estimates of that ice's thickness vary widely, and this variation may be reconciled by supposing the moon to have a layer of cold hard ice on top of a layer of warm soft ice.

Europa has a magnetic field that is induced by traveling through Jupiter's magnetic field, and that induction indicates a subsurface conductive layer. Soft ice? Liquid water?

So Europa's upper layers are:
  • Cold hard ice
  • Warm soft ice?
  • Liquid water?
The numbers I've found for these layers' thicknesses are *very* hand-wavy, and I'm reluctant to quote any of them.

The heating for the soft ice and/or liquid water would come from tidal kneading.

The rarity of impact craters suggests that the moon has been resurfaced relatively rapidly by geological standards, much like the Earth's surface. Another similarity is the possibility of tectonics on Europa's surface: spreading and subduction, much like the Earth's oceanic crust. But Europa has no analog of the Earth's continental crust. and the putative spreading and subduction features could have other origins.

There is some evidence of water-vapor plumes from Europa, much like what is known for Saturn's moon Enceladus.
 
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