Planetary GeologyJupiter & Io

Io passing in front of Jupiter

Jupiter 2

Jupiter is at a distance of about ~5 AU but is usually the 2nd brightest planet in the night sky (after Venus, though sometimes Mars can be as bright).

It has an orbital period of nearly 12 years and has a radius 11 times that of Earth.

Though it is very large and very massive, it has a mean density of only 1.3 g/cm3....which tells us that it is composed primarily of light elements and not rock and metal. Jupiter has been observed during flybys of Pioneer 10 &11 (early 1970s), Voyager 1 & 2 (late 1970s), Ulysses, Cassini, and New Horizons (2007). Only the Galileo spacecraft has orbited Jupiter.

Jupiters rings were discovered by Voyager and have since been imaged from Earth using infrared wavelengths (below).

Jupiter: A Gas Giant 3

Jupiter is the most massive planet in our Solar System, and its formation has had a profound effect the evolution of our Solar System (it affects the orbits of other planets, it was responsible for formation of the asteroid belt, it can redirect the orbits of passing objects, etc.).

It accounts for ~77% of the mass of the planets and is >300 times the mass of Earth.

Though the composition of Jupiter is very similar to the Sun (more on this in a minute) and it is very large for a planet, it is still much, much smaller than the Sun.

It is a bit larger than Saturn but has a much greater mass; this is because it is easier to compress gases/fluids and make something denser without making it much larger.

Jupiter: Composition 4

Jupiter does not have a solid surface but is instead dominated by H and He. Trace amounts of methane and ammonia are also present.

The bulk composition of Jupiter is thus very similar to the Sun, but it is not quite massive enough to ignite (does not cause fusion).

The atmosphere increases in density with depth, and at some point (though we don’t know exactly where) the material must become a fluid in which H behaves as an electrically conducting metal.

Beneath this, there may lie a core composed of heavier elements.

Together, these properties somehow produce a very intense magnetic field, though the exact process is unknown.

Jupiter: Internal Structure 5

The upper atmosphere contains ammonia (NH3) crystals and clouds, whereas ice clouds and water droplets are present below.

At some depth, the hydrogen starts to behave like fluid metallic hydrogen.

The amount of water in Jupiter is unknown, but this will be measured by an upcoming mission.

In addition, we know the general aspects of the interior structure of Jupiter (as shown in these diagrams), but we need more information on the details (where do phase transitions take place, where/how is the magnetic field generated, etc.).

Jupiter: Magnetic Field 6

The magnetic poles of Jupiter are shifted by about 10° from the rotational pole, similar to Earth.

In addition to the strong magnetic field, Jupiter also exhibits a plasma torus.

This plasma torus is produced by Io; volcanic activity on Io releases S, Cl, O, and Na in to the atmosphere, and some of this escapes and interacts with Jupiter’s magnetic field, where it is ionized and produces the torus.

The plasma torus is in the same rotational plane as the magnetic field, thus Io actually goes above the plasma torus at some points in its orbit and below it at others.

The magnetic field of Jupiter is ~14 times stronger than that of Earth, and it is the strongest in our Solar System.

Jupiter: Magnetic Field 7

Aurorae on Jupiter are 10-100 times brighter than the ‘northern lights’ seen on Earth.

They are always present on Jupiter and are caused by interaction of charged particles from the Sun (solar wind) with the strong magnetic field of Jupiter, and this interaction causes gases in the upper atmosphere to fluoresce near the magnetic poles

The image to the right shows the aurorae as seen in ultraviolet images captured by the Hubble Space Telescope. The background image was acquired at a visible wavelength of light.

The Jovian Atmosphere 8

The colorful bands that are observed on Jupiter are all atmospheric effects; none of these are surface features (there is no observable ‘surface’ for a gas giant like Jupiter).

As on Earth, different parts of the atmosphere move in different directions, as shown in the diagram on the right.

The brown and reddish colors are believed to be the result of ultraviolet (UV) radiation interacting with materials as they rise into the upper atmosphere; the exact composition of these phases is unknown but they could be hydrocarbons, S, or P.

Storms on Jupiter 9

The ‘Great Red Spot’ on Jupiter is an anticyclonic storm in the southern hemisphere of the planet.

- Oval in shape

- 2-3x the size of Earth!

- Represents a large storm in the atmosphere

- Red color could indicate the presence of complex organic molecules (from UV interaction)

- Spot takes ~6 days for full rotation

- Spot moves across planet and does not remain ‘fixed’ relative to any stationary point that would be below

- It has been present for at least a few hundred years and may be a permanent feature on Jupiter.

Storms on Jupiter 10

A new red spot appeared on Jupiter in 2000. This spot is referred to as ‘Oval BA’ and formed when 3 white spots combined (see image on bottom right).

It lies south of the Great Red Spot and has passed by it unperturbed several times since its initial appearance.

It is unclear how long this new storm will last or how big it may become, but it is currently about half the size of the Great Red Spot.

Storms on Jupiter 11

Then a third red spot appeared!

However, this one is in the same latitude zone as the Great Red Spot and has been affected by that large storm as it passed by (see images below).

It is likely that this smaller spot will be consumed by the Great Red Spot, and this may be a reason why the giant storm has persisted for so long.

Impacts on Jupiter 12

We can also image Jupiter at thermal infrared wavelengths to understand the thermal structure in the atmosphere and the distribution of debris.

In these two images, the results of an impacting object can be seen as a bright spot in the lower left.In the later image (right), the material has been shattered and pulled apart. Eventually the effects of this impact will no longer be apparent. Bright tones in these images indicate more debris.

July 20, 2009 August 16, 2009

Lightning on Jupiter 13

The Galileo, Cassini, and New Horizons spacecraft confirmed that Jupiter has lightning in its atmosphere.

In the images below, the bright flashes are lightning in ongoing storms. The image on the bottom right was taken by the Cassini spacecraft during a flyby and shows that the lightning is visible at night (left half) in the spots where storms are clearly visible during the day (right half).

The images on the bottom left were acquired by the Galileo spacecraft on the nightside of Jupiter, and the atmospheric clouds are faintly visible due to moonlight from Io.

The lightning is more or less similar to what happens on Earth, but the flashes are ~10 times brighter.

The presence of lightning suggests the presence of water, because you need a dipolar molecule like water to cause a separation of electrical charge in order to have lightning.

The Juno Mission to Jupiter 14

The Juno mission launched August 5, 2011 and arrived at Jupiter in July 2016.It is only the 2nd spacecraft to orbit Jupiter.

Some of the questions that this mission will try to answer are:

- How did the giant planets form? (get better estimates of core mass) - Does Jupiter have a rock-ice core, and if so how large is it? - Is the composition of Jupiter different from the original solar nebula? If so, why? - How deep into the atmosphere do atmospheric features (e.g., great red spot) go? - How does the dynamo on Jupiter work? - What is the abundance of water on Jupiter?

The Juno Mission to Jupiter 15

The Moons of Jupiter 16

Io

Europa

Ganymede

Callisto

Jupiter has 63 moons, though the 4 largest moons are the most well known. Several dozen of these were only discovered in the past 10 years.

The 4 largest moons are believed to have been first observed by Galileo Galilei in 1610, thus they are often called the ‘Galilean moons’.

Io is a volcanically active moon, whereas the other 3 large moons are dominated by ice and may contain liquid water oceans beneath their icy crust.

Ganymede is the largest moon and is actually larger than Mercury!

These 4 moons are spheroidal and would be considered dwarf planets if they orbited the Sun directly (like Ceres).

The Galilean moons are believed to have formed by accretion of dust and gas in a disk surrounding Jupiter, similar to a protoplanetary disk. They are not consistent with captured objects.

Galilean Moons 17

Io Europa Ganymede Callisto

Io Europa

Ganymede Callisto

The 4 Galilean moons are quite diverse in their geologic histories, but they do share some similarities as well.

They are all differentiated, and several likely have distinct cores.

Three of the four are believed to have significant amounts of water beneath the outer crust, and this may be in the form of liquid water oceans. This has opened up the possibility that these moons may be good places for life beyond Earth.

Io: A Volcanically Active Moon 18

‘True’ Color Image of Io

Voyager image of Pele region on Ioshowing an active eruption.

Io is perhaps one of the most fascinating moons in the outer solar system because of its extreme volcanic activity. This means that the surface is continuously being covered with new material and that it is a geologically young surface (impact craters are extremely rare).

Io is one of only several objects in the solar system that exhibits current volcanic activity.

To the naked eye, Io would have a yellowish hue marked with red and black spots.

Io: Tidal Heating 19

Io exhibits an orbital resonance with Jupiter and gravitational pull from Europa, Ganymede, and Callisto, and this causes the orbit of Io to be elliptical.

The elliptical orbit means that the gravitational pull on Io from Jupiter changes depending on where Io is in its orbit.

This causes the moon to flex in and out (kind of like squeezing a tennis ball).

This flexing produces internal friction, which releases energy in the form of heat.

This continual tugging by Jupiter has kept the moon warm its entire life, and this has sustained volcanic activity on Io.

If Io had a more circular orbit and less tidal heating, then it would likely be volcanically dead today, more like Earth’s moon.

Io: Internal Structure 20

Io has the highest mean density of all moons in the Solar System (3.53 g/cm3).This tells us that it is composed mostly of silicates (rock) and metal; thus it is closer in composition to the terrestrial planets than the other moons of Jupiter.

The crust of Io is silicate-rich and contains numerous sulfur compounds.Beneath the crust is a a silicate-rich mantle, and beneath that is an Fe or Fe-S rich core.

The Galileo mission did not detect an intrinsic magnetic field at Io, thus there appears to be no obvious convection.

The mantle is olivine-rich and likely has more Fe than the Moon or Earth.

The volcanic activity on Io requires that at least 10% of the mantle is molten, though it could be much higher. The heat source on Io is from tidal heating, not radioactive decay as is the case for Earth.

Global Mosaic of Io 21

The extensive volcanic activity on Io and the presence of numerous sulfur compounds gives Io an amazing diversity of color.

The silicate-rich magmas are dark (like basalt), whereas the sulfur compounds produce yellow, red, orange, brown, and whitish tones. Coatings of S-compounds on dark silicate lava flows can often produce greenish tones.

Color Variations on Io 22

The diversity of surface compositions is more striking in false-color images such as this one, where the circular features surround volcanic sources and the red/yellow tones indicate sulfur-rich compounds.

Most of the lava flows are mafic or ultramafic (Mg/Fe rich), but large amounts of sulfur are released during the volcanic eruptions.

The gases and particles from the eruptions can reach heights of 100s of kilometers and form large umbrella shaped plumes. This spreads the sulfur-rich material over the surface.

Sulfur on Io 23

Sulfur gas consisting of pairs of sulfur atoms (S2) is ejected from the hot vents of Io's volcanoes (green arrow). The sulfur gas lands on the cold surface, where the sulfur atoms rearrange into molecules of three or four atoms (S3, S4), which give the surface a red color. Eventually the atoms rearrange into their most stable configuration, rings of eight atoms (S8), which form ordinary pale yellow sulfur.

Io: Volcanic Eruptions 24

Because Io is volcanically active, and volcanic eruptions tend to be hotter than surrounding terrain, it is useful to image the surface of Io at thermal infrared wavelengths.

In the image below (on the right), measurements at thermal wavelengths (5 µm in this case) can act as temperature maps, showing ongoing volcanic activity.

We can compare this thermal map to images acquired at visible wavelengths (image on left) and see which geographic features are associated with the current eruptions.

Io: Volcanic Eruptions 25

Io: Volcanic Eruptions 26

Galileo images also captured active lava flows, and it was shown that these are sometimes preceded by fire fountain eruptions.

In the image on the left, the red/yellow colors are actually a sketch of where the fire fountain is; the image on the right shows the actual lava flow that followed.

Io: Volcanic Eruptions 27

The surface of Io exhibits numerous volcanic vents and depressions, such as those shown to the right and below.

Less common are shield volcanoes, but one is visible in the image to the right adjacent to the depression.

It’s rather rare to observe shield volcanoes on Io because the lavas tend to run out in thin, long flows (low viscosity).

Therefore, it’s difficult for lava flows to build up and form large shield volcanoes (compare this to the giant shield volcanoes on Mars). The brighter flows shown here could be sulfur-rich, whereas the darker flows may be more silicate-rich.

Mountains on Io 28

Io also has mountains, as shown in this image acquired by the Galileo spacecraft in 2000.The mountain ridge along the left side of the image rises ~7 km above the plains.

Most of the mountains on Io are not related to volcanoes but are instead believed to result from tectonic activity, likely from uplifting of the crust along thrust faults. Younger mountains are more jagged and angular, whereas older mountains have subdued topography (top middle of image).

Uplifting and compressional stresses can lead to thrust faults, which can in turn lead to mountain ridges.

The fault may not always be visible at the surface, in which case it is called a ‘blind’ thrust fault.

Io: Volcanic Eruptions 29

The New Horizons mission also captured these images during a flyby of Io.

These images were acquired when Io was in the shadow of Jupiter (an eclipse), so only hot regions are visible, including lava flows, active volcanoes, and the very tenuous atmosphere that surround Io.

The New Horizons mission to Pluto has also imaged active volcanic activity on Io (left, below), especially in the north polar region (‘Tvashtar’ region).

Global Mosaic of Io 30

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The extensive volcanic activity on Io and the presence of numerous sulfur compounds gives Io an amazing diversity of color.

The silicate-rich magmas are dark (like basalt), whereas the sulfur compounds produce yellow, red, orange, brown, and whitish tones. Coatings of S-compounds on dark silicate lava flows can often produce greenish tones.

Io: Tohil-Culann Region 31

This region on Io (Tohil-Culann region) exhibits interconnected mountains and volcanoes, and it is also one of the more colorful regions on Io.

The center of the volcano (the patera, seen at the top center) is associated with bright orange/red deposits.

Just to the south are the Tohil mountains.

The volcanoes produce both reddish sulfur-rich lavas and dark silicate-rich lavas, and the white regions are likely cooler flows rich in SO2.

Mountains and Erosion on Io 32

Another image showing the Tohil mountains; the lighting at this time of day highlights the rugged nature of the mountains and the variation in topography.

Some mountain ridges, such as the ones below, exhibit slumping. On Earth this type of erosion could be caused by wind or water, but on Io it is driven by gravity.

Geologic Map of Io 33

Map by D. Williams (ASU)