Download Planets Notes 5 - 1 Notes 5: Planetary Interiors 5.1 Layers The

Notes 5: Planetary Interiors
5.1 Layers
The interiors of planets can be divided into three basic parts, though it will depend upon which
type of planet you are talking about.
Terrestrial – core, mantle, crust
Giants – core, mantle, gas layer
The term “mantle” is loosely used here and would be an intermediate density, liquid-like layer
that is a transition between the core and the top most (surface) layer.
Of course you can’t see the interior of a planet directly, so how do we determine what the inside
of a planet is like? First of all we have some observational information such as the mass, radius,
rotation period, oblateness, gravity field, magnetic field (or not), energy output (or not), and
surface layer composition. The mass and radius can be used to provide information about the
density, and these other attributes tell us about interior conditions based upon what is revealed.
For example, what if a planet has a high density but no magnetic field? How is that reconciled?
The fact that planets are still giving off energy also tells us something about their interior, how
easily energy flows, the methods that are available for the energy flow and likely ramifications of
energy loss. All of these observable features and the rules of fun physics give us an idea about
what it is like inside of these objects and I’ll get to them later.
But first a word of caution about density. Often you usually calculate a mean or average density,
which is basically mass/volume. This is the compressed density since the mass of a planet will
compress the interior and influence the value for the density of that central material. The
uncompressed density of the material would be less than this value, so that has to be considered
when using density to determine the chemical/mineral composition of the interior of a planet.
The Earth has the highest compressed density, but Mercury has material that has a greater
average density when it isn’t compressed. This can be seen in the high pressure that is estimated
for the Earth’s center (3.6 Mbars), compared to the center of Mercury (0.4 Mbars). Typically the
average, compressed density is given since that is easier to calculate, though it may not give a
realistic view of the composition of a planet’s interior.
Typically in the solar system if the density is <1000 kg/m3, the object is rich in ices, or is
relatively porous, or has a gaseous composition. If the density is around 3000 kg/m3, the
composition is mainly rocky. For densities > 3000 kg/m3, you have some metal content (iron,
nickel, etc) included in the composition.
The shape of an object is also very revealing, since this will depend on the size, density, material
strength, rotation rate, history and environment in which the object is found. The degree that an
object is fluid (its plasticity or rheidity) will also determine the shape of the object when it is
combined with rotation or tidal forces.
5.2 Pressure and Density
Planets Notes 5 - 1
The best way to determine the internal structure of a planet once you’ve gathered up all of the
information that you can is to use hydrostatic equilibrium – the balance of gravity and pressure.
This relation can be used with stars, and can also be used with planets. The stability of an object
indicates that the two forces (gravity in, pressure out) are in balance. If that weren’t the case the
object would expand or collapse. Since planets don’t do that, they are in hydrostatic equilibrium.
While this seems like a no-brainer, you must remember that planets were at one time not as
compacted and compressed down to the sizes they are now (especially the giants). They were
not entirely in equilibrium at all times in the past.
The relation for hydrostatic equilibrium is
dP
dr
(r)
GM(r)
r2
5-1a
Even though this is a calculus formula, we won’t really do anything serious with it, but it does
show how it relates to the structure of the planet. Basically the dP and dr terms refer to how
Pressure and Radius (distance out) change relative to one another or how small changes in them
are dependent upon other characteristics. Since an object may not have the same density at every
layer, the density, , will vary with depth, so it is a function or r. The mass, M, is also a function
of depth since you have a certain amount of mass within a specific radius and that is what is
providing the gravitational influence.
The negative sign in the relation tells us that the pressure and density vary in opposite senses – if
the radius increases, the pressure decreases. This makes sense – that’s what you’d expect as you
go from the center of a planet outwards. Things are less dense further out from the center. Often
the relation is written without the (r) parts since it is just implied, and it is easier and less
confusing to write it that way.
dP
dr
GM
r2
5-1b
You just have to remember that the (r) parts are still there and the density and mass values are
varying in value with distance from the center r.
To actually use this formula, you’d have to integrate it over the radius of the star from the center
to the surface and account for every change in density along the way. Fortunately, there are
some short cuts, based on assumptions about properties, which allow us to derive relations that
can be used. One simple assumption is that the density is constant. If that assumption is made
than you could approximate the conditions at the center of the planet –
Pc
3GM 2
8 R4
5-2
Where M and R are the entire mass and radius of the planet. This relationship gives a good
approximation about the conditions in a planet’s interior, though technically it is a lower limit of
the central pressure.
Another thing that factors into the calculations of planet interiors are the equations of state,
which is the relationship between composition, P, T, and . These relationships can vary wildly,
especially in the centers of planets – and of course the planets have a wide range of materials
inside of them, from hydrogen and helium to iron and nickel.
Planets Notes 5 - 2
This also brings up another problem that we have to deal with. The extreme conditions in the
cores of planets put us into realms of physical laws that are not well defined. It is not easy to
duplicate the conditions inside of a planet’s interior in physics lab, after all pressures that are at
the level of Mbar (millions of bars) cannot be sustained for long. Also those pressures have to be
combined with very high temperatures – it is just not very easy to do in a lab.
There are also situations where the form of one material (gas, solid or liquid) would not be the
same as the form of another material under the same conditions of temperature and pressure.
The melting point of some solids may depend upon the relative amounts of the elements in the
mixture. A eutectic mixture is one that has a low melting point that depends upon the relative
amounts of the materials in the mixture. At the eutectic temperature, the material will become
crystallized from a liquid state – but this depends upon the amounts of various substances in the
mixture. At a similar temperature the material may be partly liquid, and partly solid.
Let’s look at the types of conditions that we find for all sorts of material inside of the planets.
The most common material, hydrogen and helium, can be found in a range of environments
within planets. The surface layers of the gas giants and ice giants can have temperatures of
around 50-150 K, while deep inside the temperatures get to levels of up to 10,000-20,000 K.
This large range in temperature has to also be combined with the large range of pressures, which
goes from nearly 0 at the surface, all of the way to 20-80 Mbars in the centers of the gas giants.
The large range of pressure and temperature causes hydrogen to go through various phases.
Low pressure (< 1 MB) – it is found as a solid (when you also have low temperatures), or
as a liquid H2, or gas (with high temperature)
Intermediate pressure (1 MB<P<10 MB) – a metallic solid (very low temperatures), or as
a metallic liquid (high temperature), or possibly as a gas (not likely, since it must be very
hot)
High pressure (P>10 MB) – a metallic liquid
The form that you find hydrogen in is complicated by the presence of helium, which may or may
not mix with the hydrogen. Helium becomes a liquid metal at higher pressures than are
observed, so it is likely not a liquid metal in Jupiter or Saturn. So while hydrogen may be in one
form at a particular layer, helium may be in a different form in that same layer. Generally the
pressure and temperature must both be high for the liquid helium & hydrogen to mix fully, which
would only occur well inside the planet. Outside of that region, they are separate entities in the
layers they are found in.
Water ice can take on 15 different crystalline forms with the arrangement of the water molecule
varying in each structure. These different forms of water ice also have different densities
ranging from 920 kg/m3 up to 1660 kg/m3. It is also possible for water ices to be found in
various forms at higher pressure (1 MB) and temperatures up to 200 K. Water can also be
ionized, especially at high temperature and pressure. The behavior of other types of ices, such as
methane, ammonia, CO2, etc, isn’t as well known since they are not well studied in general.
Planets Notes 5 - 3
The phase diagrams (what form a material takes) are very complex for rocks. Different rocks
with different compositions complicate the picture. In general minerals tend to change as you go
to greater and greater depths. This is seen easiest on the Earth, since we don’t have enough
information on the other worlds. The primary minerals in Earth’s upper mantle are olivine and
pyroxene. The structure of these minerals changes depending upon their environment and as the
molecules get more tightly packed with an increase in density, there can be a phase change in the
mineral.
For example, at a 400 km depth in the Earth, olivine changes its structure into a different
mineral, wadsleyite, and that at about 520 km depth it is found in the form of ringwoodite, which
has a spinel structure. At around 660 km, the spinel structure decomposes into magnesium oxide
(periclase) and perovskite, which is stable at very high pressures. Perovskite could be the
dominant rock in the core of the Earth.
Of course the core of the Earth is dominated by iron, but it can’t be composed of only iron since
it doesn’t have a density that corresponds to that. The iron is mixed with a relatively low density
material, such as sulfur, oxygen or hydrogen. Remember, that in the formation of the solar
system iron oxide, FeO, and troilite (FeS), were commonly formed? The FeO would have likely
gone into the production of olivine, and pyroxene, while the troilite, which doesn’t mix well with
others, would have likely sunk to the core. So it is likely that there is a fair amount of sulfur in
the Earth’s core.
5.3 Other Clues to Planet Interiors
Let’s finish up the list of the other characteristics of a planet that reveal something about the
interior.
Rotation – Actually this isn’t really a major part, but is can be used in other ways, particularly in
the measure of the moment of inertia, and it can also influence a planet’s oblateness. A word on
the moment of inertia first. This is the measure of the resistance to an object’s movement in an
angular way (like rotation), and depends upon how the mass is distributed, which naturally
affects the motion. A non spherical, non-uniform distribution of material will cause a planet to
rotate with a wobble, which is actually what the Earth does.
Oblateness – a rapid rotation and the degree of fluidity can affect this and make objects thicker at
the equator than in the pole to pole distance. The formula for geometric oblateness = (a-b)/a,
where a=equatorial width, b=polar width. Saturn has the largest oblateness with a value of 0.10.
Gravity field – depends upon the mass distribution of a planet. If the planet isn’t a sphere, with a
uniform distribution of material, the gravity field will not be uniform across the surface or in a
spherical form. This can also be influenced by strong density fluctuations in the interior. It is
also possible for the gravitational center of a planet to not correspond to the physical center. The
gravitational field can be very complex depending upon the amount of irregularity in the mass
and size distribution.
Planets Notes 5 - 4
Magnetic field – just having the presence or lack of a magnetic field tells us quite a bit about the
interior of a planet. There are various things that can help us determine the internal structure of a
world from the magnetic field. For example:
What is the form of the magnetic field? Is it a dipole or is it poorly defined?
What is the source material of the field? It must be some sort of electrically conducting
medium if it is formed via the dynamo mechanism.
How has the magnetic field changed over time? How has its strength or orientation
changed over time? We usually do not have enough information to determine the
magnetic field variations over time on other worlds, but can only see the current strength
and orientation. This is dependent upon the rotation of the object and the internal
alignment of structures contributing to the field. It is also possible to match up the
current magnetic field configuration to theoretical models.
Energy output – a side effect of planets cooling off over time. But what can this tell us?
How hot are they inside? What sort of temperature gradient exists within them?
Why are they hot? Usually it is a by-product of their formation or due to material that
they formed from.
How quickly are they cooling down? This will depend upon the temperature gradient,
and this influences the state of the material inside them.
How is the heat flowing? Is it via convection, conduction or radiative?
What about energy input from outside? This could be from impacts and is generally not a
current energy source for most objects. Usually the most significant external influence
on an object is tidal heating, which has been used to explain the excess energy output of
several satellites.
Surface layer composition – generally we have only a limited amount of sampling, and can only
get some information from spectra. However this really only tells us about the surface layers and
not a lot about the interior. The surface composition has likely been influenced by the interior,
but it is still probably not a good indicator about anything that is particularly deep within the
planet. The surface material may have also been influenced by external factors, and that will
also skew the data.
Seismology – There is very little information about seismic activity on worlds, which is currently
limited to observations of only the Earth, the Moon, and Mars. Seismic activity can provide
information on the flexibility of the material inside of a world, the nature of the material, its
density (which could lead to information on composition), internal structures, etc. At this time
we know that the Earth is much more seismically active compared to the Moon and Mars, with
the earthquakes coming from locations relatively close to the surface (down to only 800 km),
while on the Moon the quakes are all fairly deep in their origin. The Mars data is very limited
and shows only minor activity, possibly similar to the Moon.
5.4 Interiors of the planets - Terrestrials
Earth
The Earth’s core is comprised of 80-90% Iron (in an iron-nickel alloy). The remainder of the
material is comprised of sulfur, cobalt, etc. Even though the inner core is hottest, it is solid due
Planets Notes 5 - 5
to the high pressure. The outer core is liquid, and the motion of the material in the outer core
helps produce the Earth’s magnetic field. Material is estimated to move at a rate of 0.02 cm/s in
the outer core.
On top of the core is the rocky mantle. This is comprised of siderophile rich rocks (iron loving),
which are found in multiple layers. These layers or structures can have different mineral phases
and different rock types, such as the change of olivine at great depth. There are also likely
lateral differences in the structure of the mantle (not just radial differences). Apart from olivinerich rocks, there are pyroxene-rich rocks as well as others in the upper mantle. These rocks are
denser than rocks found on the surface and are more iron rich, however they can be erupted from
deep volcanoes and can be found on the Earth’s surface.
The upper part of the mantle includes the mobile, plastic asthenosphere, a layer that drives the
motion of the plates and leads to continents shifting around. The asthenosphere is about 100-200
km below the surface of the Earth and can be about 700 km thick. Above the asthenosphere is
the “rigid” lithosphere, which is about 100 km thick on average. Technically the lithosphere
includes both the upper most part of the mantle and the crust, but we’ll just look at the mantle
part here (crust is part of the next section of notes). Generally speaking it is the lithosphere that
is broken apart due to the motion of the asthenosphere underneath – but saying “lithosphere
tectonics” is too much of a mouthful. Within the lithosphere is a boundary between the crust and
the mantle known as the Mohorovičić, or Moho discontinuity. This marks a change in the
composition of the lithosphere from a denser material underneath to a less dense composition in
the crust.
These lithosphere motions are driven by the temperature gradient in the Earth. There are
currents in the asthenosphere and possibly also larger currents deeper in the mantle. The
temperature gradient in the Earth is between 3 and 30 K/km, which is the amount needed for
convective flow. Some features on the surface are caused by flows of material from the mantle
penetrating through the lithosphere, such as the plume that formed the Hawaiian island chain.
This type of volcanism, in the form of a hot spot, is thought to also occur on other worlds.
So you have a very fluid layer in the asthenosphere which is moving all around quite easily,
while on top you have a less fluid region that is reacting to this motion. It will bend, it will shift,
and of course at times it will also break. The lithosphere is not entirely rigid, nor is it entirely
flexible. It is just reacting to the hotter, deeper, mobile asthenosphere.
While the Earth may have earthquakes and volcanoes today, that will not always be the case. In
the past the Earth had a thicker, deeper asthenosphere, but over time this has gotten thinner, so
that it is only a relatively small part of the Earth’s upper mantle. Once the planet loses most of
its heat there will be nothing to drive the plate tectonics, volcanism, and earthquakes, so it will
probably be a much more boring place to live than it is today.
Moon
The Moon can be considered as pretty much a piece of the Earth’s mantle that has been broken
off. There is not much evidence for a significant metallic core. So either there is a very small
iron core (300-400 km), or an iron-rich core that extends out further (500-600 km). Either way,
Planets Notes 5 - 6
there isn’t much of a magnetic field today, and currently there is only evidence of a weak
remnant magnetic field (paleomagnetic). There was probably a stronger magnetic field in the
past when the object was rotating faster and had a mobile interior.
The crust of the Moon is about 60 km thick and comprised of anorthosites and anorthositic
gabbro (feldspar rich minerals). The upper layers of the crust are likely significantly fractured by
all of the impacts on the surface, while further down in the mantle, it is pretty similar to the
Earth’s mantle. Actually it is quite rigid – the depth of moon-quakes is on the order of 800-1000
km, indicating that there is not much mobility in the upper mantle layer. The fact that the surface
of the Moon has quite a few low density rocks on it indicates that there was some melting of the
Moon’s interior in the past, so that these low density minerals could float to the surface.
Gravity maps of the Moon indicate circular regions of localized high gravity over mainly the
Mare regions of the near side of the Moon. These are referred to as mascons (for mass
concentrations), since they appear to be associated with the rebound of the interior of the Moon
following the impacts that caused the basins and then the flow of basaltic material in the mare.
The lack of such features on the far side can be associated with the difference in the lithosphere
between the two sides – the near side is about 8 km thinner than the far side.
If the Moon formed from an impact, its formation would have been quick and very hot, however
it should have cooled quickly since it is such a small object. Later radioactive decay would have
heated the interior a bit but not very greatly – perhaps just enough for the material to fill in the
large impact basins (the Mare), but this would have been a slow process. The age difference
between the large impact epoch and the mare formation is seen in the age difference between the
mare basaltic material and the highland rocks.
A likely formation/evolutionary history for the Moon would have the following time-line –
4.527 Gy ago – likely formation due to impact of a large object with the Earth.
4.4 – 4.1 Gy ago – molten surface/crust slowly cools with the low density material
floating to the surface. This also is during the time of the heavy bombardment era which
created large impact features on the surface.
3.8 – 3.2 Gy ago – molten material from the interior fills in the large impact features
forming the mare. This is material that was heated by radioactive decay. Due to the
different crust thicknesses, the mare are found only in regions of relatively thin crust.
3.2 Gy ago – present – the interior continued to cool, and became more rigid. This
decreased the seismic activity on the Moon and also the chances of later volcanic activity.
There has been a revision of the interior of the Moon’s structure due to a reanalysis of the Apollo
seismic data. The new model includes the possibility of structure that is very similar to that of
the Earth, with a solid inner core, surrounded by a fluid outer core. Beyond the cores is a region
of partially melted material, and then the mantle above that. The cores may extend 240 and 330
km from the center, while the edge of the melted material is about 480 km from the center. It is
worth noting the difference between this Moon model and previous ones is the presence of a
partially molten layer. Such a layer is also not observed in the Earth. Previous models of the
Moon tended to vary in the structure of the interior. Odds are this model will change in the
future.
Planets Notes 5 - 7
Venus
Venus is not so far removed from the location of the Earth and it has a similar size, so one would
expect that it would have a similar interior structure compared to the Earth. However, it is closer
to the Sun, so it should have less volatiles, and less sulfur. This would alter the core structure of
Venus. Sulfur, when mixed with iron (iron sulfide, troilite), lowers the melting temperature of
the core. Less sulfur implies that Venus’ core may not be liquid. Also the mass is less, so
there is less internal pressure, less heating, and likely overall less iron. Are the cores of Venus
like those of the Earth? And if so, shouldn’t Venus have a strong magnetic field like the Earth?
Nope, you also need rotation, and since Venus is such a slow spinner it does not have a
measureable magnetic field. So what about the core? It seems likely that the core is not as large
as that of the Earth, nor does it appear to be comprised of a substantial liquid layer, but it appears
that there is likely a part of the core that is liquid. So not quite like the Earth, but similar. This
type of structure may also prevent it from having significant convection (circulation) that is able
to do much of anything.
The crust of Venus is quite different from the Earth’s. First of all, there does not appear to be
plate tectonics. Why? We’re not sure. Different ideas have been presented. If Venus was
formed with less water, this would raise the melting temperature in the mantle (it would not be as
mobile) and this would result in a thick lithosphere that wouldn’t move or be easy to break. On
the other side, it is significant that the high surface temperature of Venus would help to keep the
surface and lithosphere mobile, and that it thickened up due to volcanic plumes. Either way, the
crust of Venus has not been altered at the rate that the Earth’s surface has been altered. Venus’
surface has an average age of 500 My, while Earth’s crust age varies from billions of years to a
100 My.
In spite of the lack of plate tectonics, there is still heat loss on Venus primarily through hot spot
volcanoes and other volcanic forms, but those features are not associated with plate tectonics.
Since this is not a very good mechanism for heat flow, the overall effect is that the planet is
holding in its heat better than the Earth. This makes the upper mantle hotter than the lower
mantle!
While there is no organized plate motion, the large amount of volcanism can alter the surface
drastically, with the formation of large scale volcanic features (but that’s a discussion of another
section of the course).
Mars
Since Mars formed further from the Sun it was able to form with more volatiles and less metals.
So it has an overall lower density than the other terrestrial planets. This indicates that the iron
core is likely smaller compared to the other terrestrial planets. Also with less material, the
amount of energy that would have been produced by radioactive decay would have been quite a
bit less than the Earth, and odds are most of this energy has been lost by the present day.
There was a time when the asthenosphere would have been active and possibly causing surface
volcanism, but that may have been as long ago as 1.5 Gy ago. And this was major volcanism –
Planets Notes 5 - 8
those are very large volcanoes on Mars and the heat loss would have gone at a pretty good pace.
With the cooling of the planet, and the thickening of the lithosphere, plate tectonics would have
been pretty difficult. So Mars has cooled off to the point that activity has ceased – something
that is in the future for the Earth and Venus.
With the amount of iron present, there should be a magnetic field. Is there? Well, yes and no.
There is a planetary magnetic field, but it is weak and localized to various parts of the surface.
There is no overall magnetic field, or organization of the currently detected magnetic system.
Mercury
This is a very evolving section since results from the Messenger spacecraft are still being
interpreted. The basic upshot is that you can expect things to probably change over the next few
years. For the most part, Mercury has a similar history as the Moon, though it didn’t form from
an impact on the Earth of course, but was formed in the solar nebula with the other planets. Due
to its location it also formed with a large iron-nickel core. But the amount of iron in this object
is still abnormally large – about 42% of the interior is iron-nickel core. Compare that to the
volume of the Earth’s core (17%) and that’s a heck of a lot more metal than rock. The current
theory is that Mercury probably had more silicate material in its outer layers, and it was a larger
planet in the past. It may have had 2x its current mass originally, but it lost more than ½ of its
mass due to impacts, which would have mainly striped the surface layers off leaving behind an
abnormally large metal core (large given the overall size of the world). It is also possible that the
high temperature environment near the Sun could have altered the composition enough to tip it
towards a more iron-nickel rich composition, and strong solar winds could also help strip off the
lower density rocky layers. Unfortunately these theories don’t match very well with the
observations from Messenger that show relatively large amounts of sulfur and potassium on the
surface. These elements should not be retained under high temperature conditions such as that of
an impact or stellar winds. Of course a lot of our expectations are based upon our assumptions
of the formation of the planets and the composition of the solar nebula near the sun, and that may
be the source of error here. We’ll just have to wait and see if we can resolve this.
The Messenger spacecraft has also provided information about the magnetic field of Mercury
which is significantly weaker than the Earth’s but still detectable. The surprising characteristic
of the magnetic field is that it’s centered “above” or north of the center of the planet. So if you
stood at the north pole of Mercury you’d register a stronger magnetic field than if you were at the
south pole of the planet. Like the Earth, the field is likely generated by the dynamo mechanism
in the interior, which points to at least part of the core being in a molten form. This is surprising
given the small size of the planet, but you also have to consider the tidal forces it experiences as
it orbits about the Sun. These forces are likely to have helped the planet maintain a relatively
high internal temperature. And like the Earth it is also possible that element such as sulfur may
also help keep the core at least partially molten.
Unlike the Moon, Mercury shows regions of significant stress which are likely due to planetary
contraction caused by a cooling large iron core. Also there is evidence of significant volcanic
flooding on the surface, but this would have likely occurred early in the planet’s history since the
cooling would have happened early on with the crust and mantle cooled significantly.
Planets Notes 5 - 9
5.5 Interiors of Icy Worlds
The four satellites of Jupiter make up an interesting collection of worlds that share a common
origin, but have significant differences. We’ll look at them here in order of distance from the
planet, which is also the order of decreasing density and older surface features. And of course
the other large and significant worlds, Titan, Enceladus and Triton.
Io
Io likely formed without much ice so perhaps I shouldn’t talk about it in this section…just
kidding. It has the highest density (3530 kg/m3) of the four Galilean satellites, and a surface
covered with a lot of sulfur and no regular ices (water, CO2, etc). Odds are there is not much ice
to be found anywhere in Io. This is not unusual since the contracting “Jupiter nebula” would
have been very hot, and would have prevented an icy object from forming at Io’s distance. It
would have more likely formed from material similar to C-type asteroids (carbon rich material).
The active volcanism would have gotten rid of most of the volatile materials early in this world’s
history, leaving behind the higher density material. The basic model for Io has a sulfur rich
silicate crust above a silicate rich mantle. There should also be a very mobile and active
asthenosphere below the crust, since there are so many volcanoes erupting on the surface. The
asthenosphere would be rich in sulfur and sulfur dioxide, which also covers the surface of Io.
The mantle is likely to be composed of the mineral forsterite, a magnesium rich form of olivine.
The thickness of the crust isn’t known for certain, but it is likely not too thick, perhaps only 30
km. The size of the core is very uncertain since it may be influenced by the amount of sulfur in
the core – it could be anywhere from about 36% of the radius (if all iron) or about 52% of the
radius (if a iron-sulfur mix).
Europa
Even though it is a fairly low density object, Europa still has a substantial amount of silicate rock
within it. The overall lower density is helped by the large amount of low density ice and water
near the surface. There is likely to be an inner core that is composed of iron-nickel. The rocky
mantle surrounds the core, and comprises 92% of the mass of the world. Over the mantle is
~140 km water layer, comprised of both liquid water and ice. There is a magnetic field which
may be caused by the liquid water layer. The presence of a liquid water layer is not unexpected,
since Europa is able to experience tidal heating due to not only Jupiter but also Ganymede, the
largest satellite of the four. There is also evidence of rapid resurfacing events occurring with
some areas only a few million years old.
Ganymede
As the trend continues, the next satellite out from Jupiter has a lower density and more ice in its
interior than the previous two. There is still likely to be a dense metallic core composed of iron
and iron sulfide. Actually Ganymede’s iron core is likely to be a liquid iron core. Ganymede
does possess a relatively strong magnetic field which would support this model. The mantle is
composed of silicate rich material (olivine and pyroxene), and above that is an icy outer mantle.
Part of the icy mantle layer is comprised of a liquid water layer. Each layer is around 900 km
thick, though the actual sizes depend upon the amount of sulfur in the core, and the types of
minerals in the mantle.
Planets Notes 5 - 10
Callisto
The outer most of Jupiter’s large satellites has the lowest density and does not appear to have any
iron in its interior. If, by chance, it does, then it isn’t a very significant amount. Callisto is likely
to have a silicate core, which may be only 600 km in size. Above that is a layer of ice and
silicates mixed in a thick mantle, with the amount of rock increasing with depth. Below the icy
crust there is likely a liquid water layer, which could be a few 100 km thick. The presence of a
magnetic field at Callisto would support a model with a liquid water layer.
Titan
The largest satellite of Saturn is in many ways similar to Callisto. It appears that it has a large
silicate rich core, with icy layers above it. Like Callisto, Titan also appears to have a liquid layer
of water below the icy surface. It should be remembered that Titan is located further from the
Sun than the other satellites described above, and also orbits from its parent body at a greater
distance than most of those previously mentioned. Also the planet it orbits is less massive than
Jupiter, so the effect of tidal heating is less significant. The ice layers may include various things
such as ammonia which would help the water to remain in a liquid form in such extreme
environments.
Enceladus
The Voyager mission estimated that Enceladus had a nearly all-ice composition. Follow-up
observations by the Cassini mission show that there is more there than ice, likely iron and
silicates as well based upon its density. The presence of these heavier elements allows for
radioactive heating of the interior – something that usually does not occur in the icy satellites of
the outer solar system which are composed of more ice. It is likely that Enceladus has a rocky
core and icy mantle. There is also tidal heating occurring, which combines with the radioactive
decay to keep Enceladus geologically active. The geologic activity also indicates that there may
be a liquid water layer under the surface.
Triton
The largest satellite of Neptune is comparable in size to the Galilean satellites and it may be
massive enough to have been differentiated into a core, mantle and crust. It is likely similar to
the structure of Titan and Enceladus – a icy/rocky world, possibly with a liquid layer below the
surface. The composition also supports heating through radioactive decay of the interior. The
irregular orbit would also have an influence on the interior heating, though that would be
sporadic.
5.6 Interiors of Gas Giants and Ice Giants
These worlds have very distinct interiors compared to all other objects in the solar system, due to
their large liquid/gas layers. So they are pretty well differentiated and we can only guess how
they have evolved over time since all traces of their early history would have vanished long ago.
Jupiter
The core of Jupiter is similar to the composition of a terrestrial object, except that it is larger,
under greater pressure and much hotter. This material is likely composed of silicates and metals,
but the high pressures make its density insanely high. The mass of the core is estimated to be
Planets Notes 5 - 11
between 15- 30 M (overall mass of Jupiter = 318 M ), and the core is about 10% of the radius
of the planet. The temperature of the core is 22,000 K, with a pressure around 70 Mbar. Above
this is the largest part of the interior, the layer of liquid metallic hydrogen which extends out to
80% of the planet’s radius. The top of this layer is only 6800 K with a pressure of 1.7 Mbar. It
is thought that helium and neon rains through this layer, which decreases the amount of these
elements at the higher layers. Above the liquid metallic hydrogen is a layer of liquid molecular
hydrogen - this is not so unusual and can be formed in Earth labs. Above this are the visible
cloud layers which are relatively docile with temperatures of only165 K, and P=1 bar (Earth’s
pressure). The ultra strong magnetic field is thought to originate from the liquid metallic
hydrogen layer, and is helped by the very rapid rotation rate (9 hours, 50 minutes) of the planet.
Jupiter is also a very hot planet – it releases more energy than it gets from the Sun. Given its
mass, most of the internal heat is likely from its formation – basically left over gravitational
energy. It just hasn’t cooled down yet. This amount of heating is also likely to cause quite a
large amount of convective motion in the outer layers.
Saturn
It is just like Jupiter but not as much – less mass, less material, less of everything. So
structurally and composition wise, it is pretty much like Jupiter except everything is scaled
down. One unusual feature is the heating. Like Jupiter it gives off more heat than it gets from
the Sun, but it isn’t massive enough for this to be in the form of gravitational energy. The
heating is thought to be due to the release of energy caused by helium raining out of the upper
layers down towards the core. So in a way it is gravitational, but mainly from stuff falling
through the material in the interior rather than from compression. Perhaps it is better to think of
it as frictional heating and not gravitational heating.
Uranus, Neptune
Obviously we have less information about these worlds than we do have for Jupiter and Saturn.
We do know that they are richer in heavier elements than Jupiter and Saturn, but that isn’t
enough to give us a decisive model of their interiors. It is possible these worlds could be
comprised of several different things Various ices (water, methane, ammonia, H2S)
A mix of hydrogen, helium, and “rock” (silicates, irons, etc)
Or something in between
Overall both worlds have a composition which has much less hydrogen than Jupiter and Saturn,
which makes things rather confusing.
While we generally put Neptune and Uranus into a similar category, they are not entirely
identical. Neptune is 3% smaller than Uranus in terms of radius, but its mass is 15% larger than
Uranus. The density of Neptune is therefore larger (24%) than Uranus’ density. But even with
these differences, we can put forward models for each planet. For Uranus there is likely a small,
dense, rocky core, something that is about 5000 km in radius, with a pressure of around 8 Mbar,
and a temperature of about 5000 K. This core extends out 20% of Uranus’ radius. For Neptune
the rocky core is proportionally as large, about 20% of the radius, with temperatures and
pressures of the core similar to Uranus.
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For each of these worlds above the core is a mantle which is comprised of ices. This makes up
about 80% of the planet’s mass. At the top of this layer, the pressure is ~ 0.2 Mbar, and the
temperature is about 2500 K, The icy mantle extends out to about 60-65% of the radius. The
interior conditions do not allow for the existence of metallic hydrogen. Also the icy layer is
dense, hot and ionized, which would help form the magnetic field for each world. What are
these ices? Most likely they are various ions of water, methane and ammonia ices. Not very
nice ice.
Above the icy mantle is an outer layer made up of a hydrogen and helium rich atmosphere. The
top of the atmosphere has a temperature of about 80 K, and a pressure of around 1 bar.
One weird aspect of these worlds is internal heat. Uranus has no internal heat, but Neptune does.
Why? Actually both planets should have cooled off long ago. There is no good answer to this.
Also looking at the amount of surface features (storms) we can see that there are likely different
amounts of convection and turbulence in the atmospheres of these worlds. At first glance it
looks like Neptune has a more active atmosphere, which would be logical given the amount of
internal heating, but longer studies are needed to determine just how active they are. Again,
there is no good or simple answer for this at this time.
Planets Notes 5 - 13