My favourite! Largely because it involves my favourite moon in the Solar system: Titan.

Like Jupiter, Saturn also spins extremely fast, making a rotation in only 10.2 Earth hours, and it revolves around the sun in 29.5 Earth years. It’s density is only 0.69 g/cm3, meaning that if you could find a swimming pool large enough and drop Saturn in, it would float. Also like Jupiter, its atmosphere is made up mostly of hydrogen with some helium and other elements, but unlike Jupiter a haze of NH3 clouds obscures its belts and zones.

The internal structure of Saturn is similar to that of Jupiter, but its zone of liquid metallic hydrogen is proportionately smaller, so its magnetic field is considerably weaker.The small core of ice and silicates represents only a tiny portion of Saturn, but is still 17 times larger than Earth.

Like with Jupiter, the lack of a rocky surface makes this planet rather uninteresting from a geological perspective (though it makes up for this by being awesome in other areas), so I won’t put too much into it, but focus on the satellites instead.

The ring system is massive in diameter, but extremely thin, and is well organized into many consistent, separate rings, some of which are kept in place by small “shepherd” moons.

There are three theories of how the rings formed: 1. gravitational disruption of a satellite tore a small satellite to pieces when it got too close to the planet; 2. Fragmentation of moons when impacted by bolides or by each other created debris, and fragmentation has been ongoing to replenish the rings; 3. accretionary remnants never formed into a satellite but stuck around as rings because of the gravitational influence of other already formed satellites.  Considering the complexity of this ring system, any one or combination of these may have been the cause.

Titan

Titan has a density of 1.9 g/cm3, and is the only moon in our solar system with an appreciable atmosphere, which is made mostly of nitrogen with some hydrocarbons.

(Only watch up until o:43, as the rest of this video is posted in better quality below)

It actually has a hydrocarbon cycle not unlike the water cycle on Earth, with oceans, lakes, rivers, ice, rain, and snow, all made of hydrocarbons like methane.

It also has strong wind that erodes the surface and produces dunes around the equator. The presence of hydrocarbons makes Titan another point of interest in the search for life beyond our planet.

Phoebe

Phoebe orbits Saturn in the opposite direction to the other Moons, and is orbiting at an angle, suggesting it was captured by Saturn’s gravitational field and did not originate there. It is spherical in shape, which is unusual because bodies this small don’t generally have a strong enough gravitational force to pull themselves into this shape. It is of rocky silicate composition, rather than icy like most of the other moons out here.

Enceladus

Pronounced en-sell-a-dus, this one is a ball of ice with a young surface, likely resurfaced frequently by slush ice from its interior. There is active eruption on this planet’s surface, but instead of erupting hot lava and ash, it erupts H2O vapour and ice particles in cold geysers from subsurface, pressurized liquid H2O chambers. Water is able to stay liquid in spite of the freezing temperatures because it is mixed with ammonia, which lowers the melting point. It is also kept slightly warmer than it otherwise would be because of tidal friction caused by a nearby moon, Dione.  Like with Jupiter, Dione orbits Saturn exactly once for every two orbits of Enceladus, keeping the tidal flexing regular and allowing the moon’s interior to heat up.

Dione

It is much denser than other satellites around Saturn, implying a higher silicate and lesser volatile content. It has some cratered terrain, as would be expected, as well as smoother terrain, suggesting that some resurfacing has taken place, most likely from eruptions of water and frost from fractures in the icy surface some time after the cratering took place.


Jupiter is the fifth planet from the Sun, and is the first of the Jovian planets, or gas giants. It spins very fast, having a rotation period of just 10 Earth hours, and it takes 11.9 Earth years to revolve around the Sun. Being made almost entirely of gas, it has a low density of just 1.3 g/cm3. Its atmosphere is well-organized into light zones and dark belts with opposing wind directions, with shearing at the interfaces which creates vortices and storms like the giant red spot, and is dominated by hydrogen and helium. Jupiter’s magnetic field is 20,000 times more intense than Earth’s.

Below is a picture of its internal structure. The layer of liquid, metallic hydrogen below the atmosphere is highly charged, and the rapid spinning of the planet produces the huge magnetic field from it.

Because there is no rocky surface on Jupiter as there is on the terrestrial planets, we won’t focus on it too much, but instead look at four of its satellites.

Io

This is the most volcanically active body in the solar system, erupting 100 times more lava than Earth from at least 80 active volcanoes. It therefore has a very young surface with little cratering. The eruptions create lava plains and massive umbrella-shaped plumes that drop volcanic ash, sulfur, and SO2 frost and snow in rings around the volcanoes.

Volatiles produced in the eruptions, such as nitrogen gas, water vapour, and carbon dioxide gas, are lost to space upon being erupted. This incredible internal heat is created by the tidal friction caused by the gravitational pull acting on Io from Jupiter and some other nearby satellites, like Europa. Jupiter’s magnetic field may also have something to do with it, as Io is so close to Jupiter (closer than our Moon is to Earth).

Io lacks the surface ice found on other satellites around Jupiter (duh, lava), and is likely dominated by silicates, based on its density of 3.57 g/cm3. It also appears to have its own magnetic field independent of Jupiter’s.

Europa

This moon is covered in a thick layer of water ice, that may have a “lithosphere” of partially liquid, slush-like material. There are very few craters, which could be either because of slush eruptions covering them over or because of the plasticity of flowing ice smoothing them out in response to gravity. I personally like the slush eruption theory the best.

There may have been, and might still be, a huge water ocean under the ice, which makes Europa a good candidate for possible life beyond Earth. The internal structure is speculative, as it’s hard to get a look at when all you’ve got is a couple of satellites, but judging by its density of 2.97 g/cm3 it is probably dominated by silicates like Io is.  There is also a small magnetic field, that might be indicative of a small iron core, or could also be caused by Jupiter’s magnetic field interacting with a salty, subsurface ocean (the better supported of the two theories).

Magnesium sulphate salts have been detected on its surface, and while the sulphur likely comes from Io’s volcanic eruptions, there is no surface source for the magnesium.  It must therefore come from the subsurface ocean, which is likely filled with magnesium chlorides (so, magnesium plus sodium and potassium), more evidence that Europa could possibly support some kind of extant, marine life. There are also a couple of large irregularities on the ice surface (which they call “chaos terrain”, cool!) which could be a means of transferring materials (like nutrients for Europan fishes?) to the ocean below the ice.

Ganymede

This is the largest satellite in the solar system, even bigger than the planet Mercury, and also has a freaking awesome name. Ganymede…

It seems that the farther out from Jupiter you go, the less dense the satellites get.  Ganymede’s density is only 1/94 g/cm3, suggesting a composition of lesser refractory elements and more volatiles like water.  It has an icy surface, with sections of heavily cratered terrain, suggesting a surface as old as the earliest bombardment period in our solar system, and areas of bright, grooved terrain, with fractures interrupting fractures, suggesting tectonism earlier in its history.  It has a thicker ice layer than Europa that includes a mantle, and its magnetic field suggest an iron core that has been kept hot enough to produce the magnetic field by tidal heating. Ozone has been detected in the ice, but unlike on Earth, Ganymede’s ozone is created by non-biological processes, meaning that we can no longer rely on ozone as an indicator for life on other bodies (boo, Ganymede, the fun-killer).

There is a strange phenomenon involving the previous three moons around Jupiter. In the time it takes Ganymede to orbit the planet once, Europa orbits exactly twice, and Io orbits exactly 4 times, which is how the interiors of these moons are kept hot in spite of the fact that this far from the Sun, they should have cooled and solidified for good a long time ago.

Callisto

Also sporting a pretty awesome name, Callisto is the outermost satellite I will be looking at, and has the lowest density of the four at 1.86 g/cm3, further suggesting a temperature gradient moving outwards from Jupiter early in its formation. It is basically a ball of ice, with lots and lots of craters. It’s magnetic field suggests a subsurface saline ocean.


There’s a section on the K/T mass extinction event in the notes, but because I took an entire class last year on this subject, I’m not going to bother writing a blog post for study purposes.  So instead of reading my notes, please take a substantially greater amount of time to watch this amusing documentary, complete with dramatic male voice, intense music, and plenty of artistic license.

https://www.youtube.com/watch?v=EzvDat00ZzU


Meteorites

Meteorites are solid bodies of the inner solar system that have impacted Earth. Here are a few amusing links regarding the finding of meteorites on Earth’s surface:

Meteorite identification questions: http://meteorite.museums.ualberta.ca/identify.pdf

Meteorwrong: http://meteorite.museums.ualberta.ca/browseresults.php?txt=5 – a collection of things that are commonly mistaken for meteorites, but are definitely not.

Most originate from the asteroid belt, with some coming from Mars and the Moon.  There are three types of meteorites:

1. Stony meteorites: dominated by Fe and Mg, and include chondrites (contain small spherules of silicate materials within a finer matrix and are largely representative of primitive planetesimal chemistry – carbonaceous chondrites, which contain more volatiles than normal ones, are used to date the age of the solar system), and anchondrites (lack the small spherules and look very much like mafic igneous rocks – probably the product of melting to form a differentiated crust on parent planetesimals)

2. Stony-iron meteorites: rare, and probably formed at or near a mantle/core boundary and dislodged by impact.

3. Iron meteorites: Fe and Ni metal, they are very durable and therefore quite frequently recovered.  They are also the product of differentiation and cooling.

Asteroids

Main belt asteroids are located between Mars and Jupiter, and will stay there unless knocked off course.  Apollo asteroids are ones whose orbit crosses that of Earth.  They come in a range of compositions similar to meteorites.

The asteroid belt may have been formed in two ways: either it’s remnants of broken up planetesimals or (the more favoured theory) never accreted to begin with (which would explain the existence of chondrites).  The reason the asteroids in the belt never accreted may have something to do with the gravitational pull of Jupiter.


Because the notes on Mars are also freaking long (see my notes on the Moon), I will also be glossing over whatever I can here.

Basically, it’s the fourth planet from the Sun, rotates on its axis once every 24.5 Earth days, and orbits the sun in 1.88 Earth years. It’s only a little bigger than half the size of Earth, and has a density of 3.93 g/cm3. It has a thin atmosphere comprised mostly of carbon dioxide, and wind speeds at its surface make our hurricanes look like a joke. There is a weak remnant magnetic field in the crust left over from a warmer period in Martian history, when it would have had an active one.

Internal Structure

It has a very low density, which is not surprising as it formed farther from the Sun and should contain lower temperature condensates.

Here is a picture to more or less describe the internal structure, so that I don’t have to.

Geologic Provinces

There is a clear division between the northern and southern hemispheres, and a few different geologic provinces to know about.

Densely cratered highlands: Makes up the southern hemisphere, and contains several large, multiring craters in addition to numerous smaller ones.  The ejecta from Martian craters is odd, in that it sometimes appears to flow, suggesting the presence of water. The current theory is that the impact melts ice into water, which then mixes with the ejecta to produce flows.

Northern plains: Makes up the northern (duh) hemisphere, and are separated from the south by an escarpment.  There are basalts and other volcanic features, as well as stream-borne sediments and evidence of wind erosion.

Global escarpment: It encircles Mars with a few interruptions, and is thought to have been caused by a major impact. Since its creation, it has been dissected by stream erosion and mass movement or slumping, and there are some isolated blocks or mesas just north of it.

Crustal upwarps: There are two continent-sized upwarps in the northern plains, and they are covered in volcanoes and basalts.  They were likely produced by hot mantle plumes pushing up on the crust. The largest volcano in the solar system, Olympus Mons, is located here.

These volcanoes are no longer active.

Polar regions: Mars has polar ice caps like Earth does, comprised of water ice as well as dry ice in the winter. Drops in atmospheric pressure resulting from the freezing of CO2 creates a single large Hadley cell in which air is moved from the summer hemisphere to the winter one.

Volcanic and Tectonic Features

There are three major features related to volcanism on Mars: shield volcanoes, volcanic plains with basalts that are more enriched in volatiles than Earth’s basalts, and patera, which are large, low relief structures with central volcanoes, extensive flows, pyroclastics, and extensional tectonic features.

There are no plate tectonics on Mars.  Ridges are caused by the buckling of cooling lava in volcanic plains, and canyons were created by crustal expansion upward when upwelling occurred.

Water and Life on Mars

There is plenty of evidence for water on Mars, and there is no question of its presence.  The water is found in ice caps, as ice crystals in the rocks and sediments, and in the atmosphere.  There is also evidence of there once being liquid water on the surface in the form of dendritic drainage systems, outflow channels, and boulders eroded and stacked so they are tilting in the same direction, indicating a very strong flow of water.

Life, on the other hand, is a little more questionable.  For every bit of possible evidence of life, there is also something to suggest that the evidence was not created by organic processes but inorganic ones. The possibility hasn’t been rules out, but definitive evidence hasn’t yet been found either.


My notes on the Moon are hella long, so I will try to only include some of the more awesome stuff and gloss over the boring regular stuff.

Here are some of the basics on the Moon, which I will gloss over to a large extent.  Basically, the Moon is slowing down Earth’s rotation over time (theoretically), and is also gradually moving farther away from Earth (measured by shooting lasers at reflectors on the Moon to measure distance pewpew!).

The Moon rotates on its axis and orbits the Earth in exactly the same period of time, which is why we always see the same side of the Moon. Its density is 3.34 g/cm3, there is no liquid water, atmosphere, or surface pressure, and there is no magnetic field, though there might once have been.

Structure

This picture will communicate most of what you need to know about the Moon’s structure:

The density of crustal materials is 3.30 g/cm3 compared to its overall density of 3.34 g/cm3 suggests that the core is very small, if it exists at all.

Basically, the Moon is now a cold, dead object that hasn’t experienced tectonism or volcanism for over 3 billion years.

Surface Geology

There are two main divisions of lunar surface:

Terrae are characterized by bright, rugged, densely cratered areas and comprise about 60% of the near side of the Moon, and almost all of the far side. They are mostly anorthosite (remember that term from the Mercury notes?) and date to well over 4 billion years, which is consistent with how heavily cratered they are.

Mare (pronounced mar-eh) are characterized by smooth dark layers comprised of thin layers of basalt with few impact craters, and are between 4 and 3 billion years old.  They are abundant and large on the near side of the Moon, and are virtually absent from the far side.

There are some volcanic features, such as lava flows, channels and rilles formed from collapsed lava tubes or by the flow of lava over the surface, and a few lava domes and pyroclastic volcanoes (the exploding ones).

There are also some global tectonic features, like linear rilles related to thermal expansion around the time the mare basalts were formed and wrinkle ridges related to compression when the Moon cooled.

Lunar History

Like Earth, the Moon has a relative geologic time scale, pictured below so that I don’t have to type the whole thing out. I should mention before you look at the picture that it is a bit out of date, and the mare basalts should be deleted from the Eratosthenian Period.

Origin of the Moon

Now, this is the interesting stuff!

There are a number of competing theories of how the Moon may have formed, though one tends to top the rest.

Fission from Earth: Basically, the Earth was spinning so rapidly it flung some of its material into space, which was then kept in orbit by its gravitational field, becoming the Moon. There are some problems with this theory, mainly that it is highly unlikely the Earth was ever spinning this fast, as it would have had to have spun at the same frequency as the critical vibration frequency required to break a wine glass.

Capture: A body bypassing Earth was neatly caught and held in its graviational field. Also highly unlikely, as rocks that come from somewhere other than Earth have very different oxygen isotope ratios (there are different types of oxygen molecule that occur in different concentrations around the solar system), but the Moon’s oxygen isotope ratio is pretty much the same as Earth’s.  The Moon also has a coplanar orbit, suggesting a shared origin with the Earth.

Binary Accretion: The Moon and Earth accreted fromt the same material basically next to each other.  This is the most unlikely, as it would have resulted in two bodies with exactly the same density and composition (the Moon is depleted in volatiles).

Collision: A large protoplanet about the size of Mars crashed into Earth very early in its history, adding some materials to Earth and flinging others off, which would eventually accrete into the Moon, illustrated below:

Or, more artistically:

It wouldn’t have excavated deeply enough to fling off any of Earth’s core material, but would have excavated crust and mantle material instead, accounting for the Moon’s low density.  This theory also explains the similar oxygen isotope ratios, as the Moon would be made mostly of Earth’s crust and mantle, and also explains Earth’s anomalously high density, as the protoplanet would have added iron and magnesium to the system.  It can also explain how the Moon’s orbit and rotation are exactly the same.  All in all, it’s currently the most favoured theory.

Hooray for the Moon!


There have been several satellite missions to Venus, so we know quite a bit about it.  It’s the second planet from the Sun, and is relatively close to Earth in terms of its distance from the Sun.  The Earth and Venus would have therefore formed in a very similar part of the solar system, making them very similar in composition according to the Solar Nebula Hypothesis.

Its rotation and orbit are even stranger than Mercury’s.  It rotates very slowly, once every 243 Earth days, and its rotation is reversed to what is seen on other planets.  It’s orbit around the sun takes 225 days, meaning that there is less than one Venus day per Venus year.

drunkenvenus

This oddity might have been caused by a glancing impact late in it’s accretionary history.

It’s size is very similar to Earth’s, and its density is 2.24 g/cm3, compared to Earth’s 2.52 g/cm3.  Because of its thick, greenhouse gas filled atmosphere, the surface temperatures can get hot enough to melt lead. The clouds obscuring its surface are composed of a tenuous mist of H2SO4 droplets (sulfuric acid). There is no appreciable magnetic field.

Internal Structure

Because it has a similar size, density, and distance from the Sun, we would expect it to have a similar composition as well.  The lack of a magnetic field could result from the core already being solid (as convection within a liquid iron core is needed to produce a magnetic field), or it could be because the planet rotates too slowly to move the materials around.

The composition of the crust is comparable to the basalts from ocean hot spots on Earth, though the high relief of volcanoes on Venus suggest highly viscous lavas, which would normally be a different composition on Earth.  Because of Venus’ high surface temperatures, its rocks are quite weak and tend to be more ductile rather than rigid or brittle.

Atmosphere

Carbon dioxide makes up about 96% of its atmosphere, with some nitrogen and other elements, and almost no water. Even though the sulfuric acid clouds reflect all but 20% of the sunlight, the carbon dioxide atmosphere has such a strong greenhouse effect that the surface is still extremely hot. Earth would have a similar atmosphere if not for its biosphere, which has locked up almost all of its carbon in rocks. The atmospheric pressure is 92 times that of Earth.

There is no evidence of liquid water having ever existed on Venus.  The H2 enriched atmosphere (there is more H2 than H1) suggests that any H2O has been boiled off, and any suface evidence of liquid water has been subsequently covered by volcanic activity.

In spite of the sulfuric acid clouds, there is no weathering on Venus’ surface from acid rain because the surface is so hot, liquid materials evaporate before they can hit the ground.

Major Geological Provinces

The surface lacks any heavily cratered terrains, suggesting that on average it is no older than about half a billion years. There are also no very small impact craters, suggesting that the Venusian atmosphere absorbs smaller bolides before they can hit the ground.

Lowlands (Yada Planitia) – 60% of Venus, with elevations below the mean planet radius (like sea level without the sea).  They are smooth and appear to be largely covered by lavas (solidified), except for a few areas with mountain belts caused by compression.

Uplands (Yada Regio) – 30% of the planet, dominated by extension and rifting, with volcanism over mantle plumes.

Highlands (Yada Terra) – 10% of the surface, and concentrated in two areas, called the Aphrodite Terra and Ishtar Terra. These are much higher above the mean planet radius than the previous two provinces, and show a lot of compressional features, suggesting they are created by downwelling and crustal thickening, rather than subduction (see above picture).

Volcanism

There are many more types of volcanoes on Venus than there are on Earth, and they are randomly and globally distributed.

Shield volcanoes tend to occur on the plains.  Some constitute huge laval fields, and they may have concentrations of several vents. Intermediate volcanoes are higher in altitude and cover smaller areas than many other volcanoes, suggesting very viscous or stiff lavas. They tend form high, steep-sided domes that look like pancakes.  Lava pancakes. Om nom nom. Large volcanoes are very similar to shield volcanoes, except that they’re… large… er. Calderas tend to be on the small side, and are formed when the crust collapses, forming a lava-spewing depression. Coronae are unique to Venus, and are formed by the extrusion of lava and sagging of crust, resulting in a circular feature with a raised, wrinkled rim and a central sag. And finally, there are also lava channels, which are pretty self-explanatory.

We haven’t been able to catch a volcano in the act of eruption, but fluctuations in Venus’ atmosphere suggest that some volcanoes are still active.

Tectonics

Tectonics on Venus (not plate tectonics like on Earth) are mostly caused by compression and extension of the crust over mantle plumes or hot spots.  There is some horizontal movement as well, but it is fairly limited to small areas. See the above picture in the geological provinces section.

This activity is caused by the planet releasing heat from its interior by convection, and results in volcanoes and upwellings over the plumes, and ridges and faults form mountains, rilles, and grabens form elsewhere by compression to compensate for the extensional movements.

Weathering and Erosion

At higher elevations, atmospheric SO2 chemically weathers Fe in the basalts, producing pyrite (shiny!). At lower elevations where temperatures and pressures are higher, oxidation occurs producing magnetite.

There is also some wind erosion and deposition as well.  Although wind speeds are fairly low on Venus, the dense atmosphere can get things moving at 1/10 the speed required to move things on Earth.

And that’s it for Venus!