12. Terrestrial Planet Geology

 Reading: Chapter 9

 

In this section we will be considering the terrestrial (Earth-like) planets: Mercury, Venus, Earth and Mars (plus the Moon), looking for properties that they have in common and important differences between them. We will concern ourselves with the topics of



Density

The concept of density--or mass per unit volume--is exceedingly important in planetary science. The mean density is given by:

                      total mass of object
  Mean density  =  --------------------------
                     total volume of object

The above gives a mean or average density of the whole object. Note that this means parts of the object might have larger or smaller densities than the mean density. The proper metric units for density is kilograms per cubic meter (kg/m3) but density is often given in grams per cubic centimeter ( g/cm3).

The fact that water has a density of 1.0 g/cm3 allows us to compare the density of materials to water (something we have a feel for). So, rocks have densities 2-3 times that of water - and therefore sink in water. Materials with density less than 1.0 g/cm3 will float in water.

For a spherical moon or planet, we can use the formula for the volume of a sphere (4/3 R3) to rewrite the above equation:

                            3 MASS
  Mean density  =    =  ----------
                           4  RADIUS3
The Greek letter ("rho") is often used to denote density.

If we can get the MASS of a planet -- by measuring the orbital distance and orbital period of one of its moons and using Newton's Version of Kepler's 3rd Law (NVK3L) -- and measure its RADIUS, then we can then calculate the planet's DENSITY. (This is WHY we put in all that effort to learn about Newton's version of Kepler's 3rd Law.)

 

(1)(a) Order Earth, Venus, and Mars by (a) size, (b) mass and (c) density (see Table 8.1).

(b) HOW do we know the density of each planet? (Hint: Do Earth, Venus, and Mars have moons?)

(2) Densities of Moon and Mercury

    (a) Neither Mercury nor the Moon have their own moons--so how do we know their masses? Before and since the the space age?

    (b) If an object has an average density of 3.5 g/cm3, is it likely to consist of mostly iron, rock, or water?

    (c) How do we know that one object has a large core of iron underneath its rock crust while another has a very small iron core and is mostly rock?

Anatomy of a Terrestrial Planet

All of the terrestiral planets have similar structure of 3 basic regions: a core of iron (with maybe some other materials like nickel), a mantle of silicates (rock) and a lithosphere or crust. While the crust is solid, the mantle and core can be solid or liquid, depending on how hot it is and its chemical composition. The hotter the core or mantle, the more likely it is convecting--that is, flowing or turning over. A convecting mantle leads to buckling of the crust - tectonics. A convecting iron core will generate a magnetic field.

Here is a color version. The textbook has a prettier one - Figure 9.5

All five of the terrestrial bodies - Earth, Venus, Mars, Mercury and Moon - have this same 3-part structure. Below is a diagram showing the relative sizes of the planets themselves and of their cores.You can find a prettier version in the textbook Figure 9.6

(3) (a) Mercury and Earth have significant magnetic fields--Venus, Mars and Moon have negligible magnetic fields. What does this tell us about the cores of these planets?

(b) An important constraint on models of the interiors of a planet is the mean density. What are typical densities of rock and iron?

(c) Say we discover two new planets--Sunev and Thrae. They are the same size but Thrae has a mean density of 5 g/cm3 and Sunev 4 g/cm3. Which has the larger core?


Processes that Heat a Planet

Here is a cartoon. The text has a prettier one - Figure 9.7

(4) (a) Which of these 3 heating processes does NOT occur in a body that remains homogeneous - uniform in composition?

(b) Would you expect larger or smaller planets to remain homogeneous?

(c) Has the production of heat inside the planets by radioactivity increased or decreased since the planets formed?


Processes that Cool a Planet

Here is a cartoon. The text has a prettier one - Figure 9.8

(5) Large objects cool slower than small objects--can you think of some everyday examples of this? (Hint: Food often provides useful examples of planetary processes!)

 

Why is this true? Heat is contained in the volume of an object. Heat is lost through the surface. Let's look at a sphere with radius R and consider how the volume and surface area vary with the radius of the sphere.

To consider how heat is lost from an object we want to look at the ratio of the surface area to the volume. For a sphere:

SURFACE AREA      4R2           3
 ------------  =  ---------  =  ---  
    VOLUME        4/3 R3        R 

So, as the radius R of a sphere increases, the surface area per unit volume decreases.

 

(6) (a) Which cools faster--peas or a baked potato?

(b) Which animal has a hard time keeping warm --an elephant or a mouse? Which has a hard time keeping cool?

(c) Which is likely to cool faster - a mantle of liquid rock or a mantle of solid rock?



Big vs. Small

All planets are thought to have been formed as very hot bodies. Collisions by primordial proto-planetary fragments heats up the young planet. After accretion onto the planet has stopped, the planet loses this residual heat slowly as it cools off. What determines how much internal heat a planet will have left billions of years after its formation? The answer lies in the size of the planet. Small planets cool faster while large planets cool slower.

Interior Temperatures and Size

Think of planets as baked potatoes!

 

BIG planets --------------------------SMALL planets

<------------Greater Heating ------------

HOT                                      COLD INTERIOR                          INTERIOR

        -------- Faster Cooling -------
 

 


Example: Interiors of The Moon and Mercury

We learn about the bulk composition of the planet from its mean density but to determine the interior structure we need to do some detective work. First of all we make a reasonable guess about the composition, based on the abundance of elements in the solar system (i.e., we could guess that Mercury's interior is make out of gold which has a similar density to iron - but iron is much more likely). So, we assume the inner core is made up mostly of iron (density of about 8 g/cm3) and that there is a layer of rock (density of about 2.5 g/cm3) on the outside. Then we calculate the sizes of the core and mantle of these densities that would add up to the measured mean density.


Density of Moon = 3.3 g/cm3 
Density of Mercury = 5.4 g/cm3

From these densities we conclude that the Moon is nearly all rock with only a tiny iron core (<1% of the Moon's volume) while Mercury as a huge core relative to its size (40% of Mercury's volume).

 

(7) (a) Compare the size of Mercury's iron with the size of the Moon.

(b) How thick is the crust of rock wrapped around Mercury's core?

Density provides the first clues about the planet's interior. Other clues are provided by measurements of the magnetic field and, if we get the chance to land on the object put instruments on the surface, by seismometers. The process whereby a magnetic field is generated inside a planet (or stars, for that matter) is not well understood. It is known, however, that there are 3 necessary ingredients:

The result is a global magnetic field (as if there was a large bar magnet in the center) which extends well beyond the planet and can be measured by a passing spacecraft. It turns out (we have recently realized) that all of the planets (and their moons) probably have sufficient rotation so that the existance (or absence) of a magnetic field really tells us whether or not there is a substantial region of liquid iron (or similarly conducting material - rock is much less conducting than iron).

 

(8) (a) Do (i) the Moon or (ii) Mercury have magnetic fields?

(b) What does your answer to (a) tell us about the interiors of the Moon and Mercury? Is it consistent with what we infer from their mean densities?

Seismic studies from measurements made on the Earth have revealed a great deal about Earth's interior. It would be nice to have seismic stations on all the planets!

 

(9) (a) On which planetary object have seismometers been placed? Hint: Think where have spacecraft landed and spent some time.

(b) What did we learn from the measurements?

As we will discuss towards the end of the course, it is not too surprising that Mercury has a lot of iron. A bigger mystery is posed by the lack of iron in the Moon. Recent ideas about the Moon being formed in a giant impact of a Mars-sized object hitting the Earth provide an explanation for the Moon's small iron core.

 

(10) Look at this sequence from a computer simulation of a Mars-size object hitting the Earth. Blue material is denser (iron) than the brown material (rock). Follow what happens to the blue material during the impact and subsequent formation of the Moon. What happens to the blue material?



Surface Processes

Now that we have some sense of the interiors of the terrestrial planets, we will next look at their surfaces and relate what we see to the processes that sculpt them.

Impact Cratering

A quick glance at pictures of Mercury and the Moon shows that craters are the major surface features of both these objects. In fact, the surfaces are SATURATED with craters--i.e., if another impact occurs, it will probably overlap with an existing crater.

(11) (a) Look at the images at the links above and see if there are any areas on the Moon or Mercury where you could have an impact without covering up another crater.

(b) On the Moon, what is the name given to the regions that have fewer craters? Where does the name come from?

The following diagram shows what happens when a projectile--such as a meteorite hits a solid. Note that the impact is more like an explosion so that the resulting crater is round, whatever the direction the meteorite came from.

The speed of the incoming object is on the order of(*) the orbital speed of objects orbiting the Sun. Projectiles hitting a surface with such speeds deliver enormous amounts of energy--they explode a region about 40 times the size of the original projectile.

Crater Size ~ 20 x Projectile Size

(* "on the order of" is often used by scientists and engineers to mean "approximately, within about a factor of 10" - a useful "geekspeak" expression).

(12) (a) What is the orbital speed for objects at 1 A.U. from the Sun --such as the Earth? (Remember: Vcirc= (GM/R) where in this case M = Msun = 2 x 1030 kg, R is 1 A.U.= 1.5 x 1011 meters.) Or you can cheat and look at Figure 3.2.

(b) How big was the impactor that caused Plum Crater in this photo taken from the Apollo 16 mission? (Plum Crater is 40 meters across.)

While the relationship of crater size ~ 20 x impactor size holds pretty well for all sized craters, from the very small craters (e.g. Plum) to the very largest (e.g. Coloris), the shape of the craters does vary with size. Smaller craters are close to bowl-shaped, with the depth about twice the diameter. With larger craters the structures become more complex: there is often a central peak where material rebounded after the impact, there is a raised rim and a surrounding blanket of ejected material. Some of the ejected material often has enough energy to form secondary craters.

This diagram shows a cross-section of the structure of a large crater. With particularly big craters, molten rock (lava) leaks up from below the impact and floods the floor of the impact crater. This is how the lunar mare are formed. Both the Moon and Mercury suffered a huge impact which produced a multi-ring basin--Orientale on the Moon and Caloris on Mercury.

(13) (a) Find in the text examples of (i) a small, simple bowl-shaped crater, (ii) a crater with a central peak, (iii) a multi-ringed basin, (iv) a flooded crater (give the figure number and location in the figure).

(b) The crater Copernicus (yes, as you may have gathered, the lunar craters are mostly named after famous scientists) is spectacular - find it here. How far across the Moon's diameter do the ejecta from Copernicus spread? How far is this in kilometers? OR, look at the Moon and look at the Tycho crater slightly left of center at the bottom.

The surface of the Moon and Mercury are saturated with craters - why are there so few craters on Earth? Why are we not being bombarded with meteorites now? If the rate at which impactors hit the planets was constant, then the older the surface, the denser the craters (think of raindrops hitting a window when it begins to rain). If the surface is covered up with new material, there will be fewer craters on the new surface (this is like wiping the window in our raindrop analogy). But if we are not seeing very many new craters being formed now (or in the recent geological past) then there must have been a change in the rate of cratering.

The Moon has been extremely important for our understanding of how the rate of impact cratering has changed over the age of the solar system. This is because we have been able to accurately date the age of rocks brought back by Apollo astronauts from the Moon. By measure the density of craters on the Moon and then going to the Moon and collecting rocks from regions with different crater densities, we can make a plot of the rate of cratering with time. This shows that impacts were very frequent 6 billion years ago--this is the period of intense early bombardment--but the rate slowed down dramatically between 3-4 billion years ago.

(14) (a) From the diagrams, how many years ago were the lunar mare formed?

(b) When was the impact that caused the Copernicus crater (or the Tycho crater)?

This diagram of the evolution of the lunar mare shows how the flooded impact basins evolved.

We have mainly considered impact craters on the Moon and Mercury. Now, let us consider the rest of the terrestrial planets.

(15) (a) Look at the figures earlier in this Session, and in the relevant chapters in your book. List the figures with examples of impact craters for (i) Earth, (ii) Venus and (iii) Mars.
(b) What are the factors that cause fewer craters to be visible on (i) Earth, (ii) Venus and (iii) Mars?

Summary of Impact Cratering
What does the spatial density of craters tell us?

Moon - Surface largely saturated with craters except for the lunar mare - which are large crater basins that have been flooded with lava. This flooding episode must have happened about 3.8 billion years ago because dating of rocks from different regions tells us that the crating rate dropped considerably after about 3.8 billion years ago.

Mercury - Surface saturated with craters. This tells us that the surface has suffered very little reprocessing for several billion years.

Mars - The density of craters differs considerably in different regions of Mars - particularly between the north and southern hemispheres. The north is much less cratered (and hence is younger) than the south.

Venus - The crater density is fairly uniform - this tells us that most of the surface has about the same age. The density of craters allows us to estimate that the whole planet must have been re-surfaced about 800 million years ago.

Earth - On Earth most of the craters have been eroded or removed. But the remnants of about 300 craters have been found.


Volcanism

Effects of heating rocks
You generally think of rock as something that is solid, rigid, strong - unable to break and unyielding to forces applied to the rock. However, as you know from images of the ground cracking in an earthquake, rocks do yield when they are sufficiently stressed. When stresses are applied to rocks they do not always break - sometimes, rock with actually flow, particularly if the rock is heated up. The property of a substance that describes how easily or reluctantly it flows is called viscosity.

Low Viscosity = Runny and Easy to Flow
High Viscosity = Thick and Reluctant to Flow

(16) Would you describe the following as having high or low viscosity: (a) molasses, (b) cooking oil, (c) oatmeal, (d) beer?

Silly Putty is perhaps the best analogy to rocks - if you apply an intense stress it will break (e.g. hit it with a hammer and it shatters), but if you roll the silly putty into a ball and leave it on a table, the less-intense, steady force of the Earth's gravity will cause it to slowly flow, forming a "puddle" after minutes to hours.

(17) Consider what happens when you heat or cool a liquid. Think about what happens to the viscosity (a) when you heat molasses, (b) to oil in a car engine when the weather is very cold, (c) to silly putty if you put it in the freezer for half an hour.

Cooking analogies are useful when thinking about geology. Rock behaves similar to food when you heat it up - except rock generally needs to be heated to higher temperatures than used in most kitchens (hundreds to thousands of degrees).

(18) (a) If you put a chunk of cheese under the broiler what happens to it?

(b) If you leave it there longer - before it burns, it starts to bubble. Where did the gases that are in the bubbles come from?

(c) Next, we heat up some soup, gently at first. As the soup heats up, what motions do you see in the soup?

(d) If you keep heating the soup it begins to bubble and then rise up in the pan, finally overflowing the pan. If we think of this is an analogy of lava - then, is this process analagous to the lava (and the gases inside) expanding or of it contracting as it is heated?

The net effects of heating rock are:

Volcano types
Planets are hotter deeper down. Volcanism can be defined as hot, liquid rock coming to the surface of a planet. For volcanism to occur there needs to be hot underlying rock and cracks or holes in the crust where the lava can get through to the surface. The diagrams below show cross-sections of various types of volcanoes. One rule of thumb that can be applied to volcano types is that the more viscous the lava, the steeper the sides of the volcano.

 

 


Flood basalts - This is an example where basalt, a very runny lava (low viscosity) has leaked through to the surface of a planet and filled up low-lying ground in a basin (which was perhaps originally caused by an impact, such as in the case of the lunar mare).


Shield volcano - This is an example where lava which is still pretty runny pours out of a central vent and builds up to make a large, low-angled circular volcano. Often the entrance to the vent is quite large, a caldera.

Collapsed caldera - the hole in the center of the volcano - of an exinct or dormant volcano can collapse. The lava sometimes forms a "lake" which solidifies when the volcano is dormant or exinct.

Composite cone - This is a steep-sided volcano where layers of more viscous lava and ash have deposited around a central vent.

Click on volcanic plumes and domes for diagrams of the types of volcanos that are formed from the most viscous lavas - produced when a plume of lava rises up from below and pushes up the lithosphere, not always penetrating to the surface. Sometimes the lava plume later subsides, leaving a circular ring of cracks (similar to the deformation of a piece of plastic sheeting if you stick your finger almost through it).

(19) Look at the photos of volcanoes on Earth, Venus, and Mars and see if you can identify what kind of volcano is shown in each case.
(a) EARTH
(b) VENUS 1, VENUS 2, VENUS 3.
(c) MARS 1, MARS 2.

Volcano Size
A thinner crust (lithosphere) is easier for lava to break through--so you might expect a planet with a thin crust to have more volcanoes. But to have a big volcano you need to have a thick crust to hold up the weight of lava that piles up. The following diagram shows the relative heights of the highest volcanoes.

(20) (a) Which planet has the highest mountain?

(b) So, which planet do you think has the thickest crust?

Summary of Terrestrial Planet Volcanism

Moon - Very little volcanism except the lunar mare (only found on Earth-facing side) where lava leaked through and flooded large impact basins. This occurred about 3.5 billion years ago - AFTER the end of the heavy bombardment - so the mare have much less craters than the other lunar regions.

Mercury - Virtually no volcanism visible.

Mars - A few LARGE shield volcanoes (particularly Olympus Mons, the largest mountain in the solar system). These volcanoes are thought to be the result of isolated plumes of hot (relatively low viscosity) lava coming up through the mantle and eruption through a central vent onto the surface. These volcanoes have not been active for a billion years or so.

Venus - Many volcanic features are found on the surface of Venus - ranging from flooded plains (suggesting very runny lava), shield volcanoes to very strange domes and features where it seems that the crust was pushed up by a plume of viscous lava but the lava did not erupt onto the surface.  From studying the distribution of craters on the volcanic terrains, it appears that Venus suffered from a very active period of volcanism about 800 million years ago (but since there seems to have been little activity)

Earth - The Earth is still undergoing volcanic activity. Most of the volcanic activity seems to be at boundaries between plates (see next section) where the crust is weaker and the lava can leak more easily through the crust. There are some place, notably the Hawaiian islands, where lava is erupting through the middle of a plate. It is believed that there is a "hot spot" - a plume of hot mantle material, that rising under Hawaii. As the Pacific plate moves westward the plume has produced the chain of islands - Kawaii being the farthest west and oldest, the Big Island being the farthest east and still volcanically active.


Tectonics

Tectonics is basically the cracking of a planet's crust (lithosphere)--these faults can be caused by the whole planet (or regions) expanding or shrinking or because parts of the crust are being pushed or pulled because of underlying molten material (aesthenosphere) is moving and turning over (convecting).

Local Tectonics.
If the cracks are local, only extending across small regions of the planet, then we can infer that they are caused by some local process - expansion or compressing of the lithosphere caused by, for example, the uplifting of a nearby mountain range, the cooling of a lake of lava.

(21) Look at the faults on various planets and describe if they are due to expansion or shrinking (as far as you can tell) of the planet's crust, or two chunks of crust sliding past each other
(a) Earth


(b) Moon


(c) Mercury


(d) Venus 1, Venus 2, Venus 3
(e) Mars

SHRINKING
EXPANSION

Global Tectonics.

Two ways in which there can be large-scale, global tectonics are if the whole planet cools substantially and shrinks (see left graphic above) or if there is a global, planet-wide system of motion in the mantle underneath the crust which moves sections of crust (plates) around on the surface. The evidence of global plate tectonics comprises: (a) long mountain ranges (such as the Himalyas or the Andes) caused by plates colliding; (b) long ridges (such as the Mid-Atlantic ridge) where plates are spreading apart; (c) long trenches where a plate is subsiding (subducting) under another plate; (d) long fault lines where one plate is moving sideways - shearing relative to its neighbor (such as along the San Andreas fault). All of these features are LONG - that is, they extend for a substantial fraction of the planet's surface - and form a global system of plates.

(22) Look in your book for a discussion of plate tectonics on the Earth. Then look at chapters about the other terrestrial planets.
(a)Can you find evidence of plate tectonics on planets other than Earth?
(b)What do you conclude about the presence of convection in mantles of planets other than the Earth?

Summary of Terrestrial Planet Tectonics

Moon - Very little tectonics - just a few faults, mostly along the edges of mare, where lava leaked through.

Mercury - Very little tectonics - just a few places where the crust seems to have shrunk as it cooled.

Mars - Considerable tectonic activity - one large feature, Valles Marinaris, stretches about 1/3 the way around the planet. But otherwise, the tectonic features seem to be associated with relieving stresses produced by volcanic activity.

Venus - Many tectonic features. Some are large scale, some are small scale. But there is no systematic location of mountains ranges or systems of long faultlines that would indicate that Venus has undergone plate tectonics.

Earth - The Earth is still undergoing both tectonic and volcanic activity. Most of the volcanic and tectonic activity seems to be at boundaries between plates. There are places where the plates are moving apart (ocean ridges), where the plates are colliding (mountain ranges), where plates sink down into the mantle (subduction zones) and where plates slide against each other (faultlines). The plates move relative to each other at a few inches per year, re-processing most of the lithosphere over time scales of about a billion years.



Erosion

The previous geological processes--cratering, volcanism and tectonics--occur on large scales, 10s to 1000s of km (the scale of a State rather than a town). Erosion--the process of wearing down the landscape by wind, rain, snow and ice--acts on small scales, removing rock grain by grain--but over long periods of time can have large-scale effects (such as the Grand Canyon). Click on this diagram of a river drainage system for an illustration of how small scale tributaries combine to form a large scale river system.

(23) (a) List all the geological features you can think of that are caused by erosion.

(b) Which of the 4 terrestrial planets + Moon show evidence of erosion? Looking at the figures for these planets in Chapter 9.

(c) What is the main agent of erosion? In which form--vapor, liquid or solid--is it most efficient?

(d) The images below show the same region of Mars taken at two different times. The image on the right shows the appearance of enormous columns of dust that are believed to be similar to dust devils on Earth--they are formed by warm air rising above the ground and carrying with it dust in vortices. This showcases a secondary agent of erosion--wind--which carries rock-dust / sand and `blasts' rocks, eroding them and making more sand. On which planets might this be a significant form of erosion?

Summary of Erosion

Moon & Mercury - No atmosphere, no erosion.

Mars - The current atmosphere of Mars is very tenuous (low pressure). And the surface is too cold for liquid water. Any water is frozen in the polar caps or in the soil. BUT there is evidence of erosion in the past - suggesting that at one point the atmosphere was denser, the surface warmer and water flowed in rivers, canyons. There is also evidence of occasional catastrophic flooding.

Venus - no water, very little evidence of erosion (though current, radar pictures may just not be of sufficient high resolution to see evidence of erosion.). It is probable that the sulfuric acid in the clouds rains out and causes some chemical erosion of the rocks.

Earth - Plenty of evidence of erosion - practically all surfaces are eroded - mostly by water, some ice (glaciers), some wind (deserts).


Model answers - Part I to the comprehension questions.

Model answers - Part II to the comprehension questions.