Nov. 22nd - Terrestrial Worlds: Mars, Venus, Earth

Question 1

What geological processes have shaped Mars? What are our expectations?

Answer 1

• We expect Mars to be more geologically active than the Moon or Mercury but less active than Earth or Venus.
• Observations confirm this basic picture, though *Mars’s greater distance from the Sun—about 50% farther than Earth—has also played a role in its geological history.
• Interestingly, although Mars has only about one-fourth the surface area of Earth, both planets have nearly the same amount of land area, because Earth’s surface is about three-fourths covered by water.

Question 2

Impact Cratering on Mars

Southern vs. Northern

Answer 2

• Much of the southern hemisphere has relatively high elevation and is scarred by numerous large impact craters, including the very large crater known as the Hellas Basin.
• In contrast, the northern plains tend to be below the average martian surface level and show few impact craters.

Question 3

Impact Cratering on Mars: What do the differences between Northern/Southern areas tell us?

Answer 3

• The differences in cratering tell us that the southern highlands are an older surface than the northern plains, which must have had their early craters erased by other geological processes.

Question 4

Volcanism on Mars - Northern Planes

Answer 4

• Further study suggests that volcanism was the most important process in erasing craters on the northern plains, although tectonics and erosion also played a part.
• However, no one knows why volcanism affected the northern plains so much more than the southern highlands or why the two regions differ so much in elevation.

Question 5

Volcanism on Mars - Volcanoes

Answer 5

• More dramatic evidence of volcanism on Mars comes from several towering but shallow-sloped volcanoes.
• One of these, Olympus Mons, is the tallest known mountain in the solar system

Question 6

Volcanism on Mars: ongoing? How volcanically active is Mars compared to Earth?

Answer 6

• We have not witnessed any ongoing volcanic or tectonic activity on Mars
• We expect Mars to be much less volcanically active than Earth, because its smaller size has allowed its interior to cool much more.
• However, crater counts on the slopes of martian volcanoes suggest that some lava flows may have occurred as recently as tens of millions of years ago (AKA recently)

Question 7

Volcanism on Mars: Radiometric Dating

Answer 7

• Radiometric dating of meteorites that appear to have come from Mars shows some of them to be made of volcanic rock that solidified from molten lava as little as 180 million years ago—also quite recently in the 412-billion-year history of the solar system.

Question 8

Evidence of Volcanism on Mars: SUMMARY

What does this imply?

Answer 8

1. Crater counts
2. Radiometric dating (of martian meteorites)

• Given this evidence of geologically recent volcanic eruptions, it is likely that martian volcanoes will erupt again someday Within a few billion years, Mars will become as geologically dead as the Moon and Mercury

Question 9

Tectonics on Mars

Valles Marineris

Answer 9

Mars also has tectonic features, though none on a global scale like the plate tectonics of Earth. The most prominent tectonic feature is the long, deep system of valleys called Valles Marineris

No one knows exactly how Valles Marineris formed.
* Parts of the canyon are completely enclosed by high cliffs, so neither flowing lava nor water could have been responsible.
* However, extensive cracks on its western end run up against the Tharsis Bulge, suggesting a connection between the two features. Perhaps Valles Marineris formed through tectonic stresses accompanying the uplift that created Tharsis, cracking the surface and leaving the tall cliff walls of the valleys.

Question 10

Erosion on Mars

Answer 10

• Impacts, volcanism, and tectonics explain most of the major geological features of Mars, but closer examination shows extensive evidence of erosion by liquid water
• Regardless of the specific mechanism, water is the only substance that could have been liquid under past martian conditions and that is sufficiently abundant to have created such extensive erosion features.

Question 11

What geological evidence tells us that water once flowed on Mars?

Why is water unstable on Mars?

Answer 11

• There are no lakes, rivers, or even puddles of liquid water on the surface of Mars today.
• Liquid water is unstable on Mars today: If you put on a spacesuit and took a cup of water outside your pressurized spaceship, the water would rapidly either freeze or boil away, or some combination of both.

Question 12

Evidence for Ancient Water Flows (Rain/Earth) on Earth

Answer 12

1. Indistinct rims of many large craters and the relative lack of small craters: Both facts argue for ancient rainfall, which would have eroded crater rims and erased small craters altogether
2. Spectra indicate the presence of clay minerals on the crater floor, presumably deposited by sediments flowing down the river.
3. Evidence for the ocean comes from features that look like an ancient shoreline. Radar data also suggest that the rock along the proposed shoreline is sedimentary rather than volcanic,

Question 13

Surface Evidence for Ancient Water Flows

Answer 13

• All rovers have found abundant mineral evidence of past liquid water on the surface.
• However, the character of the water appears to have differed at different times in Mars’s deep past.
• Curiosity found evidence that Gale Crater once contained much purer (“drinkable”) water
• The difference in the purity of the water that apparently once resided at the Opportunity and Curiosity landing sites suggests that martian water became saltier and more acidic over time.

Question 14

Is there liquid water on Mars today?

Answer 14

• Although Mars clearly had plenty of liquid water in the distant past and still has plenty of water ice today, it is an open question as to whether there is any liquid water on Mars today
• A more promising possibility for liquid water is underground, since Mars should still have enough internal heat to keep water in liquid form in at least some places
• The bottom line is that if any liquid water is still present on Mars, it is underground and the total amount can be only a tiny fraction of the water that flowed on the surface long ago. Mars clearly was much warmer and wetter in the past than it is today.

Question 15

What geological processes have shaped Venus?

How can we see past the thick atmosphere of Venus?

Answer 15

Venus’s thick cloud cover prevents us from seeing through to its surface with visible light, but we can study its geological features with **radar mapping: bouncing radio waves off the surface and uses the reflections to create three-dimensional images

Question 16

Impact Cratering on Venus

Answer 16

• Like Earth, Venus has a fairly small number of impact craters, indicating a relatively young surface on which more ancient craters have been erased by other geological processes.
• Moreover, while Venus has a few large craters, it lacks the small craters that are most common on other worlds, probably because the small objects that could make such craters burn up completely as they enter Venus’s thick atmosphere

Question 17

Venus’ Volcanic Features

Answer 17

• Venus shows abundant evidence of volcanism with a variety of lava types: Some mountains are shallow-sloped volcanoes from mildly runny lava
• But it also has lava plains, which must have formed from a runnier lava, as well as a few steeper-sided volcanoes, indicating eruptions of a thicker lava.

Question 18

Venus’ Tectonic Features

Answer 18

• Tectonics have also been very important on Venus, as the entire surface appears to have been extensively contorted and fractured by tectonic forces.
• Some of these features, including large circular coronae (Latin for “crowns”), provide strong evidence for mantle convection beneath the lithosphere.

Question 19

What two lines of evidence suggest Venus’ recent volcanic activity?

Answer 19

• First, Venus’s clouds contain sulfuric acid, which is made from sulfur dioxide (SO2) and water. The sulfur dioxide must have entered the atmosphere through volcanic outgassing, but once in the atmosphere it is steadily removed by chemical reactions with surface rocks.
• The second line of evidence narrows the time scale further. The European Space Agency’s Venus Express spacecraft (which orbited Venus from 2006 until early 2015) detected an infrared spectral feature from rocks on three volcanoes indicating that they erupted within about the past 250,000 years (FIGURE 9.38), which is very recent on a planetary time scale

Question 20

Weak Erosion

Answer 20

• We might naively expect Venus’s thick atmosphere to produce strong erosion, but the view both from orbit and on the surface suggests otherwise.
• We can trace the lack of erosion on Venus to two simple facts:
• First, Venus is far too hot for any type of rain or snow on its surface.
• Second, as we discussed earlier, Venus’s slow rotation—once every 243 days—means it has very little surface wind.

Question 21

Does Venus have plate tectonics?

Answer 21

• Venus shows no evidence of Earth-like plate tectonics.
• On Earth, plate tectonics reshapes the surface gradually, so different regions have different ages.
• In contrast, Venus’s relatively few impact craters are distributed fairly uniformly over the entire planet, suggesting that the surface is about the same age everywhere.
• **Crater counts suggest a surface age of about 650 million years, leading us to conclude that the entire surface was somehow “repaved” at that time, erasing all craters that formed earlier. **

Question 22

Hypotheses for Venus’ lack of tectonics

Answer 22

• Earth’s lithosphere was broken into plates by forces due to the underlying mantle convection.
• Venus’s similarity to Earth in size and density leads us to expect it to have a similar level of mantle convection.
• Most scientists therefore suspect that Venus’s lithosphere resists fracturing into plates because it is thicker and stronger than Earth’s.
• Of course, this still leaves the question of why Venus’s lithosphere would differ from Earth’s.
• One possible answer invokes Venus’s high surface temperature. Venus is so hot that any water in its crust and mantle has probably been baked out over time.

Question 23

Most aspects of Earth’s geology should make sense in light of what we have found for other terrestrial worlds:

Size, Erosion, Volcanism, Outgassing, Rotation Rate

Answer 23

• Earth’s size—largest of the five terrestrial worlds—explains why our planet retains internal heat and remains volcanically and tectonically active.
• Earth’s erosion by water and wind is explained by the combination of our planet’s size, distance from the Sun, and rotation rate
• Earth is large enough for volcanism and outgassing to have produced an atmosphere, while its distance from the Sun allowed water vapor to condense and fall to the surface as rain
• Earth’s moderately rapid rotation drives wind and other weather.

Question 24

How is Earth’s surface shaped by plate tectonics?

Plate Tectonics

Answer 24

• Refers to the scientific theory that explains much of Earth’s surface geology as a result of the slow motion of plates that essentially “float” over the mantle, gradually moving over, under, and around each other as convection moves Earth’s interior rock.
• Earth’s lithosphere is broken into more than a dozen plates (FIGURE 9.41). The plate motions are barely noticeable on human time scale

Question 25

The Discovery of Continental Motion

1912: German meteorologist and geologist Alfred Wegener

How was he wrong?

Answer 25

• Promoted the idea of a continental drift, the idea that continents gradually drift across the surface of Earth.
• Despite this evidence, no one at the time knew of a mechanism that could allow the continents to move about.
• Wegener suggested that Earth’s gravity and tidal forces from the Sun and Moon were responsible, but other scientists quickly showed that these forces were too weak to move entire continents.

Question 26

The Discovery of Continental Motion

1950’s

Mid-ocean ridges, Seafloor Spreading

Answer 26

• In the mid-1950s, scientists discovered mid-ocean ridges along which mantle material erupts onto the ocean floor, pushing apart the existing seafloor on either side.
• This seafloor spreading helped explain how the continents could move apart with time

Question 27

Evidence for plate tectonics

Seafloor Crust

Answer 27

• Seafloor crust is thinner, denser, and younger than continental crust.
• No other planet shows evidence of such distinct differences in crust from place to place.
• Seafloor crust is typically 5–10 kilometers thick and is made primarily basalt (very runny, so it spreads outward when it erupts from volcanoes along mid-ocean ridges)
• Radiometric dating shows that seafloor crust is quite young—usually less than 200 million years old—indicating that it erupted to the surface relatively recently in geological history.

Question 28

Evidence for plate tectonics

What can impact craters tell us about seafloors?

Answer 28

Large impacts should occur more or less uniformly over Earth’s surface, and the oceans are not deep enough to prevent a large asteroid or comet from making a seafloor crater.

Question 29

Continental Crust

Answer 29

• Continental crust is much thicker—typically between 20 and 70 kilometers thick—but it sticks up only slightly higher than seafloor crust because its sheer weight presses it down farther into the mantle below.
• It is made mostly of rock (such as granite) with lower density than seafloor crust.

Question 30

What does the Conveyor Belt of Plate Tectonics explain?

Answer 30

The theory of plate tectonics explains:
* Continental motion
* Seafloor spreading
* The existence of two types of crust as direct results of the way plates move about on Earth

Question 31

Over millions of years, the movements involved in plate tectonics act like a giant conveyor belt for Earth’s lithosphere:

Answer 31

• Mid-ocean ridges where mantle material rises upward, creating new seafloor crust and pushing plates apart.
• The newly formed crust cools and contracts as it spreads sideways from the central ridge, giving seafloor spreading regions their characteristic ridged shape.
• Over tens of millions of years, any piece of seafloor crust gradually makes its way across the ocean bottom, then finally gets recycled into the mantle in the process we call subduction.

Question 32

What is Subduction?

Answer 32

• Subduction occurs where a seafloor plate meets a continental plate, which is generally somewhat offshore at the edge of a sloping continental shelf.
• As the dense seafloor crust of one plate pushes under the less-dense continental crust of another plate, it can pull the entire surface downward to form a deep ocean trench. At some trenches, the ocean depth is more than 8 kilometers (5 miles).