Nov. 8th - Processes on Moon & Mercury

Question 1

Volcanism - Rises for 3 reasons:

occurs when underground molten rock finds a path to the surface

Answer 1

1. generally less dense than solid rock, and lower-density materials tend to rise when surrounded by higher-density materials
2. the solid rock surrounding a chamber of molten rock can squeeze the molten rock, driving it upward under pressure
3. molten rock often contains trapped gases that expand as it rises, which can make it rise much faster and lead to dramatic eruptions

Question 2

The result of an eruption depends on…

Answer 2

…how easily the molten rock, or lava, flows across the surface

Question 3

Lava can shape three different types of volcanic features depending on whether it’s thick or runny:

Thick

Answer 3

• The thickest lavas cannot flow far before solidifying and therefore build tall, steep-sided volcanoes (technically called stratovolcanoes)
• can allow particularly great pressure, making them prone to explosive eruptions that can deposit debris over vast areas.

Question 4

Lava can shape three different types of volcanic features depending on whether it’s thick or runny:

Liquidy

Answer 4

• Somewhat runnier lavas can spread some distance before they solidify, creating volcanoes that slope more gradually (technically called shield volcanoes)
• Tall but not steep

Question 5

Lava can shape three different types of volcanic features depending on whether it’s thick or runny:

Runny

Answer 5

The runniest lavas flow far and flatten out before solidifying, creating vast volcanic plains

Question 6

All the terrestrial worlds (and at least one jovian moon) show evidence of volcanic plains and shallow-sloped (shield) volcanoes

All of which appear to be made of the fairly dense rock:

Answer 6

• Basalt, which makes up most of Earth’s seafloor and volcanic islands such as those of Hawai‘i.
• Apparently, basaltic lava is common throughout the solar system.

Question 7

Outgassing

Answer 7

• The water and gases from planetesimals (jovian/terrestrial) became trapped in the interiors of the planets in much the same way that the gas in a carbonated beverage is trapped in a pressurized bottle.
• Volcanic eruptions can release this gas in a process known as outgassing, which occurs both with dramatic volcanic eruptions and with the more gradual escape of gas from volcanic vents
• Virtually all the gas that made the atmospheres of Venus, Earth, and Mars—and the water vapor that rained down to form Earth’s oceans—originally was released from the planetary interiors by outgassing

Question 8

Tectonics

Answer 8

• In geology, tectonics refers to the “building” of surface features by stretching, compression, or other forces acting on the lithosphere
• Tectonic activity usually goes hand in hand with volcanism, because both require internal heat, and most tectonic activity is a direct or indirect result of mantle convection
• While at least some tectonic activity has occurred on every terrestrial world, it has been particularly important on Earth, where the ongoing stress of mantle convection fractured the lithosphere into more than a dozen pieces, or plates. These plates move over, under, and around each other in a process we call plate tectonics

Question 9

Erosion:

Answer 9

• Refers to the breakdown or transport of surface rock through the action of ice, liquid, or gas
• Can also build up - eg Earth’s surface rock: Over long periods of time, erosion piled sediments into layers on the floors of oceans and seas, forming sedimentary rock

Question 10

Why do the terrestrial planets have different geological histories?

Answer 10

Ultimately, we can trace the answer back to three fundamental planetary properties:
1. Size
2. Distance from the Sun
3. Rotation rate

Question 11

Geology of Moon/Mercury

Moon/Mercury’s Smalll Sizes

Answer 11

• Their small sizes mean that most of their internal heat was lost long ago, leaving them without significant ongoing volcanism or tectonics.
• Small size also explains their **lack of significant atmospheres and erosion: Their gravity is too weak to hold gas for long periods of time, and without ongoing volcanism they lack the outgassing needed to replenish gas lost in the past. **
• This overall lack of geological activity means they still retain the scars of ancient impacts, explaining why their surfaces are heavily cratered.

Question 12

What geological processes shaped our moon?

Volcanism and Tectonics in the Lunar Maria

Answer 12

• During the heavy bombardment, craters covered the Moon’s entire surface.
• The largest impacts were violent enough to fracture the Moon’s lithosphere beneath the huge craters they created.
• Although there was no molten rock to flood the craters immediately, the ongoing decay of radioactive elements in the Moon’s interior ultimately built up enough heat to cause mantle melting between about 3 and 4 billion years ago.
• Molten rock then welled up through the cracks in the lithosphere, flooding the largest impact craters with lava.

Question 13

Why are the Lunar Maria (moon) circular, dark, and flat? Why do they lack caters?

Answer 13

• The maria are generally circular because they are flooded craters (and craters are almost always round)
• Dark because the lava consisted of dark, dense, iron-rich rock (basalt).
• The flat surfaces of the maria tell us that the lunar lava spread easily and far, which means that it must have been among the runniest lava in the solar system.
• The relative lack of craters within the maria is a consequence of the fact that the lava floods occurred after the heavy bombardment subsided, and relatively few impacts have occurred since that time.

Question 14

Far side of the moon - how does it differ?

Answer 14

• The far side landscape consists almost entirely of heavily cratered highlands, with very few maria
• Indeed, the entire far side essentially has a higher altitude than the near side
• No one knows what caused this difference in altitude between the two sides, but it apparently allowed lava to well up and create maria much more easily on the near side.

Question 15

The Moon today

“Geologically dead”, Micrometeorites - why affect the moon?

Answer 15

• Today, the Moon is geologically dead
• Small impacts occur from time to time, as witnessed by ground-based monitoring telescopes, but we are unlikely ever to witness a major one.
• The only ongoing geological change on the Moon is a very slow “sandblasting” of the surface by micrometeorites, sand-size particles from space—the same type of particles that burn up as meteors in the atmospheres of Earth, Venus, and Mars but rain directly onto the surface of the airless Moon.
• The micrometeorites gradually pulverize the surface rock, which explains why the lunar surface is covered by a** thin layer of powdery “soil.”**

Question 16

What geological processes shaped Mercury?

Impact Craters and Volcanism on Mercury:

Answer 16

• Impact craters are visible almost everywhere on Mercury, indicating an ancient surface
• However, Mercury’s craters are less crowded together (opposed to the moon), suggesting that molten lava later covered up some of the craters that formed on Mercury during the heavy bombardment
• Mercury’s largest impact craters, called basins, apparently formed from impacts so large and violent that they melted surface rock that then flowed over and filled them.
• The largest of these, known as Caloris basin, spans more than half of Mercury’s radius, though it is difficult to see clearly in photographs.

Question 17

Tectonic Evidence of Planetary Shrinking

Why does Mercury have cliffs?

Answer 17

• Mercury’s most surprising feature is a set of tremendous cliffs that appear to be distributed all over the planet.
• These cliffs have vertical faces up to 3 or more kilometers high and typically run for hundreds of kilometers across the surface.
• They probably formed when tectonic forces compressed the crust, causing the surface to crumple.
• Because crumpling would have shrunk the portions of the surface it affected, Mercury as a whole could not have stayed the same size unless other parts of the surface expanded.
• However, we find no evidence of large-scale “stretch marks” on Mercury. Can it be that the whole planet simply shrank?

Question 18

Tectonic Evidence of Planetary Shrinking

Mercury’s Internal Heat

(also in relation to the cliffs)

Answer 18

• In addition to being larger than the Moon, Mercury also has a surprisingly large iron core.
• Mercury therefore gained and retained more internal heat from accretion and differentiation than the Moon, and this heat caused Mercury’s core to swell in size.
• Later, as the core cooled, it contracted by perhaps as much as 20 kilometers in radius : parts of Mercury’s crust were forced to slide under others
  ** The mantle and lithosphere must have contracted along with the core, generating the tectonic stresses that created the great cliffs. The contraction probably also closed off any remaining volcanic vents, ending Mercury’s period of volcanism.

Question 19

Although there is no active volcanism on Mercury today, its surface still displays one odd form of activity:

Answer 19

• Some crater floors appear to be releasing easily vaporized materials from the rock, causing the rock to crumble and make pits nicknamed “hollows”
• The release of the vaporized gases leaves behind a light-colored coating, whose composition remains unknown