1086 Unit #3 Chapter 9, 10, 11

Why is it hard for us to view Mercury’s surface?

Mercury always stays in the same general part of the sky as the Sun, making it difficult to observe from Earth. It also never comes closer than about 80 million km, so even powerful telescopes historically showed only pale surface details.

How is Mercury similar to the Moon?

Both are:

• Small and essentially airless

• Heavily cratered

• Covered by ancient, geologically inactive surfaces

• Affected by tidal forces

• Have some large craters that were flooded by ancient lava

How does Mercury’s surface differ from the Moon’s?

Mercury has enormous cliffs/scarps and fault systems, and about 40% of its surface is covered by volcanic plains. Unlike the Moon, Mercury lacks the obvious dark maria because its lava plains are not much darker than the surrounding surface.

What spacecraft provided us with the most information about Mercury?

MESSENGER. It orbited Mercury from 2011–2015, mapped the entire surface, and collected data on surface composition, polar materials, magnetic fields, and Mercury’s core.

Which is longer on Mercury: a day or a year?

A day is longer.

• Mercury year = 88 Earth days

• Mercury solar day = 176 Earth days

So, one Mercury day equals two Mercury years.

Is Mercury’s surface hot or cold? Why?

Both—extremely hot during the day and extremely cold at night. Temperatures range from about +427°C to −173°C because Mercury is close to the Sun, has very long days, and has almost no atmosphere to redistribute or retain heat.

What is the origin of Mercury’s atmosphere?

Mercury has an extremely thin exosphere made mainly from atoms blasted off its surface by the solar wind. The particles quickly escape into space, so the atmosphere must constantly be replenished.

What evidence shows that lava flowed on Mercury?

Mercury has inter-crater plains and smooth plains with fewer craters than surrounding terrain, suggesting younger lava flows buried older cratered surfaces. The Caloris Basin is also partly filled with lava.

Why does Mercury have a low albedo?

Albedo: How much sunlight a surface reflects.

Mercury reflects only about 10% of incoming light. The exact reason is uncertain, but the chapter suggests a coating of carbon-rich dust from comet impacts may darken the surface.

If Mercury has no plate tectonics, why does it have faults and scarps?

They probably formed as Mercury cooled and shrank. Its diameter may have decreased by at least 10 km, compressing and wrinkling the crust and producing large thrust faults and scarps.

How are Mercury’s hollows formed?

Hollows are pits thought to form when volatile material beneath the surface is vaporized and escapes, often after asteroid impacts expose or heat it.

How does Mercury’s interior compare with Earth’s?

Mercury is almost as dense as Earth but has an enormously larger core relative to its size. Its iron-rich core makes up about 75–85% of the planet’s mass, leaving only a relatively thin rocky mantle and crust. At least part of the core is probably still molten because Mercury has a magnetic field. Chapter 9 - Mercury

Why might Mercury have such a large core?

One hypothesis is that a giant impact stripped away much of Mercury’s lower-density crust and mantle, leaving its dense metallic core behind. However, the high abundance of volatile elements such as sulfur creates problems for this idea.

What is the geological history of Mercury?

1. Formation: Mercury formed near the Sun; a giant impact may have stripped away some outer rock.

2. Heavy bombardment: intense cratering occurred early, including formation of the Caloris Basin.

3. Volcanism: lava flooded low areas and created inter-crater and smooth plains.

4. Slow evolution: Mercury cooled and contracted, producing scarps; today change is mainly from micrometeorites, rare impacts, and extreme heating/cooling.

What two elements are surprisingly common on Mercury’s surface?

Sulfur and magnesium. Mercury has about 10 times more sulfur than Earth, and its surface rocks are also unusually rich in magnesium. Chapter 9 - Mercury

Is there water on Mercury? How do we know?

Yes—water ice exists near Mercury’s poles. Earth-based radar detected highly reflective material, and MESSENGER detected high levels of hydrogen and reflectance consistent with water ice.

Why hasn’t the Sun evaporated Mercury’s polar ice?

Mercury’s axis is tilted less than 1°, so the floors of some deep polar craters are permanently shadowed and never receive sunlight. Temperatures there can remain below −150°C, allowing ice to survive. Chapter 9 - Mercury

Why is it hard for us to view Venus’ surface?

Venus is permanently covered by a thick cloud layer, so telescopes see the tops of the clouds rather than the solid surface. Radar and spacecraft are needed to study the ground beneath them.

What are our 3 main sources of information about Venus?

1. Radar observations through the clouds

2. Orbiting/flyby spacecraft such as Magellan and Venus Express

3. Landers, especially the Soviet Venera missions, which returned surface data and images

Why does Venus rotate clockwise, opposite to most planets?

The chapter suggests Venus probably originally rotated in the same direction as the other planets but was knocked over by a very large impact late in its formation. Its axial tilt is about 177°, so it is essentially rotating upside down.

Which is longer on Venus, a year or a day?

A rotation day is longer than a year.

• Rotation period = about 243 Earth days

• Orbital period/year = about 225 Earth days

What is Venus’ cloud layer made of? What is the main component of its atmosphere?

• Clouds: mainly droplets of sulfur/sulfuric acid

• Atmosphere: about 96.5% carbon dioxide, with about 3.5% nitrogen

How do Venus’ clouds and atmosphere affect its climate?

Its CO₂-rich atmosphere produces an extremely strong greenhouse effect. Water and sulfur dioxide also help block escaping infrared radiation, producing a nearly uniform surface temperature of about 462°C.

What is the greenhouse effect?

Sunlight enters the atmosphere and warms the surface. The surface releases energy as infrared radiation, but greenhouse gases such as CO₂ absorb/trap some of this outgoing energy, warming the planet. Chapter 10 - Venus

How does the greenhouse effect differ on Venus and Earth?

Both planets have a greenhouse effect, but Venus has an enormous amount of atmospheric CO₂, producing a runaway greenhouse effect. Earth’s oceans removed much of its CO₂ from the atmosphere, preventing this extreme warming.

Which is hottest: Mercury, Venus, or Earth?

Venus, with a surface temperature around 462°C. It is even hotter than Mercury because of its powerful greenhouse effect. Chapter 10 - Venus

Has Venus ever had water? What is the evidence?

Yes. Evidence includes:

• Venus has about 150 times more deuterium relative to ordinary hydrogen than Earth.

• Venus Express detected OH, a breakdown product of water.
  These suggest Venus once had much more water, possibly enough for a planet-wide ocean about 25 m deep.

What happened to Venus’ water?

As Venus warmed, its water evaporated. Ultraviolet radiation split water molecules, and much of the hydrogen escaped into space. Without oceans, Venus could no longer remove atmospheric CO₂, strengthening the greenhouse effect further. Chapter 10 - Venus

Earth and Venus have similar total amounts of carbon. Why doesn’t Earth have Venus’ runaway greenhouse effect?

Earth stayed cool enough to maintain liquid-water oceans. Water dissolved atmospheric CO₂, and much of that carbon became stored in oceans and rocks such as limestone. Venus lost its oceans, leaving much more CO₂ in its atmosphere.

Why are there fewer impact craters, especially small ones, on Venus than on the Moon or Mercury?

Venus has a very dense atmosphere. Small impactors burn up through friction before reaching the surface, so surviving craters tend to be medium or large.

What evidence shows that Venus’ surface is young?

• Venus has relatively few craters.

• The craters are rarely filled with lava and appear rough, suggesting they formed after a relatively recent resurfacing.

• Crater counts indicate most of the surface is only about 500 million years old, despite Venus itself being about 4.5 billion years old.

What volcanic features exist on Venus?

Venus has:

• Small volcanic domes

• Huge volcanoes such as Maat Mons

• Coronae

• Caldera-like volcanic craters

• Pancake volcanoes

• Shield fields

• Lava-flow channels thousands of kilometres long

What may cause a global volcanic event on Venus?

Venus lacks plate tectonics, so internal heat may build up beneath the crust rather than escaping gradually. Eventually the planet may “boil over,” producing massive volcanic eruptions that resurface much of the planet.

What evidence suggests Venus experienced a global volcanic resurfacing?

Crater counts indicate that most of Venus’ surface is about 300–500 million years old, suggesting widespread basaltic lava flows resurfaced the planet around that time.

Why do rocks erode faster on Earth than on Venus?

Earth rocks contain water, which weakens rock and promotes weathering and erosion. Venusian rocks are extremely dry, making them stronger and allowing very sharp ridges and cliffs to survive.

What causes erosion on Venus?

Since Venus has no liquid water, erosion must mainly be caused by wind. Dust and debris patterns around impact craters and volcanoes show evidence of prevailing winds.

What causes Venus’ weak magnetic field? How is this different from Earth’s?

Venus’ weak magnetic field is not generated by its interior. It is produced by the interaction between the solar wind and Venus’ upper atmosphere.

Earth’s field, in contrast, is generated internally by a dynamo involving its solid inner core and liquid outer core.

What are the proposed stages of Venus’ geological history?

The other rocky bodies generally went through:

1. Differentiation

2. Heavy cratering

3. Lava flooding

4. Slow surface evolution

Venus probably began similarly, but its later evolution was unusual because it may undergo planet-wide resurfacing events instead of ordinary slow surface evolution.

What supports Venus having a partially liquid core?

Venus has a similar size and density to Earth, and its extensive volcanism shows that its interior is still very hot. This makes a molten or partly molten core plausible.

What argues against Venus having a partially liquid core?

If Venus had an Earth-like solid inner core and liquid outer core with sufficient differential rotation, we might expect an internally generated magnetic field, but none has been detected. The chapter notes Venus’ extremely slow rotation could also explain why a dynamo does not operate, so the core’s state remains uncertain.

Why is there no active plate tectonics on Venus?

Two main reasons are proposed:

• Venus’ crust is extremely dry and relatively low-density/buoyant, so it resists sinking into the mantle.

• The crust is very hot and not rigid enough to form stiff plates like Earth’s lithosphere.

Instead, convection beneath Venus’ crust creates features such as coronae, mountains, and volcanism.

Do canals exist on Mars? Why were people excited about them?

: No. The “canals” were optical illusions caused by poor telescopic views. Schiaparelli called them canali (“channels”), but this was mistranslated as “canals,” which made people think they were artificial waterways built by intelligent Martians

Is Mars smaller or larger than Earth?

Smaller. Mars is about 0.53 times Earth’s diameter, roughly half Earth’s size.

Does Mars have polar caps?

Yes. Mars has permanent polar caps containing water ice and carbon dioxide ice.

Does Mars have a greenhouse effect?

Yes. Mars has a CO₂-rich atmosphere, and its early atmosphere was thick enough to produce a stronger greenhouse effect that allowed liquid water to remain stable on the surface. Today, the atmosphere is much thinner.

Is there evidence for life on Mars?

No confirmed evidence of life has been found. Viking found no obvious life or organic material, although some soil experiments produced unusual reactions. Curiosity has detected organic molecules, but these do not by themselves prove life.

Does Mars have satellites? Are they round? Why or why not?

Mars has two moons, Phobos and Deimos. They are small and irregular rather than round. The chapter describes Phobos as loaf-shaped and Deimos as very small.

Does Mars have seasons? Why?

Yes. Mars has an axial tilt of about 25°, similar to Earth’s 23.5°, so its hemispheres alternately tilt toward and away from the Sun.

Which is longer on Mars, a year or a day?

A year is much longer.

• Day = about 24 hours 40 minutes

• Year = about 1.88 Earth years / 687 Earth days

What spacecraft have explored Mars from its surface?

Viking 1 & 2, Pathfinder, Spirit, Opportunity, Phoenix, Curiosity, Perseverance, and Zhurong.

What are Curiosity’s main objectives?

To investigate:

• Whether Mars could or did support microbial life

• The presence of water

• Martian climate

• Martian geology

What kinds of evidence does Curiosity look for?

It analyzes rocks, soil, minerals, atmospheric composition, hydrated materials, sedimentary rocks, and organic molecules for evidence of past water, climate, and habitability.

Does Mars have an atmosphere? What is it made of?

Yes, but it is very thin. It is approximately:

• 95% CO₂

• 3% nitrogen

• 1.6% argon

• trace gases

Does the Martian atmosphere contain water?

Yes, but only tiny traces of water vapour.

How has Mars’ atmosphere affected its soil?

UV radiation breaks apart water molecules. Hydrogen escapes, while oxygen reacts with iron in the soil, producing iron oxides (rust) that give Mars its red colour.

How did the atmosphere of Mars form?

Mars originally had a primitive atmosphere of mainly hydrogen and helium, but its low escape velocity caused much of this to escape. It was replaced by a secondary atmosphere from volcanic outgassing, mainly CO₂ and water vapour.

How is Mars’ atmospheric history similar to Venus and Earth?

Mars, Venus, and Earth all developed similar secondary volcanic atmospheres. Earth later developed a different “tertiary” atmosphere because of life, oxygen production, liquid water, and carbon storage in rocks.

How is Mars different from Mercury?

Mars has a genuine but thin secondary atmosphere. Mercury’s extremely thin exosphere is mainly made of atoms knocked off its surface, rather than a substantial volcanically produced atmosphere.

What are the implications of high vs. low escape velocity?

• High escape velocity: gases are harder to lose, so atmospheres are retained more easily.

• Low escape velocity: gases escape more easily.

Mars has an escape velocity of only about 5 km/s, less than half Earth’s, so it loses gases much more readily.

How does planet size affect atmospheric composition and retention?

Larger planets have stronger gravity and higher escape velocities, so they retain gases more easily. They also cool more slowly, maintaining internal heat, volcanism, and sometimes magnetic fields longer. Smaller planets cool faster, produce less volcanic gas, and lose atmospheres more easily.

How do gas giants compare with terrestrial planets in atmosphere retention?

Gas/ice giants have strong enough gravity to retain their primitive hydrogen/helium atmospheres, while terrestrial planets generally lost those early gases and developed secondary atmospheres.

How can cooling affect a planet’s atmosphere?

As a small planet cools, volcanism declines, so fewer gases are released. Cooling can also shut down a core dynamo and magnetic field, allowing the solar wind to strip atmospheric gases more easily.

Can water escape Mars’ atmosphere? How is it lost?

Whole water molecules are heavy enough to be retained, but UV radiation breaks H₂O apart. The light hydrogen escapes to space, while oxygen reacts with surface materials.

Was there ever liquid water on Mars?

Yes. The chapter says ancient Mars had a thicker atmosphere and stronger greenhouse effect, allowing liquid water to exist on the surface.

What evidence do we have for weather, storms, and wind on Mars?

Evidence includes:

• Clouds and ground fog

• Frost

• Global dust storms

• Small dust devils

• Dunes

• Wind-driven saltation of sand grains

What evidence shows that water exists or once existed on Mars?

Evidence includes:

• Gullies and flow features

• Hydrated salts

• Ancient river channels and deltas

• Sedimentary rocks

• Deuterium enrichment

• Water-bearing minerals and meteorites

• Polar ice and subsurface ice

What large-scale features show ancient liquid water?

Evidence includes a possible ancient northern ocean, huge river outflow channels, and a clear river delta in Eberswalde crater.

Where does water currently exist on Mars?

• Polar ice caps

• Permafrost/subsurface ice

• Water chemically bound in rocks and minerals

• Tiny amounts of atmospheric water vapour

• Temporary frost/ice deposits

Compare plate tectonics on Mars, Earth, and Venus.

• Earth: active plate tectonics with multiple moving plates

• Mars: one-plate planet; no active plate tectonics

• Venus: also lacks active plate tectonics and behaves broadly as a one-plate planet

Why doesn’t Mars have the thick regolith seen on the Moon?

Even Mars’ thin atmosphere protects the surface from many micrometeorites that would otherwise grind rocks into powder. Dust storms can also remove fine material and leave larger rocks exposed.

Why are there few craters in Mars’ northern lowlands?

The northern lowlands were resurfaced, probably about a billion years ago. Possible causes include volcanic flooding or an ancient ocean of liquid water covering/burying older cratered terrain

How is this similar to Venus?

Venus also has relatively few craters because large-scale volcanic resurfacing covered older cratered surfaces.

What surface features tell us about Mars’ crust?

• Olympus Mons is so large yet has not sunk deeply, suggesting Mars has a thick crust.

• Valles Marineris formed from large crustal faults, showing the crust was once more tectonically active.

• The Tharsis Rise shows major crustal uplift

What do sulfates in Martian soil tell us?

Sulfates are evaporite minerals, so they indicate that liquid water once stood and later evaporated, leaving dissolved salts behind

What do iron oxides tell us about Mars?

Red iron oxides indicate oxidation of iron, likely promoted by episodes of moisture in an otherwise dry environment. They give Mars its red colour.

What are Martian “blueberries” and what do they indicate?

“Blueberries” are small hematite (iron oxide) spheres thought to have formed through gentle movement of water. They support the idea that liquid water once existed in the region.

Is Mars’ core molten?

Partly. The chapter says at least the outer part is molten, and NASA data suggest the core is not completely solid but more like a thick “goopy” liquid.

Is Mars’ core metallic?

Yes. It is an iron-rich core, but it also contains lower-density materials such as iron sulfide and magnetite.

Does Mars have a magnetic field?

Mars has no current global magnetic field. However, ancient parts of the crust are magnetized, suggesting Mars once had an internal dynamo and stronger planetary magnetic field.

How did Mars’ giant volcanoes form?

They are shield volcanoes formed by low-viscosity lava rising above long-lived mantle plumes. Because Mars has no plate tectonics, the crust stayed over the same plume, allowing lava to pile up repeatedly and create enormous volcanoes such as Olympus Mons

What are the 4 developmental stages of Mars?

1. Differentiation — Mars separated into core, mantle, and crust; it likely once had a magnetic field.

2. Cratering — heavy bombardment produced ancient cratered terrain and large impact basins.

3. Flooding — widespread lava flows and abundant liquid water reshaped the surface; oceans, rivers, and valley networks likely existed.

4. Slow surface evolution — Mars cooled, lost atmosphere and internal heat, plate tectonics stopped, and the planet became cold, dry, and mostly geologically inactive.