Astronomy Test 2

Why Doesn’t the Moon Fall to Earth?

• The moon is falling toward Earth, but it’s also moving sideways fast enough that it misses the Earth.

• Since the Earth is round, the moon keeps "falling around" it instead of crashing into it.

• This combination of gravity (pulling it down) and inertia (moving sideways) is what keeps the moon in orbit.

• The same idea applies to satellites, space shuttles, and anything else in orbit.

Tides (How the Moon Affects Earth's Water)

• The moon’s gravity pulls on Earth’s oceans, causing water to bulge out on the side closest to the moon and on the opposite side.

• Because the Earth rotates, these bulges move, creating high tides every 12 hours (2 per day).

• The sun also causes tides, but its effect is about half that of the moon.

• Biggest tides (Spring Tides) happen when the sun, Earth, and moon are aligned (New Moon or Full Moon).

• Smallest tides (Neap Tides) happen when the sun and moon are at 90° angles from each other.

What Causes the Seasons?

Earth’s Tilt? ✅ (Correct)

• Earth is tilted 23.5°, so different parts of the planet get more or less sunlight at different times of the year.

• This explains why the Northern and Southern Hemispheres have opposite seasons.

• The sun’s height in the sky and the amount of daylight change throughout the year, which makes seasons happen.

If tilt causes seasons (which it does), one hemisphere should always have opposite seasons from the other.

Summer vs. Winter

• In summer, the sun is higher in the sky (74°), and its light is more direct, meaning more heat.

• In winter, the sun is lower in the sky (27°), and its light is spread out more, meaning less heat.

• The Southern Hemisphere has the opposite seasons from the Northern Hemisphere (when it's summer for us, it's winter in Australia).

• Direct sunlight hits a small area and heats it more efficiently.

• Indirect sunlight is at an angle, so it spreads out over a larger area, making it less intense.

Why Are the North and South Poles So Cold?

• They always get indirect sunlight, even in summer.

• The sunlight is spread over a large area, so there’s less heat per square foot.

Bottom Line: Earth’s tilt is the main reason we have seasons, not how close we are to the sun! 🌎☀

Earth’s Gravity & Interior Composition

• Gravity helps scientists figure out Earth's mass and what’s inside it.

• Earth’s gravity is affected by its density (how tightly packed its material is).

Earth’s Stats:

• Diameter: 13,000 km

• Mass: 6 × 10²⁴ kg

• Density: 5.5 g/cm³

Newton’s Law of Universal Gravitation

• Every object in the universe pulls on every other object.

• The force of gravity depends on:

  1. Mass – Bigger objects pull harder.

  2. Distance – The farther apart objects are, the weaker the pull.

• Formula:

  F=Gm1m2r2F=Gr2m1​m2​​

  Where:

  • F = gravitational force

  • G = gravitational constant (6.674 × 10⁻¹¹)

  • m₁ & m₂ = masses of two objects

  • r = distance between them

What’s Inside the Earth?

• Heavy materials sink, lighter ones rise.

• Core: Iron (very dense, 10-12 g/cm³).

• Mantle: Denser silicate rock (3-5 g/cm³).

• Crust: Light silicate rock (3 g/cm³).

• Earth’s crust floats on the mantle → continental drift.

• Seismic waves (from earthquakes) help scientists study Earth’s interior.

How Old is the Earth?

• Earth is 4.5 billion years old (measured using radioactive decay).

• It took less than 100 million years for Earth to separate into layers.

The Moon

• The Moon is Earth's only natural satellite.

• Formed when a large body hit Earth, ejecting material that later formed the Moon.

• Density (3.3 g/cm³) → Mostly rock, very little iron.

• Mass is 1/80 of Earth’s mass.

Why is the Moon’s Gravity So Weak?

• Gravity on the Moon is 1/6 of Earth’s gravity.

• If you weigh 180 lbs on Earth, you’d weigh 30 lbs on the Moon.

• Result: It’s easy to jump on the Moon!

Why Doesn’t the Moon Have an Atmosphere?

• Weak gravity → Can’t hold onto gases.

• No atmosphere means extreme temperatures:

  • Day: +130°C

  • Night: -110°C

Is There Water on the Moon?

• Yes! Found in deep craters near the poles where sunlight never reaches.

• NASA confirmed frozen water ice in these craters

• So oxygen can be pulled from the water

Moon’s Surface & Craters

• Very old rocks (over 4 billion years).

• Maria ("seas") → Formed 3-4 billion years ago from lava flows.

• Heavy asteroid impacts occurred about 4 billion years ago.

Mercury

• Closest planet to the sun

• Mercury takes 88 Earth days to complete one orbit around the Sun.

• Very dense (5.4 g/cm³) → Mostly iron with a thin rocky crust.

• Extreme temperatures:

  • Day: +400°C

  • Night: -180°C

Weird Rotation (3:2 Spin-Orbit Resonance)

• Mercury spins 3 times for every 2 orbits around the Sun.

• One solar day on Mercury = 176 Earth days!

• Caused by the Sun’s gravitational pull on Mercury’s elliptical orbit.

Mysterious Surface Features

• Mercury has been hit by unknown blue and red objects (cause still being studied).

Maria on the Moon 🌑

• "Maria" (Latin for "seas") are large, dark, flat plains on the Moon.

• Early astronomers thought they were actual seas, but they’re solidified lava flows from ancient volcanic activity.

• Formed between 3-4 billion years ago when molten rock rose from beneath the Moon’s surface and filled impact craters.

• Maria are younger than the Moon’s highlands (which are more heavily cratered).

• Less cratered = younger surface because lava covered up older impact marks.

• Mostly made of basalt, a type of volcanic rock that cooled into a smooth, dark surface.

Why Did Lava Flow on the Moon?

• The Moon’s interior was still hot back then, and large asteroid impacts cracked the surface, allowing magma to escape.

• This stopped about 3 billion years ago as the Moon’s interior cooled down.

Where Can You See Maria?

• The dark patches on the Moon’s near side are maria!

• Some famous ones:

  • Mare Imbrium (Sea of Rains) 🌧

  • Mare Tranquillitatis (Sea of Tranquility) – where Apollo 11 landed! 🚀

  • Mare Serenitatis (Sea of Serenity) 🌊

Venus: Earth's Twin, but a Hellish World 🌋🔥

Venus might be similar in size and density to Earth, but its extreme runaway greenhouse effect makes it the hottest planet in the Solar System!

Why is Venus So Hot? ☀

• Thick Atmosphere: 96% carbon dioxide (CO₂) traps heat.

• Runaway Greenhouse Effect: CO₂ lets sunlight in but traps heat, raising the surface temperature to 480°C (896°F)—hot enough to melt lead!

• No Carbon Cycle: Unlike Earth, Venus lacks a way to remove CO₂ from the atmosphere. No oceans, no life, and no plate tectonics to recycle carbon.

• Sulfuric Acid Clouds: Venus has a thick yellowish cloud layer made of sulfuric acid, which reflects 75% of sunlight (high albedo).

Venus vs. Earth: Atmosphere Comparison 🌍🆚🪐

Gas	Venus (%)	Earth (%)
CO₂	96%	0.04%
Nitrogen	4%	78%
Oxygen	~0%	21%
Argon	0.007%	1%

🔴 High CO₂ = Intense Greenhouse Effect = Extreme Heat

Venus Rotates Backwards! 🔄

• "Retrograde rotation" – Venus spins clockwise (opposite of most planets).

• Likely due to tidal effects from the Sun slowing its spin and reversing it over time.

• A Venus day (one full rotation) is longer than its year!

  • 1 Venusian day = 243 Earth days

  • 1 Venusian year = 225 Earth days

Yeah, it sounds wild, but it’s true! Here’s how it works:

- A Venusian "day" (the time it takes to rotate once on its axis) is 243 Earth days long.

- A Venusian "year" (the time it takes to orbit the Sun) is 225 Earth days long.

- This means a single full day on Venus is longer than its entire year!

### Why Does This Happen?

1. Venus spins VERY slowly—it barely rotates at all compared to Earth.

2. Venus spins in the opposite direction ("retrograde rotation"), meaning if you were standing on Venus, the Sun would rise in the west and set in the east.

3. Because of its slow spin + opposite rotation, the time from one sunrise to the next (a solar day) is actually about 117 Earth days—still longer than its year, but shorter than its full rotation time.

### What This Means on Venus

- If you were on Venus, a full "day" (sunrise to sunrise) would take about 117 Earth days.

- But the planet itself takes 243 Earth days to complete one full rotation relative to the stars.

- In the time it takes Venus to rotate once, it has almost completed an entire orbit around the Sun!

 But…recent computer simulations suggest that Venus’s spin slowed down and reversed from the Sun’s tidal effect on Venus’s atmosphere. Me saying the atmosphere wants to maintain its orientation towards the sun. 

 

Why Did Venus Get So Messed Up? 😵‍💫

1⃣ Too Close to the Sun → More heat, no liquid water to trap CO₂.
2⃣ No Plate Tectonics → No subduction to recycle CO₂ into the mantle.
3⃣ No Life → No plants to absorb CO₂ and produce oxygen.
4⃣ Atmospheric Pressure 90x Earth’s → Equivalent to being 1 km (0.6 miles) underwater!
5⃣ No Protection from Heat → Thick clouds trap and amplify the heat.

What Happened to Space Probes on Venus? 🛰🔥

• Soviet Venera landers melted within 30 minutes due to the heat and pressure!

• The acidic atmosphere corroded equipment quickly.

• NASA and ESA are planning new Venus missions to study its past habitability and climate evolution.

What causes atmospheric pressure?

It’s the weight of air above the surface, caused by gravity and density of the air. 

How the Greenhouse Effect Works

1⃣ CO₂ gas is transparent to sunlight 🌞

• Sunlight passes through CO₂ in the atmosphere and reaches the surface.

2⃣ Sunlight heats the surface, which then releases heat (infrared radiation) ♨

• The ground absorbs sunlight and later radiates heat back into the air.

3⃣ CO₂ traps heat radiation, preventing it from escaping into space 🚫🌌

• Unlike visible sunlight, heat radiation gets blocked by CO₂, trapping warmth in the atmosphere.

Greenhouse gases that trap heat

• CO₂ (carbon dioxide) – Major heat-trapping gas.

• CH₄ (methane) – Traps even more heat than CO₂ but is less abundant.

• N₂O (nitrous oxide) – Another strong heat-trapping gas.

• H₂O (water vapor) – The most powerful greenhouse gas, but it cycles in and out of the atmosphere quickly.

🔎 Examples in real life:

• Greenhouse glass – Lets sunlight in but traps heat inside.

• Cars with closed windows in summer – Sun heats the inside, and heat can’t escape easily.

• CO₂ acts just like this, but in the atmosphere instead of a greenhouse.

Why Earth Stays Balanced, But Venus Overheats

✅ Earth’s Greenhouse Effect = Life-Friendly 🌍

• Earth is warmed by ~32°C (58°F) from greenhouse gases, making it habitable.

• Without CO₂ and H₂O vapor, Earth would be a frozen wasteland.

• Earth has natural CO₂ cycles to regulate temperature.

🔥 Venus’s Greenhouse Effect = Runaway Hellscape 🌋

• Way too much CO₂ (96%) → Traps too much heat → Hottest planet in the Solar System (480°C / 900°F).

• Venus lost its ability to regulate CO₂ like Earth does.

• No plate tectonics = No way to bury excess CO₂.

Earth’s Two CO₂ Control Systems

🌎 1) Geological Cycle (Long-Term CO₂ Removal)

• CO₂ dissolves in water and reacts with rock.

• It forms seashells, sediments, and limestone.

• Subduction (plate movement) pushes CO₂ into Earth’s crust and mantle.

• Volcanoes later release some CO₂ back into the air.

• 🌍 This cycle helps Earth "breathe" CO₂ in and out.

🌱 2) Life Cycle (Short-Term CO₂ Regulation)

• Plants take in CO₂ and release oxygen.

• Dead plants store carbon in the ground (fossil fuels, soil).

• Erosion & ocean absorption remove CO₂ from the air.

🔁 These cycles keep Earth’s CO₂ in check!

Mars is names after

• the Roman god of war (Greek: Ares)

• Reason for Name: The planet's red color, caused by iron oxide (rust) on its surface, resembles blood.

Physical Characteristics

Feature	Mars	Earth
Diameter	0.5 of Earth (6779 km)	1 Earth diameter
Mass	0.1 of Earth	1 Earth mass
Distance from Sun	1.52 AU	1 AU
Orbital Period	1.9 years (almost 2 Earth years)	1 year
Density	3.9 g/cm³ (between Earth's and the Moon’s)	5.5 g/cm³
Temperature	-130°C to 20°C (Very Cold!)	-89°C to 58°C

Mars' Atmosphere

• Thin atmosphere mostly made of carbon dioxide (CO₂).

• Atmospheric pressure is only 1/100 of Earth's, making it difficult to retain heat.

• Very little oxygen (O₂) compared to Earth.

Why is Mars' Atmosphere So Thin?

• Low Gravity → Mars is smaller, so it couldn’t hold onto its atmosphere over time.

• Less Solar Energy → Being farther from the Sun (1.52 AU), Mars gets less heat, so CO₂ and water freeze and fall to the ground, thinning the atmosphere.

• No Magnetic Field → Unlike Earth, Mars doesn’t have a strong magnetic field to protect its atmosphere from being stripped away by solar winds.

Surface & Climate

• Mars has the most Earth-like surface but is too cold for liquid water.

• Water and CO₂ are frozen in the polar ice caps and underground.

• Dust storms can cover the entire planet for weeks.

Why is Mars Red?

Iron oxide (rust) covers the surface, giving it a reddish color.

Key Concepts to Remember 4 Mars

✅ Mars is cold and dry because of its thin atmosphere and distance from the Sun.
✅ Most of Mars’ water is frozen in its polar ice caps or underground.
✅ Mars has low gravity, so it lost much of its atmosphere to space.
✅ Mars' red color comes from iron oxide (rust).

Mars' Core & Interior

• Core: Takes up just under 50% of Mars' diameter (~3300 km).

• Composition: Liquid iron, surrounded by a thin layer (150 km) of liquid rock.

• Discovered by: The InSight mission, using seismometers to study Mars' interior.

Geological Features

• Mars has large volcanoes and canyons, showing past geological activity.

• Largest volcano: Olympus Mons (3x taller than Mauna Loa, the largest volcano on Earth).

• Largest canyon: Mariner Valley (Valles Marineris) was formed by the crust rising and buckling, not by water flow.

• No Plate Tectonics: Like Venus, Mars doesn’t have moving tectonic plates.

Mars' Weather

• Main weather event: Dust storms, some of which can cover the whole planet for weeks.

• Driven by: The evaporation and condensation of CO₂, similar to how dust storms work on Earth.

Water on Mars

• Huge reserves of frozen water exist beneath the polar ice caps.

• Mars Express found a frozen sea covered in dust near the equator.

• Mars rovers (Opportunity & Spirit) have found evidence of past liquid water in Meridiani Planum and altered rocks from flowing water.

• In 2025, China’s Zhurong rover found possible evidence of ancient ocean shorelines (3.6 billion years old) in Utopia Planitia, suggesting that an ocean once covered 1/3 of Mars.

• Mars Reconnaissance Orbiter discovered seasonal channels with hydrated salts (perchlorates) that lower the freezing point of water (-70°C) and prevent quick evaporation.

🚀 Why Isn’t Liquid Water Possible on Mars Today?

1. Thin atmosphere → Not enough pressure to keep water in liquid form.

2. Cold temperatures → Water would freeze almost instantly.

Signs of Life on Mars?

• A Mars meteorite found in Antarctica contains possible nanobacteria fossils (disputed).

• Some regions on Mars release methane, which could suggest microbial life.

• In 2025, Curiosity rover found large organic molecules in 3.7-billion-year-old rocks, indicating the potential for ancient life.

Terrestrial Planets Overview

	Distance from Sun (AU)	Diameter (Earth = 1)	Density (g/cm³)	Core Type
Mercury	0.4 AU	0.38	5.4	Iron
Venus	0.7 AU	0.95	5.3	Iron
Earth	1.0 AU	1.0	5.5	Iron
Mars	1.5 AU	0.5	3.9	Iron & Rock

• Similarities:

  • All close to the Sun (0.4 – 1.5 AU).

  • Small and dense (3 – 6 g/cm³), meaning they’re rocky planets.

  • Iron cores surrounded by rocky crusts.

What are Seismometers?

are instruments that detect and measure seismic (ground) vibrations, such as marsquakes, meteor impacts, and underground activity.

📌 On Mars, the InSight lander used a seismometer to study the planet’s interior.

• It measured marsquakes to determine the size and composition of Mars' core, mantle, and crust.

• Helped confirm that Mars has a liquid iron core surrounded by a thin layer of molten rock.

• Showed that Mars is still geologically active, even though it doesn’t have plate tectonics like Earth.

🔥 Bonus Fact: Seismometers work on Earth, the Moon, and Mars to reveal what’s happening inside a planet—like a doctor using an X-ray for bones! 🩻🌍🔴