Terrestrial Planets: Mercury, Venus, Earth, Mars

Mercury

• Closest to Sun (0.38 AU), orbits in 88 days

• No moons!

• Moon-like, no atmosphere

• lots of craters from impacts

• Slow rotation - 59 days

• no delay temp change, equator temperature day/night

• Absurdly large metal core

• 80% planet radius!

• low mass, surface gravity 38% earth’s

Venus

• Closer to Sun (0.72 AU), 224 day orbits

• Rotates slower than Mercury (243 days) and slower than orbits, also opposite direction

• No moons!

• Sometimes called “Earth’s sister”

• similar size, density, gravity, has atmosphere, clouds

• hottest surface, very strong greenhouse effect

• thick atmosphere carbon dioxide, like being 1km (3000ft) under the ocean

• Very young surface: 300-500 million years

• Probably active volcanoes still!

Earth

• Largest terrestrial planet

• Wet! 70% surface covered in water

• only planet with surface liquid, active chemistry, life

• Most geologically active planet

• surface <1 billion years old, constantly renewed

• plate tectonics, volcanism, erosion, weathering

• large elevation map, age of continental plates and age of ocean plates

• record of ancient impacts almost entirely erased, heat map of every meteorite strike ever

• Thick atmosphere, nitrogen and some oxygen

• protects surface from high energy light

• Strong global magnetic field

- protects atmosphere, sometimes changes polarity

• Absurdly large natural satellite, our moon

• ¼ size of earth, ½ size of mars

Mars

• Farthest terrestrial from Sun (1.5 AU)

• Two tiny asteroids moons

• Small with surface gravity 38% Earth’s, but record for largest volcano and canyon

• Geologically inactive, but similar to Earth

- similar day, axial tilt and seasons

- ice caps, clouds, dust storms, dust devils

• Thin atmosphere, carbon dioxide

• Wet past!

- active volcanoes in the past!

- water hiding beneath the surface, under ice caps of dry ice

Terrestrials cool from 

Formation

Terrestrials heat from

radioactive decay

Earth and Venus

Lots of internal energy

Mars and Mercury 

cooled off

Magnetic fields

• Earth’s is strong (liquid core + rotation)

• Mars doesn’t have one (not enough core liquid)

• Mercury’s very weak (partially melted by heat crystallization, Earth’s solid core already crystallized)

• Venus doesn’t have one (slow rotator)

• Atmosphere bound by gravity, replenished by volcanoes, but magnetic field helps protect

• particles from solar wind of sun, also explosions on surface of Sun (CME’s)

• high-speed charged particles from supernova in galaxy

• magnetic field funnels particles to poles, where slam into atmosphere, excite gas, cause aurora

Atmospheres

• Composition

• Venus and Mars: carbon dioxide

- gasses from volcanoes

• Earth: 70% nitrogen, 28% oxygen

- changed over time

- carbon dioxide locked up (plate tectonics) nitrogen built up

- algae “polluted” with oxygen, almost caused own extinction

• Different thickness due to gravity, surface temp, volcanic activity

- Earth: 14.6 pounds per square inch

- Venus: 92xEarth’s

- didn’t lock any up in rocks, has gravity to hold onto it all

- Mars: 1/1000 x Earth’s

- too low gravity, losing atmo

- Mercury: none

- too low gravity, surface too hot!

Greenhouse effect

• Good: Earth is warmer by 30k (50 degrees F) than expected temp

• Bad: Venus is blistering 737K (867 degrees F), hotter than Mercury

• Surface temp is balance incoming and outgoing energy

- incoming sunlight adds energy, heats surface (also heat from still cooling)

- planet loses energy by emitting infrared (nothing to do with volcanoes!)

• Greenhouse gases (ggs) delay escape of infrared light, insulation blanket holds onto thermal energy longer

- Earth natural balance of carbon dioxide, methane, water in atmosphere

- released from volcanoes, decay of dead things

- trapped in oceans, rock, living plants, frozen tundra

• Change ggs, change balance

- Venus closer to Sun, oceans evaporated, active volcanoes = thick ggs

Seasons

• Change in sunlight from tilted rotation axis

- summer warmer, winter cooler regardless of atmosphere composition or surface feature

• Mercury: axis perpendicular = no seasons

• Venus: axis perpendicular = no seasons

- also, very little sunlight reaches surface

• Earth (23.5 degrees) and Mars (25 degrees) tilted = seasons

- Mars winter: 25% atmosphere condenses onto polar ice cap, air pressure plummets

- Mars spring/summer: dust storms, can grow to cover entire planet

• Mars orbit more eccentric, unequal length seasons compared to Earth

Impacts

• Some refresh surface, hide impacts

- Venus paved over by volcanoes

- Earth’s renewed by active geology

• Early solar system full of debris, cleared out when hit objects

• Fewer impacts now, but still happen

Liquid water

• Water delivered to every planet by comet, asteroid impacts

• Earth is liquid water king!

• Mercury too close to sun, too small

- ice hiding in deep polar craters, permanent shadow keeps it very, very cold

• Venus too hot, lost its water

- didn’t lock up carbon dioxide, runaway Green house effect

- if ocean, eventually water evaporated into atmosphere

- broken down by sunlight (into hydrogen, oxygen), leaked into space

- maybe viable for few billion years

• Mars too cold now, but water in the past

- when volcanoes still active, Mars had thicker warmer atmosphere

- planet cooled, volcanoes and magnetic field died

- solar wind stripped atmosphere, water evaporated, leaked into space

- planet died after 1 billion years, became cold dry dessert