the terrestrial planets chapter 6 getting to know our first cousins
TRANSCRIPT
Topics
• Solar System--the big picture
• Earth, Moon, Mercury, Venus, Mars
• How do we know?
• Why do we care?
• What is common about the terrestrial planets?
• What is peculiar to each of these planets?
Models
• The test of all knowledge is experiment.• We use models to understand how we think the
Solar System, including the Sun and planets, formed.
• Models can be used to make predictions.• Ultimately the accuracy of the predictions reveal
the efficacy of our models.• As we discuss “what happened” remember that
these are based on models. Perhaps at some point, experiments will point us to new models.
Contents of the Solar System
• All masses that orbit the Sun plus the Sun!• One star - called the Sun• nine planets
– Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto
• more than 60 moons (often called natural satellites)
• tens of thousands of asteroids• countless comets• dust and gas• Our Sun constitutes nearly 99.44% of the mass of the
Solar System
Terrestrial planets (Earth-like):Mercury, Venus, Earth, Mars
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What makes them similar?
small--1/100 radius of the Sun
orbit at 0.4 to 1.5 AU
few
none
Size
Location
Moons
Ringsdense rock and metalcomposition
Density
Density of water = 1.0 g/cm3
Density of wood = 0.5 g/cm3
Density of silicate rock = 3.0 g/cm3
Density of iron = 7.8 g/cm3
density = mass/volume
Composition?
Density
Mercury 5.4 g/cm3
Venus 5.2 g/cm3
Earth 5.5 g/cm3
Mars 3.9 g/cm3
So what are these planets mostly made of?
Earth
• Mass and radius give mass/volume = bulk density, about 5.5 times water
• Key to composition, internal structure, verified by seismic waves
• Metals: bulk density about 8 g/cm3; rocks: about 3 g/cm3; earth: about 50-50 metals/rocks
How do we measure density?
• Mass & spherical shape (Newton’s law of gravitation)
• Radius (from angular size and distance)
• Bulk density (mass/volume) => infer general composition
Evolution of a planet -internal effects
• Energy flow from core to surface to space
• Source: Stored energy of formation, radioactive decay
• Results in volcanism, tectonics
Evolution of a planet -external effects
• Impact cratering: Solid objects from space
• Bomb-like explosion; many megatons (H-bomb!)
• Creates circular impact craters on solid surfaces
Aurora
• caused by charged particles emitted from the Sun interacting with the Earth’s atmosphere
• charged particles are most highly concentrated near the poles due to their motion in the earth’s magnetic field.
Craters
• Barringer meteor crater
• Largest, most well-preserved impact crater
• Fist crater recognized as an impact crater (~1920s)
• 49,000 years old
Earth’s layers
• Core (metals)
• Mantle (dense rocks)
• Crust (less dense rocks)
• Partially or fully melted material separates by density (differentiation)
• Age of earth ~ 4.6 Gy ~age of meteorite material and lunar material
Astronomy: The Evolving Universe, Michael Zeilik
Earth’s age
• Radioactive dating: Decay of isotopes with long half-lives; for example, uranium-lead, rubidium-strontium, potassium-argon.
• Gives elapsed time since rock last melted and solidified (remelting resets clock)
• Oldest rocks about 4 Gy + 0.5 Gy for earth’s formation => about 4.5 Gy for earth’s age
Earth’s Tides
• due to the variation of the gravitational force of the moon on the earth
• two tides per day
TidesThe Sun also has an effect on the tides.
Eventually the earth and moon will slow down and the moon will recede.
Moon
• Origin– fission?– capture?– condensation?– ejection of a gaseous ring?
• maria• craters• similar in density to
Earth’s mantle but proportion of elements is not exactly like the Earth’s
Mercury
• rotational period is 2/3 of its orbital period -- hot and cold
• hard to view from Earth• highly elongated orbit• iron core• small magnetic field• thin atmosphere, mostly
sodium• it looks like the Moon
Venus• ...where the skies are cloudy all
daayyyy.• dense atmosphere, mostly CO2
• high surface pressure and temperature
• rotation (117 E-days), revolution (225 E-days)
• rotates about its axis in the “wrong direction”
• similar density and size as Earth• two continents, one continental
plate• no moons
Mars
• small in size• two moons• thin atmosphere, mostly
CO2
• 4 seasons (why?)• smaller density (what
would this mean?)• polar caps (mostly CO2,
some water)• canyons (evidence of
flowing water?)
What’s important?
• similarities of terrestrial planets
• peculiarities of terrestrial planets
• how we know things like the period of rotation, composition, and age of a planet, to name a few