# AST121 Introduction to Astronomy

The Terrestrial Planets Chapter 6 Getting to know our first cousins 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

QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime anddecompressor a TIFF (Uncompressed) are needed to see this picture. What makes them similar? Size

small--1/100 radius of the Sun orbit at 0.4 to 1.5 n o i t a

c Lo AU s few n o o M none s g n i

R n o i t i s o p m co dense rock and metal

Density density = mass/volume 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 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 (Newtons 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 (Hbomb!) Creates circular impact craters on solid surfaces

Earth Composition Volcanism Plate tectonics Atmosphere

Craters Magnetic field Aurora caused by charged particles emitted from the Sun interacting with the Earths atmosphere charged particles are most highly concentrated near the poles due to their motion in the earths magnetic field.

Craters Barringer meteor crater Largest, most wellpreserved impact crater Fist crater recognized as an impact crater (~1920s) 49,000 years old Earths 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

Earths 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 earths formation => about 4.5 Gy for earths age Earths Tides due to the variation of the gravitational force

of the moon on the earth two tides per day Tides The 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 Earths mantle but proportion of elements is not exactly like

the Earths 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?) Whats 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 For Practice Looking through this chapter, make a list of similar features and different features of the terrestrial planets.

Identify each instant where the book described something we know about a planet and how we know it.

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