ASTR-1010ASTR-1010Planetary AstronomyPlanetary Astronomy

Day - 25

AnnouncementsAnnouncements

Smartworks Chapter 6: Due Today, March 22.

Smartworks Chapter 7: Due Friday, March 26.

1st Quarter Observing Nights:

Tues & Thurs, March 23 & 25 -- 7:30pm

Lab this week: Earthquakes

Lab next week: Rotation Rate of Mercury

Chapter 7Chapter 7Lecture OutlineLecture Outline

The Terrestrial Planets The Terrestrial Planets and Earth’s Moonand Earth’s Moon

ClassAction StuffClassAction Stuff

• SS Splash page questions

• Terrestrial Planets

• Iceland Volcano

Two broad categories of planets:Two broad categories of planets:Earthlike and JupiterlikeEarthlike and Jupiterlike

• All of the planets orbit the Sun in the same direction and in almost the same plane

• Most of the planets have nearly circular orbits

DensityDensity

V

mD

• The average density of any substance depends in part on its composition

• An object sinks in a fluid if its average density is greater than that of the fluid, but rises if its average density is less than that of the fluid

• The terrestrial (inner) planets are made of rocky materials and have dense iron cores, which gives these planets high average densities

• The Jovian (outer) planets are composed primarily of light elements such as hydrogen and helium, which gives these planets low average densities

The Terrestrial PlanetsThe Terrestrial Planets

• The four inner planets are called terrestrial planets– Relatively small (with diameters of 5000 to 13,000 km)– High average densities (4000 to 5500 kg/m3)– Composed primarily of rocky materials

Seven large satellites are almost as Seven large satellites are almost as big as the terrestrial planetsbig as the terrestrial planets

• Comparable in size to the planet Mercury• The remaining satellites of the solar system are much smaller

Hydrogen and helium are abundant on the JovianHydrogen and helium are abundant on the Jovianplanets, whereas the terrestrial planets areplanets, whereas the terrestrial planets are

composed mostly of heavy elementscomposed mostly of heavy elements

Similar but DifferentSimilar but Different

• Terrestrial planets:– Mercury– Venus– Earth– Mars– Earth’s Moon (or simply, the Moon)

• All are rocky/metallic, dense.

• Smallest two have little/no atmosphere.

Mass is KeyMass is Key

• The differences between the planets are largely driven by mass.

• Different processes depend on the mass of the planet.

Mass ratioto Earth

Moon 0.012

Mercury 0.055

Mars 0.11

Venus 0.82

Earth 1.00

Comparative PlanetologyComparative Planetology

• We can learn a lot by comparing the planets.

• The same processes operate on each planet:– Tectonism (moving crustal plates)– Volcanism (volcanoes)– Impacts (cratering)– Gradation (smoothing by weathering and erosion)

• These processes are stronger or weaker on the different planets.

Cratering on planets and satellites is the resultCratering on planets and satellites is the resultof impacts from interplanetary debrisof impacts from interplanetary debris

• When an asteroid, comet, or meteoroid collides with the surface of a terrestrial planet or satellite, the result is an impact crater

• Geologic activity renews the surface and erases craters, so a terrestrial world with extensive cratering has an old surface and little or no geologic activity

• Because geologic activity is powered by internal heat, and smaller worlds lose heat more rapidly, as a general rule smaller terrestrial worlds are more extensively cratered

ImpactsImpacts

• Craters on the Moon are relics of the last phase of planetary accretion.

• All terrestrial planets experienced this.

• Venus and Earth have few craters.

• Subsequent tectonism and gradation erases the craters.

• Some large impacts on the Earth have influenced the evolution of life.

Cratered Cratered Region Region

on the Moonon the Moon

NASA/JSC

Radioactive DatingRadioactive Dating

• Some elements can decay from one to another (e.g., uranium to thorium).

• These changes take place at known rates.

• Parent element declines, daughter element accumulates.

• Ratio of parent to daughter abundance gives the age of the rock.

• Age = time since rock was last molten.

On the MoonOn the Moon

• Rocks returned in the Apollo missions (1969-1972) give ages.

• Rocks from different places show rate of accretion in the early Solar System.

• Accretion rate fell sharply after a billion years.

• Older surfaces have more craters because they were formed when the cratering rate was higher.

Cratering RateCratering Rate

NASA/JPL/Caltech

Concept Quiz Concept Quiz The Moon Long The Moon Long AgoAgo

Imagine taking a picture of the Moon about 2 billion years ago. What would you expect to see?

A. It would have many fewer craters.B. It would have many more craters.C. It would have about as many craters as it does now.

Hadean Earth, Dawn of LifeLate Heavy Bombardment – ~3.9 Gyr ago

Relatively quiet between formation and LHB Since then, protected by Jupiter

Sterilizing Impacts 350-400 km in diameter (Fig 4.13)

Completely vaporize the oceansGlobal surface temperature rise 2000 C (3600 F)Last ~4.2-3.8 Gyr ago

How Old is the Earth?

Age of the Earth – oldest rocks 4.0 Gyr “Zircons” ~4.4 Gyr

Suggest the crust separated from interior ~4.5 Gyrs

Moon rocks ~4.4 GyrTherefore, Moon existed by this time.

Formation of the MoonFormation of the Moon

• Moon formed in large collision between Earth + Mars-sized protoplanet.

• The collision scattered material into Earth orbit; this collected by accretion to form the Moon.

• Composition of Moon is like that of Earth’s crust.

• Dark areas on Moon (maria) are ancient lava flows from later large impacts.

Impact Energiesand these are the

small ones

A Model of the EarthA Model of the Earth

• We model the Earth’s interior by studying earthquakes.

• Sound moves at different speeds through different materials.

• P (primary) waves travel through solids and liquids.

• S (secondary) waves go through solids only.• Earth’s layers are: crust, mantle, liquid outer

core, solid inner core

The Earth’s InteriorThe Earth’s Interior