The Terrestrial Planets

Astronomy 311Professor Lee

CarknerLecture 9

Where did the Earth come from?

a) It accreted from rocky planetesimalsb) It accreted from rocky and icy planetesimalsc) It rapidly condensed out of the solar nebulad) It was captured by the Sun’s gravitye) It was ejected from a collision of Jupiter and

Saturn

Why do we think the planet Jupiter formed quickly?

a) Samples from Jupiter indicate it is older than the other planets

b) It is mostly made of hydrogen gas which was only around in the early solar system

c) We see many Jupiter-type planets around very young stars

d) Jupiter’s orbit is right at the spot where the solar system was born

e) Jupiter has a very rapid rotation rate

Early Missions to the Inner Planets 1962 -- Mariner 2

Venus Fly-by

1964 -- Mariner 4 Mars Fly-by

1970 Venera 7 Venus lander first successful landing

on another planet

1973 Mariner 10 Venus/Mercury Fly-by

1975 Viking 1 and 2 Mars lander first successful landing

on Mars

Planetary Missions First wave of exploration from 1960-1979

Very large number of Soviet missions, most

were failures Venus: 15 successes, 31 missions

Smaller number of US missions, but higher success rate Mercury: 1 success, 1 mission Venus: 6 successes, 7 missions

We are now starting to see other countries get more involved with space exploration Most notably Japan and The European Union

Sources of Information for the Inner Planets

Mercury: Mariner 10 --

Venus: Soviet Venera landers -- surface conditions Magellan --

Mars: Viking, Pathfinder, Spirit, Opportunity --

Viking, Global Surveyor, Odyssey, Recon Orbiter -- maps of the surface

Inner Planet Facts

Mercury Diameter: 0.38 Mass: 0.06 Orbital Radius: 0.4

Venus Diameter: 0.95 Mass: 0.82 Orbital Radius: 0.7

Earth Diameter: 1 Mass: 1 Orbital Radius: 1

Mars Diameter: 0.53 Mass: 0.11 Orbital Radius: 1.5

Determining Planetary Properties

Mass

Distance Can find directly with radar

Diameter Can get from the angular diameter

and the distance

Determining Planetary Properties (cont.)

Average Density

Atmospheric composition take a spectrum of the atmosphere,

look for the spectral signature of elements

Scale Models

We want to make a scale model to try to understand astronomical distances

Need to find the scalescale = (real size) / (model size)

example: miles per inch or light years per cm

Once you have the scale you can find the model size for any real object

(model size) = (real size) / scale

The Planets That Weren’t There should have been 2 other inner

planets

A planet about the size of Mars may have hit the Earth a few billion years ago, the debris formed into the Moon

Jupiter’s gravity disrupted the planetesimals

between Mars and Jupiter so they never formed a planet

The Moon Most of our information comes from the

6 Apollo landings (11-17, excluding 13) Moon facts

Diameter: 0.27 Mass: 0.01 Orbital Radius (from Earth): 0.003

Moons of the Inner Planets Venus and Mercury have no moons Earth has one large moon

Mars has two moons, Phobos and Deimos

Inner planets may be too small to capture moons easily It is difficult to gravitationally capture something

Asteroids Millions of small bodies orbit the Sun, most

between Mars and Jupiter (the asteroid belt)

Our information comes from 2 sources:

Pieces of asteroids that have fallen to Earth

For example: NEAR orbiting Eros Hayabusa landing on Itokawa

Asteroid Facts Asteroids

Diameter: <0.14 Mass: <0.02 Orbital Radius: 2.8

Most have orbits within the asteroid belt (~2-3.5 AU)

Sizes of the Inner Planets Sizes relative to Earth

Earth: 1 (diameter = 13,000 km) Venus: 0.95 Mars: 0.53 Mercury: 0.38 Moon: 0.27 Asteroid: <0.01

All are small compared to the gas giants (Neptune is ~4 times the diameter of the Earth and ~64 times the volume)

Atmospheres

Mars Surface pressure = Composition = 95 % CO2, 3 % N (also water vapor,

oxygen) Venus:

Surface pressure = Composition = 96 % CO2, 4 % N (also sulfur

compounds such as sulfuric acid, H2SO4)

Atmospheres (cont.) Earth:

Surface pressure = Composition = 77 % N, 21 % O2 (also water

vapor, CO2, trace elements) Why are the atmospheres of Venus,

Mars and the Earth so different? The Earth can regulate its atmosphere

through the carbonate-silicate cycle, the other planets cannot

The Carbonate-Silicate Cycle

Water+

CO2

(rain)

Ocean

Carbonate + silicate(Sea floor rock)

CO2

Volcano

Atmosphere

Carbonate+ water(stream)

CO2 + silicate(subvectivemelting)

CO2 and Greenhouse Effect

Water washes CO2 out of atmosphere where it is eventually deposited as rock

CO2 is a greenhouse gas

More CO2 = higher temperature

Carbonate-Silicate Feedback

Hot

more CO2 washes out cools off

Cool

less CO2 washes out heats up

CO2 and the Inner Planets Venus:

all the water boiled off and was disassociated

thick CO2 atmosphere and high temperatures

Mars: no way to get CO2 out of rocks

thin CO2 atmosphere and low temperatures

Earth:

mild temperature and atmosphere

Composition

Density of rock (silicates) ~3000 kg/m3

What makes up the difference? Iron

“Rocky” planets could also be called the “metal” planets

Interior Structure

Composition (cont.)

Earthquake studies indicate that the Earth has a iron core Earth has a density gradient, heavier

materials near the center, lighter near the surface

We believe that the other inner planets have a similar structure

Next Time

Read Chapter 8 but just the Mercury parts

Summary Inner or Terrestrial region

4 planets (Mercury, Venus, Earth, Mars) 1 large moon (The Moon) thousands of asteroids

Information from 30 years of space missions Size

Earth and Venus about the same Mars, Mercury, the Moon, 1/2 -1/4 size of the

Earth Asteroids few km

Summary (cont.) Composition

silicate rock crust iron-silicate mantle iron core each planet has different proportions of each

Atmosphere Mercury, Moon, asteroids -- none Venus -- no water means CO2 is in atmosphere

Mars -- no plate tectonics means CO2 is in rocks Earth -- carbonate-silicate cycle balances greenhouse

effect