The Terrestrial Planets Astronomy 311 Professor Lee Carkner Lecture 9

Where did the Earth come from? a)It accreted from rocky planetesimals b)It accreted from rocky and icy planetesimals c)It rapidly condensed out of the solar nebula d)It was captured by the Sun’s gravity e)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  Mariner 2 Venus Fly-by   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   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  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 scale scale = (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): 

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 % CO 2, 3 % N (also water vapor, oxygen)  Venus:  Surface pressure =  Composition = 96 % CO 2, 4 % N (also sulfur compounds such as sulfuric acid, H 2 SO 4 )

Atmospheres (cont.)  Earth:  Surface pressure =  Composition = 77 % N, 21 % O 2 (also water vapor, CO 2, 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 + CO 2 (rain) Ocean Carbonate + silicate (Sea floor rock) CO 2 Volcano Atmosphere Carbonate + water (stream) CO 2 + silicate (subvective melting)

CO 2 and Greenhouse Effect  Water washes CO 2 out of atmosphere where it is eventually deposited as rock   CO 2 is a greenhouse gas   More CO 2 = higher temperature

Carbonate-Silicate Feedback  Hot   more CO 2 washes out   cools off  Cool   less CO 2 washes out   heats up

CO 2 and the Inner Planets  Venus:   all the water boiled off and was disassociated   thick CO 2 atmosphere and high temperatures  Mars:   no way to get CO 2 out of rocks  thin CO 2 atmosphere and low temperatures  Earth:   mild temperature and atmosphere

Composition   Density of rock (silicates) ~3000 kg/m 3  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 CO 2 is in atmosphere  Mars -- no plate tectonics means CO 2 is in rocks  Earth -- carbonate-silicate cycle balances greenhouse effect