Chapter 7 Earth and the Terrestrial Worlds Understanding the similarities and differences between the planets of the solar system, in particular, the four terrestrial planets, can tell us how Earth becomes the way it is today. The similarities and differences of the terrestrial worlds. The small terrestrial worlds: Mercury and our Moon. The large terrestrial worlds: Mars, Venus, and Earth. What makes the environment on Earth suitable for life? Mars? What’s the future of Earth?

Similarities and Differences of the Terrestrial Worlds From a distance, they appear very similar… rocky and small (we really can’t see the surface of Venus directly) ! No or few moons No rings Examined close-up, They are very different… Mercury and Earth’s Moon are airless and barren Mars has a very thin atmosphere Earth has oxygen, water, and life! Venus has a thick atmosphere and very hot!

What makes the Earth hospitable to life? Internal Structure Surface Features Atmosphere What makes the Earth hospitable to life? Venus Mars Global Warming?

Internal Structure of the Terrestrial Planets The internal structure of the terrestrial planets are similar. They all have Core – High density metal Mantle – Medium density rocky materials, such as silica (SiO2), hot, semi-solid Crust – lowest density rocks, such as granite and basalt (black lava rock…) The layering of different density materials occurs due to differentiation – heavy materials sink to the bottom while lighter material rise to the top… Lithosphere: The coolest and most rigid layer of rock near a planet’s surface. Molten lava of Earth exists at a very narrow region beneath the lithosphere

Inside the Earth: More Details Lava comes from a thin layer under the lithosphere… Illustration from USGS http://pubs.usgs.gov/gip/dynamic/inside.html Illustration by J. C. Butler http://www.uh.edu/~jbutler/physical/chapter19.html

Heating of the Terrestrial Planets The interiors of the terrestrial planets are heated by Gravitational potential energy of the accreting planetesimals are converted into thermal energy. Radioactive heating Radioactive Heating Radioactive materials (e.g., uranium, potassium, thorium) decay by emitting subatomic particles (alpha particle—nuclei of helium, beta particle– electrons or positron, neutron, proton, etc.) and often gamma-ray, which collide with surrounding atoms, heating them up. Potassium-40 → Argon-40 Uranium-234 → ……. → Lead-206 Internal heating causes Mantle Convection---hot rock rises to the top and cools off, cool rock sinks to the bottom, resulting in the cooling of the planet…

Earth’s Magnetic Field Another important characteristics of the Earth is its magnetic fields, which shield us from the bombardment of the high-energy charged particles, mostly from the Sun. The rapid rotating liquid outer metal core of Earth generate magnetic field. The charged particles from the Sun must move along the magnetic field lines, and are directed to the north and south polar regions. The interactions between the charged particles and the molecules of the atmosphere cause the glow of atmosphere near the north and south poles  aurora borealis and aurora australis Without magnetic field, solar wind can strip much of the Earth’s atmosphere…

Building a Magnet We can generate magnetic field by circulating electric charges (running a electric current) in a spiral path. We do not have a complete theory of how the magnetic field of the Earth is formed yet…but generally, it is believed to form by the rotation of the Earth carries the electrically conducting molten metals in the core around, generating Earth’s magnetic field. Lines indicate points with equal magnetic field strength

Reaching Inside the Earth We can study the interior structure of the Earth by studying how seismic waves travel through Earth… Seismic waves propagate through Earth in two modes: P wave: Primary (Pressure, or Pushing) wave P wave can travel through any material. S wave: Secondary (Shear, or side-to-side) wave. S wave cannot travel through liquid.

Internal Structure Surface Features Atmosphere What makes the Earth hospitable to life? Global Warming?

Surface Features Processes shaping the surface of the planets Impact cratering: the blasting of bowl-shaped impact craters by asteroids or comets striking a planet’s surface. Volcanism: the eruption of molten rock, or lava, from a planet’s interior onto its surface. Tectonics: the disruption of a planet’s surface by internal stresses. Erosion: the wearing down or building up of geological features by wind, water, ice, and other phenomena of planetary weather.

Impact Crater in Arizona

Volcanism Volcanism occurs when underground molten rock finds a path through the lithosphere to the surface In addition to shaping the surface of the planet, it explains the existence of our atmosphere and ocean…water and gases trapped in the interior of Earth are released into the atmosphere through volcanic activities… The best results…the formation of our island paradise…Hawaii!

Plate Tectonic Tectonics is particularly important on Earth, because the underlying mantle convection fractured Earth’s lithosphere into more than a dozen pieces, or plates. These plates move over, under, and around each other, leading to a special brand of tectonics that we call plate tectonics.

Erosion Erosion is a blanket term for a variety of processes that break down or transport rock through the action of ice, liquid, or gas. The shaping of valleys by glaciers (ice), the carving of canyons by rivers (liquid), and the shifting of sand dunes by wind (gas) are all examples of erosion. Mauna Kea and Mauna Loa are very tall, but as we move toward the west, the volcanoes gets lower and lower…

Internal Structure Surface Features Atmosphere What makes the Earth hospitable to life? Global Warming?

The Atmosphere of the Terrestrial Worlds According to the Nebular Theory, the terrestrial planets were formed by metallic and rocky planetesimals. So, Where did the gas come from? The gases came from comets and asteroids impact during the period of heavy bombardment. The gases are trapped in the interior of the planets, later released through volcanic out-gassing. But, why are their atmosphere so different? How come Earth has so much H2O? How come Earth don’t have much CO2? How come Earth has so much O2?

Why Mercury and the Moon don’t have an Atmosphere? Mercury and the Moon don’t have an atmosphere because they are too small. Their weak gravitation field is not enough to keep the gas. Given the same temperature… Light gases escape easily, Heavier gases are trapped by gravity… Given the same temperature, the thermal velocity of lighter gases are higher compared with velocity with heavier gases. Therefore, the lighter gases have better chance of acquiring a speed greater than the escape velocity of the Earth and escape…(remember that the escape velocity of the Earth is about 14 km/sec, and independent of the mass of the escaping object).

Mercury and Earth’s Moon Similarities between Mercury and the Moon: Size No Atmosphere Dense impact craters on the surface  No geological activities to alter the surface features after the period of heavy bombardment – they are geologically ‘dead’ long time ago… Large day/night temperature difference Surface of Mercury looks very similar to the Moon The similarities between these two worlds can be explained by their small sizes: Small size  low surface gravity  low escape velocity  gas cannot be trapped by gravity on the surface. No atmosphere  large day/night temperature difference Small size  small initial heat content  they cool off fast  low level of geological activities

Venus, Mars and Earth Venus and Earth are very similar in size, and Mars is a little smaller. Their surface features are somewhat similar also – they both have few impact craters, and they all have volcanoes and evidence of tectonic activities. Although there is a very large difference in the amount of gases on Venus and Mars, their chemical compositions are very similar – high percentage of CO2 Among the three large terrestrial worlds, the surface environment of Earth is very unique. The surface of the Earth is characterized by Abundant surface liquid water Abundant atmospheric oxygen Plate tectonics Climate stability These are features that are very important to support life on Earth.

Surface Temperature of the Terrestrial Worlds The “No Greenhouse Temperature” of a planet depends on its distance from the Sun, and its albedo (or the reflectivity of a surface or a body)… However, the presence of an atmospheres can drastically change the surface temperature of a planet.

The “No Greenhouse Temperature” Without an atmosphere, the surface materials of the planets absorb some of the visible light from the Sun. The temperature of the surface material increases, depending on the amount of energy it absorbs (the albedo). The planet surfaces re-radiates the absorbed energy in the form of thermal radiation. The re-radiated energy is equal to the absorbed energy. The amount of energy the planet surface radiates depends on its temperature. The equilibrium temperature is around a few hundred degree Kelvin… Therefore, the surface of the planets emits in the infrared wavelength range… Objects with temperature of a few hundred degree Kelvin emit thermal radiation in the IR

How does the Atmosphere Affects the Environment on the Surface? X-rays are absorbed by the atoms and molecules of the atmosphere at high altitude… UV is absorbed by the ozone (O3) in the stratosphere… Visible light reaches the ground and warms the surface. IR radiation is absorbed by the water vapor… Radio waves are not affected by the atmosphere…

The Effects of the Atmosphere on Planet Surface Temperature Depending on the composition of the atmosphere, the effect can be very different Venus: 96% CO2, 3.5% N2  T = 740 K. Earth: 77% N2, 21% O2, 1% Argon (‘dry air’), Variable H2O (~ 10%...order-of-magnitude) Small amount of CO2 (~0.03%)  288 K. Why? Greenhouse Effect!