The Terrestrial Planets

The Terrestrial Planets

A Study in Contrasts • Mercury in many ways is similar to Earth’s moon. • Venus and Mars both have properties more like Earth’s, as well as drastic differences. • Earth - vibrant, teeming with life. • Venus - uninhabitable inferno. • Mars - dry, dead world.

Which object is pictured below? • Mercury • Venus • Earth • Mars • Earth’s Moon

Physical Properties • Surface gravity - strength of the gravitational force at the body’s surface. • Escape speed - speed required for any object to escape forever from the body’s gravitational pull. • The moon’s gravitational pull is much weaker than Earth’s (Moon’s gravity is 1/6 that of Earth). • Using radar and lasers, the distance of the moon from the earth is known to within about 2 cm. The semi-major axis of the moon’s orbit about the earth is roughly 384,000 km.

Overall Structure • Earth has 6 main regions: • Core - central region of Earth, surrounded by the mantle. • Mantle - layer of Earth just interior to the crust. • Crust - layer of Earth which contains the solid continents and seafloor. • Hydrosphere - layer of Earth that contains the liquid oceans and accounts for roughly 70% of Earth’s total surface area. • Atmosphere - layer of gas confined close to planet’s surface by force of gravity. • Magnetosphere - zone of charged particles trapped by planet’s magnetic field, lying above the atmosphere.

Overall Structure • Moon: • Lacks a hydrosphere, an atmosphere, and a magnetosphere. • Not as well studied as Earth’s since less accessible. • Crust, mantle, and core are present but differ from those of the earth.

Tides • Definition - the daily fluctuation in ocean level. • The magnitude of the fluctuation (height of the tides) can vary from a few centimeters to many meters depending on location and time of year. Average 1 m in the open sea. • An enormous amount of energy is contained in the daily tidal motions of the oceans. • Tidal bulge - elongation of the earth caused by the difference between the gravitational force on the side nearest the moon and the force on the side farthest from the moon. The long axis of bulge points toward the moon. • The tidal bulge effect is greatest in the oceans since liquid can move around easier. • Tidal force - the variation in one body’s gravitational force from place to place across another body (doesn’t necessarily have anything to do with ocean tides!).

Is the moon the only object that causes the tides on Earth? • Yes, tides are caused only by the moon. • Tides are also caused by the sun. • Tides are caused by all objects that exert a force on the earth.

Tides • We also have a tidal bulge that points in the direction of the sun. • The interplay between the tidal force due to the sun and the tidal force due to themoon accounts for the varying tide heights over the course of a month or year (depends on alignment of sun, earth, and moon).

Tidal Locking • The moon rotates once on its axis in 27.3 days - the exact time it takes to complete one revolution around the earth. This is what’s called a synchronous orbit. • So, the same side of the moon always faces the earth. • This is due to the tidal interaction of the moon and Earth. • Tidal locking is a common occurrence with moons in our solar system.

Earth’s Atmosphere • Composition (% by volume) • Nitrogen - 78% • Oxygen - 21% • Argon - 0.9% • Carbon dioxide - 0.03% • Water vapor content varies, ranging from 0.1 to 3%.

Mercury’s Atmosphere • Has no appreciable atmosphere. • Can trap gas from the solar wind (hydrogen and helium) for a matter of weeks at a time. • High surface temperatures (up to 700 K at noon on the equator) are reason for lack of atmosphere. • Surface temperature falls to 100 K at night. • Largest temperature range of any planet or moon in the solar system. • Polar regions remain cold (as low as 125 K) and may contain thin sheets of permanently frozen water ice.

Venus’s Atmosphere • Spacecraft measurements indicate a surface temperature near 730 K. • Venus’s atmosphere is much more massive than Earth’s and extends further out from the planet’s surface. • The surface pressure is 90 times that of Earth - equivalent to being 1 km under water on Earth (humans can only dive to 100 m safely). • 96.5% atmosphere is carbon dioxide. • Remaining 3.5% mostly nitrogen. • Venus is similar to Earth in mass, radius, and location in the solar system. So we assume it must have started out much like Earth. • Venus however has no water - so if it had some at its beginning, something has happened to it. • Clouds are composed not of water vapor, but sulfuric acid droplets. • Upper level winds reach speeds of 400 km/h relative to the planet. • Below the clouds is a layer of haze extending down to an altitude of 30 km. Below 30 km, the air is clear.

Mars’ Atmosphere • Atmosphere is quite thin. • Atmosphere composed primarily of carbon dioxide (95.3%), 2.7% nitrogen, 1.6% argon, plus small amounts of oxygen, carbon monoxide, and water vapor. • Surface pressure is 1/150 that on Earth. • Average surface temperatures are about 70 K cooler than on Earth.

Earth’s Atmosphere • Layers • Troposphere - portion of the atmosphere from the surface to about 15 km. • Stratosphere - lying above the troposphere, extending up to an altitude of 40 - 50 km. • Mesosphere - lying between the stratosphere and the ionosphere, 50 - 80 km above Earth’s surface. • Ionosphere - above 100 km where the atmosphere is significantly ionized and conducts electricity. • Convection occurs in the troposphere. It is the constant upwelling of warm air and the concurrent downward flow of cooler air. • The ozone layer is contained in the stratosphere. Ozone is a form of oxygen. In this layer, incoming solar ultraviolet radiation is absorbed by atmospheric oxygen, ozone, and nitrogen. It acts as a “planetary umbrella,” protecting us from damaging radiation.

The Development of Earth’s Atmosphere • Primary atmosphere - atmosphere it had when it formed. • Consisted of light gases such as hydrogen and helium, methane, ammonia, and water vapor. Nothing like what we have today. • This atmosphere would have escaped - Earth’s gravity is not strong enough to hold onto the light gases. • Secondary atmosphere - released from planet’s interior as a result of volcanic activity (outgassing). • Rich in water vapor, carbon dioxide, sulfur dioxide, and compounds containing nitrogen. • Water vapor condensed forming the oceans as the planet cooled. • Much of the carbon dioxide and sulfur dioxide dissolved in the oceans or combined with surface rocks. • UV radiation liberated nitrogen from its chemical bonds with other elements, forming a nitrogen-rich atmosphere. • Oxygen is so reactive that any free oxygen which appeared would have been removed as quickly as it formed. • Life appears in the oceans - living organisms produce atmospheric oxygen and eventually the ozone layer formed. • The fact that oxygen is a major constituent of our present atmosphere is a direct consequence of the evolution of life on Earth.

The greenhouse effect is … • Bad • Good • Neutral • Depends on the situation.

The Greenhouse Effect • Definition - The partial trapping of solar radiation by the atmosphere, similar to the trapping of heat in a greenhouse. • Sunlight that is not reflected by the clouds reaches Earth’s surface, warming it up. Infrared radiation reradiated from the surface is partially absorbed by carbon dioxide (and also water vapor) in the atmosphere, causing the overall surface temperature to rise. • More and more evidence is pointing to humans increasing this effect (increasing CO2 levels), which could have catastrophic consequences (rising ocean levels, etc.).

The Runaway Greenhouse Effect on Venus • Venus is hot because of the greenhouse effect - 730 K surface temperature. • Greenhouse gases, mainly water vapor and carbon dioxide, warm our planet. • Venus’s atmosphere is made up almost entirely of a greenhouse gas - carbon dioxide. This thick blanket absorbs 99% of all radiation released by the surface. • Why is Venus’s atmosphere so different from Earth’s? • On Earth, much of the greenhouse gases left the atmosphere (carbon dioxide dissolved in oceans or combined with surface rocks). This did not happen on Venus. If it hadn’t happened on Earth, we’d look a lot more like Venus. • Venus is closer to the sun, so it has a higher temperature initially. This could have been too high for water vapor to condense into oceans. Full greenhouse effect would have gone into operation immediately following outgassing. • If oceans did form, then the temperature must have been high enough to allow a process called the runaway greenhouse effect to come into play.

The Runaway Greenhouse Effect on Venus • Runaway greenhouse effect: • Imagine placing Earth at Venus’s orbit. • The amount of sunlight hitting Earth’s surface would nearly double, causing the planet to warm. • More water would evaporate to the atmosphere. • Ability of oceans and surface rocks to hold carbon dioxide would diminish, and more carbon dioxide would enter the atmosphere. • Greenhouse heating would thus increase. • Planet warms even more, resulting in the release of more greenhouse gases, and so on. This process would “run away” or snowball. • Eventually, all the greenhouse gases would return to the atmosphere.

The Runaway Greenhouse Effect on Venus • Greenhouse effect was even more extreme in Venus’s past when the atmosphere contained water vapor. • Water vapor helped temperature reach probably twice as high as present. • Water vapor rose high in the atmosphere due to extremely high temperatures, where UV radiation broke it apart into hydrogen and oxygen. • Hydrogen escaped the atmosphere and oxygen quickly combined with other atmospheric gases. • All water on Venus was lost forever. • If Earth was located between 0.7 and 1 AU, it would have experienced the same fate as Venus. • It is highly unlikely that global warming due to human activities will ever send Earth down the path taken by Venus (that doesn’t mean it will have no effect, however).

Evolution of the Martian Atmosphere • Presumably, Mars had first a primary and then a secondary (outgassed) atmosphere early in its history. • 4 billion years ago, atmosphere rich in carbon dioxide, perhaps even blue skies and rain. Sun was less luminous at this time, so conditions could have been fairly comfortable - above freezing temperatures possible due to thick atmosphere. • During next billion years, most of the Martian atmosphere disappeared. • Leaked away due to planet’s low gravity. • Expelled by impacts with large bodies in the early solar system. • Mars cooled faster than Earth and never developed large-scale plate tectonics. So Mars had less volcanism. So more carbon dioxide was depleted than replenished (not as much outgassing). The level of carbon dioxide steadily declined. • Planet cooled as greenhouse gases diminished. • Could have depleted atmosphere of carbon dioxide in a few hundred million years. • Water froze out of the atmosphere as the temperature continued to fall. • Eventually, even the remaining carbon dioxide began to freeze out, particularly at the poles. • Mars is now a cold, dry planet with most of its original atmospheric gases now residing in or under the barren surface.

Evolution of the Martian Atmosphere • Mars continues to cool, slowly losing what remains of its thin atmosphere, and no natural event can reverse the process. • Scientists are considering “terraforming” Mars - transforming its present inhospitable climate into an Earth-like environment. • Would have to introduce greenhouse gases to reverse the chain of events just described. • Melt polar caps using giant orbiting mirrors. • Deflect ammonia-rich asteroids to collide with Mars. • Build greenhouse emitting factories on Mars. • Process (which is not feasible with today’s technology) would take several centuries for natural processes to take over. • Would then take up to 2 thousand years to reach an Earth-like climate.

Lunar Air? • The moon has no atmosphere - it escaped a long time ago! • The moon’s escape speed is only 2.4 km/s (compared to the earth’s 11.2 km/s), so it couldn’t hang on to any atmosphere it may have once had. • Due to the lack of atmosphere (which helps moderate temperatures), the moon experiences a wide range of surface temperatures, from 100 K at night to over 400 K during the day (from well below water’s freezing point to well above waters boiling point). • While most of the moon’s surface is bone dry, a good amount of water ice might be located at the lunar poles, where the sun never gets very high above horizon, making temperatures there much cooler (never exceeds about 100 K).

Seismology • Earthquakes - sudden dislocation of rocky material near Earth’s surface. • Seismic waves - systematic waves that move outward from the site of an earthquake, which can be measured using a seismograph. • P-waves - pressure waves, which expand and compress the core or mantle as they move at speeds of 5 to 6 km/s. They travel through both liquids and solids. • S-waves - shear waves, cause side to side motion as they move at speeds of 3 to 4 km/s. They cannot travel through liquid.

Seismology • Using the different arrival times of the waves, geologists can infer the density of matter in the interior. • This is how we know what the interior of the earth is like.

Modeling Earth’s Interior • The outer core is molten - as evidenced by the lack of s-waves which pass through from one side of the planet to the other after an earthquake. The radius of this outer core is about 1300 km. • The mantle is about 3000 km thick and accounts for the bulk of Earth’s volume. • Average thickness of crust is only 15 km. • Density and temperature both increase with depth. • High central density implies the presence of large amounts of nickel and iron. • Inner core is solid and metallic, outer core is molten and metallic. • Mantle is mostly rocky (compounds of silicon and oxygen). • We have not been able to drill more than 10 km, so we don’t have a direct mantle sample. However, lava brings up mantle material in volcanoes. Therefore, mantle material probably resembles the basalt found near volcanoes.

Differentiation • Earth is not a homogeneous ball of rock - it has a layered structure. • Low-density rocky crust at the surface. • Intermediate-density rocky material in the mantle. • High-density metallic core. • Variation in density and composition is known as differentiation. • Why not uniform composition? • In distant past, much of Earth was molten. • Higher-density material settled to the core, with lighter-density material rising to the top. • Heating sources: • Bombardment of interplanetary debris when Earth was young, which would have generated enough heat to melt the planet. • Radioactivity (release of energy by certain unstable elements such as uranium) built up heat in the interior as it takes a long time to travel to the surface through the rock. Enough of the early planet was radioactive to at least keep the planet at most semi-solid for a billion years.

The Lunar Interior • The low average density of the moon suggests it contains fewer heavy elements (like iron) than does Earth. • Seismic instruments were left on the moon by astronauts and indicate only weak “moonquakes” deep within the lunar interior. These contain about as much energy as a firecracker and have little effect. • The moon is “geologically dead.” • From the weak moonquakes, it has been inferred that: • Moon is almost uniform in density, but is chemically differentiated (chemical properties change from core to surface). • Central core - 300 km in radius. • 400 km thick inner mantle of semisolid rock similar to Earth’s upper mantle. • 900 km thick outer mantle of solid rock. • 60 to 150 km thick crust. • Core is probably more iron rich than the rest (nothing compared to Earth’s though). • Central temperature is too cool to melt rock, although there is evidence that the inner core may be partially molten. • The crust is thicker on the far side of the moon - the earth’s gravity pulled more mantle to the near side of the moon, leaving the far side with more crust as the moon cooled and solidified.

Surface Activity on Earth • Earthquakes • Volcanoes • Wind erosion • Water erosion

Continental Drift • Plates (slabs of Earth’s surface) are constantly in motion - called continental drift. • The study of plate motion is called plate tectonics. • Plates and continents are not the same thing! • Plate motions have created mountains, oceanic trenches, and other large scale features. • When crustal rock shifts, we have earthquakes. • When mantle material upwells, we have volcanoes.

Continental Drift • Plates move at an extremely slow rate (few cm per year). Over a period of 200 million years (5% of Earth’s age) two continents could separate by 4000 km (width of the Atlantic Ocean). • When plates collide, mountains form. • Plates can also slide along one another (rather than colliding head-on). This motion causes fault lines and earthquakes (motion is jerky rather than smooth). • Plates can also move apart. When this happens, new mantle material wells up between them, forming midocean ridges. This is why the Atlantic seafloor is slowly growing in size since the North and South American plates are moving away from the Eurasian and African plates.

What Drives the Plates? • Convection! • Each plate is made up of crust plus small portion of upper mantle. • Below the plates, at a depth of maybe 50 km, the temperature is high enough that the mantle is soft enough to flow, very slowly, although it is not molten. • We have warm matter underlying cool matter, perfect conditions for convection. • The circulation is extremely sluggish - semisolid rock takes millions of years to complete one convection cycle. • Early on, geologists believe we had one massive supercontinent - Pangaea. • This process has likely happened several times in the past, and will probably repeat in the distant future as the landmasses continue to move.

Lecture Tutorial: Earth’s Changing Surface (p. 99) • Work with a partner! • Read the instructions and questions carefully. • Discuss the concepts and your answers with one another. Take time to understand it now!!!! • Come to a consensus answer you both agree on. • If you get stuck or are not sure of your answer, ask me or another group.

Plate Tectonics on the Moon • No evidence of plate tectonics on the moon today. • No obvious fault lines. • No significant seismic activity. • No ongoing mountain building. • Plate tectonics requires a relatively thin outer rocky layer and a soft convective region under it to make the pieces move. The moon has neither. • Thick crust and solid upper mantle of moon make it impossible for pieces of the surface to move relative to one another.

Magnetospheres • Definition - the region around a planet that is influenced by that planet’s magnetic field. • Forms a buffer zone between the planet and the high-energy particles of the solar wind.

Earth’s Magnetosphere • Earth’s magnetic field extends far above the atmosphere and completely surrounds our planet. • The magnetic field lines, which indicate the strength and direction of the field at any point in space, run from north to south. • The north and south magnetic poles, where the axis of an imaginary bar magnet within our planet intersects Earth’s surface, are very roughly aligned with Earth’s spin axis.

Earth’s Magnetosphere • Van Allen Belts • Most pronounced near Earth’s equator and surround the planet. • Two doughnut shaped zones in Earth’s inner magnetosphere of high-energy charged particles. • Located at 3000 km and 20,000 km above the surface. • Particles that make up the belts originate in the solar wind. Charged particles will be attracted by Earth’s magnetic field, which can then trap them.

Earth’s Magnetosphere • Aurora • Particles from the Van Allen Belts escape from the magnetosphere near Earth’s north and south magnetic poles. • At these locations, the magnetic field lines cross the atmosphere. • As the charged particles collide with air molecules, we get a light show called an aurora (called the Northern Lights in the northern hemisphere). • Collisions excite atmospheric atoms, which then emit light as they go back to their ground states.

Earth’s Magnetosphere • The stream of solar wind particles can affect the magnetosphere. • Toward the sun, the magnetosphere is squeezed inward toward Earth’s surface. • The opposite side has a long tail extending hundreds of thousands of km into space. • The magnetosphere shields us from the potentially destructive charged particles of the solar wind. Without the magnetosphere, life may not have been able to exist here. • Earth’s magnetic field is not an intrinsic part of our planet. • It is continually generated in Earth’s core and exists only because the planet is rotating. • Earth’s magnetism is produced by the spinning, electrically conducting, liquid metal core. • Both rapid rotation and a conducting liquid core are needed to produce this magnetism.

Lunar Magnetism • No Earth-based observation or spacecraft measurement has ever detected any lunar magnetic field. • As just discussed, planetary researchers believe planetary magnetism requires a rapidly rotating liquid metal core. • The moon rotates slowly and the core is likely neither molten nor rich in metals, so we would not expect to see a lunar magnetic field.

Formation of the Earth - Moon System • 4.6 billion years ago, the Earth formed by accretion in the solar nebula. • The earth and moon are too dissimilar in both density and composition to have formed from the same pre-planetary matter (coformation theory). • The mantles of the two however are quite similar, so they did not form totally independently of one another either (capture theory). • Favored theory of the day - impact theory.

Formation of the Earth - Moon System • Impact theory: • Glancing collision between Mars-sized object and a young molten Earth. • Most of the bits of Earth that would have been blasted into space could have recombined into a stable orbit (as evidenced by computer simulations). • This material would have then coalesced into our moon.

Evolution of the Earth - Moon System • Earth and Moon were at least partially solidified 4.4 billion years ago (have found rocks of that age). • 1st billion years - Earth at least partially molten. • 3.9 billion years ago - heavy bombardment ceased. • Earth cooled from the outside in (can transfer heat to space more readily closer to the surface). • Moon (being smaller in size) lost its heat rapidly to space. • 1st half-billion years - heavy bombardment kept much of the moon molten (not the deep interior as rock doesn’t conduct heat very well). • When heavy bombardment ceased, moon was left with a solid crust with numerous large basins. • Crust became the highlands. • Basins flooded with lava and became the maria. • Between 3.9 and 3.2 billion years ago, lunar volcanism filled the basins with the basaltic material we see today. • Lunar volcanism ceased 3.2 billion years ago. • Not all basins were filled by lava - some still are simply craters. • Due to Earth’s gravitational pull, the lunar crust is thicker on the far side as compared to the near side. As a result, little volcanic activity occurred on the far side (more crust for lava to travel through). • The lunar crust is now too thick for volcanism or plate tectonics to occur. • The lunar surface has not changed much in the past 3 billion years. Only changes that have occurred were due to erosion from meteoritic bombardment.

Volcanism on Mars • Contains largest known volcanoes in the solar system. • Largest is Olympus Mons - 700 km in diameter at its base - slightly smaller than the state of Texas. Rises to a height of 25 km above surrounding plains. • Do not know if any are still active, but from cratering rates, some erupted as recently as 100 million years ago. • Great height of volcanoes is a result of planet’s low surface gravity. A mountain can only be as tall as it can support (lower gravity means the lava weighs less). Martian surface gravity is 40% of Earth’s, letting its volcanoes rise 2.5 times higher.

The Martian Grand Canyon • Valles Marineris • Running water played no part in its formation (not like an Earth canyon!). • Formed by same crustal forces that pushed the Tharsis region upward, causing the surface to split and crack. • Cratering studies suggest it’s at least 2 billion years old. • Runs for almost 4000 km, extends 1/5 of the way around the planet. • Widest point is 120 km across, gets as deep as 7 km. Earth’s Grand Canyon could fit into one of its side “tributary” cracks. • Large enough to be seen from Earth. • The crustal forces however did not develop into full-fledged plate motion as on Earth (not plate tectonics!).

The Terrestrial Planets