1445 Introductory Astronomy I Chapter 6 Earth and Moon R. S. Rubins Fall, 2010
The Earth 2 2005 photograph by Rosetta taken from 600,000 km.
Earth and Moon 1992 photograph by Galileo taken from 600,000 km.
About the Earth 1 • Mass ≈ 6.0 x 1024 kg. • Radius ≈ 6400 km (≈ 4000 mi), diameter ≈ 8000 mi. • Average distance to Sun is 1 AU ≈ 150 million km (93 million mi) or 8.3 light-minutes.. • Mean surface temperature = 290 K (63oF). • Siderial periods: revolution ≈ 365 d, rotation ≈ 1 d. • The Earth is the largest terrestrial planet, but much smaller than the gas giants: Jupiter, Saturn, Uranus and Neptune. • The presence of the Moon, at an average distance of roughly 384,000 km (239,000 mi) or 1.3 light-seconds, stabilizes the Earth’s rotation, preventing tumbling and the periodic reversals of its north and south poles.
About the Earth 2 • Water covers about 71% of the Earth’s surface. • The Earth is geologically active with frequent earthquakes and volcanoes. • Rain and snow clear the atmosphere of dust particles. • The main components of the atmosphere at sea-level is about 78% nitrogen (N2) and 21% oxygen (O2), or roughly 4 parts of N2 to 1 part of O2. • There are also small quantities of argon, CO2 and ozone, plus a very variable quantity of water molecules of up to 3%. • The unusually large concentration of atmospheric O2, the large quantity of water, and the temperate climate, all help to make the Earth unique in the solar system for its hospitality to myriads of life-forms.
The Earth’s First Atmosphere • The Earth’s first atmosphere consisted mainly of hydrogen molecules (H2) and helium atoms (He) in the approximate ratios 4 parts of H2to 1 part of He by weight. • This approximate ratio was produced during the Big Bang, and billions of years later in the formation of the Solar System. • Eventually, these very light molecules escaped into space, and different molecules made up the new atmosphere.
Methods of Atmospheric Escape 1 • 1. Jeans escape This method, described in the early 20th century by James Jeans, is analogous to the firing of a spaceship, with an initial speed fast enough for it to escape the Earth’s gravity. For particles above about an altitude of about 500 km, the air is so dilute, that gas particles rarely collide. At that altitude, the average speed of a hydrogen atom is about 5 km/s, which is below the escape velocity of roughly 11 km/s. However, the H atoms have a distribution of speeds, with some of them going faster than the escape velocity. If one such atom has a velocity directed away from the Earth’s surface, it will leave the Earth.
Methods of Atmospheric Escape 2 • The remaining methods are less obvious, but often more important than Jeans escape. • 2. Hydrodynamic escape In this method, the upper atmosphere absorbs UV radiation from the Sun, warms and expands, pushing air upwards at ever faster speeds. The best evidence for this escape method has come from observations of the extrasolar planet HD 209458b. Jeans escape Hydrodynamic escape
Methods of Atmospheric Escape 3 • 3. Electron-stealing escape In this case, a positively charged ion, following a magnetic line, collides with a neutral atom, and steals an electron from it. The now neutral atom is not constrained to the field line, and breaks free.
Methods of Atmospheric Escape 4 • 4. Open-field line escape In this case, an ion follows a magnetic field line which does not return back to Earth. • Comet or Asteroid Collision • When acomet or asteroid strikes a planet, the explosion sends rocks, water and air into space.
The Earth’s Second Atmosphere • The light elements were replaced by carbon dioxide (CO2) and water (H2O), with some nitrogen (N2), all of which escaped from volcanoes and fissures. • Consisting mainly of CO2, this atmosphere (similar to the present atmosphere on Venus) was about 100 times denser than our current atmosphere, and it stored the Sun’s heat through the greenhouse effect.
The Earth’s Third Atmosphere • Eventually, the atmospheric water condensed to form oceans, which dissolved roughly half the carbon dioxide (CO2) . • Organic life-forms, beginning with cyanobacteria, is thought to have converted most of the rest to O2. • This accumulation of oxygen is thought to have begun about 2.4 billion years ago. • The small percentage of N2present in the 2nd atmosphere became the primary component of the 3rdatmosphere, where it was joined by O2 , ultimately giving us our present atmosphere. • The balance of O2 and CO2 in our atmosphere is maintained through photosynthesis in plants, which converts CO2 to O2.
The Earth’s Distant Future The ultimate loss of the Earth’s water should occur through the breakdown of water into its components, hydrogen and oxygen. The hydrogen would tend to move towards the upper atmosphere, leaving the oxygen behind. After about 3.8 billion years, the remaining water should be confined to the polar regions. In 3,8 billion years
Earth’s Atmosphere: Temperature Profile Ionosphere or
Troposphere • The troposphere extends from the ground to about 11 km (7 mi or 36,000 ft), the height at which commercial jets fly. (The height of Mt. Everest is 29,000 ft.) • 75 % of the mass of the atmosphere lies in this layer. • Because heat generated by the absorption of IR radiation is greater at lower elevations, the temperature of the troposphere drops steadily from an average 290 K (63oF) at ground level to 218 K (– 67oF) at 11 km. • Because of convection, which causes the hotter air near the ground to rise, and the cooler, denser air to fall, all the Earth’s storms occur in this layer. • Near ground level the temperature drops about 1oF for each increase of about 300 feet in altitude.
Stratosphere, Mesosphere and Ionosphere • The stratosphereextends from 11 km to 50 km. • This is the realm of the ozone layer, which absorbs solar UV radiation, thus preventing most of this dangerous radiation from reaching the ground. • Because ozone (O3) is an efficient absorber of UV radiation, more absorption occurs at higher altitudes, causing the temperature to increase with altitude to a maximum of about 285 K (50oF) at 50 km. • The mesosphereextends from 50 km to 80 km, with the temperature dropping to a minimum of about 200 K ( 100oF) at 80 km. • The temperature rises again in the ionosphere, where atoms are ionized and heated by the Sun’s UV light.
Noctilucent or Night-Shining Clouds 1 • Noctilucent or night-shiningclouds observed from the International Space Station occur just above the mesosphere, which is 50 miles (80 km) above the Earth’s surface. • For comparison, common high altitude cirrus clouds occur at a height of 11 miles (18 km), which is low in the stratosphere. • The water vapor which produces clouds at such high altitudes apparently comes from both the rising warm air in the tropics, and the breakdown of methane in the atmosphere. • This phenomenon was first reported in Nature magazine, after the eruption of Krakatoain 1883. • They are observed above the north pole from May to August, and the south pole from November to February.
Noctilucent or Night-Shining Clouds 2 Top of mesosphere
The Earth’s Interior 1 • In the originally molten Earth, the denser elements migrated towards the center, while lighter elements rose towards the surface. • The final result was a layered structure, consisting of a very dense central core of iron (mainly) and nickel, surrounded by a less dense mantle, and a thin outermost crust . • The Earth’s temperature now varies from about 6000 K at its center to roughly 300 K at the surface. • The inner core, out to nearly 800 mi, is solid because of the very high pressure, while the outer core, from 800 mi to about 2000 mi, is liquid. • The mantle, extending from 2000 mi almost out to surface (about 4000 mi from the center) consists largely of minerals rich in iron, calcium and magnesium.
The Earth’s Interior 2 • The mantle is made of hot plastic-like rock, which tends to flow very slowly towards the surface. • As it moves towards the crust, the pressure is reduced, and it becomes molten rock, or magma, in the crust, occasionally erupting as lavain volcanic activity. • The motion of the mantle continually reshapes the Earth’s surface through plate tectonics. • Periodically, the mantle breaks the crust in a process known as rifting. • The rift valley in the Atlantic Ocean has caused the separation of the Americas from Europe and Africa. • The crust (only about 60 mi thick) contains mainly compounds of the lighter elements, particularly the compound silica SiO2, which occurs as sand.
Seismic Waves 1 • Much of the knowledge of the Earth’s interior is obtained from the study of seismic waves, produced by earthquakes. • The two main types of seismic wave are P (longitudinal) waves and S (or transverse) waves, where i. P waves can travel through all materials, ii. S waves can travel only through solids. • Liquid material is known to exist in the Earth, since only P waves are observed from earthquakes occurring on the side of the Earth opposite to where the measurements were made.
Ancient Rocks Near the Earth’s Surface The Jack Hills zircons in Australia are the oldest rocks known.
The Earth’s Magnetic Field • The Earth’s magnetic field is produced by the motion of charged particles, particularly iron, in the Earth’s molten outer core, caused by the Earth’s rotation. • It behaves as if a bar magnet were placed inside the Earth, with itssouthpole pointing towards the magnetic north. • The magnetosphereextends beyond the Earth’s atmosphere, behaving as a protective bubble around the Earth, which diverts most of the ionized particles coming from the Sun – the solar wind – towards the Van Allen Belts. • Ionized particles, leaking into the upper atmosphere near the poles, cause fluorescencein these gases, which we observe as the aurora borealis (northern lights) and aurora australis(southern lights), which occur typically between 100 km and 400 km above the Earth’s surface.
Moon Data • The mass of the Moon is roughly 1/100 that of Earth, while its diameter is a little less than 1/3, giving the Moon an average density of only about 1/2 that of Earth, but similar to that of the Earth’s crust. • Surface gravity on the Moon is 1/6th that at the Earth’s surface, so that a person weighing 180 lb on Earth would weigh 30 lb on the Moon. • The Moon orbits the Earth with a siderial period of just over 27 days. • The Moon’s distance from the Earth varies from about 360,000 km to 400,000 km each month, which causes its angular diameter to vary by about 10%.
Moon’s Surface • Moon-rocks as old as 4.4 billion years have been found by radioactive dating. • The Moon’s surface (especially its farside) is covered with craters, most of which formed from 4.2 to 3.9 billion years ago. • Mountain peaks in the centers of large craters were caused by the reaction of the ground to the depressions produced on impact. • Prominent features of the nearside of the Moon are large dark gray plains, known as maria(singular mare). • Maria were formed from 3.8 to 3.1 billion years ago, when molten lava filled the floors of large craters. • While the Moon’s south pole was thought to contain significant ice deposits in the Shackleton crater, a 2008 Japanese lunar explorer satellite, could not find any exposed deposits.
Mountains inside Large Craters These peaks are formed by the rebound of the ground following a very large meteoric impact.
Lunar Mare Rilles are lunar canyons, probably carved by lava flows.
Lunar Footprint Powdered rock (theregolith), built up over billions of years, covers the surface of the Moon. Although not moist, it sticks together underfoot like wet sand.
Water on the Moon • When NASA’s LCROSS probe slammed into the south polar region in 2009, an appreciable quantity of water ice was ejected. • In 2010, a NASA radar instrument on an Indian moon-probe found millions of tons of water ice on the bottoms of craters at the lunar north pole. These pebble-like beads observed by the Apollo missions of the 1960s and 1970s were found in 2008 to contain about 46 parts per million of water, indicating that water was a part of the Moon’s early existence.
Peaks of Eternal Light: Artist’s Impression • Chosen as a Scientific American “Wonder of the Solar System”, the Peaks of Eternal Life near the Moon’s north pole, is the only known region in the solar system where the Sun never sets.
Extreme Temperatures on the Moon • Night-time surface temperatures on the Moon inside the coldest craters in the northern polar region have been measured to dip as low as 26 K (– 247 oC) near the winter solstice. • During the day, temperatures at the equator can reach 400 K (127 oC), which indicates that the Moon has one of the most extreme thermal environments of any object in the solar system. • Calculations indicate that one would have to travel well beyond the orbit of Neptune to find a surface so cold.