Chapter 8 The Earliest Earth: 2,100,000,000 years of the Archean Eon
The Archean Eon The Archean Eon is the oldest unit on the geologic time scale. It began 4.6 billion years ago and ended 2.5 billion years ago. The Archean lasted for 2.1 billion years (2,100,000,000 years).
Earth's Oldest Rocks • Earth's oldest rocks are found in Canada. They are about 4.04 billion years old. • But there are even older mineral grains. Sand-sized zircon grains in metamorphosed sedimentary rocks from Australia are 4.4 billion years old.
Earth's Oldest Rocks There are no rocks on Earth that date back to 4.6 billion years ago because the Earth is geologically active, and the oldest rocks have been recycled by plate tectonics or by weathering and erosion as part of the rock cycle. Much of our knowledge of the Earth's earliest history comes from indirect evidence - meteorites.
The Precambrian The Archean and Proterozoic Eons comprise the Precambrian, which spans 87% of the geologic time scale.
Earth in Space The origin, history, and characteristics of Planet Earth need to be considered in the context of the Universe.
Galaxies • The Universe hosts billions of galaxies. A galaxy is an aggregate of stars, planets, dust, and gases. • Planet Earth orbits the Sun, a dwarf star, that belongs to the Milky Way galaxy. Whirlpool galaxy. Image courtesy of NASA and The Hubble Heritage Team (STScI/AURA)
The Solar System The Sun and the planets, moons, asteroids, comets and other objects that orbit it, comprise the Solar System.
Origin of the Universe • The Earth is part of the Solar System; the Solar System is part of the Milky Way galaxy; and the Milky Way galaxy is part of the Universe. • The story of the origin and history of the Earth requires that the origin and history of the Universe and Solar System must be considered.
Origin of the Universe Evidence to be considered when interpreting the history of the Universe: • Galaxies are rapidly moving apart (Hubble's Law). Suggests that galaxies were closer together in the past. Discovered by Edwin P. Hubble in 1929. • Observed temperature of the Universe today (background microwave radiation) 3 degrees above absolute zero. • Present abundances of hydrogen and helium.
Origin of the Universe Interpretation: • The Universe is expanding. • Everything began together at a point. • A big explosion occurred, which astronomers call the Big Bang. • This explosion caused everything in the Universe to begin moving rapidly apart.
How do we know the galaxies are moving apart? • Red shift.In 1914, W.M. Slipher first noted that galaxies displayed the red shift.Their light is shifted toward the red (or long wavelength) end of the spectrum. • Colors of the spectrum ROYGBIV
What the Spectrum Reveals The spectrum of a star reveals: • The star's composition by means of absorption lines. Various elements in the star's atmosphere absorb parts of the light of the spectrum. • Whether it is moving toward or away from the Earth (and at what speed).
How do we know the galaxies are moving apart? • Light reaching us from distant receding galaxies has its absorption lines shifted toward the red end of the spectrum. This indicates that the galaxy is moving away from the Earth. • The red shift indicates that the universe is expanding.
The Big Bang Calculations indicate that the Big Bang occurred 18-15 billion years ago. The Big Bang marked the instantaneous creation of all matter in the Universe.
Variations on the Big Bang Theory • Steady-state cosmology- Universe will continue to expand forever; new matter is formed in spaces between galaxies at about the same rate that older material is receding. Density of matter in the universe remains relatively constant. • Oscillating Universe cosmology - The Universe expands (like with the Big Bang) and then contracts. Expansion and contraction alternate.
Lines of evidence that must be considered for any hypothesis on the origin of the Solar System • Planets revolve around sun in same direction - counterclockwise (CCW) • Planets lie roughly within sun's equatorial plane (plane of sun's rotation) • Solar System is disk-like in shape
Planets rotate CCWon their axes, except for: • Venus - slowly clockwise • Uranus - on its side • Pluto - on its side • Moons go CCW around planets(with a few exceptions)
Distribution of planet densities and compositions is related to their distance from sun • Inner, terrestrialplanets have high density • Outer, jovianplanets have low density • Age - Moon rocks and meteorites are as old as 4.6 billion years
Solar Nebula Hypothesis or Nebular Hypothesis • Cold cloud of gas and dust contracts, rotates, and flattens into a disk-like shape. • Roughly 90% of mass becomes concentrated in the center, due to gravitational attraction. • Turbulence in cloud caused matter to collect in certain locations.
Solar Nebula Hypothesis or Nebular Hypothesis • Clumps of matter begin to form in the disk. • Accretionof matter (gas and dust) around clumps by gravitational attraction. Clumps develop into protoplanets. • Solar nebula cloud condenses, shrinks, and becomes heated by gravitational compression to form Sun.
Solar Nebula Hypothesis or Nebular Hypothesis • Ultimately hydrogen (H) atoms begin to fuse to form helium (He) atoms, releasing energy (heat and light). The Sun "ignites". • The Sun's solar wind drives lighter elements outward, causing observed distribution of masses and densities in the Solar System.
Solar Nebula Hypothesis or Nebular Hypothesis • Planets nearest Sun lose large amounts of lighter elements (H, He), leaving them with smaller sizes and masses, but greater densities than the outer planets. Inner planets are dominated by rock and metal. • Outer planets retain light elements such as H and He around inner cores of rock and metal. Outer planets have large sizes and masses, but low densities.
How old is the Solar System? Based on radiometric dates of moon rocks and meteorites, the Solar System is about 4.6 billion years old.
Meteorites: Samples of the Solar System • Meteors = "shooting stars". The glow comes from small particles of rock from space being heated as they enter Earth's atmosphere. • Meteorites = chunks of rock from the Solar System that reach Earth's surface. They include fragments of: • Asteroids • Moon rock • Planets, such as Mars (i.e., "Martian meteorites")
Types of Meteorites • Ordinary chondrites • Carbonaceous chondrites • Achondrites • Iron meteorites • Stony-iron meteorites
Meteorites: Ordinary Chondrites • Most abundant type of meteorite • About 4.6 billion years old, • May contain chondrules - spherical bodies that solidified from molten droplets thrown into space during Solar System impacts
Meteorites: Carbonaceous Chondrites • Contain about 5% organic compounds, including amino acids – the building blocks of proteins, DNA, and RNA • May have supplied basic building blocks of life to Earth • Contain chondrules
Meteorites: Achondrites • Stony meteorites without chondrules, resembling basalt
Meteorites: Iron Meteorites • Iron-nickel alloy • Coarse-grained intergrown crystal structure (Widmanstatten pattern) • About 5% of all meteorites
Meteorites: Stony-iron Meteorites • Composed partly of Fe, Ni and partly of silicate minerals, including olivine (like Earth's mantle). • About 1% of all meteorites. Least abundant type.
The Sun • The Sun is a star • Composition: • 70% hydrogen • 27% helium • 3% heavier elements • Size: About 1.5 million km in diameter • Contains about 98.8% of the matter in the Solar System.
The Sun • Temperature:may exceed 20 million oC in the interior. • Sun's energy comes from fusion, a thermonuclear reaction in which hydrogen atoms are fused together to form helium, releasing energy. • The Sun's gravity holds the planets in their orbits.
Sun's energy is the force behind many geologic processes on Earth • Evaporation of water to produce clouds, which cause precipitation, which causes erosion. • Uneven heating of the Earth's atmosphere causes winds and ocean currents. • Variations in heat from Sun may trigger continental glaciations or change forests to deserts. • Sun and moon influence tides which affect the shoreline.
The Planets • Mercury • Venus • Earth • Mars • Jupiter • Saturn • Uranus • Neptune • Pluto
Terrestrial planets: Small Dense (4 - 5.5 g/cm3) Rocky + Metals Mercury, Venus, Earth, Mars Jovian planets: Large Low density (0.7 - 1.5 g/cm3) Gaseous Jupiter, Saturn, Uranus, Neptune The Planets • Other: • Small • Low density • Pluto
Mercury • Smallest of the terrestrial planets • Revolves rapidly around the sun; its year is 88 Earth days • Densely cratered • Thin atmosphere of sodium and lesser amounts of helium, oxygen, potassium and hydrogen • Weak magnetic field and high density suggest an iron core • No moons
Mercury This mosaic of Mercury was taken by the Mariner 10 spacecraft during its approach on 29 March 1974. The mosaic consists of 18 images taken at 42 s intervals during a 13 minute period when the spacecraft was 200,000 km (about 6 hours prior to closest approach) from the planet. Image courtesy of NASANSSDC Photo Gallery
Venus • Similar to Earth in size, mass, volume, density and gravity • No oceans or liquid water • Very high atmospheric pressure • Atmosphere is 98% carbon dioxide • Dense clouds of sulfuric acid droplets in atmosphere • Greenhouse effect causes temperature on planet's surface to reach 470°C, hot enough to melt lead
Venus • Rotates once on its axis (one day on Venus) in 243 Earth days • Rotates on axis in opposite direction to other planets, possibly due to collision with other object • Has volcanoes • Has craters • Surface rocks resemble basalt • No moons
Venus Ultraviolet image of Venus' clouds as seen by the Pioneer Venus Orbiter (Feb. 5, 1979). Image courtesy of NASA NSSDC Topographic Map of Venus from Pioneer Venus (Mercator Projection). Image courtesy of NASA NSSDC
Earth • Diameter = nearly 13,000 km (8000 mi) • Oceans cover 71% of surface • Atmosphere = 78% nitrogen and 21% oxygen • Surface temperature approx. -50 and +50 oC • Average density = 5.5 g/cm3 • Surface rock density = 2.5-3.0 g/cm3 Earth - The Blue Marble. Credit: NASA Goddard Space Flight Center Image by Reto Stöckli. Visible Earth.
Earth • Core about 7000 km in diameter; • Mantle surrounds core. Extends from base of crust to depth of 2900 km. • Geologically active. Plate tectonics. • Only body in the Universe known to support life. Earth - The Blue Marble. Credit: NASA Goddard Space Flight Center Image by Reto Stöckli. Visible Earth.
Factors that make Earth hospitable for life • Distance from Sun maintains temperatures in the range where water is liquid. • Temperature relatively constant for billions of years. • Rotation allows all sides of Earth to have light and heat. • Atmosphere absorbs some heat from the Sun and reflects some solar radiation back to space. • Magnetic field protects life from dangerous high energy particles and radiation in the solar wind.
Earth's Moon • Diameter = about 1/4 that of Earth. • Density = about 3.3 g/cm3 (similar to Earth's mantle). • Rotates on its axis at same rate as it revolves around Earth (29.5 days). Results in same side of Moon always facing Earth. • Far side of moon is more densely cratered • No atmosphere. • Ice is present at the poles.
Geology of the Moon • Dominant rock type is anorthosite (related to gabbro; rich in Ca plagioclase feldspar). • Basalt is also present. • Two types of terrane • Lunar highlands • Maria (singular = mare)