Further Information
http://solarsystem.nasa.gov (click on Venus)
www.space.com/venus/
www.nasm.si.edu (click on Venus)
© Springer International Publishing Switzerland 2016
John WilkinsonThe Solar System in Close-UpAstronomers' Universe10.1007/978-3-319-27629-8_6
6. Earth: The Planet of Life
John Wilkinson1
(1)Castlemaine, Victoria, Australia
Highlights
The interaction of the Earth and the Moon slows the Earth’s rotation by about 2 ms per century.
In 2010 radar onboard India’s Chandrayaan-1 space probe detected up to 600 million tonnes of water ice in craters at the Moon’s north pole.
In 2009, the LCROSS spacecraft was deliberately crashed into the lunar south pole. Water vapour and other molecules were detected in the debris plume.
Recent space probes have mapped mass concentrations (mascoms) on the Moon’s surface. In 2012, scientists determined how these mascons formed.
In December 2013, China successfully carried out the world’s first soft landing of a space probe on the Moon in nearly four decades.
Earth is the third planet from the Sun, orbiting at a average distance of 150 million km from the Sun. This distance is also known as one astronomical unit (1 AU).
Earth is the fifth largest planet with a diameter of 12,756 km. This makes Earth slightly larger than Venus. Earth orbits the Sun between Venus and Mars and is 1.4 times the distance from the Sun as Venus.
Earth is thought to have formed at the same time as the other planets in the Solar System about 4.5 billion years ago. Scientists known the age of the Earth because the oldest rocks ever discovered are 4.3 billion years old (determined from radioactive dating). Earth is the largest of the four rocky or terrestrial planets (Mercury, Venus, Earth and Mars). Like Mercury and Venus, Earth formed from a hot and molten state before it cooled to become a solid planet. Even though it has cooled since formation, Earth is currently the most geologically active planet, and its interior is still quite hot. The main feature that separates Earth from the other planets in the solar system is that it is the only planet to contain water in the liquid state (Fig. 6.1 and Table 6.1).
Fig. 6.1The Earth as seen from a satellite about 36,000 km out in space. Water covers about 70 % of the Earth. Land covers only about 30 % (Credit: NASA).
Table 6.1Details of Earth
Distance from Sun
149,600,000 km (1.0 AU)
Diameter
12,756 km
Mass
5.97 × 1024 kg (1.0 Earth’s mass)
Density
5.52 g/cm3 or 5520 kg/m3
Orbital eccentricity
0.017
Period of revolution
365.3 Earth days (1.00 year)
Rotation period
1.0 Earth days (24 h)
Orbital velocity
107,244 km/h
Tilt of axis
23.5°
Day temperature
15 °C
Night temperature
10 °C
Number of Moons
1
Atmosphere
Nitrogen, oxygen
Strength of gravity
9.8 N/kg at surface
Early Views About Earth
Earth is the only planet whose English name has not been derived from Greek or Roman mythology. The name comes from Old English and Germanic. There are other names for Earth in other languages.
In Roman mythology, the goddess of the Earth was Tellus—the fertile soil. In Greek, Gaia means terra mater—Mother Earth.
Ancient understandings of Earth varied often according to religious views. For a long time people thought the Earth was flat because it seemed that way. It was not until the time of Copernicus in the sixteenth century that it was understood that the Earth was just another planet.
Views of the Earth from space probes have confirmed that it is indeed a spherical body.
Probing Earth
Human exploration of the solar system began with the Earth. The first spacecraft were small-unmanned craft launched into the Earth’s atmosphere. Improvements in space technology eventually led to manned craft orbiting Earth in a period of time known as the ‘space race’.
Early in the space race the USSR was active with its Sputnik, Vostok, Voskhod and Soyuz spacecraft. Early USA spacecraft placed in Earth orbit included Explorer and those of the Mercury and Gemini programs. These early missions were the pioneers of future exploration of the Moon and other planets. For example, the 10 manned Gemini missions between 1964 and 1966 involved rendezvous between spacecraft in orbit, space walks, and even dockings with unmanned target vehicles. The American Apollo program contained many spacecraft missions that eventually resulted in humans visiting the Moon (Earth’s only satellite).
Today there are thousands of satellites in orbit around Earth, each belonging to particular countries. These are being used for many purposes, including communications, defence, science, GPS navigation, map making, environmental and weather monitoring. Orbiting observatories, such as the International Space Station are the largest and most complicated of all scientific satellites. Such satellites contain many types of scientific instruments that provide valuable research data.
Most Earth observation satellites orbit above 500 km in order to avoid the significant air-drag that occurs at low altitudes. The Earth observation satellites ERS-1, ERS-2 and Envisat of European Space Agency as well as the MetOp spacecraft of the European Organisation for the Exploitation of Meteorological Satellites are all operated at altitudes of about 800 km. Many are operated in a Sun synchronous polar orbit so as to get better global coverage. Such an orbit will have a period of roughly 100 min.
Spacecraft carrying instruments (e.g. meteorological satellites) for which an altitude of 36,000 km is suitable sometimes use a geostationary orbit. Such an orbit allows uninterrupted coverage of more than 1/3 of the Earth. Three geostationary spacecraft at longitudes separated with 120° can cover the whole Earth except the extreme polar regions.
The Landsat program of NASA offers the longest continuous global record of the Earth’s surface (over 40 years); it continues to deliver visually stunning and scientifically valuable images of our planet. Currently in operation are Landsat 7 and 8.
The National Oceanic and Atmospheric Administration of the USA (NOAA) has a number of environmental satellites that monitor the Earth from space. They are also used to analyse the coastal waters, relay life-saving emergency beacons, and track tropical storms and hurricanes. NOAA satellites also monitor conditions in space and solar flares from the Sun help us understand how conditions in space affect the Earth. The NOAA system uses both polar and geostationary satellites.
The Geostationary Operational Environmental Satellite system (GOES), operated by the United States National Environmental Satellite, Data, and Information Service, supports weather forecasting, severe storm tracking, and meteorology research. The National Weather Service (NWS) uses the GOES system for its United States weather monitoring and forecasting operations, and scientific researchers use the data to better understand land, atmosphere, ocean, and climate interactions. The GOES system uses geosynchronous satellites. Currently in operation are GOES 13, 14 and 15.
NASA’s Earth Observing System (EOS) is a coordinated series of polar-orbiting and low inclination satellites for long-term global observations of the land surface, biosphere, solid Earth, atmosphere, and oceans. As a major component of the Earth Science Division of NASA’s Science Mission Directorate, EOS enables an improved understanding of the Earth as an integrated system. More information can be found on their website. On 2 July 2014, NASA launched the Orbiting Carbon Observatory-2 (OCO-2) mission. OCO-2 is the first NASA satellite dedicated to monitoring carbon dioxide, and it will do so with greater precision and detail than current instruments.
The UCS Satellite Database is a
listing of the more than 1000 operational satellites currently in orbit around Earth. It includes basic information about the satellites and their orbits, but does not contain the detailed information necessary to locate individual satellites.
Position and Orbit
Earth orbits the Sun in a slightly elliptical orbit. Its mean distance from the Sun is just over 149 million km. At closest approach (perihelion) it is 147 million km from the Sun and its furthest distance (aphelion) is 152 million km.
The Earth like the other planets has three motions. It spins like a top on its axis, it travels around the Sun, and it moves through the Milky Way galaxy with the rest of the Solar System. The Earth’s axis is an imaginary line that connects the North and South poles. The spinning motion causes day and night on Earth, since only one side faces the Sun at any one time.
The Earth takes just over 365 days, 6 h and 9 min to orbit the Sun once; this length of time is called a sidereal year. Its axis is inclined at 23.5° off the perpendicular to the plane of its orbit. A single rotation on its axis takes 23 h 56 min and 4 s; this length of time is called a sidereal day. Earth rotates from west to east on its axis and this makes the Sun appear to move across the sky from east to west each day. The tilt of the Earth’s axis and its position around the Sun cause the seasons.
The interaction of the Earth and the Moon slows the Earth’s rotation by about 2 ms per century. Research indicates that about 900 million years ago there were 481 eighteen-hour days in a year.
Density and Composition
The Earth has a slightly larger mass, diameter and average density than Venus. Because of this, the strength of gravity on Earth is slightly more than that of Venus. A 75 kg person on Earth would weigh 735 N, but on Venus they would weigh only 607 N. Earth is also the densest planet in the Solar System, due to the fact that it has a large nickel-iron core.
Scientists know a lot about the interior of the Earth from their study of earthquake (seismic) waves and volcanoes. Earthquakes release enormous amounts of energy that travel through the Earth and along its surface as waves.
The interior of the Earth is divided into three main layers, which have distinct chemical and seismic properties: the core, mantle and crust. The core has an inner and outer layer. The inner core makes up only 1.7 % of the Earth’s total mass and is solid iron and nickel. It is extremely hot (about 5000 °C) but remains a solid because of the pressure from surrounding layers. At the centre the pressure is about four million times greater than at the surface. Surround the inner core is a liquid outer core containing about 30 % of the Earth’s mass. The outer core has a temperature of about 4100 °C. Convection currents in the liquid outer core are thought to produce electrical currents that generate the Earth’s magnetic field.
The outer core is surrounded by the mantle, which contains 67 % of the Earth’s mass. The mantle is 2820 km thick and contains iron/magnesium silicate minerals and oxygen that are kept in the solid state by high pressures. Nearer the surface of the mantle there are some molten regions that are liquid enough to flow. Sometimes this molten material rises to the surface and forms volcanoes and lava flows.
Surrounding the mantle is a thin outer layer or crust about 40–70 km thick that accounts for about 0.4 % of the Earth’s mass. It is made of granitic and basaltic rocks, which contain silicon dioxide (quartz) and other silicates like feldspar. The surface of the crust is covered by oceans (70 %) and continental land (30 %).
The crust varies in thickness. It is thicker under the continents and thinner under the oceans (Table 6.2 and Fig. 6.2).Table 6.2Chemical composition of the Earth
Element
Proportion (%)
Iron
34.6
Oxygen
29.5
Silicon
15.2
Magnesium
12.7
Nickel
2.4
Sulfur
1.9
Titanium
0.05
Fig. 6.2The interior structure of Earth.
Unlike the other terrestrial planets, Earth’s crust is divided into several separate solid plates, which move around on top of the hot mantle material. The continents are attached to the plates. Some plates are moving towards each other, and when they collide, material builds up to form fold mountains (such as the Himalayas). When plates are moving apart, molten material from the mantle often gets drawn up creating new crust; this occurs along the mid-Atlantic ridge between South America and Africa. Other plates are sliding past each other in a slow but jerky motion; such as the San Andreas Fault in California. Sometimes an oceanic plate slides underneath a continental plate, causing a deep trench (subduction zone) and mountain ranges; the Andes Mountains in South America were formed this way. Where plates interact, earthquake and volcanic activity occurs along the boundaries. Some volcanoes and earthquakes do not occur along plate boundaries, but in places are called ‘hot spots’—this accounts for the occasional earthquake in Australia, which rides in the middle of a plate.
The slow movement of plates (a few centimetres a year) is driven by convection currents and tectonic forces in the mantle (see Fig. 6.3).
Fig. 6.3Map showing the major plate boundaries of Earth.
The Surface
The crust of the Earth is a thin layer containing rock material.
The surface of the crust consists of mountain ranges, valleys, flat plains, deserts, and vegetated areas of varying density. From space, it can be seen that oceans of water cover much of the surface. The landscape has been shaped over millions of years by tectonic forces and volcanic action and erosion by the wind, water and glaciers.
The four terrestrial planets—Mercury, Venus, Earth and Mars—contain evidence of volcanic action having occurred in their past. In the case of Earth, some volcanoes are still active and these often erupt, releasing molten rock material into the air or surrounding areas. There are about 500 volcanoes on Earth, most are found along plate boundaries.
A key feature of the Earth is the presence of liquid water on its surface. This water is found in the oceans, lakes, and rivers. Water also occurs in the polar ice-caps, and as vapour in the air. Water allows living things to survive on Earth.
The highest land feature on Earth is Mount Everest at 8848 m above sea level. The lowest land feature is the shore of the Dead Sea at 399 m below sea level. The deepest part of the ocean is an area in the Mariana Trench in the Pacific Ocean southwest of Guam, 11,030 m below the surface. Average ocean depth is 3795 m.
The largest impact structure discovered on Earth is the Chicxulub Crater. It is hidden under sediments on the coast of the Yucatan Peninsula. The crater is a circular structure about 180 km wide and was discovered when instruments detected variations in the Earth’s gravitational and magnetic field. The meteorite that made the structure has been estimated to be about 10 km across.
The Atmosphere
The Earth’s atmosphere contains 7 % nitrogen, 21 % oxygen, with traces of argon, carbon dioxide and water.
In recent years the amount of carbon dioxide in the atmosphere has been increasing, due mainly to the burning of fossil fuels. At the same time people are cutting down forest trees that use carbon dioxide. The increase in carbon dioxide levels has resulted in higher average temperatures via the greenhouse effect. Carbon dioxide gas allows sunlight to reach the surface but prevents heat from escaping, in this way the atmosphere warms up. However, without the greenhouse effect surface temperatures would be much colder than they currently are.
Oxygen began to accumulate in the atmosphere when primitive life forms (first bacteria and plants) began to photosynthesis. Increases in oxygen levels allowed more forms of life to evolve.
Only a small amount of heat given off by the Sun enters the atmosphere; most is lost in space. About 34 % of the sunlight that enters the atmosphere is reflected back into space by clouds. Only 19 % of the sunlight that enters the atmosphere heats it directly. The remaining 47 % heats the ground and seas. Re-radiated hea
t from the ground and seas causes most of the warming of the atmosphere.
Temperature and Seasons
The Earth is heated by the Sun and has an average surface temperature of 15 °C. The highest recorded temperature on the surface is 58 °C at Al Aziziyah in Libya; the lowest temperature recorded is −89.6 °C at Vostok Station in Antarctica.
Varying temperatures and pressures in the atmosphere create strong winds.
The Earth has seasons because of its slightly elliptical orbit around the Sun and the fact that its axis is tilted at 23.5 °C off the perpendicular to the plane of its orbit (see Fig. 6.4).
Fig. 6.4The seasons on Earth.
The four climatic seasons are called summer (hot conditions), autumn (mild conditions), winter (cold conditions) and spring (mild conditions). The seasons in the northern hemisphere are opposite to those in the southern hemisphere. Today the Earth has a fairly stable climate with a narrow range of temperatures. In the past, however, there may have been rapid and dramatic climate change. Such changes may have resulted from changes in the position of the Earth in its orbit around the Sun, increased volcanic activity, changed atmospheric composition or large meteorite impacts.
The Solar System in Close-Up Page 10