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Physics 217 An Introduction to Modern Astronomy Part 6: Planets Figure 1: A schematic layout of solar system. Note sizes and distances are not to scale. 1 Introduction The term solar system refers to the sun and all the objects orbiting around it. The solar system can be divided into three main regions: • The innermost region region containing the Sun and the small, terrestrial and rocky planets. • The Gas giants. Large balls of gas orbited by their own systems of moons • The outer limits, inhabited by large empty wastelands with a scattering of icy worlds both large and small. The Solar system is, as the name suggests, dominated by the Sun which contains 99.86% of the mass of the Solar System, mostly in the form of ionised Hydrogen and Helium. The gas giants account for about 90% of the remaining mass with a tiny fraction left for the terrestrial planets. To illustrate how small the planets are in comparison to the Sun imagine if the Sun were modelled as a beach ball, Jupiter, the largest planet would be a golf ball about 150 metres away and Earth would be smaller than a pea and about 20m away. A few things to note about the solar system: • The orbits are roughly circular, apart from Mercury and Pluto. 2 Figure 2: A schematic of the original solar nebula. • The orbits are mostly coplanar, i.e. the planets all orbit in the same plane. • Most of the movements and spins are in the same direction. • It obeys Keplers Laws 2 The formation of the solar system As mentioned above, the major planets follow almost circular orbits, all going around in the same direction and in almost the same plane. This suggests a common origin from a flat disk-shaped gaseous cloud that encircled the Sun. A collapsing protostar during its formation needs to shed angular momentum, i.e. reduce its rotational inertia. One possible way is to develop a disk. Such disks appear to encircle a number of newly formed stars observed today. It is therefore very likely that a disk formed around the sun. This disk nebula, or solar nebula, was composed of the same material as the Sun, i.e. 98% hydrogen and helium, 2% heavier elements (i.e. metals in astronomy lingo). Isolated from the central heating within the protoSun, the disk began to cool. Elements with the highest temperatures of vapourisation, particularly iron and silicates, condensed first, thereby forming swarms of tiny grains within the solar nebula Like the dust in interstellar space, these grains, although accounting for only a tiny fraction of the mass, would render the solar nebula opaque. Thus, 3 shielded from the heat of the proto-Sun, the cooling in the outer part of the disk accelerated. The condensation of ices may have made the grains sticky since in time they adhered to one another forming tiny bodies or planetesimals. Those bodies that grew large enough used their mild gravitational forces to attract their fellows. In time, each radial zone of the solar nebula became dominated by a single proto planet that used its substantial gravity to sweep up its competitors. Yet, in competition to the force of gravity were thermal motions. In the outer part of the nebula, the light hydrogen atoms had thermal velocities of a few kilometres per second. Since this was soon less than the escape velocities of the outer proto-planets, the hydrogen was accumulated by those proto-planets - likewise the helium. In the inner part of the nebula, the thermal velocities of hydrogen and helium were around, ten kilometres per second, greater or equal to the escape velocities of the proto-planets. Consequently the inner proto-planets were unable to draw in hydrogen or helium (other than very small amounts) - even though this made up 98% of the mass. The four giant outer planets are therefore similar in composition to the Sun. The four inner planets, by comparison, are midgets - made almost entirely of the 2% heavy elements. The gaseous remains of the inner solar nebula have long since been blown away. The above discussion is the generally accepted scenario for the distinction between the four small inner planets and the four giant liquid planets. It has been somewhat thrown by the discovery of large Jupiter-like planets very close to their suns in other solar systems. Whilst each of the four inner terrestrial planets swept up all competitor proto-planets (and a large collision may have knocked the material out of the Earth to make the Moon), no fifth planet came together. Perhaps the radial “zone” was too large for gravity to bring together a single body. Instead that zone is filled by minor planetary bodies - the asteroid belt - of similar composition to the terrestrial planets. In similar fashion, there is no fifth outer giant planet, but there appears to be a Kuiper belt of smaller bodies - icy in composition. Pluto, its moon, and other minor planets so far detected out there seem to be members of the belt. Further out still, there may be a halo, the Oort Cloud, a reservoir of small icy bodies that provide the seeds for comets. The process of the formation of the solar system was repeated on reduced scales as satellite systems formed around the giant outer planets. Obviously such lesser bodies were either heavy elements or icy in composition. Each outer planet today has a ring system - it is not clear whether this is material 4 that never formed into satellites (due to the severe tidal forces) or whether it originates from a satellite that broke up close to the planet. An empirical relationship, the Bode-Titius law, gives the relative distances of the planets from the Sun. Take the series 0, 3, 6, 12, 24, 48, 96, 192, ... , add 4 and divide by 10. The latter results give very good comparison to the mean distance (in A.U.) of the planets (including the dwarf planet Ceres), except it does not work for Neptune and Pluto. 3 A short tour of the solar system Our solar system consists of the Sun, eight planets, moons, many dwarf planets (or plutoids), an asteroid belt, comets, meteors, and others. The sun is the centre of our solar system; the planets, their moons, a belt of asteroids, comets, and other rocks and gas orbit the sun. The eight planets that orbit the sun are (in order from the sun): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune. Another large body is Pluto, now classified as a dwarf planet or plutoid. A belt of asteroids (minor planets made of rock and metal) lies between Mars and Jupiter. These objects all orbit the sun in roughly circular orbits that lie in the same plane, the ecliptic (Pluto is an exception; it has an elliptical orbit tilted over 17 degrees from the ecliptic). The largest planet is Jupiter. It is followed by Saturn, Uranus, Neptune, Earth, Venus, Mars, Mercury, and finally, tiny Pluto (the largest of the dwarf planets). Jupiter is so big that all the other planets could fit inside it. A gas giant (sometimes also known as a Jovian planet after the planet Jupiter, or giant planet) is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in our Solar System: Jupiter, Saturn, Uranus, and Neptune. Many extrasolar gas giants have been identified orbiting other stars. Planets above 10 Earth masses are termed giant planets. Below 10 Earth masses they are called super earths or, sometimes probably more accurately for the higher mass examples, Gas Dwarfs. Objects large enough to start deuterium fusion (above 13 Jupiter masses) are called brown dwarfs and these occupy the mass range between that of large gas giant planets and the lowest mass stars. The 13 Jupiter-mass cut-off is a rule of thumb rather than something of precise physical significance. 5 Figure 3: Visible image of the sun with sunspots. 3.1 The Sun The Sun is the nearest star to Earth and the centre of our Solar system. It is a large ball of gas composed of 74% Hydrogen, 24% Helium, 2% heavier elements by mass. It has a diameter of about 1,392,000 km, about 109 times that of Earth, and its mass is about 2 × 1030 kilograms, 330,000 times that of Earth. The Sun is about 150,000,000 km from the earth. It takes sunlight about 8 minutes to reach the earth. The Sun, like most stars shines incredibly brightly, the energy it emits is generated by nuclear fusion in the centre of the Suns core. This fusion is combining Hydrogen nuclei (essentially protons) to form Helium nuclei. The energy released from these reactions is in the form of photons, which after moving slowly to the surface of the sun are eventually converted into the photons we see on earth. The Sun’s surface temperature is about 6000 K. The Sun possesses a generally weak magnetic field, but complications arise because the Sun does not rotate as a solid body. The equatorial regions rotate once in about 25 days, whereas polar regions rotate in about 30 days. The gaseous material of the Sun, being ionised, drags the magnetic field with it and the uneven rotation tangles the field as suggested in 6 Figure 4: Sunspots up close. See granular surface of sun. figure ??. The “active bands” move slowly towards the Equator. Over an 11-year cycle, new bands start around latitudes of about 40 degrees north and south, grow to a maximum, and eventually fade out at latitudes of about 5 degrees north and south, just as the next new pair of bands begin. Much of the disturbance in the active bands consists of magnetic loops breaking the surface of the Sun. If the magnetic field in these loops is strong enough it will divert the heat flow from the interior resulting in cooler regions on the surface. These cooler regions appear darker; thus a pair of sunspots, with opposite magnetic polarities, is created. The sense of polarity is reversed from northern to southern hemisphere, and one cycle to the next. The last solar maximum - when the greatest number of sunspots are visible on the photosphere - was be around 2013/14. The magnetic disturbances extend well above the photospherc - great prominences, some showing the loop structure, can occur. Energy is also released, partly as short-duration flares of light - or by heating the outer corona of the Sun to temperatures of millions of degrees. At such temperatures, the tenuous gas is no longer held by the Sun’s gravitational attraction, but streams outward through the solar system. In general this solar wind may “blow” at speeds of a few hundred kilometres a second, but can be enhanced in speed and strength during “storms”. Occasionally, flares may be followed by mass coronal ejections, with enormous bubbles of material erupting from the solar surface. Very energetic charged particles originating from such ejections can disrupt satellite communications on Earth as well as power grids. As these charged particles reach the earth, the earth’s magnetic field channels them onto it’s magnetic poles. As the particles crash into the the atmosphere, 7 Figure 5: Poster of the Aurorae Borealis (northern lights). they ionize the gas atoms in the atmosphere. When these ions recombine with electrons light is emitted. This hazy light called the aurora, can be seen near from high latitude regions. 3.2 The inner planets versus the outer planets The inner planets (those planets that orbit close to the sun) are quite different from the outer planets (those planets that orbit far from the sun). • The inner planets are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons. • The outer planets include: Jupiter, Saturn, Uranus, Neptune. They are mostly huge, mostly gaseous, ringed, and have many moons. 3.3 Mercury Mercury is the smallest planet and also the closest to the Sun, it takes about 88 Earth days to orbit the Sun. Mercury is quite bright when viewed 8 Figure 6: Mercury. from earth but it is not very easy to see as it is seldom very far from the Sun. Compared to the other planets very little is known about Mercury. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of the planets surface from 1974 to 1975. The second is the Messenger spacecraft, which mapped a further 30% during its flyby of January 14, 2008. Messenger’s last flyby took place in September 2009 and it was scheduled to attain orbit around Mercury in 2011, where it began mapping the rest of the planet. Mercury looks very similar to the Moon and is heavily cratered with some regions of smooth plains. It also has no natural satellites and no substantial atmosphere. However, unlike the Moon, it has a large iron core, which generates a small magnetic field. Surface temperatures range from about 90 to 700 K (-183 C to 427 C). 3.4 Venus Venus is the second planet from the Sun, its orbit takes about 225 Earth days. After the Moon it is the brightest natural object in the night sky. Venus reaches its maximum brightness shortly before sunrise or shortly after sunset, which is why it has been known as the Morning Star or Evening Star. 9 Figure 7: Venus. Top images show thick cloud layer. Bottom image shows surface of the planet mapped using radar on the Magellan probe. 10 Figure 8: Image of the earth from space, taken by Apollo 17 mission. Venus is classified as a terrestrial planet and it is sometimes called Earth’s “sister planet” due to the similar size, gravity, and composition. Venus is covered with an thick layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. Venus has the densest atmosphere of all the terrestrial planets in our solar system, consisting mostly of carbon dioxide. Venus’s surface is a dusty dry desert scape with many slab-like rocks, periodically refreshed by volcanism. The atmospheric pressure at the planet’s surface is 92 times that of the Earth. The Venusian surface was a subject of speculation until some of its secrets were revealed by planetary science in the twentieth century. It was mapped in detail by Project Magellan in 1990. The ground shows evidence of extensive volcanism, and the sulphur in the atmosphere may indicate that there have been some recent eruptions. The planet has few impact craters, demonstrating that the surface is relatively young, approximately 300 to 600 million years old. 3.5 Earth Earth is the third planet from the Sun and the densest of the eight planets in the Solar System. It is also the largest of the Solar System’s four terrestrial planets. Home to millions of species including humans, Earth is currently the only 11 Figure 9: The moon. astronomical body where life is known to exist. The planet formed 4.54 billion years ago, and life appeared on its surface within a billion years. Earth’s outer surface is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. About 71% of the surface is covered with salt water oceans, the remainder consisting of continents and islands which together have many lakes and other sources of water contributing to the hydrosphere. Earth interacts with other objects in space, especially the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 366.26 times it rotates about its axis, which is equal to 365.26 solar days, or one sidereal year. 3.6 The Moon - Earth’s natural satellite. The moon is Earth’s only natural satellite. It is a cold, dry orb with a surface that is riddled with craters and strewn with rocks and dust (this dust is called regolith). The moon has no atmosphere. Recent lunar missions indicate that there is some frozen ice at the poles. As mention before the moon always show the same side towards the Earth due to its synchronous rotation. The far side of the moon was first observed by humans in 1959 when the unmanned Soviet Luna 3 mission orbited the moon and photographed it. Neil Armstrong and Buzz Aldrin (on NASA’s Apollo 11 mission, which also included Michael Collins) were the first people to walk on the moon, on July 20, 1969. If you were standing on the moon, the sky would always appear dark, even during the daytime. This is because the moon has no atmosphere. 12 Also, from any spot on the moon (except on the far side of the moon where you cannot see the Earth), the Earth would always be in the same place in the sky. The rotation of the earth will be apparent when viewed from the moon, and it it will going through different the phases similar to the lunar phases. The moon is about 384,000 km from Earth on average. At its closest approach (the lunar perigee) the moon is 356,410 km from the Earth. At its farthest approach (its apogee) the moon is 406,700 km from the Earth. The moon revolves around the Earth in about one month (27 days 8 hours). It rotates around its own axis in the same amount of time. The same side of the moon always faces the Earth; it is in a synchronous rotation with the Earth. The Moon’s orbit is expanding over time and its rotation rate is also slowing down. As a fact of interest Earth’s rotation rate is also slowing down. For example, a billion years ago, the Moon was much closer to the Earth (roughly 200,000 kilometres) and took only 20 days to orbit the Earth. Also, one Earth ‘day’ was about 18 hours long (instead of our 24 hour day). The tides on Earth were also much stronger since the moon was closer to the Earth. 3.7 Mars Mars is the fourth planet from the Sun in the Solar System. It is often described as the “Red Planet”, as the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the volcanoes, valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, as is the tilt that produces the seasons. Mars is the site of Olympus Mons, the highest known mountain within the Solar System, and of Valles Marineris, the largest canyon. Until the first flyby of Mars occurred in 1965, by Mariner 4, many speculated about the presence of liquid water on the planet’s surface. Geological evidence gathered by unmanned missions suggest that Mars once had largescale water coverage on its surface, while small geyser-like water flows may have occurred during the past decade. In 2005, radar data revealed the presence of large quantities of water ice at the poles, and at mid-latitudes. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008. Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids. 13 Figure 10: Mars. Mars is currently host to three functional orbiting spacecraft: the Mars Reconnaissance Orbiter, the Mars Orbiter Mission (Mangalyaan) and MAVEN (Mars Atmosphere and Volatile EvolutioN mission). On the surface are the two Mars Exploration Rovers (Spirit and Opportunity) and several inert landers and rovers, both successful and unsuccessful. The Phoenix lander completed its mission on the surface in 2008. Mars can easily be seen from Earth with the naked eye. 3.8 Asteroids and meteorites Tens of thousand of minor planetary bodies, concentrated in the asteroid belt, have been detected. Towards 20 000 asteroids with known orbits are named and catalogued. They avoid orbits with periods 1/2, 1/3 etc. of Jupiter’s period - the Kirkwood Gaps (likewise the gaps in rings of Saturn due to innermost moon). The exception are the Trojan asteroids which have same period as Jupiter but keep 60 degrees ahead or 60 degrees behind the planet. A few Apollo asteroids have eccentric orbits that bring them within the Earths orbit - it is not known what perturbed them into such orbits. In a relatively short time ( million years) they will collide with one of the inner 14 Figure 11: The asteroid Gaspra. planets. Smaller examples of Apollo asteroids are large meteors which survived the fiery passage through the Earth’s atmosphere. Once landed they are termed meteorites. They are divided into different types/classes depending on composition: the Chondrites (stoney, 86%), the Achrondrites (also stoney, 7%), the Stoney-Irons (2%) and the Irons (5%). The latter number statistics apply to those seen to fall. In comparison, most fallen meteorites discovered are irons which is by far the easiest to recognise or detect. The iron meteorites must be fragments of a larger body or bodies wish was sufficiently big for differentiation to have occurred. The achrondrites may be the mantle fragments Chrondrites are characterised by tiny round bodies, chrondrules, packed together in a grey silicate matrix. Chrondrules vary in size from that of a pinhead to a pea. Their presence implies no differentiation and it is possible that they are original planetesimals. 3.9 Jupiter Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets. The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after the Roman god Jupiter. When viewed from Earth, Jupiter is the third-brightest object in the night sky after the Moon and Venus. 15 Figure 12: Jupiter. Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. The outer atmosphere is divided into several bands at different latitudes, resulting in turbulence and storms along their boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding the planet is a faint planetary ring system and a powerful magnetosphere. There are also at least 63 moons, including the four large moons called the Galilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Plutobound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa. 16 Figure 13: Saturn. 3.10 Saturn Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn. Saturn has an average radius about 9 times larger than the Earth’s. While only 1/8 the average density of Earth, due to its larger volume, Saturn’s mass is just over 95 times greater than Earth’s. Because of Saturn’s large mass and resulting gravitation, the conditions produced on Saturn are extreme if compared to Earth. The interior of Saturn is probably composed of a core of iron, nickel, silicon and oxygen compounds, surrounded by a deep layer of metallic hydrogen, an intermediate layer of liquid hydrogen and liquid helium and finally, an outer gaseous layer. Wind speeds on Saturn can reach 1,800 km/h, significantly faster than those on Jupiter. Saturn has nine rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. 62 known moons orbit the planet; fiftythree are officially named. This is not counting hundreds of “moonlets” within the rings. Titan, Saturn’s largest and the Solar System’s second largest moon (after Jupiter’s Ganymede), is larger than the planet Mercury and is the only moon in the Solar System to possess a significant atmosphere. 3.11 Uranus Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Though it is visible to the naked eye like the five classical planets, it was never recognized as a planet by ancient observers because of its dimness and slow orbit. Sir William Herschel announced its discovery on March 13, 1781, 17 Figure 14: Uranus. expanding the known boundaries of the Solar System for the first time in modern history. Uranus was also the first planet discovered with a telescope. Uranus is similar in composition to Neptune, and both are of different chemical composition than the larger gas giants Jupiter and Saturn. Uranus’s atmosphere, while similar to Jupiter and Saturn’s in its primary composition of hydrogen and helium, contains more “ices” such as water, ammonia and methane, along with traces of hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (224 degrees Celsius). It has a complex, layered cloud structure, with water thought to make up the lowest clouds, and methane thought to make up the uppermost layer of clouds. In contrast the interior of Uranus is mainly composed of ices and rock. Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among the planets because its axis of rotation is tilted sideways, nearly into the plane of its revolution about the Sun. As such, its north and south poles lie where most other planets have their equators. Seen from Earth, Uranus’s rings can sometimes appear to circle the planet like an archery target and its moons revolve around it like the hands of a clock. The wind speeds on Uranus can reach 900 km/h. 18 Figure 15: Neptune. 3.12 Neptune Neptune is the eighth and farthest planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than Uranus. On average, Neptune orbits the Sun at a distance of 30.1 AU, approximately 30 times the EarthSun distance. Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Calle within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet’s remaining 12 moons were located telescopically until the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. Neptune is similar in composition to Uranus, and both have compositions which differ from those of the larger gas giants Jupiter and Saturn. Neptune’s atmosphere, while similar to Jupiter’s and Saturn’s in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, contains a higher proportion of ”ices” such as 19 Figure 16: The comet Hale-Bopp. water, ammonia and methane. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Traces of methane in the outermost regions in part account for the planet’s blue appearance. In contrast to the relatively featureless atmosphere of Uranus, Neptune’s atmosphere is notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet’s southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching 218 degrees Celsius (55 K). Temperatures at the planet’s centre, however, are approximately 5,400 K (5,000 C). Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2. 3.13 Icy bodies and comets There is apparently a vast reservoir of small icy bodies in the Kuiper Belt and Oort Cloud. On occasions, these are somehow perturbed and fall in towards the Sun. They follow a virtual parabolic orbit around the Sun and return to the outer regions. In certain cases, an encounter with a planet may slow down the body such that it ends up in a short period eccentric orbit - such as the nucleus of Comet Halley with a period of only 75 years. 20 These object are low density, partly uffy in composition. Once described as “dirty snowballs”. The 2005 impact of a spacecraft with Comet Tempel 2 indicates that “snowy dirtball” might be more appropriate. During the time when the body is close to the Sun, the heat of the sun sublimates the ice (turns directly into water vapour). Hampered by the build up of rocky debris on the surface, the water vapour so formed may blast out through crevices in the crust carrying ner particles with it. Halley has now shrunk to about 11 × 7 km in size and probably loses about 6m of surface ice each perihelion passage. Other cometary nuclei that pass closer to the Sun may not even survive the passage intact. The water vapour and dust also form a spherical tenuous coma, sometimes millions of kilometres across during this phase in the comets orbit. The coma is the most obvious feature of a visible comet. Dissociation of water molecules releases hydrogen that forms an even larger cloud. From the fragile coma, two forms of tails develop. Solar radiation pushes the dust to form an amorphous yellow tail. The solar wind drives ionised gas to produce a structured blue tail. Obviously, both tails point away from the Sun. Over time (maybe several orbits) after which most of the water/ice has been removed, eventually what will remain is a swarm of particles. At times the Earth has encountered such swarms (they can even be predicted) and a meteor shower is observed. Since they all come from a common direction, they will appear to come from a common radiant in the sky. None of these particles is large enough to reach the Earth’s surface. Comet-planet collisions can and doe occur. At least one comet (Shoemaker Levy 9) was captured in orbit around Jupiter and eventually collided with that planet. It is not known if the explosion in the atmosphere over Siberia in l908 was caused by the impact of a small comet or asteroid. Many comets fall straight into the Sun. Comets have a wide range of orbital periods, ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper belt, or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort Cloud. The long-period comets plunge towards the Sun from the Oort Cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune), or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space along hyperbolic trajectories. Currently there are more than 5000 known comets. This number is steadily increasing. However, this represents only a tiny fraction of the 21 Figure 17: A schematic of the interior of terestrial planet. total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular. 4 4.1 Planetary structures Internal composition and structure The scenario presented above indicates that planets can be classied as either “giant” or “terrestrial” - the latter class can include the large numbers of Moons in the solar system. Planets differ greatly in composition - an indication is provided by their average density. It is also possible to gauge the internal mass distribution from their period of rotation and the resulting degree of attening. Terrestrial Planets The inner planets of the solar system are composed almost entirely of heavy elements - including some radioactive isotopes, with very long half lives. The decay of these isotopes has released a great deal of heat - for example, the centre of the Earth is believed to have a temperature of 6600 K. Under such conditions the interiors are molten and differentiation can occur whereby elements with higher density (chiefly iron and nickel) sink to the centre to form a core (see figure 17). Slow convective currents in the core then generate weak magnetic elds (much weaker than those of the giant planets). We would expect Venus to 22 Figure 18: A schematic of the interior of an icy moon. have a core similar to the Earth, but it lacks a magnetic field and its very slow rotation rate does not allow us to gauge its internal mass distribution. Mercury has a large core, almost as if part of its outer mantle had been lost. Mars has a weak cores and the Moon no core. That the density and composition of the Moon seems to match that of the Earth’s outer mantle supports the theory that the material used to make the Moon broke from the Earth’s mantle, possible during a catastrophic collision. Europa, Ganymede and Callisto (the three biggest moons of Jupiter) lie in a much cooler part of the solar system. They appear to have silicate cores, but surrounded by substantial mantles of water ice (or perhaps water, in the case of Europa and Ganymede). Ganymede is the largest moon in the Solar System, larger even than planet Mercury. Lying closer to Jupiter, tidal stresses have heated the interior of Io - so much so that it has lost its water mantle and is subject to severe volcanism that is constantly “turning the moon inside out”. The density of most of the moons of Saturn (Titan excluded) Uranus and Neptune suggests that they are almost completely water ice - or rather “dirty ice”. So peculiar is the appearance of Miranda (a moon of Uranus), that some researchers have even suggested that this moon broke apart into a number of pieces and then re-assembled. Gas planets The two largest planets, Jupiter and Saturn, are mainly hydrogen and helium in liquid form (and the description “gas giant”, often applied, is misleading) - what heavy elements they have are congregated in the centres; Under the extreme pressures towards the centre, the liquid hydrogen behaves as a metal (i.e. electrical currents can ow in their cores). Jupiter 23 Figure 19: A schematic of the interior of gas giant planet. and Saturn are “would be stars”; their interiors are hot and they radiate out to space about twice the heat they receive from the Sun - although it is obviously in the far infrared. The flow of heat outward sets up convective motions within the planets’ interiors. The details are not understood, but the motions within the metallic region are thought to generate the very strong magnetic fields possessed by these planets. Uranus and Neptune are much smaller and their interiors much cooler. Some models suggest that their cores could be surrounded by ices. 4.2 Atmospheres Terrestrial planets The primary atmosphere formed with the planet - hydrogen and hydrogen compounds (methane, ammonia etc.) - but was gradually lost to space because the thermal molecular motions approach escape velocity - especially molecular hydrogen and helium. The Moon’s gravity was too low to retain any atmosphere, virtually the same for Mercury. The secondary atmosphere, formed from outgassing from the crust (volcanism), is almost entirely carbon dioxide - as in atmospheres of Venus 24 and Mars - with a small amount of nitrogen. The Earth has lost 99% of its atmosphere - almost all carbon dioxide washed out by rain to form carbonate rocks. Only nitrogen remains, plus oxygen from plant photosynthesis. Weather is common phenomenon in terestrial planetary asmospheres. Hot air rises where the solar heating is greatest (near equators) and descends at higher latitudes. This circulation is forms Hadley cells (seen on Earth and Venus). On Earth, variations in weather are largely driven by the heat stored in the oceans. Both Earth and Venus show weather patterns rotating faster than the planet. Violent atmospheric motions can occur if substantial energy can be directly inserted into atmosphere. This happens when condensing water releases large amount of latent heat which drives thunderstorms, hurricanes etc. Also dust particles suspended in atmosphere absorb solar heat directly which leads to sandstorms. In 1971 and 2002, the whole of Mars was covered by a single sandstorm. Gas Planets The gaseous outer regions of the giant planets are, of course, mainly hydrogen and helium. However, a variety of other components create cloud layers and colours. The deep blue colour of Neptune is from methane absorption, but the reddish colours in the clouds of Jupiter and Saturn are not yet explained. Rising convective motions push clouds higher (lighter bands on Jupiter and Saturn), sinking motions make clouds lower (darker bands on Jupiter and Saturn). Why these should form longitudinal bands, each rotating at a speed different to each other, is not properly understood either. The driving forces may be cyclonic storms, the most conspicuous being the great red spot of Jupiter (which has been there since telescopes were first used) the great dark spots of Uranus and Neptune and the white spots on Jupiter and Saturn (which come and go). How these storms are powered is not known. 4.3 Magnetospheres Planets with magnetic fields (Mercury, Earth, Jupiter, Saturn, Uranus and Neptune) are surrounded by magnetospheres. The stronger the eld, the more extensive the magnetosphere. Magnetospheres are bounded and deformed by the solar wind (charged particles) blowing past. Surges in the solar wind may compress the magnetosphere and set up massive electrical currents and causing aurora displays. 25 Figure 20: A schematic of the magnetosphere of a planet. Magnetospheres serve as a shield to cosmic rays - but such charges particles tend to accumulate in particle belts (van Allen belts). Such regions are dangerous to humans and (in the case of Jupiter and Saturn) even spacecraft. In the case of Io (innermost of Galilean moons) moves through magnetosphere and stirs up particle belts causing radio radiation. 4.4 Surface gravities The force of gravity experienced on the surface of a planet varies according to the mass of the planet and the inverse square of its radius. Consequently the surface gravities of the Moon, Mercury and Mars are much lower than the Earth’s, while Jupiter’s is much higher. 5 Planetary surface features The four terrestrial planets and all the moons and minor planets of the Solar System have hard surfaces exhibiting many common features. Mercury is very cratered, Venus has elevated “continents”, volcanoes and some impact craters. The earth have moving continents, mountain ranges, and surface largely sculpted by water, deserts and ice sheets. Mars has an elevated southern hemisphere heavily cratered, has desert and ice sheets. The moon, like Mercury, is heavily cratered and show volcanic maria. The moons of Jupiter shows a wide variety of surface features. lo has volcanoes, while Europa has an ice covered ocean. Ganymede has a thick 26 Figure 21: A schematic of the crater formation process. ice cover and Callisto is heavily cratered. Titan shows dry rivers and lakes while Triton have icy geysers. 5.1 Impact cratering Impact craters are the most ubiquitous of surface features on planets, Some planets, like the Moon, have so many impact craters that there is no place on their surface that is not an impact crater, and newer craters obliterate older craters. As such they represent the “sweeping up” of minor bodies in the solar system. Their ages conrm that most of the cratering (including some major impacts) took place during the first billion years of the solar system. The cratering rate has declined dramatically as the population of minor bodies was decimated. However, even today, cratering continues - though the last major impact on Earth was about 50 000 years ago. The source for the impacting material is apparently the asteroid belt. Figure 21 illustrates what happens during an impact. All planets is believed to have received similar numbers of impacts - yet on some planets, erosion has removed the craters. The Earth, with only one intact impact crater (in Arizona), is an example of severe erosion, though Io has none. Many planets, like Venus or Mars, show limited numbers of impact craters. The southern hemisphere of Mars has much less cratering than the Moon, while Venus only shows cratering from the last half billion 27 Figure 22: A schematic showing difference in structure between craters and volcanoes. years. Thus the degree of cratering on a planetary surface can he used as an indication of the age of the surface (the heavier the cratering, the older surface). Stratigraphy concerns the relative chronological sequence of surface features and is particularly applicable to the Moon and Mars. A newer feature will cover or partially obliterate an older feature. Also older craters are more subdued and dark. Fresh craters show sharp proles, rugged appearance, light materials or ray systems. 5.2 Volcanism Volcanic vents emit lava and gas. Thick lava builds elevated shield volcanoes, especially on Venus, Earth and Mars. The largest is Olympus Mons on Mars - almost 25 km high. Thinner lava seems to have gradually ooded large impact basins, forming maria basins, the most conspicuous features on the Moon’s near side also found on Mercury and Mars. The flows are complex, building up over millions of years. The thin lava comes to the surface either through fractures or from craters that mimic impact craters, but are different in their cross sections (see figure 22). High speed ejection ( 1 km/s) is constantly occurring from several volcanoes on Io. Much sulphur is emitted and rains down on the surface almost completely covering the planet. Lakes of hot liquid sulphur and lava ows are apparent; it seems that the underlying mechanism must be different to terrestrial volcanoes. 5.3 Erosion Planets with no atmosphere suffer bombardment from tiny meteors, which tends to pulverise surface layers - hence making a regolith resembling dry Portland cement. It also contains stones and microscopic glass beads (solidified from drops of molten material flung skyward during impacts). The 28 beads are an excellent light backscatterer (esp. rays from craters on Moon best visible near Full Moon). Planets with some atmosphere - wind erosion results from abrasive action of suspended particles. Sand shifts with wind patterns, but collects to form dune fields in low lying basins. Water is by far the most powerful erosive agent. The surfaces of the continents of the Earth are subject to severe water erosion. Smaller features (e.g. impact craters) are rapidly erased, even major mountain ranges only survive for perhaps only a tenth of the Earth’s lifetime. Sedimentary basins; later elevated, are signicant features (e.g. Karoo). 5.4 Valleys and Rilles Rivers rapidly carve valleys or, in dryer climates, canyons with characteristic dendritic pattern - esp. Earth, Mars and Titan. The climate of Mars is currently so cold that virtually no water exists in liquid form; apparently it was warmer in the past. The surface of Titan is so cold that water only exists as hard “rocks”, but liquid methane was probably responsible for the valleys and lakes (now dry). The Moon, Venus and Mars possess many valleys clearly not made by running liquid, at least not on the surface. Rilles - some straight and some sinuous - are somehow related to volcanism, partly by collapse. Africa has a giant rift valley where the continent is currently splitting into two pieces. Mars has gigantic rift valley (Valles Marinus), thousands of kilometres long and up to 6 km deep, a great split in its surface. In places it is modified by water erosion, but it is mainly growing by the surrounding elevated plateau collapsing. 5.5 Water and Polar Caps These are abundant stores of water ice. Mars had a wet episodes in its past, the low lying northern hemisphere (devoid of ancient craters) may have at times been a shallow ocean. Currently Martian water is frozen in the soil, and thick layers of ice appear to have accumulated at the poles. These are covered by sand. The conspicuous white polar caps on Mars that grow and shrink with the seasons are formed from thin coatings of carbon dioxide (when about a quarter of the Martian atmosphere condenses). Earth has ice caps kilometres thick on Antarctica and Greenland. 29 Figure 23: A schematic of a continent. 5.6 Crustal Materials The crusts of terrestrial planets are predominantly silicates. Silicon atoms are usually linked by oxygen atoms (silica - acidic), otherwise metal ions intervene (olivines - basic). There are intermediate cases, such as feldspars and pyroxenes. There is a general tendency for original crustal materials to be acidic or intermediate. Materials deeper down, brought up in lava, tend to be basic. The marias of the Moon are basalts (basic) rich in iron and titanium. Materials elsewhere on the Moon are complex - usually feldspathic breccias (partial melting and recrystallization) as expected from impact ejecta. Mars also has basalts. The high iron content in Mars’ crustal materials gives its rocks and sand a reddish colour. 5.7 Continents Both Earth and Venus have continents - bodies of material elevated above the surrounding crust - apparently masses of lighter material being in isostatic equilibrium and oating higher. On Earth, it is of course the continents that generally project above the surface of the oceans (see figure 23). Most continents have a hard core termed a craton (heavy stippling in figure 23) surrounded by softer orogenic zones (light stippling). 30 Figure 24: A schematic of Earth’s tectonic plate movements. The tectonic plates of the Earth The thin ocean beds are not permanent but are constantly replenished and replaced by new material from the Earth’s interior. The new ocean bed pushes and spreads sideways from mid-oceanic ridges. Old ocean beds are subducted back into the Earth’s interior in deep trenches close to the continents. The surface of the Earth is fragmented into segments, the tectonic plates (see figure 24). The continents ride with the plates sometimes separating and sometimes colliding with one another - the soft orogenic zones getting crumpled and the continents getting sutured together (c,g. the collision between India and Asia causing the Himalayas - a collision still in progress). The Urals were formed when Europe and Asia were sutured, the Appalachians when Africa and North America collided. At intervals of a few hundred million years, the continents tend to gather into a single supercontinent. Some 300 million years ago, most of the continental masses came together to form Pangea, Pangea became unstable, possibly because too much heat built up beneath it, and split into Laurasia and Gondwanaland. Laurasia split into North America and Eurasia, Gondwanaland into South America, Africa, Antartica, India and Australia. Africa is soon to split again (along the rift valley). The Atlantic Ocean (only 150 million years old) is rapidly opening up - the Pacific is being 31 closed on all sides - its plate is slowly being subducted. In less than another 200 million years, this is where all the continents will regroup to form a new supercontinent. And so the cycle will repeat, as it must have already done so many times in the Earth’s history. Note that oceans are relatively shallow over the mid-oceanic ridges where new material comes up. The mid-Atlantic ridge actually rises above sea level at one point - Iceland. One other place of interest is California where the infamous San Andreas fault marks the boundary between the Pacific plate (moving laterally to the north) and the N. American plate (moving to the south). The Earth is, of course, the only planet known to have this plate motion. 6 Extrasolar planets An extrasolar planet, or exoplanet, is a planet outside the solar system. It is now known that a substantial fraction of stars have planetary systems, including at least around 10% of sun-like stars. The true proportion may be much higher. It follows that billions of exoplanets must exist in our own galaxy alone. Even more exciting are that current estimates suggest that about 10% of all sun-like stars probably harbour earth-size planets in the habital zone. Broadly speaking, the habital zone is defined as the region around a star where the conditions (e.g. planetary surface temperature) would be suitable for liquid water (and thus making life as we know it possible) to exist. Extrasolar planets became an object of scientific investigation in the nineteenth century. Many astronomers supposed that they existed, but there was no way of knowing how common they were or how similar they might be to the planets of our solar system. The first confirmed detection was made in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of an exoplanet orbiting a main sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi. The frequency of detections has tended to increase on an annual basis since then. The number of exoplanets detected and confirmed is now well over the 1000 mark. There are at least another several thousand more planet candidates awaiting confirmation by more detailed investigations. Exoplanets have been detected through various techniques. The most successful of these detects a planet indirectly, due to its (very small) gravitational influence on its host star. The planet causes a slight wobble in the motion of the host 32 star, which is detected as a very small line-of-sight velocity change. The technique obviously favours massive Jupiter-like planets close to their parent stars, and these are the majority found. For example our Sun moves in a circle at 16 metres a second as Jupiter (by far the most massive planet in our Solar system) orbits around it. Some extra-solar planets are found because their orbits are seen edgeon and they “eclipse” their parent star, causing a small drop in the light we receive. The Kepler mission has found thousands of such planets using this technique, many of which are are of super-earth size and smaller. For more on this exciting field in modern astronomy explore the website exoplanets.org. 33