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Mercury Mercury is mountainous and cratered. Caloris Basin (1300 km across) is surrounded by mountains 2 km high! When the planet is at perihelion the temperature can reach 450ºC . Mercury has very little atmosphere. There is a magnetic field present. Its density equal to that of the Earth and it has a large iron-rich core.
Mercury Mercury can be seen in the morning or evening twilight. Occasionally it passes (transits) in front of the Sun. Sunspot Mercury
Missions to Mercury Mariner 10 mapped 45% of the surface of Mercury from 1974 to 1975.
Missions to Mercury MESSENGER mapped 30% of Mercury’s surface during it’s flyby on 14th Jan 2008. It will pass Mercury again in Sept 2009 and will survey and map the whole planet in 2011.
Venus Venus can approach the Earth to within 40 000 000 km. The planet has a dense atmosphere consisting mainly of CO 2 along with H 2 SO 4 . The average surface temperature is 480ºC . No measurable magnetic field is present. Venus spins in the opposite direction to all the other planets. The upper clouds spin more quickly than the surface, with a rotational period of only 4 days . UV image of Venus by Pioneer
Venus When Venus and Earth were formed 4600 million years ago the Sun wasn’t as powerful. As it became brighter the planets began to evolve differently. Earth was moving at a safe distance from the Sun unlike Venus which was slowly getting scorched. The oceans boiled away due to a runaway greenhouse effect and carbonates were released from the rocks into the atmosphere. Venus Earth
Venus Venus is brightest shortly before sunrise or just after sunset, as a result of this it is called the Morning Star or the Evening Star. Venus’ orbit is slightly tilted relative to Earth’s orbit so it doesn’t always cross in front of the Sun. The last transit Venus made was in 2004, the next will be in 2012.
Venus Reconstructed image of the surface from radar maps. A radar map of the surface shows plains, highlands and lowlands (which make up 65% of the surface). There are two main highland areas: Ishtar and Aphrodite. Ishtar is the size of Australia. The highest mountains adjoining Ishtar are 11 km high. There are many volcanoes on Venus, spewing sulfur into the atmosphere.
Missions to Venus Mariner 2 (1962) became the world’s first successful interplanetary mission, passing 34 833 km above the surface of Venus. The Russian probe Venera 4 entered the atmosphere and found it to be 95% CO 2 . At the same time Mariner 5 collected data on different properties of the atmosphere and the Russian and Americans analysed the data together.
Missions to Venus In 1974 Mariner 10 passed Venus on its way to Mercury, taking over 4000 photos of the planet. In 1978 the US Pioneer Venus Orbiter mapped the surface with radar for over 13 years. The Pioneer Venus Multiprobe measured different properties of the atmosphere.
Missions to Venus Magellan was launched in 1989 to map the surface of Venus with radar. It produced high- resolution images of 98% of the surface and mapped 95% of it’s gravity field. In 1994 it was sent into Venus’ atmosphere to measure the density, after which it was destroyed.
Missions to Venus ESA (the European Space Agency) released the Venus Express probe in 2005; it entered a polar orbit around Venus in April 2006. It is studying in detail the atmosphere and clouds. NASA’s MESSENGER passed Venus in Oct 2006 and June 2007 and collected data on the planet.
Mars The closest approach of Mars to the Earth is 60 000 000 km. It has a thin atmosphere and an average temperature of -60ºC although the surface is not permanently frozen. The poles are covered with white caps, the sizes of which vary according to the seasons; in winter they are largest and summer they are smaller. They are composed of water ice and carbon dioxide ice .
Mars The axial tilt of Mars is similar to the Earth and so has similar seasons (which last longer as it has a greater orbital period). The atmosphere of Mars is mainly CO 2 . There are no obvious craters and no sign of life past or present. The sky is pink and has clouds. Dust storms occur.
Mars Mars has two satellites, Phobos and Deimos . They are less than 30 km and 20 km in diameter. They are irregular in shape, heavily cratered and rocky. They may be asteroids captured by Mars in the past.
Mars People may be sent to Mars in the future. To achieve this target the rocket would have to be accelerated so that it swings outwards reaching the orbit of Mars at a certain point. Most of the journey would be done without fuel as the rocket falls under gravity. It would take months to reach Mars. A base would have to be constructed there for refuelling and other supplies. Astronauts would have to adapt to the weaker gravitational field on Mars (which is only 38% of that on Earth) and subsequently the stronger pull after returning to Earth.
Mars Martian sunset as seen by the Mars Rover Opportunity
Missions to Mars In 1965 Mariner 4 passed by at a distance of 10 000 km. It discovered that the apparent dark surface features were regions where red dust had been swept away exposing the darker rocks.
Missions to Mars Mariner 9 (1971) discovered Olympus Mons (Mount Olympus), a volcano 25 km high with a huge central crater. The surface has many volcanoes and basins; Hellas is 200 x 1800 km in size.
Missions to Mars The Valles Marineris (Mariner Valley) is 4500 km long and 7 km deep and is referred to as the ‘Grand Canyon’ of Mars.
Missions to Mars In 1976 the Viking landers sent back images of the surface. The surface material was analysed.
Missions to Mars In 1997 Pathfinder landed in Ares Vallis. The rover Soujourner crawled out and analysed the rocks. The area was a valley that may have contained water in the past.
Missions to Mars In1996 Mars Global Surveyor entered a closed orbit around Mars and mapped the surface. It showed that parts of the southern polar ice cap have been receding. In 1998 the Mars Climate Orbiter failed due to miscommunication between two NASA teams: one team used imperial units and the other used metric! The orbiter ended up crashing into the surface.
Missions to Mars In July 2008 water ice was discovered on Mars by the Phoenix Mars Lander . It has since completed its mission.
Missions to Mars Current orbiting spacecraft: Mars Odyssey , Mars Express (released failed Beagle 2 lander in Dec 2003), Mars Reconnaissance Orbiter . Mars Odyssey Beagle 2 (unsuccessful)
Missions to Mars Current Mars Exploration Rovers surveying the landscape: Spirit and Opportunity . Icy rocks have been found confirming the presence of water.
Jupiter and the Asteroids In the 18th century astronomers predicted the presence of a planet beyond Mars using a pattern called Bode’s law: 0 3 6 12 24 48 96 +4 1 7 10 16 28 52 100 Make Earth-Sun distance = 10, all other planets follow this scale: Mercury 3.9, Venus 7.2, Mars 15.2, Jupiter 52, Saturn 95.4 No planet at 28, however Ceres (an asteroid) is at 27.7 Law breaks down for Neptune and Pluto.
Asteroids In 1801 Piazzi (Sicily) discovered Ceres , a chunk of rock 90 km across. Pallas, Juno and Vesta were also found soon after. Currently there are over 13 000 asteroids orbiting the Sun between Mars and Jupiter. Very few are more than 50 km in size and only Vesta is visible with the naked eye. No large planet could form in this region as Jupiter’s strong gravitational tidal forces would tear it apart.
Asteroids Some small asteroids move in eccentric orbits. Eros (< 30 km) can approach Earth to within 25 million km. In 1999/2000 it was surveyed by NEAR Shoemaker . In 1937 Hermes passed us at only twice the distance of the Moon. Earth was probably hit by an asteroid 65 million years ago. Icarus and Phaethon get closer to the Sun than Mercury. The Trojan asteroids move in the same path as Jupiter. Hidalgo travels as far out as Saturn; Chiron passes mostly within Saturn and Uranus.
Jupiter Jupiter is the largest planet in the Solar System. It has a rocky core with a temperature of 30 000ºC , surrounded by layers of liquid hydrogen and a hydrogen and helium atmosphere. Jupiter produces it’s own energy but the core is not hot enough to start fusion. It has a strong magnetic field and is surrounded by zones of strong radiation. Jupiter is a strong source of radio emission. Jupiter is an oblate spheroid like the Earth. It has a short rotation period of less than 10 hrs so the equator bulges out. Also the equatorial zone has a period 5 min shorter than the rest of the planet (differential rotation). Other areas have differing periods so drift around in longitude (around the surface). Dark and bright belts are a result of descending and rising regions of gas. The clouds consist of ammonia . Spots seen on the planet are frequent and short-lived. The Great Red Spot has a longer lifetime. It’s red colour is due to copious amounts of phosphorus and the spot is a whirling storm larger than the surface area of the Earth.
Jupiter Four bright satellites were discovered by Galileo in 1610 and were subsequently called the Galileans . Io is slightly larger than the Moon, Ganymede and Callisto are much larger and Europa is smaller. Ganymede and Callisto are icy and cratered, Europa is icy and smooth and Io is red with a sulfur-covered surface as a result of volcanic activity. A strong electrical current connects Io with Jupiter and it’s orbital position affects Jovian radio emission. Europa may have liquid water under the ice. 12 other satellites exist, all are too small and faint to be seen with small telescopes. The outer 4 orbit Jupiter in the opposite direction to the others, these may be captured asteroids. There is also a thin dark ring of dust.
Missions to Jupiter Voyager 1 passed Jupiter in 1979. It used Jupiter to swing itself into orbit towards Saturn, passing the planet in 1980. Voyager 2 imaged Jupiter after Voyager 1, continuing on to the outer planets.
Missions to Jupiter In 1990 Ulysses was launched to observe the Sun. It travelled to Jupiter first and in 1992 Jupiter’s gravity was used to push the spacecraft into an orbit inclined to the ecliptic by 80º. It then entered an orbit around the north and south poles of the Sun. Further distant images of Jupiter were taken in 2004 when Ulysses was at aphelion.
Missions to Jupiter In 1995 Galileo went into orbit around Jupiter. It stayed there for 7 years making flybys of all the Galilean moons. It also observed the impact of comet Shoemaker-Levy 9 in 1994. Galileo was deliberately steered into the planet in 2003 at a speed of over 50 km s -1 to prevent it contaminating Europa which may harbour water.
Missions to Jupiter In 2000 Cassini approached Jupiter and took many images. Dark belts were previously thought to be areas of descending gas but they have rising cells of gas. The bright zones also have sinking gas.
Missions to Jupiter In Feb 2007 New Horizons on it’s way to Pluto took images of Jupiter. The spacecraft used Jupiter’s gravity to increase it’s speed and adjust it’s trajectory toward Pluto saving years of travel.
Missions to Jupiter Future missions include the Jovian Europa Orbiter which will look for water on Europa and Juno is planned for 2011 to go into polar orbit around Jupiter and investigate it’s origins and structure.
Saturn Like Jupiter Saturn has dark belts and bright zones and the occasional bright white spot. It has a solid core , surrounded by liquid hydrogen and a gaseous atmosphere. The average density of Saturn is less than water! It also has a magnetic field and radiation zones.
Saturn Saturn has 3 main rings , 2 bright and 1 semi-transparent. The bright rings A and B are separated by the Cassini division , ring C is called the Crepe ring. They are made up of icy particles . The diameter of the whole ring system is 275 000 km , and the thickness is 1 km . There are actually thousands of narrow ringlets and minor divisions. Several new faint rings were found outside the main system, possibly due to the gravitational effects of the satellites.
Saturn Saturn has 18 known satellites. The outermost moon, Phoebe (220 km) moves in the opposite direction to the rest and may be a captured asteroid. Titan is 5000 km in size and has a dense atmosphere of nitrogen and methane . Liquid ethane may be present. The temperature here is -180ºC; this is the triple point of methane where the 3 states solid, liquid and gas coexist.
The changing aspect of Saturn’s rings: it takes 14 years to get from edge to edge orientation. In 2009 they will be seen edge on.
Missions to Saturn In 1980 Voyager 1 imaged Saturn. Surfaces of the moons were seen for the first time. No surface details of Titan were observed due to it’s dense atmosphere. In 1981 Voyager 2 took more images of the planet. Saturn’s gravity was used to propel the spacecraft towards Uranus.
Missions to Saturn In 1979 Pioneer 11 passed to within 20 000 km of the planet. Low resolution images were sent back of the surface and some of the moons. Pioneer discovered that the dark ‘gaps’ in between the rings were bright when viewed towards the Sun and so contained material.
Missions to Saturn In 2004 Cassini-Huygens entered into orbit around Saturn. It sent back images and data about the moons, Phoebe and Titan. Radar images of Titan showed the existence of large lakes and mountains. The Huygens probe landed on Titan in Jan 2005 measuring it’s atmosphere while it descended.
Uranus Uranus was discovered by Herschel in 1781 . It is half the size of Saturn, has less hydrogen and more ammonia and water . It is tilted as much as 98º to the orbital plane. In 1977 it was discovered to have thin dark rings (no ice). A star being occulted flickered before and after Uranus passed in front of it. A magnetic field exists with an axis inclined to the rotational axis by 60º. The planet has radiation zones, cloudy features, and no strong source of internal heat. The lowest temperature recorded in Uranus’ atmosphere is 49 K (-224ºC) making it the coldest planet in the Solar System.
Uranus Uranus has 20 observed moons: Titania is 1500 km across. Miranda, Ariel, Umbriel and Oberon have icy cratered surfaces, Miranda has ridges and ice cliffs. In the 19th century mathematicians investigated the perturbations in Uranus’ position and predicted the presence of another planet - Neptune.
Missions to Uranus Voyager 2 flew past Uranus in 1986 and Neptune in 1989. A Gravity assist technique was used to propel it forwards along the right path (like a slingshot). This was only possible in the late 70s because the planets were arranged in a curve. This arrangement won’t happen again for over a century. Voyager 2 studied the structure and composition of the atmosphere and the unusual weather system due to its high axial tilt.
Neptune Neptune is of similar size to Uranus but has more mass. It has a heat source and is more active than Uranus. There is a huge dark spot (the Great Dark Spot ) above which are clouds of methane , 50 km above the main cloud layer. Other dark spots are visible and strong winds blow in the opposite direction to the planet’s rotation (retrograde direction) at 1000 km hr -1 . Neptune is a source of radio emission. It has a magnetic field with an axis offset from the rotational axis. There are 3 rings and 1 broad sheet of particles , all of which are dark.
Neptune Triton , a satellite of Neptune is 2720 km in diameter and moves in a retrograde orbit. There are few craters but many irregular formations a few km across. Triton has an atmosphere of nitrogen and methane , it’s surface is nitrogen and methane ice . Below the surface there may be liquid nitrogen which may explode to create icy volcanoes from which dark streaks can be seen.
Missions to Neptune Voyager 2 passed over north pole of Neptune in 1989 at a distance of 5000 km. It made a close flyby of Triton.
Pluto Pluto’s presence was predicted by the mathematician Le Verrier in the 1840s and in 1930 it was discovered by Clyde Tombaugh in America. Pluto is a mixture of rock and ice. It has a thin atmosphere, and is smaller and less massive than the Moon. One of it’s defining features is it’s highly eccentric orbit relative to all the other planets. It overlaps Neptune’s orbit however it’s high inclination prevents a collision. When Pluto was at perihelion 1979 - 1999 Neptune was the outermost planet.
Pluto Charon , one of Pluto’s moons is 1199 km across. Charon’s orbital period around Pluto is equal to Pluto’s rotation so to an observer on Pluto Charon would appear to be fixed in the sky ( geosynchronous ).
Pluto originated from the Kuiper Belt and has since been demoted from the Solar System.
On 24th Aug 2006 the International Astronomical Union (IAU) re-classified Pluto as a dwarf
planet due to it’s failure to meet one of the 3 conditions for an object to be classified as a
The object must be in orbit around the Sun
The object must be massive enough to be a sphere (from its own gravity)
It must have cleared the neighbourhood around its own orbit
Pluto fails (3) as its mass is only 0.07 times that of the mass of the other objects in its orbit.
Missions to Pluto Launched in 2006 New Horizons is on it’s way to Pluto. It used Jupiter’s gravity in 2007 to increase it’s speed and adjust it’s trajectory toward Pluto saving years of travel. It will arrive at Pluto in July 2015 and will then continue to the Kuiper Belt.
Planetary Data 0.248 17º12’ 6d 9h 247.7 yrs 4425 7375 5900 Pluto 0.009 1º45’ 16h 3m 164.8 yrs 4456 4537 4497 Neptune 0.047 0º48’ 17h 14m 84 years 2735 3004 2870 Uranus 0.056 2º29’ 10h 38m 29.5 years 1347 1507 1427 Saturn 0.048 1º18’ 9h 50m 30s 11.9 years 741 816 778 Jupiter 0.093 1º51’ 24h 37m 23s 687 days 207 249 227.9 Mars 0.017 0 23h 56m 4s 365.3 days 147 152 149.6 Earth 0.007 3º24’ 243.2 days 227.7 days 107.4 109 108.2 Venus 0.206 7º 58.6 days 88 days 45.9 69.7 57.9 Mercury min max mean Orbital eccentricity Orbital inclination Rotation period Sidereal period Sun from the (10 6 km) Distance Planet