Terrestrial Planets – Begin With Mercury 1 2 Guiding Questions 1. What makes Mercury such a difficult planet to see? 2. What is unique about Mercury’s rotation? 3. How do the surface features on Mercury differ from those on the Moon? 4. Is Mercury’s internal structure more like that of the Earth or the Moon? 88 days 3 4 Solar Transit Earth-based optical observations of Mercury are difficult Transits occur • At its greatest eastern and western elongation, about Mercury is never more than 28° from the Sun twelve • It can be seen for only brief periods just after times a sunset or before sunrise century when the sun, Earth and Mercury are There was a aligned transit on 5 November 8, 6 2006 1 Earth-based Views of Mercury Mercury rotates slowly and has an unusual Difficulties observing Mercury from Earth led early spin-orbiting coupling astronomers to incorrectly decide that Mercury always kept the same face towards the sun in synchronous orbit Note phases like the moon 7 8 Radio telescope observations from sites such as Arecibo gave evidence of a non-synchronous orbit 9 10 •Mercury • Strong tidal spins 1 ½ effects, Mercury’s times on slightly its axis elongated for every shape and its complete very orbit eccentric •Mercury orbit cause spins this strange 3-to-2 orbit three times •A “day”of solar light on during Mercury every two would be 88 orbits11 earth days 12 2 MESSENGER Spacecraft Images from Mariner 10 revealed Mercury’s heavily cratered surface • Most of our detailed information about Mercury’s surface is from the Mariner 10 Image from Flyby #3 - 2009 flyby mission in 1974/1975. • Mariner only saw one side of the planet. • The MESSENGER mission spacecraft is now at Mercury. – It is adjusting its flight path for an ultimate orbit of Mercury – 3 flybys to date – Will achieve orbit in 2011 13 14 •Heavily cratered surface Note • Less how dense much cratering more than moon densely •Gently the rolling craters plains occur on •Scarps the •No Moon’s evidence of surface. tectonics 15 16 Scarps are cliffs This one is more than a km high They probably formed as the planet cooled and shrank 17 18 3 The seismic waves from •The the impact Caloris that caused the Caloris Basin is Basin caused evidence this of a large deformation on the impact opposite side of Mercury 19 20 Mercury has an iron core and a surprising magnetic field • Most iron-rich planet in the solar system with a core that is 75% of the diameter • The earth’s core is 55% of its diameter and the moon’s core is 20% of its diameter • Highest density for the planets • Weak magnetic field indicating part of the core is liquid • Magnetic field causes a magnetosphere similar to Earth’s but weaker This may be evidence of ice at 21 22 Mercury’s North Pole. 23 24 4 The magnetosphere blocks the solar wind from reaching the surface of the planet 25 26 Cloud-Covered Venus 27 28 • At its greatest Guiding Questions eastern and western elongations, Venus 1. What makes Venus such a brilliant “morning star” or “evening star”? is about 47° from 2. What is strange about the rotation of Venus? the Sun 3. In what ways does Venus’s atmosphere differ • It can be seen for radically from our own? several hours after 4. Why do astronomers suspect that there are active sunset or before volcanoes on Venus? sunrise 5. Why is there almost no water on Venus today? Why do astronomers think that water was once very common on Venus? 6. Does Venus have the same kind of active surface geology as the Earth? 29 30 5 The surface of Venus is hidden beneath a thick, In 1962 the unmanned U.S. spacecraft highly reflective cloud cover Mariner 2 made the first close flyby of Venus •Venus is similar to the Earth in its size, mass, average density, and surface gravity •It is covered by unbroken, highly reflective clouds that conceal its other features from Earth-based observers 31 32 Venus’s rotation is slow and “retrograde” Venus has a hot, dense atmosphere and corrosive cloud layers • Spacecraft measurements reveal that 96.5% of the Venusian atmosphere is carbon dioxide • Most of the balance of the atmosphere is nitrogen. • Venus’s clouds consist of droplets of concentrated sulfuric acid. • The surface pressure on Venus is 90 atm, and the surface temperature is 460°C • Venus rotates slowly in a retrograde direction with a solar day of 117 • Both temperature and Earth days and a rotation period of 243 Earth days pressure decrease as altitude increases • There are approximately two Venusian solar days in a Venusian year. 33 34 The upper cloud layers of the Venusian atmosphere move rapidly around the planet in a retrograde direction, with a period of only about The circulation of the Venusian atmosphere is dominated by two huge 4 Earth days convection currents in the cloud layers, one in the northern hemisphere 35 and one in the southern hemisphere 36 6 Volcanic eruptions are probably responsible for Venus’s clouds •Venus’s clouds consist of droplets of concentrated sulfuric acid •Active volcanoes on Venus may be a continual source of this sulfurous material The density of craters suggests that the entire surface of Venus is no more than a few hundred million years old. According to the equilibrium resurfacing hypothesis, this happens because old craters are erased by 37 38 ongoing volcanic eruptions The climate on Venus followed a different evolutionary path from that on Earth • Venus’s high temperature is caused by the greenhouse effect, as the dense carbon dioxide atmosphere traps and retains energy from sunlight. • The early atmosphere of Venus contained substantial amounts of water vapor • This caused a runaway greenhouse effect that evaporated Venus’s oceans and drove carbon dioxide out of the rocks and into the atmosphere • Almost all of the water vapor was eventually lost by the action of ultraviolet radiation on the upper atmosphere. • The Earth has roughly as much carbon dioxide as Venus, but it has been dissolved in the Earth’s oceans 39 and chemically bound into its rocks 40 The surface of Venus shows no evidence of plate tectonics • The surface of Venus is surprisingly flat, mostly covered with gently rolling hills • There are a few major highlands and several large volcanoes • The surface of Venus shows no evidence of the motion of large crustal plates, which plays a major role in shaping the Earth’s surface 41 42 7 43 44 Venusian Surfaces 45 46 What I’ll Talk About Mars • Some history – a view at the start of the 20th century • Mariners to Mars • Viking Mission – in search of life of Mars •A meteorite – in search of life in a rock • Some latest views from Mars •Conclusions – keeping it simple 47 48 8 The High Hopes • “The planet Mars, on the other hand, exhibits in the clearest manner the traces of adaptation to the wants of living beings such as we are acquainted with. Processes are at work out yonder in space which appear utterly useless, a real waste of Nature’s energies, unless, like their correlatives on earth, they subserve the wants of 49 organized beings.” [Richard Proctor, 1902] 50 From Schiaparelli… To Percival Lowell • As seen by telescopes from Earth • Percival Lowell – An orange-red orb, with (1855-1916) some darker patches and bright polar caps sometimes – appointed MIT visible astronomy professor • Giovanni Virginio in 1902 Schiaparelli (1835-1910) – published books – 1876 announced discovery • Mars (1895) of “canali” (channels) on • Mars and its Canals Mars (1906) – misreported as canals • Mars as the Abode (artificial) by the press 51 of Life (1908) 52 More Historical Background Lowell’s Observations and Explanation • At the turn of the 20th century: •No canals – publication offered a reward for anyone • human brain tendencies coming forth with proof of life on another • connect unrelated planet or anywhere in space EXCEPTING points together by lines Mars • Recent theory – just about every major observatory had • Lowell’s telescope acted as released hand paintings of Mars and some an ophthalmoscope were even releasing photographs as • caused Lowell to see astrophotography was in its infancy the reflection of the • no two drawings could agree on the radial pattern of his own formations on the planet's surface retinal blood vessels •they showed a Mars with a varied surface possessing darker and lighter areas, as well as the polar caps 53 54 9 Mariner 4, 6 and 7 Mariner 4 Photographs •Mariner 4 – Mars flyby mission – closest approach came on July 15, 1965 – pictures from this mission showed no canals and a surface that was disappointingly looking like that of the moon, quite LIFELESS • In 1969 the United States launched Mariner 6 (February) and Mariner 7 (March) • At closest approach (July for Mariner 6 and August for Mariner 7) both craft were at a distance of approximately 3400 kilometers 55 56 Mariners 6 and 7 A Time to Fail and Succeed • The Mariners (6 & 7) contained: • In 1969 – narrow and wide angle cameras – two unsuccessful attempts by the Russians – infra-red radiometer • In 1971 – both Americans and Russians had unsuccessful – infra-red spectrometer missions to Mars – ultra-violet spectrometer – Russian Mars 2 and Mars 3 • Temperature, pressure and atmospheric • both equipped with lander modules but neither lander was successful constituents were analyzed – Americans Mariner 9 • Pictures were still anything but • reached Mars during a global dust storm – the storm did eventually subside and the mission was spectacular enough of a success so as to provide pictures for the 57 choosing of a site for landing the upcoming Viking missions 58 Mariner’s Atmosphere Mariner 9 Photographs