Published on March 11, 2008
ASTR 330: The Solar System: ASTR 330: The Solar System Mars Facts and Figures Dr Conor Nixon Fall 2006 Mars orbits the Sun at a distance of 228 million km, or 1.52 AU. Can you calculate it orbital period in Earth years? Use Kepler’s third law? Mars is a medium-sized terrestrial planet. With a diameter of 6787 km, Mars is half the size of the Earth, and falls in between the larger Earth and Venus and the smaller Moon and Mercury. Its mass is 11% of the Earth’s mass, or nine times as much as the Moon. Can you calculate the density? (Answers: 1.88 Earth Years, 3.9 g/cm3) Figure credit: Albert T Hsui, Univ. Ill ASTR 330: The Solar System: ASTR 330: The Solar System Lecture 16: Mars I Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL - Viking Mars historical perspective ASTR 330: The Solar System: ASTR 330: The Solar System Overview Dr Conor Nixon Fall 2006 Mars certainly has the most magnetic appeal for most Earthlings, due to countless fictionalizations in novels, comic books, TV shows, radio dramatizations, and of course, movies. As with the Earth, a single lecture barely begins to scratch the surface of what we now know about Mars. In this lecture we will concentrate on: Picture credit: NASA Exploration of Mars, from the 19th century canal-watchers, to 21st century rovers. The major terrain types on Mars. Volcanism and tectonics; soil and rock. Atmosphere. ASTR 330: The Solar System: ASTR 330: The Solar System Telescopic Observations Of Mars Dr Conor Nixon Fall 2006 As seen by most telescopes from the Earth, Mars is an orange-red orb, with some darker patches and bright polar caps normally visible. Seen through the Hubble Space Telescope (HST) – right – the shapes of the major terrains and the largest geographical features begin to appear. (Compare to the Viking image on the first slide). In the 19th century however, this level of detail would not have been visible. ASTR 330: The Solar System: ASTR 330: The Solar System The Mars Saga: 1877-1920s Dr Conor Nixon Fall 2006 The Mars controversy began in 1877 with the observations and notes of the Italian astronomer Giovanni Schiaparelli (1835-1910). Schiaparelli claimed sightings of faint dark linear markings on the surface, which he referred to as ‘canali’, the Italian word for ‘channels’. In English translation of course, these quickly became ‘canals’, with all the consequent connotations of industry by intelligent beings. The most famous proponent of the Martian canals was the American Percival Lowell (1855-1916), who took up astronomy after reading Schiaparelli, and founded an observatory in Flagstaff, AZ to study the canals. Picture credits: SPL/Photo Researchers. Lowell Observatory ASTR 330: The Solar System: ASTR 330: The Solar System Lowell’s Contribution Dr Conor Nixon Fall 2006 Lowell, with his 24 in refractor was able to see a great many canals, intersecting at junctions he referred to as ‘oases’. Lowell published 3 books of drawings describing the canals, and even made similar claims regarding Venus! Many astronomers doubted Lowell however: most could not see the canals. Perhaps a warning sign was that observers using smaller telescopes were better able to see the features. ASTR 330: The Solar System: ASTR 330: The Solar System No canals, but… Dr Conor Nixon Fall 2006 We now attribute Lowell’s sightings to a trick of the mind, connecting unrelated points together by lines, like a picture outline of dots - remember what constellations are? In the case of Venus, a recent theory is that Lowell’s particular telescope settings acted as an ophthalmoscope, allowing him to see the radial pattern of blood vessels in his own retina, backlit by the bright Venus. However, as the 20th century progressed, darker areas on Mars were definitely seen to change shape over the Martian year: proof some said of seasonal growth of vegetation. ASTR 330: The Solar System: ASTR 330: The Solar System Early Spacecraft visits Dr Conor Nixon Fall 2006 Of the over 40 attempts to send spacecraft to Mars, starting in 1960, only around a third have had real success. Many early missions failed before leaving the Earth, as their rockets exploded or didn’t ignite. Mariner 4 (1965) was the first successful US attempt, sending back the blurry TV pictures of craters (rather than lush vegetation), and determining the surface pressure of the atmosphere to be around 0.01 bar. Picture credits: NASA/NSSDC ASTR 330: The Solar System: ASTR 330: The Solar System Mariner 9 Dr Conor Nixon Fall 2006 After Mariner’s 4, 6 and 7 which saw only craters, the prevailing view of Mars was of a geologically inactive, heavily cratered world like the Moon. By pure bad luck, the most interesting features had been entirely missed! When Mariner 9 arrived at Mars in 1971, the planet was encompassed in one of its trademark global dust storms. However, as the dust storm finally subsided, four giant ‘craters’ began to emerge. Soon, it became clear that these were not surface impact features, but calderas on top of immense mountains: the first volcanoes discovered outside the Earth. Picture credits: NASA/NSSDC ASTR 330: The Solar System: ASTR 330: The Solar System Vikings 1 & 2 Dr Conor Nixon Fall 2006 One of the most ambitious and costly ($1bn) unmanned planetary missions ever was also one of the most successful. The dual Viking orbiters/landers (2 of each) arrived at Mars in 1976. The huge landers (600 kg each, and the size of a subcompact car) contained entire weather stations which remained active for 6 years (Viking 1) and 4 years (Viking 2), much longer than designed for. Picture credits: NASA/NSSDC ASTR 330: The Solar System: ASTR 330: The Solar System Mars Density and Composition Dr Conor Nixon Fall 2006 The density is 3.9 g/cm3, or 3.8 g/cm3 uncompressed. Compare this to the Earth (4.5 g/cm3) and we expect a lower ratio of iron to silicates (rocks). Figure credit: Albert T Hsui, Univ. Ill We do believe Mars has a core of FeS (iron sulfide), with a diameter 40% of Mars: a similar proportion to the Earth’s core. However, the lower density of FeS compared to the Earth’s Fe and Ni leads to a lower overall density. ASTR 330: The Solar System: ASTR 330: The Solar System Interior and Core Dr Conor Nixon Fall 2006 Our calculations predict that the core is solid, not liquid, so we do not expect a strong magnetic field, which requires a spinning liquid core. However, magnetometers have discovered a weak magnetic field over certain regions of the planet. We guess that Mars did in fact have a liquid core and magnetic dynamo in the past, and that this has permanently magnetized some rocks. These magnetic rocks are very old, suggesting the field was only ‘on’ for the first few hundred million years of Mars’ history. Mars is of course differentiated, with a mantle and crust: we do not know much about them for certain. Figure: PSRD Hawaii, Brook Bays ASTR 330: The Solar System: ASTR 330: The Solar System Quick Tour 1: Olympus Mons Dr Conor Nixon Fall 2006 Olympus Mons (Mount Olympus) is the largest of the four great volcanoes, seen by Mariner 9. Aptly named after the mythological seat of the Greek gods, Olympus Mons is the largest volcano in the entire solar system. Olympus Mons is nearly 27 km high and 700 km wide at the base! Figure credit: NASA ASTR 330: The Solar System: ASTR 330: The Solar System Quick Tour 2: Valles Marineris Dr Conor Nixon Fall 2006 In a rare event, the giant canyon system discovered by Mariner 9 was named after the spacecraft! The ‘Mariner Valleys’ stretch more than 4000 km in length, 500 km wide, and up to 8 km deep: this would swallow up the Grand Canyon many times over. Figure credit: NASA/USGS ASTR 330: The Solar System: ASTR 330: The Solar System Quick Tour 3: Hellas Dr Conor Nixon Fall 2006 Hellas is the largest impact basin on Mars, about 2000 km across and 5 km below the average Martian surface level. Hellas often collects clouds in its interior. Hellas was produced by … you’ve guessed it, a giant impact during the Late Heavy Bombardment stage of the solar system formation, 3.9 Gyr ago. Figure credits: (left) NASA/JPL (right) MGS/MOLA Hellas has a relatively simple form, with a single rim of mountains, parts of which are missing, eroded. ASTR 330: The Solar System: ASTR 330: The Solar System Scale of Terrain on Mars Dr Conor Nixon Fall 2006 We have just seen that topography on Mars can be on a huge scale. The figure below compares Olympus Mons with Everest (fold mountain) and Mauna Loa (shield volcano,wrongly labeled) on Earth. Calculations show that on Earth, and Venus, mountains can only rise 10-15 km before the rock begins to deform under its own weight. Now can you guess why mountains on Mars can get so big? Answer: the Martian gravity is only 2/5 (40%) that of the Earth. Figure credit: Universiity of North Dakota ASTR 330: The Solar System: ASTR 330: The Solar System Mars: North and South Dr Conor Nixon Fall 2006 The northern and southern hemispheres of Mars are very different: Elevation: the north is much lower than the south, by about 6 km. Roughness: the northern hemisphere is fairly flat and smooth: the southern hemisphere is rougher. Color: the southern hemisphere is darker. Cratering: the southern hemisphere is more cratered, probably older. If we believe that the southern hemisphere accurately reflects older, original terrain, then what happened to lower the north so much? And how did the north become flatter? This is one of the greatest riddles of Mars. ASTR 330: The Solar System: ASTR 330: The Solar System Craters on Mars Dr Conor Nixon Fall 2006 Craters on Mars look much like the ones on airless planets, with raised and terraced rims, flat floors, and central peaks. However, the ejecta patterns are quite different from the lunar variety. Lunar craters have a rough, hilly blanket close to the rim, surrounded by radial streaks. Craters on Mars however display a more fluid ejecta pattern, such as the ‘flower’ form (crater Yuty, 18 km, left) at lower latitudes, or the ‘pancake’ form (crater Arandas, 28 km, above) closer to the poles. The explanation is that the Martian ejecta flowed along the surface rather than being flung through the air, probably due to melting of crustal ice. Figure credit: NASA ARC/CMEX ASTR 330: The Solar System: ASTR 330: The Solar System Argyre Dr Conor Nixon Fall 2006 At the opposite end of the crater size scale, we have huge impact basins such as Hellas and Argyre. Argyre is some 700 km wide, including a 300-km wide smooth central plain surrounded by a 200-km thick rugged rim. This picture is a cleverly shaded altimetry map, not a real image. Note the Uzboi valley entering or exiting to the north (top): evidence of ancient inflow or outflow. Figure credit: MOLA Science Team and G. Shirah, NASA GSFC ASTR 330: The Solar System: ASTR 330: The Solar System Craters, Basins and Ages Dr Conor Nixon Fall 2006 Mars has many more craters than the Earth or Venus, so the highland terrain is fairly old: at least 3.6 Gyr. Mars has fewer of the large impact basins than the Moon, despite its larger surface area. This is a fact we should explain. Our theories suggest that the crust of Mars probably stabilized (geologically) later than the Lunar surface. Mars is larger then the Moon and therefore took longer to cool, and so remained geologically active until near the end of the Late Heavy Bombardment. This allowed some of the basins which had formed before that time to be erased. ASTR 330: The Solar System: ASTR 330: The Solar System Martian Timeline from Crater Counts Dr Conor Nixon Fall 2006 Craters on Mars were caused by a similar population of impactors as craters on the Moon. By counting craters on Mars and comparing the numbers to the areal density of lunar craters, we can derive the age of various terrains. We must remember to take account of some factors which may differ between planets, such as gravity, which affects the number and speeds of impactors. Table: Morrison and Owen ASTR 330: The Solar System: ASTR 330: The Solar System The Volcanoes Dr Conor Nixon Fall 2006 The three central Tharsis volcanoes are each about 400 km across and 25 km in height. Even more modest shield volcanoes elsewhere are 100 km across. Figure credit: ESA/DLR/FU Berlin (Neukum) Each volcano has a caldera, formed when the magma retreats and the peak partially collapses. The caldera of Olympus Mons (left) is 80 km across, shows multiple episodes of collapse, but no erosion. Note the lava flow channels. Animation of Tharsis Caldera ASTR 330: The Solar System: ASTR 330: The Solar System The Volcanoes (contd.) Dr Conor Nixon Fall 2006 The shield volcanoes are predicted to be basaltic, like those in Hawaii. On the flanks of the volcanoes, lava channels about 100m in width carve the shallow broad slopes. The slope of 4° indicates a low viscosity and a large volume of outflow. However we do not see lava rivers as on Venus. Some volcanoes have steeper sides, indicating more viscous outflow, but we do not see the ‘pancake domes’ of Venus. ASTR 330: The Solar System: ASTR 330: The Solar System Tharsis Bulge Dr Conor Nixon Fall 2006 Tharsis is a massive uplifted region the size of North America, right between the northern plains and southern uplands. The Tharsis area bulges 10 km above its surroundings (figure right) and is one of the least cratered (youngest) terrains on Mars. In Tharsis are 3 of the 4 great volcanoes, and also the Valles Marineris. Olympus Mons is offset on the NW slope. Figure credit: NGDC/USGS ASTR 330: The Solar System: ASTR 330: The Solar System Tectonics and Tharsis Dr Conor Nixon Fall 2006 What is the cause of the Tharsis bulge? Tharsis appears to be due to two reasons: firstly, an actual bulge of the crust due to a mantle plume, and second, a build-up of layers of lava. How do we know? Extensive fracturing of the crust occurs in a radial pattern like spokes of a wheel. The fractures can be 100s of km in length and several km in width. We believe that the uplift began about 3 Gyr ago, and continued to 1 Gyr ago, before the formation of the actual volcanoes. On Earth and Venus, compressional forces produce uplifted terrain, such as Tibet and Lakshmi, with high mountains. On Mars however, the highest features are volcanic, not compressional in origin. ASTR 330: The Solar System: ASTR 330: The Solar System Canyons On Mars Dr Conor Nixon Fall 2006 What is the origin of the great canyon systems on Mars, including the Valles Marineris? Were they carved by rivers like the Grand Canyon? These features are in fact tectonic in origin: originally huge cracks in the crust, which were later widened and shaped by erosion. Figure credit: NASA/JPL. Viking mosaic of Western Candor Chasma ASTR 330: The Solar System: ASTR 330: The Solar System Canyon Widening Dr Conor Nixon Fall 2006 This image shows a closer view of the white box region on the previous slide. Here, the edges of the canyons, which appear similar to landslides, are clearly visible. We believe that landslides, possibly lubricated by undercutting water springs or melted ice, took the main role in widening the canyons. But where has the material gone to? Possible explanations include dust removal by wind, and ice itself which has run off and perhaps evaporated. Figure credit: NASA/JPL. Viking image of Western Candor Chasma ASTR 330: The Solar System: ASTR 330: The Solar System Channels Dr Conor Nixon Fall 2006 The Martian channels are quite distinct from the canyons. The runoff channels are positively caused by running water. The channels caused great excitement when discovered: no-where else in the solar system other than the Earth has evidence for running water been found. This Viking orbiter image shows an area between the Lunae Planum and Chryse Planitia, just west of the Viking 1 lander site. The image is 300 km across. The channels here are due to outflow. Image credit: NASA/JPL ASTR 330: The Solar System: ASTR 330: The Solar System Channel Morphology and Origin Dr Conor Nixon Fall 2006 The water channels can be split into three types: Runoff channels Outflow channels. Gullies. Runoff channels are similar to terrestrial dry river beds, found only in the cratered uplands of the southern hemisphere. They are often seen on the steep sides of crater walls, and are 10s to 100s of meters wide and 10s of km long. The runoff channels are old, as old as the cratered highlands, putting their age around 4 Gyr. This is long before the formation of the Tharsis bulge or northern plains. Clearly, the conditions for liquid water to exist on the surface have long since passed. Mars must have had a thicker, warmer atmosphere in the past. ASTR 330: The Solar System: ASTR 330: The Solar System Outflow and Floods Dr Conor Nixon Fall 2006 The outflow channels are much larger and less common than the runoff channels, and are found in the equatorial regions. Such channels are at least 10 km wide and 100s of km long. We believe these channels to have been caused by intermittent or periodic flooding. A mechanism could be the breaking of an ice dam, holding back a lake, with catastrophic results. Characteristic features of the outflow terrain includes teardrop islands, seen in the Viking image (right), terraced walls, and sandbars. These islands were carved by the flood of water rushing over original plateau terrain, descending from the uplands into the Chryse basin. Image credit: NASA/JPL ASTR 330: The Solar System: ASTR 330: The Solar System Chaos Terrain Dr Conor Nixon Fall 2006 Chaos is the name given to the jumbled mixture of hills and valleys we see in certain parts of the cratered uplands. Image credit: ESA/DLR/FU Berlin This image of the Iani Chaos comes from the HRSC on ESA Mars Express. It lies east of the Valles Marineris, and is composed of mesas 1 to 8 km across and up to a km high. ASTR 330: The Solar System: ASTR 330: The Solar System Chaos and Floods Dr Conor Nixon Fall 2006 We believe the chaos terrains are associated with the major floods and outflow channels, about 3.5 Gyr ago, after the runoff channels had been created, and about the same time as the Tharsis uplift. Underground water was probably the source of the floods, but what was the exact mechanism? Three have been proposed: Melting of sub-surface ice by volcanic activity. Chemical release of water bound to the Martian soil. Movement of liquid water, due to the uplifting of Tharsis. We do not currently know which was the culprit: perhaps all three were involved. ASTR 330: The Solar System: ASTR 330: The Solar System Gullies Dr Conor Nixon Fall 2006 Martian gullies are the best evidence we have for liquid water on Mars today. These fresh-cut features are found on the inner walls of some craters and the old runoff channels. For water to produce these features, it must have been released in a torrent, for a slow trickle will not suffice. This MGS/MOC image shows recent gullies in a crater wall. Image credit: NASA/JPL/MSSS ASTR 330: The Solar System: ASTR 330: The Solar System Gullies: explanation Dr Conor Nixon Fall 2004 The gullies are found only at high latitudes, in the shaded walls of craters and channels, some of the coldest places on Mars. Why? Our theory starts with the idea that there is permafrost under the surface at the high latitudes. Now imagine that a hot magma plume approaches from below. The permafrost is melted, perhaps even causing steam. If the water cannot reach the surface, due to over-lying rock layers, it will try to escape sideways. Some will probably re-freeze as it nears the channel or crater wall, and will slowly build up into an ice plug. Eventually, the ice plug could be released explosively as the pressure gets too much, and the liberated water will gush out. ASTR 330: The Solar System: ASTR 330: The Solar System Quiz-Summary Dr Conor Nixon Fall 2006 Are there canals on Mars? How did the idea start, and how was the issue resolved? What features were discovered by Mariner 9? What was the greatest accomplishment of the Viking missions? Does Mars have any large impact basins? Describe the main geologic features of the Tharsis uplift. How was Tharsis produced? Compare Martian volcanoes to terrestrial and Venusian mountains. Are the Valles Marineris on Mars bigger versions of the Earth’s Grand Canyon? ASTR 330: The Solar System: ASTR 330: The Solar System Quiz-Summary Dr Conor Nixon Fall 2006 Are craters on Mars the same as those on the Moon and Mercury? If not, what differences are there? What differences are there between the northern and southern hemispheres on Mars? What types of rocks were found on Mars. Is this what we expected? What three types of channels are found on Mars, and what caused them? Is there liquid water on the surface of Mars today? Compare the Martian atmosphere to that of (i) Venus (ii) the Earth. What missions are currently underway to explore Mars?