ASTR 330: The Solar System

1. ASTR 330: The Solar System • 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) Mars Facts and Figures Dr Conor Nixon Fall 2006 Figure credit: Albert T Hsui, Univ. Ill

2. ASTR 330: The Solar System Lecture 16: Mars I Mars historical perspective Dr Conor Nixon Fall 2006 Picture credit: NASA/JPL - Viking

3. ASTR 330: The Solar System 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: Overview • 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. Dr Conor Nixon Fall 2006 Picture credit: NASA

4. ASTR 330: The Solar System 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. Telescopic Observations Of Mars Dr Conor Nixon Fall 2006

5. ASTR 330: The Solar System 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. The Mars Saga: 1877-1920s Dr Conor Nixon Fall 2006 Picture credits: SPL/Photo Researchers. Lowell Observatory

6. ASTR 330: The Solar System 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. Lowell’s Contribution Dr Conor Nixon Fall 2006

7. ASTR 330: The Solar System 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. No canals, but… Dr Conor Nixon Fall 2006

8. ASTR 330: The Solar System 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. Early Spacecraft visits Dr Conor Nixon Fall 2006 Picture credits: NASA/NSSDC

9. ASTR 330: The Solar System 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. Mariner 9 Dr Conor Nixon Fall 2006 Picture credits: NASA/NSSDC

10. ASTR 330: The Solar System 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. Vikings 1 & 2 Dr Conor Nixon Fall 2006 Picture credits: NASA/NSSDC

11. ASTR 330: The Solar System 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). 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. Mars Density and Composition Dr Conor Nixon Fall 2006 Figure credit: Albert T Hsui, Univ. Ill

12. ASTR 330: The Solar System 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. Interior and Core Dr Conor Nixon Fall 2006 Figure: PSRD Hawaii, Brook Bays

13. ASTR 330: The Solar System 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! Quick Tour 1: Olympus Mons Dr Conor Nixon Fall 2006 Figure credit: NASA

14. ASTR 330: The Solar System 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. Quick Tour 2: Valles Marineris Dr Conor Nixon Fall 2006 Figure credit: NASA/USGS

15. ASTR 330: The Solar System 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. Hellas has a relatively simple form, with a single rim of mountains, parts of which are missing, eroded. Quick Tour 3: Hellas Dr Conor Nixon Fall 2006 Figure credits: (left) NASA/JPL (right) MGS/MOLA

16. ASTR 330: The Solar System 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. Scale of Terrain on Mars 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. Dr Conor Nixon Fall 2006 Figure credit: Universiity of North Dakota

17. ASTR 330: The Solar System • 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. Mars: North and South Dr Conor Nixon Fall 2006

18. ASTR 330: The Solar System 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 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. Dr Conor Nixon Fall 2006 Figure credit: NASA ARC/CMEX

19. 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. ASTR 330: The Solar System Argyre Dr Conor Nixon Fall 2006 Figure credit: MOLA Science Team and G. Shirah, NASA GSFC

20. ASTR 330: The Solar System 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. Craters, Basins and Ages Dr Conor Nixon Fall 2006

21. ASTR 330: The Solar System 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. Martian Timeline from Crater Counts Table: Morrison and Owen Dr Conor Nixon Fall 2006

22. ASTR 330: The Solar System 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. 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. The Volcanoes Animation of Tharsis Caldera Dr Conor Nixon Fall 2006 Figure credit: ESA/DLR/FU Berlin (Neukum)

23. ASTR 330: The Solar System 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. The Volcanoes (contd.) Dr Conor Nixon Fall 2006

24. ASTR 330: The Solar System Tharsisis 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. Tharsis Bulge Dr Conor Nixon Fall 2006 Figure credit: NGDC/USGS

25. ASTR 330: The Solar System 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. Tectonics and Tharsis Dr Conor Nixon Fall 2006

26. ASTR 330: The Solar System 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. Canyons On Mars Dr Conor Nixon Fall 2006 Figure credit: NASA/JPL. Viking mosaic of Western Candor Chasma

27. ASTR 330: The Solar System 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. Canyon Widening Figure credit: NASA/JPL. Viking image of Western Candor Chasma Dr Conor Nixon Fall 2006

28. ASTR 330: The Solar System 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. Channels Dr Conor Nixon Fall 2006 Image credit: NASA/JPL

29. ASTR 330: The Solar System • 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. Channel Morphology and Origin Dr Conor Nixon Fall 2006

30. ASTR 330: The Solar System 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. Outflow and Floods 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. Dr Conor Nixon Fall 2006 Image credit: NASA/JPL

31. ASTR 330: The Solar System Chaos is the name given to the jumbled mixture of hills and valleys we see in certain parts of the cratered uplands. 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. Chaos Terrain Dr Conor Nixon Fall 2006 Image credit: ESA/DLR/FU Berlin

32. ASTR 330: The Solar System • 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. Chaos and Floods Dr Conor Nixon Fall 2006

33. 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. ASTR 330: The Solar System Gullies Dr Conor Nixon Fall 2006 Image credit: NASA/JPL/MSSS

34. ASTR 330: The Solar System 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. Gullies: explanation Dr Conor Nixon Fall 2004

35. ASTR 330: The Solar System • 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? Quiz-Summary Dr Conor Nixon Fall 2006

36. ASTR 330: The Solar System • 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? Quiz-Summary Dr Conor Nixon Fall 2006