Lecture 22: Mars Percival Lowell 1 Martian Water Mystery •Lots of interesting connections to this course: –Comparative planetology of Earth/Mars –Insights into the origin of life –Application of the scientific method –Examination of the Copernican Principle –Study of crater production and meteorite orbits Mars Rovers •Evidence for Martian water! •Mars Rover Website 2 Martian Landers Martian Topography •Mars Orbiting Laser Altimeter •Mars Global Surveyor Images Water Erosion on Mars 3 Geological Variety on Mars 4 Orbit of Mars •The semi-major axis of the Martian orbit is a = 1.52 AU Mars Sun a •The eccentricity of Mar’s orbit is e = 0.093 Orbit of Mars Mars Sun a •Due to the rather large eccentricity, the distance between Mars and the Sun varies substantially during the Martian year: D = 1.38 AU (perihelion) D = 1.66 AU (aphelion) •The ratio of these distances is about 1.20 Orbit of Mars •Since the ratio of the perihelion to aphelion distances is about 1.20 Mars is much closer to the Sun during perihelion •The solar flux therefore varies by about 45% during the Martian year •This causes complications in the Martian seasons and climate Mars Mars (perihelion) Sun (aphelion) Earth 5 Orbit of Mars •At favorable opposition, the Earth-Mars distance is only about 34.65 million miles •This happened in 2004, for the first time in 60,000 years Mars Earth (favorableMars Sun opposition)(opposition) Earth Earth Mars (conjunction) Orbit of Mars •The minimum Earth-Sun distance at (favorable) opposition is 1.38 AU – 1 AU = 0.38 AU = 35 million miles •The maximum Earth-Sun distance at (unfavorable) opposition is 1.66 AU – 1 AU = 0.66 AU = 61 million miles •Hence Mars is about 1.7 times closer during favorable opposition Orbit of Mars •Kepler’s third law relates the semi-major axis a to the orbital period P 2 3 ⎛ P ⎞ = ⎛ a ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ years⎠ ⎝ AU ⎠ •Solving for the period P yields 3/ 2 ⎛ P ⎞ = ⎛ a ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ years⎠ ⎝ AU ⎠ •Since a = 1.52 AU for Mars, we obtain P = 1.87 Earth years or P = 682 Earth days 6 Bulk Properties of Mars •We have for the radius and mass of Mars Rmars = 3,397 km = 0.53 Rearth 26 Mmars = 6.4 x 10 g = 0.11 Mearth •The volume of Mars is given by 4 V = π R 3 mars 3 mars 26 3 •Hence the volume of Mars is Vmars = 1.6 x 10 cm Bulk Properties of Mars •The average density of Mars is therefore M × 26 ρ = mars = 6.4 10 g mars × 26 3 Vmars 1.6 10 cm •We obtain ρ = −3 mars 3.9 g cm •This is much lower than the density of the Earth •Hence Mars probably does not contain a great deal of iron, or a large, dense core •The lack of a dense core is consistent with the absence of a magnetic field Surface of Mars •The surface of Mars appears quite reddish •This indicates that much of the iron remains at the surface, and the color is due to iron oxide •Hence, very little differentiation occurred, and the iron did not sink to the center of the planet •Mars must have solidified before differentiation could be completed •This is probably because the planet is so small that it cooled despite the radioactive heating 7 Rotation of Mars •The surface of Mars is visible because the atmosphere is very thin •Hence the rotation of Mars can be measured by tracking surface features: •The sidereal day is 24.6 hours long (vs. 24 hours for Earth) •The rotation axis is tilted by 25.2o with respect to the orbital plane (the angle is 23.4o for Earth) Highlands and Lowlands •There are large differences between the terrain in the Northern and Southern hemispheres North: Lava-filled lowlands; 3 x 109 years old South: Heavily cratered highlands; 4 x 109 years old Surface of Mars •The surface of Mars has many interesting features, such as Volcanoes Canyons Dunes Evidence for water erosion 8 Surface of Mars •The lava flows probably occurred about 3 x 109 years ago •There was little plate tectonic activity, because the planet is small and cooled quickly •The only “continent” is the Tharsis bulge on the equator •The height of Tharsis is 10 km, and it’s about 2-3 billion years old Formation of Tharsis •How did Tharsis form? Probably due to upward convection in the mantle, which ended soon after Tharsis was uplifted Valles Marineris •Valles Marineris, located on the Tharsis bulge, is the largest canyon in the solar system •This canyon is 3,000 km long, 600 km wide, and 8 km deep (over 4 times larger than the Grand Canyon on Earth) •Valles Marineris is a tectonic crack formed during the uplifting of the Tharsis bulge (unlike the Grand Canyon, which was formed by water erosion) 9 Martian Topography •Martian Topography Movie Martian Volcanism •The largest volcano is Olympus Mons: Height = 25 km (3 times taller than Everest) Base = 700 km (about the Texas) •There is no evidence for ongoing plate tectonics •The Martial volcanoes are shield volcanoes sitting on top of hot spots in the mantle 10 Seasons on Mars •The seasons on Mars are complicated by the large eccentricity of the orbit, which causes substantial variations in the solar flux on the surface •Mars is closest to the Sun during summer in the Southern hemisphere Mars (perihelion) Sun Earth Seasons on Mars •Mars is farthest from the Sun during winter in the Southern hemisphere •Mars is closest to the Sun during summer in the Southern hemisphere •Therefore, the seasons in the South are extreme and the seasons in the North are mild Mars Mars Sun (perihelion) (aphelion) Earth Seasonal Variation of Polar Caps carbon dioxide and water ice at polar caps 11 Atmosphere of Mars •The surface temperature on Mars was measured by instruments on board the Viking landers •They indicate a low, variable temperature •This is due to the thin atmosphere Atmosphere of Mars 12 Atmosphere of Mars •Atmospheric composition: •Venus: Carbon Dioxide -- 95.3% Carbon Dioxide: 96.5% Nitrogen -- 2.7% Nitrogen: 3.5% Argon -- 1.6% Trace gases: < 0.01% Oxygen -- 0.13% Almost no oxygen CO -- 0.07% Practically zero water H2O -- 0.03% Clouds are sulfuric acid Water ice clouds •Though somewhat similar in composition, the atmosphere of Mars is about 10,000 times thinner than Venus’ (150 times thinner than Earth’s) •Mars is much colder (250 K) than Venus (750 K) 13 Water on Mars •Liquid water once existed on the surface of Mars •We see runoff channels and outflow channels •There was a huge flood in the distant past from south to north •The flow rate was equivalent to 100 Amazon Rivers! •All of this water is probably now locked up in the soil as permafrost •Mars Rover Opportunity finds Evidence for Ancient Sea on Mars 14 Martian Topography 15 16 Martian Atmosphere •The primary atmosphere (mostly hydrogen) was lost to space due to the low mass of Mars •When the secondary atmosphere formed, water and CO2 were out-gassed from volcanoes •As the planet cooled, the water froze out of the atmosphere onto the surface •About 109 years later, volcanoes melted the ice, creating a gigantic flood of water and lava from south to north Martian Atmosphere •The flood ran from the highlands in the South to the lowlands in the North •The volcanic activity stopped, and the CO2 was absorbed by the water •This was followed by a period of strong cooling, and the development of the permafrost •We call this process the “Reverse Runaway Greenhouse Effect” •Mars is now in a permanent deep-freeze, but the atmosphere could rise again if heating occurs… Life on Mars? •Many potato-sized meteorites from Mars have been found in Antarctica 17 Life on Mars? •Gases trapped in bubbles in the rock are identical in composition to samples obtained by Viking in 1976 •Hence the meteorites are almost certainly from Mars Life on Mars? •The rocks contain organic polycyclic aromatic hydrocarbons (PAH’s), often found in decaying microbes on Earth •There are also small structures in the meteorites that look like micro-fossils… Astrobiology Website Scenario for Life on Mars •Microscopic life began on Mars when the atmosphere was thick, warm, and dense •Liquid water was still abundant on (or just under) the surface •Some of the microbes became fossilized in rock •A large meteor or asteroid impact on Mars launched the rock into space •The meteorite traveled through the solar system for 15,000,000 years •It landed in Antarctica about 13,000 years ago 18 Questions •Is the rock really from Mars? ¾Probably, based on the composition of the trapped gas •Could the rock have been contaminated with organic materials while on Earth? ¾Probably not, since the concentration of the materials increases towards the center of the sample •Were the polycyclic aromatic hydrocarbons produced in space via radiation exposure? ¾Probably not, because the composition is different from chemicals seen in other meteorites Questions •Why are the fossils so small? ¾We’re not sure •Why do we see similar structures in lunar rock samples brought back by the Apollo astronauts? ¾This is a strong argument against evidence for life •The debate continues ¾Several new missions are planned or already on their way to Mars to try to answer these questions •There may also be other places in the solar system where we should look for life (perhaps Saturn’s moon Titan?) Martian Moons •Mars has two very small moons, Phobos (“fear”) and Deimos (“panic”) Phobos is 28 km long Deimos is 16 km long •Both are probably captured asteroids •They orbit very close to Mars: Phobos at distance 9,378 km from Mars Deimos at distance 23,459 km from Mars Recall that Rmars = 3,397 km •Phobos is so close to Mars that it travels faster than Mar’s spin rate •Hence Phobos rises in the West and sets in the East! 19 20.