Mars ( ) The Sun and Planets Lecture Notes 6. Spring Semester 2018 Prof Dr Ravit Helled

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The Sun and Planets Lecture Notes 6. Spring Semester 2018 Prof Dr Ravit Helled Mars ( ) Mars is the fourth planet from the Sun and the outermost terrestrial planet. It has a density of 3.

It takes Mars 687 days to go around the Sun, or about twice as long as it takes the Earth. Its diameter is just over half that of the Earth (D = 60 787 km). Mars is much colder than Earth.

1 The Sun and Planets Lecture Notes 6. Spring Semester 2018 Prof Dr Ravit Helled Mars ( ) Mars is the fourth planet from the Sun and the outermost terrestrial planet. It has a density of 3.93 g/cm3, which is somewhere between the density of the Earth and Moon. Its rotational period is 24 hours and 40 minutes nearly the same as the Earth. It takes Mars 687 days to go around the Sun, or about twice as long as it takes the Earth. Its diameter is just over half that of the Earth (D = km). Mars is much colder than Earth. As a result, most of the water on Mars is frozen in subsurface permafrost. Its surface atmospheric pressure is so low that liquid water cannot exist on the surface. However, there are indications of cyclical climatic variations in the past, presence of surface runoffs, and intense volcanism. Mars is the only planet (besides Earth) showing evidence of having had pools of liquid water on its surface. 1

Atmosphere Mars has a thin atmosphere. The atmospheric pressure at the Martian surface is less than 1% of that at Earth s surface. This low pressure renders liquid water unstable on its surface.

2 Atmosphere Mars has a thin atmosphere. The atmospheric pressure at the Martian surface is less than 1% of that at Earth s surface. This low pressure renders liquid water unstable on its surface. The Martian atmosphere is composed mostly of carbon dioxide (CO 2 ; 95.4%), followed by nitrogen (N 2 ; 2.7%), argon (Ar; 1.6%), and carbon monoxide (CO; 0.07%). No sulfuric or other acids have been detected. Figure 1: Martian topography Why is Mars sky red? The air on Mars scatters incoming sunlight, scattering blue light just as on Earth. However, since there is less air, it scatters less light than on Earth. By itself, this scattering would make the Martian sky a deep blue. Martian winds and dust storms bring dust into the air. Dust has the opposite effect. It absorbs blue light. By itself, the dust will create a sky with brown-pink color. The actual color of the Marian sky comes from the combination of these two effects. Most of the time, the daytime sky has yellow-brown color. Sometimes, the sky has a different color such as blue or green particularly in mornings and evenings. Climate The global average temperature on the Martian surface is 50 C ( 58 F). Mars has seasons like Earth due to its similar axis tilt, but they last twice as long because the Martian year lasts roughly two Earth years. The seasons on Mars create weather (e.g., 2

3 winds blowing from the summer pole to the winter pole). The eccentric shape of Mars orbit plays an important role in its climate. Because Mars orbit is more elliptical, it travels relatively closer to and farther from the Sun. Therefore, Mars has more extreme seasons. Weather Pole-to-Pole Winds Temperatures at the winter pole reach 130 C, at which temperature carbon dioxide (CO 2 ) condenses into dry ice at the polar cap. At the summer pole, higher temperatures cause all of the frozen carbon dioxide to sublimate into its gaseous state. This causes the atmospheric pressure to increase at the summer pole, resulting in pole-to-pole winds (winds from the high-pressure summer pole towards the lower-pressure winter pole). Dust Devils The strong winds discussed above can carry and shift the Martian surface dust. One side effect of this interaction between dust and wind are dust devils. A dust devil is a strong, well-formed, and relatively long-lived whirlwind. They range in scale from small (half a meter wide and only a few meters tall) to large (10+ meters wide and meters tall). Eventually this dust settles back onto the surface and changes the color of different areas. Obliquity Theoretical calculations suggest that Mars obliquity (i.e., axis tilt) varies from 0 to 60 or more it is currently 25 on timescales of thousands to millions of years. Mars obliquity changes due to the tiny but persistent gravitational tugs of the other planets. However, only Jupiter has a significant effect on Mars tilt, due to its mass and proximity. While Earth s tilt is stabilized by the gravity of the Moon, Mars moons (Diemos and Phobos) are far too small to stabilize its tilt. Changes in the tilt affect the seasons and the global average temperature. When Mars tilt is small, the polar caps stay frozen year round and the atmosphere becomes thinner, lowering the pressure and weakening the greenhouse effect. When the axis is highly tilted, the summer pole becomes quite warm, and water ice sublimates along with carbon dioxide (CO 2 ) the atmospheric pressure increases, and Mars becomes warmer. The Martian polar regions show layering of dust and ice that probably reflects changes in climate due to the changing axis tilt. 3

Geology The Martian core is 2 400 km in diameter, or 40% of the diameter of planet (this is the same as the Earth in relative terms).

4 Geology The Martian core is km in diameter, or 40% of the diameter of planet (this is the same as the Earth in relative terms). The Martian core is composed of iron sulfide (FeS) rather than an iron-nickel alloy (FeNi). The core is solid rather than liquid and therefore Mars lacks a strong magnetic field. While Mars has tectonic features, there is no global tectonic activity as on Earth. On Mars, the southern hemisphere is characterized by high elevation and is scarred by many impact craters. In contrast, the northern plains show a few impact craters and tend to be below the average Martian surface level. The prevalence of impact craters in the southern highlands can be explained by the fact that the southern highlands are older. Volcanism Volcanism is the most important process in regards to erasing ancient craters on Mars. The tallest volcano in the Solar System is on Mars and is known as Olympus Mons. It is 600 km across (the size of Arizona) and rises 26 km above the surface. Figure 2: Olympus Mons Olympus Mons and other volcanoes are located in a region known as the Tharsis Bulge, an area that was probably formed by a plume of rising mantle material. This rising material provided the molten rock for the volcanoes. 4

5 Are Mars volcanoes still active? Mars is expected to have less volcanism than Earth due to its smaller size and colder interior. Impact craters on the volcanoes slopes suggest that the volcanoes have not been active in the last ten million years. However, 10 million years is not very long in geological timescales. In addition, meteorites coming from Mars show evidence for molten lava that is only 180 million years old. This suggests that active volcanism is still possible! Mars interior keeps cooling and its lithosphere is thickening. Therefore, Mars will become geologically dead in a few billion years. Topography Apart from Earth, Mars has the most highly varied and interesting surface of any of the terrestrial planets. Much of it is quite spectacular. For example, Olympus Mons is the largest mountain in the Solar System and Valles Marineris is the largest canyon system in the Solar System. Earth and Mars have roughly the same amount of land area (surface not covered by water in the case of Earth). On Mars, the southern hemisphere is higher than northern hemisphere by roughly 6 km. The southern region is more heavily cratered. Much of the Martian surface is very old and cratered, but there are also much younger rift valleys, ridges, hills and plains. A few of the most interesting geological features on Mars are: The Tharsis Bulge is a huge bulge on the Martian surface that is about km across and 10 km high. Valles Marineris is a system of canyons km long and 2 7 km deep. It is tectonic in origin and not a result of flowing water like the Grand Canyon on Earth. Hellas Planitia is an impact crater in the southern hemisphere that is over 6 km deep and km in diameter. Olympus Mons is the tallest mountain in the Solar System. For reference, the largest shield volcano on the Earth is Mauna Loa, which is only about 200 km across its base, and the 25 km height of Olympus Mons is about 2.5 times the height of Mount Everest. Note the tall cliff around its rim and the central volcanic crater from which lava erupted. 5

6 Figure 3: Valles Marineris Water Today, Mars has no liquid water on its surface. It s too cold liquid water would freeze immediately to ice. Close to the equator, where temperatures are warmer, liquid water would evaporate due to the low air pressure. Liquid water is unstable on the Martian surface. However... While impacts, volcanism, and tectonics explain most of the major geological features on Mars, a closer look reveals erosional features. Erosionally created channels appear to have been carved by running water, although we do not know whether the water came from runoff after rainfall, from erosion by water-rich debris flows, or an underground source. This implies that water was once present on the Martian surface but where is the water now? Most of the water was probably lost to space. However, a significant amount of water still remains, frozen at the polar caps and in the top meter of the surface soil. Water ice and, pontentially, liquid water may lie deep underground. Liquid water can be near sources of volcanic heat, providing a potential home to microscopic life. It is still possible that water ice melts and flows for a short time due to volcanic heat until it either freezes or evaporates. 6

7 Outflow Channels Outflow channels are thought to be the result of catastrophic flows, usually occuring in equatorial regions. The larger and more catastrophic channels usually run from the southern highlands (old) into the northern plains (young). These outflow channels are more than 10 km in width and hundreds of kilometers long. Streamlined flow features in these channels argue for a flowing water origin. The scale of these features indicate catastrophic flows. This large outflow channel, Ma adim Vallis, is at least 700 km long. Ma adim Vallis formed in the ancient Martian highlands and may be as much as 3.5 billion years old. Figure 4: Ma adim Vallis Why did Mars change? Mars changed from a world of flowing water to the cold and dry planet we see today. What happened? In the past (billtions of years ago), Mars had a much denser and warmer atmosphere. In contrast, for Mars to support liquid water again, it would need 400 times its current level of CO 2 (to drive a greenhouse effect and increase the global average temperature). It is possible that, in the past, volcanoes on Mars could have supplied the atmosphere with a sufficient amount of CO 2, as well as a significant amount of water. However, Mars lost its atmosphere until it reached it s current frozen state. Some of the CO 2 froze onto the poles or was bound into surface rocks but most of it escaped to space. The reason Mars atmosphere escaped relatively quickly is related to its magnetic field. Namely, its magnetic field is far too weak today. In the past, Mars most likely had a molten core which generated a global magnetic field. The magnetosphere supported by this magnetic field would have protected the atmosphere. But, as Mars cooled, the magnetic field weakened and core convection eventually ceased. At that point, solar wind particles stripped gas from Mars atmosphere. 7

Spirit & Opportunity Opportunity, 2004 2014, NASA Opportunity landed in the Meridiani Plana.

8 Spirit & Opportunity Opportunity, , NASA Opportunity landed in the Meridiani Plana. Rocks near its landing site were found to contain tiny spheroids ( blueberries ), which are thought to have formed in standing water, or possibly by groundwater percolating though rocks. Analysis shows that the blueberries contain minerals that form in water, possibly in an environment such as a sea or ocean. Moreover, layering of the sedimentary rocks suggest a changing environment resulting from waves and/or winds. The measurements made by Oppurtunity provide convincing evidence for water in Mars past! Spirit, , NASA Spirit landed three weeks before Oppurtunity and on the other side of the planet. In 2009, the rover became stuck and its last communication with Earth was in However, before its mission ended, it also uncovered potential evidence of water on Mars. The Spirit science team analyzed results from the rover and hypothesized that water, perhaps as snow melt, trickled into the subsurface fairly recently and on a continuing basis. Figure 5: Opportunity looking back at what remains of its descent vehicle (a.k.a. a selfie on Mars). 8

9 Figure 6: Robotic missions to Mars. 9

Lecture Outlines. Chapter 10. Astronomy Today 8th Edition Chaisson/McMillan Pearson Education, Inc.

Lecture Outlines. Chapter 10. Astronomy Today 8th Edition Chaisson/McMillan Pearson Education, Inc. Lecture Outlines Chapter 10 Astronomy Today 8th Edition Chaisson/McMillan Chapter 10 Mars Units of Chapter 10 10.1 Orbital Properties 10.2 Physical Properties 10.3 Long-Distance Observations of Mars 10.4