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Venus -- Our “sister” planet. Stark atmospheric / surface differences contrast with an interior that’s quite similar to Earth’s. Terra not-so-Firma The Earth’s lithosphere is fractured into several plates and the distortions in the crust happen at the edges of these plates. These plates drift slowly relative to each other over time. Continental Drift -- Fossil and Geological evidence of overlapping continents. Why do the continents remain on the surface of the Earth for long (Gyr) periods of time? A. Being rocky, they stay in one place. B. The terrestrial magnetic field keeps them at the surface. C. The ocean plates are pushed under the continents by the force of the oceans. D. The moon’s tidal forces pull them upward. E. They are less dense than the stuff beneath them. Why do the continents remain on the surface of the Earth for long (Gyr) periods of time? A. Being rocky, they stay in one place. B. The terrestrial magnetic field keeps them at the surface. C. The ocean plates are pushed under the continents by the force of the oceans. D. The moon’s tidal forces pull them upward. E. They are less dense than the stuff beneath them. The Greenhouse Effect •This is a misnomer. Agricultural greenhouses work by inhibiting convection, not what we’ll discuss. •The Earth’s atmosphere contains greenhouse gases such as water (H2O) and carbon dioxide (CO2) which are defined by their absorption of infrared light. Venus Earth Mars Visible Light incoming Infrared Light 10.6 Greenhouse Effects •Some degree of greenhouse effect is essential for life on Earth. The equilibrium temperature of Earth without any greenhouse is -15 °C. Instead it’s a balmy +15 °C •On Venus, the greenhouse effect results in almost 500 °C difference between equilibrium and actual temperatures. Losing Atmospheres Some gas is always escaping from the exosphere of the planet through thermal escape. Unless replenished, most of the atmospheres will eventually boil away. Lighter gases (e.g. He) leave terrestrial planets easily. What would be not be a consequence of the Earth’s atmosphere’s temperature increasing? A. More water would evaporate into the atmosphere. B. Fewer water molecules could undergo thermal escape. C. Helium would continue to escape the planet. D. More massive molecules would break free of Earth’s gravity. E. The atmosphere would expand slightly. What would be not be a consequence of the Earth’s atmosphere’s temperature increasing? A. More water would evaporate into the atmosphere. B. Fewer water molecules could undergo thermal escape. C. Helium would continue to escape the planet. D. More massive molecules would break free of Earth’s gravity. E. The atmosphere would expand slightly. Other losses •Solar wind blows off outer atmosphere •Giant impacts can blow off the gas layer •Chemical or phase transition onto the surface. 1 bar = standard earth atmosphere = 101,325 Pa Linking Geology to Atmospheres There is a tight link between geological activity and atmospheres mediated by two effects. 1.Volcanic activity provides outgassing, increasing the atmosphere. 2.A molten core creates a magnetic field which shields the atmosphere from solar wind, reducing the losses. As Mars dies, geologically, these two effects slow down and the planet loses its atmosphere. Losing Water Without an ozone layer, UV light dissociates water into O + H + H. Since H is low mass, it’s thermal velocity is larger than oxygen and is preferentially lost to thermal escape. The left-over oxygen then combines with other atoms to form other molecules (CO2, FeOx) Gaining atmosphere •Outgassing (the primary source) •Evaporation / chemical release •Volatile impactors such as comets. Magnetic fields Of all the terrestrial planets, only Earth has a significant magnetic field that shields it from the Solar Wind. Aurora Borealis (North) Aurora Australis (South) Planetary Magnetism •To have a strong magnetic field, three ingredients are key: a (1) rotating (2) liquid (3) metallic core. •Such a core drives the formation of a magnetic field through some seriously badass physics (dynamo theory) A crude dynamo simulation The Solar Cycle in X-ray Emission Sunspot number over time When will the thermosphere of the Earth reach its maximum temperature next? A. 2010 B. 2013 C. 2016 D. 2018 E. 2020 When will the thermosphere of the Earth reach its maximum temperature next? A. 2010 B. 2013 C. 2016 D. 2018 E. 2020 Based off solar magnetic activity, the aurora activity can be predicted. Storms happen at peak of solar activity cycle. spaceweather.com spaceweather.ca My Mass Budget: A. < 1000 kg B. 103 - 104 kg C. 104-105 kg D. 105-106 kg 6 7 E. 10 -10 kg The Jovian Planets The Jovian Planets (to scale) The big difference in composition of the Jovian planets is the dominance of H (specifically H2) and He. Jupiter and Saturn have nearly stellar compositions (70% H, 28% He, 2% other) Uranus and Neptune are mostly Hydrogen compounds (H2O, CH4, NH3) with pure H2 and He forming a smaller fraction. Note, C, N, O as basis for compounds. Jovian Planet Structure Saturn is the least dense planet (0.73 g/cm3) because it’s made of light elements (H, He). Jupiter is richer in light elements but more dense (?). This is because Jovian planets are mostly gas and so their structure is like that of terrestrial atmospheres: pressure balances gravity! What is one reason why Jupiter could be smaller, but more massive than Saturn? A. Jupiter is warmer. B. Jupiter’s composition is made of lighter elements. C. Jupiter has a liquid centre, but Saturn is a gas. D. Jupiter has a stronger magnetic field compressing it. E. Jupiter’s gravity is the strongest of all the planets. What is one reason why Jupiter could be smaller, but more massive than Saturn? A. Jupiter is warmer. B. Jupiter’s composition is made of lighter elements. C. Jupiter has a liquid centre, but Saturn is a gas. D. Jupiter has a stronger magnetic field compressing it. E. Jupiter’s gravity is the strongest of all the planets. Maximum size planet For Hydrogen and Helium planets... The structure of Jupiter is determined by the need to support its own weight and its composition. At high pressure, hydrogen becomes a metal! This means, among other things, that it conducts electricity well. Large metal core? Rapidly rotating? Magnetic Field!!!! Jupiter has a magnetic field >104 times stronger than Earth’s, i.e. a lot. The structure of Jupiter’s atmosphere. Thermosphere is heated by X-ray and UV ionization (atoms) The stratosphere is heated by UV absorption and dissociation (molecules) Convection establishes the temperature gradient in the troposhere. Bands on the Jovian planets form from the Coriolis effect breaking up convection cells. Since the Coriolis force is stronger (faster rotation) there are more bands. Neptune also shows strong storms and banding, including the “Great Dark Spot” Saturn’s weather is like that of Jupiter (high / low cloud layers). The banding is less striking since all the cloud layers occur deeper in the atmosphere. The winds on Saturn are the stronges in the Solar System but there are very few obvious storms (less internal heating?) Even Uranus shows some storms, but the weather is weak most of the time. Because of the ~90° tilt of the axis, the seasons are thus quite extreme. Recent observations indicate new storms have arisen, probably because of the “sunrise” for the Northern hemisphere. Clouds form at unique combinations of pressure and temperature. Jupiter has three main cloud “decks.” Ammonia Ammonium hydrosulfide Water Convection cells on Earth. Hot air rises and the water condenses as the air cools, forming clouds. Io Ammonia Ammonium Hydrosulfide et al. We don’t see deep enough to see water. The outer planets, Uranus and Neptune lack the hot temperature structures in their tropospheres to form Ammonia or NH4SH clouds. Instead, we see methane cloud layers, the coldest possible cloud layers. Charged particles are trapped in the magnetic fields from the planets. This creates auroras and a belt of plasma (charged particles) around the planet. All of the Jovian planets have extensive moon systems. These form out of the disks channelling material onto the forming Jovian planets. Larger than Mercury! All massive moons are spherical because of gravity, just like planets. Their geology is similar to terrestrial planets, but contain large fractions of ices because they form past the frost line! Planets have also gravitationally captured some debris. Not massive enough to become spherical. The most geologically active object in the solar system. The volcanos outgas sulfur compounds, SO2. This accounts for the yellow colour. Tidal forces with Jupiter and the other massive moons keep Io geologically active. Volcanos! Io (Jupiter) An elliptical orbit prompts tidal stretching and squishing of Io. The orbit’s ellipticity is maintained by orbital resonances with Europa and Ganymede Europa (Jupiter) has an incredibly new, geologically rich surface that appears to be mostly water ice. Geological features suggest liquid water underneath! - The surface looks a lot like glacial fracturing. - Evidence also for liquid water volcanos. - Tidal stretching keeps the interior hot and liquid. Titan (Saturn) has is the only moon to have a thick atmosphere. The atmosphere is largely nitrogen (N2) and hydrocarbons (CH4, C2H6 etc.) The hydrocarbons are significant greenhouse gases, so the surface is a little warmer than expected (-170° C). Atmospheric pressure is 1.5 times that of Earth (which is close!). The Huygens probe was dropped onto Titan to explore it. Why? Titan shows a complex chemistry; plenty of pre-biotic hydrocarbons. Moreover, there’s ample evidence for liquid-based geology on Titan, like erosion from liquid methane.