The Sun and Planets Lecture Notes 6.

Transcription

1 The Sun and Planets Lecture Notes 6. Lecture 6 Venus 1 Spring Semester 2017 Prof Dr Ravit Helled

2 Cover photo: Venus in true color (Courtesy of NASA) Venus Properties Venus is the second brightest natural object in the night sky after the Sun and Moon. The brightness of Venus is caused by its relatively small distance to the Sun and the fact that it s surrounded by thick clouds that reflect 75% of the sunlight (i.e., it has a high Albedo). The semi-major axis of Venus (i.e., its distance from the Sun) is km (0.723 AU). It is the second closest planet to the Sun and the closest planet to Earth. One year on Venus (its orbital period), the time it takes to complete one orbit around the Sun, is 243 days. A day on Venus (its rotation period), or the time it takes to complete one rotation about its axis, is 225 days (almost the same as its year). Venus is very similar to Earth in terms of size (only 5% smaller), mass, density, and composition. The surface gravity on Venus is almost the same as on Earth (0.9g). Internal Structure The internal structure of Venus is remarkably similar to Earth. Venus has a core, mantle, and crust. Venus core is at least partially liquid. Venus smaller size relative to the Earth means that the pressure is 24% lower in its deep interior. Figure 1: Interal structure of Venus compared to that of Earth. 2

3 Surface Venus has 167 large volcanoes that are over 100 km across. The only volcanic complex of this size on Earth is the Big Island of Hawaii. This is not because Venus is more volcanically active than Earth, but rather because its crust is older. Earth s oceanic crust is continentally recycled by subduction at the tectonic plate boundaries and has an average age of 100 million years. Venus surface is a few hundred million years old. Venus surface consists of impact craters, mountains, and valleys. Special geological features on Venus include: Flat-topped volcanic features called farra, which resemble pancakes with diameters of km and are meters high. Radial, star-like fracture systems called novae. Spider web-like features, known as arachnoids. Circular rings of fractures, sometimes surrounded by a depression, called coronae. All of the above features are volcanic in origin. Volcanism Are Venus volcanoes still active? Some volcanoes should still be active on Venus, although we have not observed any eruptions directly. However, the presence of sulfur dioxide (SO 2 ) in Venus atmosphere suggest that volcanic outgassing is still occurring on Venus at least on geological timescales (i.e., within the past 100 million years). SO 2 is gradually removed from the atmosphere by chemical reactions with surface rocks. In 2008/2009, ESA s Venus Express spacecraft detected hotspots (at infrared wavelengths) that are associated with lava flows resulting from volcanic eruptions. Observations of the Venusian Surface Venus thick clouds prevents us from seeing its surface with visible light. Therefore, geological features on Venus surface are discovered using radar, since radio waves are able to pass through the clouds. Between 1990 and 1993, the Magellan spacecraft used radar to map the surface of Venus. 3

4 Figure 2: Topographic map of the Venus as mapped by the Magellan spacecraft (Mercator projection). Atmosphere Composition The atmosphere of Venus is 96.5% carbon dioxide (CO 2 ), 3.5% nitrogen, and traces of several other elements and molecules (see Figure 3). Sulfuric acid is present in the atmosphere as the result of volcanic outgassing in the absence of rainfall. Temperature & Pressure The mean surface temperature on Venus is an unpleasant 735 K (462 C). At the surface, the pressure is a crushing 90 bars (roughly equivalent to an ocean depth of 886 meters). However, at an altitude of 50 km, the temperature and pressure are Earth-like. Weather The weather on Venus is nearly unchanging. The slow rotation of the planet leads to little wind, with maximum wind speeds only reaching 6 km/h. Venus has no seasons because the planet s rotational axis is not tiled relative to the Sun (ψ = 0). Therefore, temperatures remain the same all year. 4

5 Figure 3: Elemental composition of Venus atmosphere measured in PPM (Parts Per Million). An atmosphere is a layer of gases surrounding a planet or other planetary object. This gas is held in place by the gravity of the object. Since a collection of molecules may be moving at wide range of velocities, there will always be some molecules travelling fast enough to escape the object (v molecule > v escape ). This produces a slow leakage of gas into space. An atmosphere is more likely to be retained if its gravity is high and the atmospheres temperature is low. Sources of atmospheric gas: 1. Outgassing: Volcanoes (and possibly accretion of volatile planetesimals) 2. Evaporation/sublimation: evaporation of surface liquids and ices 3. Surface ejection: tiny impacts of micrometeorites Losses of atmospheric gas: 1. Condensation: e.g., rain and snow 2. Chemical reactions: iron rust removing oxygen from the atmosphere (a) Solar wind stripping: Without a magnetosphere, particles from the solar wind will gradually carry away gas particles from the upper atmosphere. (b) Thermal escape: If a gas atom/molecule reaches a high enough velocity (v molecule > v escape i.e., it becomes hot enough), it can escape the planet. 5

6 How do atmosphere s affect planets? Atmospheres create pressure that determines whether or not liquid water (H 2 O) can exist on a planet s surface. Atmospheres absorb and scatter light. Scattering makes daytime skies bright on planets with atmospheres, and absorption prevents (some) radiation from reaching the surface. Atmospheres can create wind and weather. Interactions between atmospheric gases and the solar wind creates a protective magnetosphere around planets with strong magnetic fields. Atmospheres can make planetary surfaces warmer than they would be otherwise via the greenhouse effect. The Greenhouse Effect The main heat source for the Venusian atmosphere is radiation from the Sun (this is true for all planets in the Solar System). The radiation from the Sun is primarily at visible wavelengths, where the Sun is the brightest (recall blackbody radiation and Wien s law). Cooling of the atmosphere occurs via radiation to space (radiative cooling). outgoing radiation is primarily at infrared wavelengths. This The final temperature is determined by the equilibrium point, whereby the heating rate is balanced by the cooling rate. Certain gases act like a planet-sized blanket, in that they act to keep the planet warm by partially preventing radiative cooling (at infrared wavelengths). These are known as the greenhouse gases e.g. carbon dioxide (CO 2 ), water vapor (H 2 O), and methane (CH 4 ). The magnitude of warming due to these greenhouse gases is called the greenhouse effect. On Earth, the greenhouse effect is 30 C, whereas on Venus it is much stronger at 400 C. The surface temperature on Venus is 467 C. This makes Venus surface temperature the highest in the Solar System even hotter than Mercury despite being more than twice the distance from the Sun (and therefore receiving only 25% of Mercury s solar irradiance). 6

7 The Greenhouse Effect on Earth The water vapor (H 2 O) and carbon dioxide (CO 2 ) that is found naturally in Earth s atmosphere keeps Earth warmer than it would otherwise be. Our relatively clear atmosphere allows incoming sunlight (at visible wavelengths) to penetrate the atmosphere and warm Earth s surface. At the same time, the surface radiates energy as infrared radiation, which is absorbed by the water vapor and CO 2 in the atmosphere. Figure 4: Illustration of the Greenhouse Effect on Earth. Why did Venus get hot? Is it hot because of its small distance to the Sun? No. Despite its small distance to the Sun, its high Albedo (due to its clouds) would make Venus cold. Venus became too hot to develop liquid oceans. Without oceans to dissolve outgassed CO 2 and subsequently lock it into carbonate rocks all of the CO 2 remained in the atmosphere, which resulted in its intense greenhouse effect. 7

8 The Evolution of Venus Atmosphere Billions of years ago, liquid water could have been existed on Venus surface as oceans. During this time, CO 2 would have able to dissolve in the oceans (removing it from the atmosphere). N 2 would have been in the atmosphere. However, an increase in the Sun s radiation led to an increase in surface temperature, which in turn led to the evaporation of the oceans. The result was that water vapor (H 2 O) and carbon dioxide (CO 2 ) moved into the atmosphere. Remember that both of these are greenhouse gases. Photodissociation in the upper atmosphere by solar radiation leads the thermal escape of atomic hydrogen, e.g. H 2 O + hγ (sunlight) 2H + O The water (H 2 O) molecule is split into two hydrogen atoms (H) and an oxygen atom by sunlight. Whereas a H 2 O molecule is too heavy to escape the planet, the two individual hydrogen atoms can easily escape. Therefore, when water is in the atmosphere, some of it will be destroyed (photodissociated) and the resulting hydrogen will escape to space. Deuterium ( 2 H) is a stable isotope of hydrogen, but is heavier than atomic hydrogen. Therefore, the lighter atomic hydrogen will escape more easily to space than deuterium. Therefore, we use the deuterium-to-hydrogen ratio as a measure of how much hydrogen has been lost to space as a result of photodissociation and thermal escape. This has led to a D/H (Deuterium-to-Hydrogen) ratio on Venus that is 100 higher than that on Earth. This means that more than 99% of Venus water has been lost! Therefore, Venus is no longer habitable despite its similarity to Earth. 8