Today in Astronomy 111: Venus and Earth Earth as a planet Venus, Earth and atmospheric circulation: Hadley cells. Venus, Earth and the greenhouse effect: one dead planet, global warming, and ocean acidification. Distinctive features of Earth Earth and Venus, from • Life and its influence on the Galileo and geology and the atmosphere Magellan missions, • Plate tectonics respectively (JPL/NASA). Click to • The core and the global spin the planets. magnetic field 22 September 2011 Astronomy 111, Fall 2011 1 Tomorrow afternoon, at 1:04 PM EDT – very close to true noon – the Sun crosses the celestial equator again: it reaches the autumnal equinox. Photo: Joe Orman Within a few minutes: what will the sidereal time be, at true midnight (1AM EDT) tomorrow night? A. 0h B. 3h56.4m C. 6h D. 1h04m E. 12h F. 23h 22 September 2011 Astronomy 111, Fall 2011 2 Mass 5.9736× 1027 gm Earth’s Equatorial radius 6.3781× 108 cm vital statistics Average density 5.515 gm cm-3 Moment of inertia 0.33MR2 Full Earth, seen from Clementine (US DoD) Albedo 0.37 Effective temperature 254.3 K 1.49597887147× 1013 cm Orbital semimajor axis (1.00000011 AU) Orbital eccentricity 0.01671022 Sidereal 365.256 days revolution period Sidereal 23.9345 hours rotation period Length of day 24.0000 hours Magnetic field 0.5 gauss 22 September 2011 Astronomy 111, Fall 2011 3 Venus, Earth and atmospheric circulation In 1735, George Hadley, who was interested in explaining the direction and steadiness of the trade winds, extended an earlier suggestion by Edmund Halley. Warm air is less dense than cold air; if one makes a warm (cold) “bubble” of air somewhere in the atmosphere, it will rise (sink) through the rest of the atmosphere. The ground, and the air adjacent to it, is warmer at the equator than at the poles, because of the different incidence angles of sunlight. Therefore there should be N-S circulation, with warm air at high elevations flowing toward the poles, and cooler air at the surface flowing toward the equator. 22 September 2011 Astronomy 111, Fall 2011 4 Atmospheric circulation (contd.) Cool air sinking N. pole From Sun Equator Warm air rising 22 September 2011 Astronomy 111, Fall 2011 5 Atmospheric circulation (contd.) So far, this is Halley’s idea, and doesn’t explain the direction of the trade winds, which tend to be easterly between the tropics. But Hadley realized that the N-S flow must be modified, since the planet and atmosphere are rotating about the axis, at higher speeds closer to the equator, this speed given by: 2π R⊕ vr = cosλ (λ = latitude) day • At the equator (λ = 0) Earth’s rotational speed is 0.46 km/sec, but at λ = ±45º, it’s down to 0.33, and at ±75º, 0.12 km/sec, decreasing to zero at the poles. 22 September 2011 Astronomy 111, Fall 2011 6 Atmospheric circulation (contd.) The Earth rotates counterclockwise, as viewed from the North. As warm air is pushed north or south, it finds itself moving into slower air, and each bubble of it moves out somewhat ahead of the normally-rotating airmass. Thus the warm flow turns toward the rotation direction as it goes. Underneath, the cooler return flow goes the other way. Warm Cool (high- (surface) altitude) flow flow Direction of rotation 22 September 2011 Astronomy 111, Fall 2011 7 Atmospheric circulation (contd.) Between cooling and turning, the warm high-altitude flow only makes it to latitude ±30º before sinking, driving surface flow back the way the warm flow came. This circulation pattern is called a Hadley cell. Taking advantage of the extreme difference between rotational speed and solar heating from the poles to latitudes just below, another Hadley cell, usually called the polar cell to distinguish it from the equatorial one, works the same way between latitudes ±60º and the poles. In between (i.e. in the temperate zones) a circulation pattern is driven by the Hadley cells that has the opposite sense of these two, counter to rotation. This one’s called the Ferrel cell. 22 September 2011 Astronomy 111, Fall 2011 8 Atmospheric circulation (contd.) The Northern hemisphere prevailing-wind system (SPaRCE/EVAC/U. Oklahoma). 22 September 2011 Astronomy 111, Fall 2011 9 Aside: Columbus actually knew some of this As we’ve discussed in recitation, Columbus made very good time – considering the nature of his ships – and made nearly perfect use of the trade winds (outbound) and westerlies (return), indicating that he knew to expect these patterns to persist all the way around the globe. Voyage #1, for example: Westerlies 30ºN Calspace/UCSan Diego 22 September 2011 Astronomy 111, Fall 2011 10 Another aside: Hadley was on to something Hadley turned out to be correct about the effects of the Earth’s rotation on circulation. About 100 years later, it was described theoretically in accurate detail by Gaspard Coriolis, and we’ve called it the Coriolis force ever since. The Coriolis force, like the more-familiar centrifugal force, is the effect of inertia and motion within an accelerating reference frame. It’s given by ω Fv=2m( ×ω ) = angular velocity of reference frame Coriolis v = velocity within reference frame As such, the Coriolis force is fictitious: it’s an artifact of the acceleration of one’s surroundings. 22 September 2011 Astronomy 111, Fall 2011 11 Atmospheric circulation (contd.) All this makes for a famous distinction of Venus. It turns out that the faster a planet rotates, the more bands of alternating Hadley and Ferrel cells are obtained in the atmosphere. By the same token, a slow enough rotator would have only one, ideal-looking, Hadley cell per hemisphere, stretching all the way from the equator to the poles. This is the situation of Venus, the slowest rotator among the planets. • Most of the air circulation on Venus is north-south: by far the simplest atmospheric structure of the planets. • This is why the surface temperature on Venus is so uniform –as hot at the poles as at the equator, and as hot at night as during the day. 22 September 2011 Astronomy 111, Fall 2011 12 Venus, Earth and the greenhouse effect Venus is also famous for having a dense, very dry atmosphere that makes it the exemplar of the greenhouse effect, as was first realized by Rupert Wildt (1940) and first explained in detail by Carl Sagan (early-mid 1960s). Venus probably started off with the same ingredients as Earth, meaning that it had water after the surface cooled. As we’ve mentioned, the water was mostly from asteroids and comets, rather than its original ingredients, or the pre-solar nebula. But the atmospheres of Earth and Mars also endow their planets with a substantial greenhouse effect: their surfaces are also warmer than they would be just from solar illumination and blackbody radiation. 22 September 2011 Astronomy 111, Fall 2011 13 What constituent of Earth’s atmosphere makes the largest contribution to the greenhouse effect? A. Carbon dioxide BB.. Methane C. Ozone D. Nitrogen E. Water F. Argon 22 September 2011 Astronomy 111, Fall 2011 14 The greenhouse effect (continued) 1 L 4 278 K T = = , 16πσ r2 r[AU] so the terrestrial planets emit most of their light at infrared wavelengths. They would all be brightest near a wavelength of 10 µm. Solar heating arrives mostly at visible wavelengths, where the Created for Global Warming Art by Robert atmosphere is transparent. A. Rohde 22 September 2011 Astronomy 111, Fall 2011 15 The greenhouse effect (continued) Infrared light is absorbed very strongly by molecules in the atmosphere, notably by water and CO2. Light can only escape directly to outer space through “windows”, of which the most important lie at wavelengths 8-13, 4.4-5, 3-4.2, 2-2.5, 1.5-1.8, and 1-1.4 µm. Created for Global Warming Art by Robert A. Rohde 22 September 2011 Astronomy 111, Fall 2011 16 The greenhouse effect (continued) Hotter blackbodies shine more at shorter wavelengths, so if not enough light escapes at 3-5 and 8-13 µm, the surface heats up until enough of the emission leaks out in the shorter- wavelength windows. This effect warms all three of the atmosphere- bearing planetary Created for Global Warming Art by Robert surfaces. A. Rohde 22 September 2011 Astronomy 111, Fall 2011 17 The greenhouse effect (continued) If there’s liquid water on the surface, the greenhouse effect can be self-stabilizing, as water droplets form clouds that reflect sunlight. (CO2 forms neither droplets nor clouds.) If temperature rises, → more water evaporates into atmosphere → more clouds form → albedo increases → less sunlight reaches surface → temperature drops. And vice versa. But on Venus, sunlight and the greenhouse effect was sufficient to evaporate all of the water, leaving no liquid bodies on the surface. 22 September 2011 Astronomy 111, Fall 2011 18 The greenhouse effect (continued) Liquid water dissolves carbon dioxide, both from the atmosphere and from rocks, creating carbonic acid: +- H23 CO (in solution, H3 O+ HCO 3 ). From there the carbon can be incorporated in carbonate minerals that can form readily in liquid water. • These days, this is done most readily on Earth by ocean-dwelling organisms, creating CaCO3 . Thus if there is a lot of liquid water, carbon from CO2 will eventually be locked up in carbonate minerals, rather than allowed to be present in the atmosphere. • This is the case, for example, on Earth. • On Venus, though, the lack of liquid water let the CO2 remain in the atmosphere. 22 September 2011 Astronomy 111, Fall 2011 19 Carbon, as currently imprisoned on Earth Flows (arrows) in petagrams (1 Pgm = 1015 grams, about 1 billion US tons) of C per year.