The Terrestrial PlanetsII Venus and Mars
Topics • Atmospheres • Planetary Atmospheres • Earth • Venus • Mars
Atmospheres • Atmospheres • Consist of atoms and molecules moving about in random directions, that is, gases. • Pressure • Is the force per unit area. It arises from collisions between the atoms and molecules • Temperature • Is a measure of the average kinetic energy of the particles, that is, the energy of motion.
Energies of Gas Molecules • Kinetic energy (m = mass, v = speed) mv2 / 2 • The average kinetic energy per gas molecule is 3kT / 2 • k = 1.381 x 10-23 J / K is Boltzman’s constant • T is the absolute temperature in Kelvin
Temperature • In science temperature is measured in • Kelvin (K) • The scale starts at the lowest possible temperature, called absolute zero • 0 0C corresponds to 273 K • 27 0C corresponds to 300 K • 100 0C corresponds to 373 K • Temperatures in the universe range from • 3 K to several billion K
What is the Speed of Gas Molecules? • Speed of Molecules • Equate the formula for kinetic energy to that for the average kinetic energy per molecule • Example: At T=290 K oxygen molecules move at ~ 500 m/s • Lower mass molecules move faster than higher mass ones
Escape of Planetary Atmospheres • The Escape Speed • The escape speed, V = √(2GM/R), is the minimum initial speed required for an object to escape from another of mass M and radius R. • For Earth, this speed is 11km/s. • Therefore, if a molecule is moving upwards at a speed greater than 11 km/s, then unless it is impeded it will escape into space. • We expect that planets, like Earth, with large escape speeds will have denser atmospheres than those with low escape speeds such as Mars.
Planetary Atmospheres • Terrestrial Planets • The atmospheres of the Terrestrialplanets are rather different from that of the Sun, which is composed mostly of hydrogen and helium. • It is likely that these planets began with hydrogen-rich atmospheres, called primary atmospheres, which somehow were replaced by those present today, called secondary atmospheres. • Jovian Planets • Jupiter and Saturn retained their primary atmospheres, while Venus, Earth and Mars did not. • This suggests that a planet’s mass is an important factor in determining a planet’s atmospheric composition.
Why are Venus and Earth so Different? • Venus and Earth • They have similar masses and radii, but their atmospheres are quite different. • Volcanism • On both planets, volcanic activity released huge amounts of CO2 and H2O to form their primordial secondary atmospheres. • Distance from Sun • But Venus is closer to the Sun than Earth and so receives more energy per unit area. The difference in heating could explain the different evolution of the two atmospheres.
Evolution of Atmospheres • Earth • Water rained down to form the oceans • Erosion and water chemistry removed some of the CO2 from the atmosphere to form carbonates (in rocks such as limestone) • The evolution of life further altered the atmosphere • Venus • Being hotter, water re-evaporated, leaving the surface very dry and the CO2 and water still in the atmosphere. This triggered a runaway greenhouse effect
The Greenhouse Effect • Sunlight • When sunlight falls on an object the latter heats up and radiates some of the energy as infrared radiation. • Infrared Radiation • Infrared radiation cannot pass easily through certain substances, such as carbon dioxide. • Therefore, carbon dioxide in an atmosphere tends to trap heat near a planet’s surface.
The Greenhouse Effect – II • Greenhouse Effect is Good! • Were it not for the greenhouse effect, the Earth would be about 33o C cooler. • But! • If the concentration of carbon dioxide is too great this triggers a runaway greenhouse effect. Venus, with its dense atmosphere of carbon dioxide, appears to have suffered a greenhouse catastrophe that has pushed its surface temperature to about 750 K.
Mars • 1877 • Giovanni Schiaparelli announced he saw canali on Mars. This was mistranslated from Italian into English as canals. • 1892 • The Percival Lowell built an observatory at Mars Hill, Flagstaff Arizona to look for life on Mars. • 1897 • H.G. Wells published the novel The War of the Worlds. • 1938 • Just before the outbreak of the Second World War a broadcast by Orson Welles frightened millions of Americans who believed that Martians were in fact invading!
Mars • 1907 – Alfred Russell Wallace critiqued Lowell's work: “Any attempt to make that scanty surplus [of water], by means of overflowing canals, travel across the equator into the opposite hemisphere, through such terrible desert regions and exposed to such a cloudless sky as Mr. Lowell describes, would be the work of a body of madmen rather than of intelligent beings. It may be safely asserted that not one drop of water would escape evaporation or insoak at even a hundred miles from its source.”
Mars • 1976 • July 20, Viking 1 landed on Mars after a one-and-a-half year journey of about 100 million km.