Chapter 7 Atmospheres of Venus Earth and Mars Sun radiates energy and charged particles charged particles follow magnetic field lines into Earth s upper atmosphere Solar wind distorts Earth s magnetosphere atmospheric formation gases in a protoplanetary disk mostly hydrogen and helium were captured by young planets forming primary atmospheres sunlight heated the atmospheres this rapid thermal motion caused the primary atmospheres to escape volcanoes released gases larger planets gravity could retain an atmosphere Venus Earth Mars smaller planets grav ity could not retain an atmosphere Mercury Moon 7 1 Atmospheres Are Oceans of Air atmosphere formed in phases young planets captured hydrogen and helium that filled the protoplanetary disk surrounding the Sun until the supply of gas ran out this gaseous atmos phere is called the planet s primary atmosphere terrestrial planets had small masses and weak gravity no ability to hold onto light gases gas began leaking back into space because they exceeded escape velocity even if of the same mass hotter molecules move faster than cooler ones so intense solar radiation primary source of thermal energy in terrestrial planets raises the kinetic energy of at mospheric atoms and molecules enough to escape the gravitational field the kinetic energy of a molecule is determined by its mass and speed when a volume of air contains molecules with different masses the average kinetic en ergy tends to be distributed equally among the different types so every molecule from lightest to most massive will have the same average energy but if each has the same average energy the less massive molecules must be moving faster than the more massive ones giant planets are more massive in cooler environment this stronger gravity and lower temperature allowed them to retain their massive primary atmospheres secondary atmosphere caused by accretion volcanism and impacts the accretion process had minerals containing water carbon dioxide and other volatile matter in the Earth s interior the interior heated up and these gases were released from the minerals volcanism brought these gases to the surface they accumulated to form our secondary atmosphere impacts by comets and asteroids have also caused gases as the giant planets grew to ma turity they perturbed the orbits of comets and asteroids some were scattered into the inner parts of the Solar System and could strike the terrestrial planets bringing water carbon monox ide methane and ammonia water from these impacts mixed with water from volcanism on Earth maybe Mars water vapor condensed as rain and flowed into the lower areas to from the earliest oceans other molecules changed from their original form UV light from the Sun fragmented as ammonia and methane ammonia breaks down to form hydrogen and nitrogen the lighter hy drogen atoms escape to space the heavier nitrogen atoms combine to form more massive nitro gen molecules even less likely to escape to space this ammonia turned nitrogen is the pri mary source of molecular nitrogen in the atmospheres of terrestrial planets and on Saturn s moon Titan Mercury s small mass and proximity to Sun made it lose all of secondary atmosphere to space even molecules massive as CO2 can escape from a small planet if temp is high enough intense UV radiation breaks up molecules lost to space even quicker Moon is cooler than Mercury but Moon s mass is so small that molecules escape even at low temperatures so small mass proximity to Sun doomed Mercury and Moon airless 7 2 A Tale of Three Planets The Evolution of Secondary Atmospheres Venus Earth Mars have similar geological histories all had volcanism at some point shared cometary showers early secondary atmospheres must have also been similar Venus and Earth are similar in mass and composition both have orbits less than 0 3 AU apart Mars is similar in composition but has a mass about a tenth of Earth and Venus atmospheres of Venus and Mars extremely similar mostly CO2 and a little nitrogen but vastly different amounts of atmosphere atmospheric pressure on Venus is 100x greater than Earth s on Mars less than a hundredth of a our planet s pressure Earth is different in that its atmosphere is made of primarily nitrogen and oxygen only a trace of CO2 ture and on evolution of atmosphere differences in present day mass of atmospheres have a large effect on surface tempera we can calculate temp of a planet by finding the equilibrium between the amount of en ergy it radiates and the amount it receives for Earth and Venus the answers are far from mea sured for planets without atmospheres we get good results this is because of the atmospheric greenhouse effect which traps solar radiation the Atmospheric Greenhouse effect in planetary atmospheres and the conventional green house effect operate in different ways conventional like in a car on a sunny day sunlight pours in and heats the interior air temp hot air is trapped and temp can climb as high as 180 degrees atmospheric gases like nitrogen oxygen CO2 and water vapor transmit visible light allowing the sun to warm the planet s surface the surface radiates excess energy in the infrared but CO2 and water vapor absorb infrared radiation and convert it to thermal energy these molecules reradiate this thermal energy in all directions some of it continues into space but much goes back to the ground therefore the surface receives energy from both the Sun and atmosphere Conventional greenhouse effect infrared radiation is trapped by glass transparent to visi ble but opaque to infrared the trapped infrared causes the temp of the greenhouse to rise to a new equilibrium temp Atmospheric greenhouse effect greenhouse gases such as water and carbon dioxide ab sorb infrared radiation and reradiate it in all directions slowing the transport of energy out of the atmosphere this causes the temperature of the planet to rise to a new equilibrium visible sunlight passes through the atmosphere and heats the ground the warm ground ra diates in infrared some infrared radiation escapes to space but some is trapped by greenhouse gases and is reradiated back to the ground heating it further the temp climbs until the escaping infrared radiation balances the absorbed sunlight greenhouse gases such as water vapor and CO2 transmit visible radiation but absorb infrared radiation thus causing a planet s surface temperature to rise until the surface gets hot enough that the fraction of infrared radiation leaking out through