Terrestrial Atmospheres
Why Is There Air? An atmosphere is a layer of gas trapped by the gravity of a planet or moon. Here s Earth s atmosphere viewed from orbit:
Why Is There Air? If atoms move faster than the escape speed, they can leave the planet forever. Those atoms are lost: Lighter gases (like hydrogen and helium) move the fastest, so they are the most easily lost. Heavier gases (like CO 2, N 2, and O 2 ) move more slowly, so they stick around. It is also easier to escape from a world with weak gravity (low escape speed): more atoms move fast enough to escape the gravity of the planet.
The Airless Worlds Mercury and the Moon do not have enough gravity to hold onto atmospheres. Most atoms of gas escape to space. From the surface of the Moon, the sky is black. Why aren t they visible in this picture?
Atmospheres and Surfaces An atmosphere changes the surface: winds cause erosion, move sand and dust clouds reflect sunlight back into space atmo. pressure can permit liquid water or ice Airless worlds have none of these features: surface changes are slow: volcanism & cratering surface temperatures are extreme volatiles escape to space from high temperatures (volatiles freeze/evaporate in narrow temp. range)
Earth s Atmosphere Composed of: 77% nitrogen (N 2 ) and 21% oxygen (O 2 ) a variable amount of water vapor (about 1%) less than 1% argon a small quantity of carbon dioxide The atmosphere undergoes variations in temperature, water vapor content, wind speed. We call this day-to-day change the weather. Weather averaged over many years is the climate.
Water in Earth s Atmosphere Most of the water vapor in Earth s atmosphere is at low altitudes (under 15 kilometers). Here is a view of the atmosphere from orbit. The upper atmosphere is cloud-free. Clouds are limited to the lower atmosphere.
Water in Earth s Atmosphere Temperature decreases with height, so water is trapped near the surface. Water vapor at high altitude cools and turns to liquid or ice. Droplets / crystals are heavier than air and fall back to the surface.
Convection Warm, humid air close to the ground is buoyant and rises. It is replaced by sinking drier, colder air. This process is called convection. Convection is responsible for transporting heat and water from the ground to the atmosphere and drives most weather on Earth. Water condenses into rain or ice, releasing heat. Heat transported into upper atmosphere. Water evaporates from oceans and land into dry air. Sunlit warm water and land heats air.
The Atmosphere of Venus Early astronomers saw that Venus is totally shrouded in clouds. Not able to see the surface, they imagined lush jungles teeming with life. It was not until the 1960 s that radio telescopes were used to measure Venus temperature: a blistering 750 Kelvin (900 o F).
The Atmosphere of Venus Very high pressure: 90 times pressure at Earth s sea level carbon dioxide: 96.5% and nitrogen: 3.5% Global cloud layer 50 70 km above surface Clouds are sulfuric acid not water
The Atmosphere of Venus If Earth and Venus are so similar, where did Venus get all of its carbon dioxide? Actually, Earth has as much carbon dioxide. But Earth s CO 2 is in carbonate rocks like limestone. The White Cliffs of Dover are massive layers of carbonate rock.
early Venus vs. today s Earth A cooler, early Venus may have had liquid water, just as Earth does today. Liquid water is essential in forming carbonate rock. On Earth, this process is known as the CO 2 cycle.
The Atmosphere of Venus The surface temperature on Venus is extremely uniform from day to night and pole to equator. The thick atmosphere traps nearly all heat that reaches the surface. Imagine an oven on the highest setting Since the temperature is nearly the same at all locations on the surface and the planet rotates so slowly, there is really no weather or winds. The Venera landers in 1980 s measured winds of 3-4 miles per hour (walking speed). Truly hell
Surface Conditions on Venus Weather report: Very hot and 100% cloud cover Venera 13 and Venera 14 landing images (1982)
The Martian Atmosphere Martian atmosphere is mostly carbon dioxide, and very thin (0.006 Earth) Too thin to retain much heat; very little greenhouse effect Sharp drop in temperature at night. Dust can greatly heat atmosphere.
Polar Caps Temperatures on Mars are too cold for liquid water over most of the planet: 150-280K. In fact, it is so cold that carbon dioxide freezes at the poles. The polar caps are carbon dioxide ice, but there is a deeper layer of water ice. The Mars Express radar system has detected deep deposits of water ice buried in the soil around the polar caps. There is enough water buried under just the south pole to completely cover all of Mars in a layer of liquid water 10 meters deep. Today, that water is frozen.
Mars Express Radar visible polar cap Mars Express (orbiter) has also mapped the south pole and found deep layers of water ice below the surface.
Mars Reconnaissance Orbiter Radar MRO has mapped the north pole and also finds deep layers of water ice below the surface. visible polar cap
Water on Mars: Glaciers Mars Reconnaissance Orbiter radar system has found water ice closer to Mars equator, too: smooth aprons are water ice covered in soil and dust.
Polar Terrain seen by Phoenix Lander On May 25, 2008, the Phoenix Lander landed on the northern plains of Mars (latitude 68 o North).
Phoenix Lander seen by MRO 3 days later, Mars Recon. Orbiter flew over Phoenix Lander and saw this:
Layers of Ice seen by Phoenix Lander Looking underneath the lander on Day 8, camera on the robotic arm took this picture of smooth, white patches uncovered by rocket exhaust.
Definitely Struck Ice This Time Twenty days after landing, Phoenix extended its arm and began to dig. Little pebbles are visible in the lower-left of the trench. They disappeared 4 days later.
Water on Mars: Not IF, but HOW MUCH The question at this point is not IF there is water on Mars. We now have a definite answer: YES. The new questions that need to be answered is how much water is still present and what caused the planet s climate to change in the ancient past. We now have clues to both these questions from Phoenix Lander and the Mars Exploration Rovers.
Ancient Mars vs. Today s Mars Signs of liquid water long ago are evidence that atmosphere must have been thicker in the past. Today, the surface of Mars is cold and dry. Mars is smaller than Venus or Earth: 1. Mars lost its internal heat more quickly, so volcanic activity stopped and its core stopped generating a planetary magnetic field. 2. Its gravity was weaker, so gases were able to escape from the planet.
Stripping by the Solar Wind The absence of a magnetic field allowed the solar wind to strike Mars atmosphere directly:
Seasonal Changes Mars undergoes seasons for the same reason as Earth: the tilt of the planet s axis. However, Mars orbit is also more elliptical:
Seasonal Changes Mars atmosphere is composed of carbon dioxide. CO 2 freezes in fall and sublimates in spring, so there are strong winds during spring and fall. Sunlight
Polar Caps Grow and Shrink Here s the northern polar cap in northern spring. The large area of CO 2 ice/snow is sublimating permanent cap Dust storms
Polar Cap in Summer Here s a view from orbit looking down at the northern polar cap during northern summer. The layers of ice and soil are thought to record the history of changing climate on Mars. Just like ice cores from Earth s polar caps, these layers would tell the story of Mars climate.
Global Dust Storms Migration of gas from atmosphere to polar cap can stir up dust storms that circle the planet. Large storms developed in 2001, 2004, and 2007. This is a view from the Hubble Space Telescope of the 2001 global dust storm.
Global Dust Storms Mars maps show before and after images in 2007: