The Layered Atmosphere:

The Layered Atmosphere:

The Earth s Atmosphere Like all the planets, the Earth s atmosphere is highly distinct. What makes it different from the other terrestrial planets?

Comparative Planetology The basic parameters of the three terrestrial planet atmospheres are very different. Venus and the Earth have comparable amounts of N 2, Venus atmosphere is about 90 times thicker. Mars atmosphere is only about 1% as thick as the Earth. Both Venus and Mars atmospheres are made up mostly of CO 2, while the Earth has none. The surface temperature of Mars never rises above 32 F, while on Venus it stays above 870 F!

Basic Elements The atmospheric variations between Mars, Venus, and the Earth are mainly due to 3 of our 4 physical parameters. 1. Composition and density: What gasses are present and how much of them are there? (We ve just seen how they re different) 2. Energy Input: How much energy delivered to an atmosphere and where? (Venus receives 2x the energy from the Sun that the Earth does, while Mars receives ½ as much. ALL terrestrial planet atmospheres get >99% of their energy from the Sun) 3. Gravity: How well does a planet hold onto its atmosphere and how extended is it in altitude? (Venus is similar in size to Earth, but Mars is about 11% as large, with ¼ the gravity.)

Earth vs. Venus The major factors affecting the Earth and Venus are composition, density, and energy input. A factor of two in energy input cannot explain the temperature difference at the surface. In fact, as seen from space Venus is actually cooler than the Earth (why? ). We now think that Venus and Earth s atmospheres were very similar early on. Over the past 4.0 billion years Venus and Earth have evolved in very different ways. Two factors were important on Earth. 1. Rain washed CO 2 out of the Earth s atmosphere and locked it up in sedimentary rocks (limestone-caco 3 ). 2. Living things (plants) converted the rest of the CO 2 into O 2 (which we breathe). (why is this important? )

Global Warming Much is made of global warming in the press. CO 2 is what we call a greenhouse gas. It allows visible light through, but doesn t allow heat to escape. On Venus, this process condemned the planet. Before the CO 2 could be removed entirely via water, the temperature rose enough to boil the oceans away (if oceans formed at all! ). The same thing could happen on Earth.

Mars vs. Earth The two primary differences between the Earth and Mars are geology and gravity, with gravity. The weaker gravity on Mars was not strong enough to hold its atmosphere as well. Eventually the atmosphere became too thin to support liquid water, which then boiled away. Stronger gravity on the Earth could hold its atmosphere for billions of years, allowing liquid water to exist over all that time. The lack of plate tectonics on Mars may have trapped CO 2 in rocks.

The Goldilocks Syndrome The lessons of Mars and Venus are: The modern atmosphere is a product of long evolution. Our circumstances are not a unique outcome of that evolution. Small changes in external or internal conditions can have profound effects on our climate. It pays to understand the nature of our fragile atmosphere and how it is changing with time and response to human activity.

Atmospheric Layers on the Earth

Atmospheric Layers As we move from the surface of a planetary body outward into space the characteristics of the atmosphere change. The temperature and temperature gradient (change with altitude) changes. The composition and charge state changes. The type, rate, and location of energy input changes. Altitude regions where these parameters uniformly characterize the atmosphere are referred to as atmospheric layers.

Atmospheric Layers Each planetary body has a distinct layering scheme. Not all planets have the same layers. Some sets of governing parameters produce similar conditions in all atmospheres where they occur. We define 7 such regions of which the Earth has all 7 (not coincidence!): The Troposphere: 0-10km (Turbulent weather, decreasing Temp) The Stratosphere: 10-50 km (stable with increasing Temp) The Mesosphere: 50-80 km (highly chaotic, decreasing Temp) The Thermosphere: 100-600 km (Very hot and variable) The Ionosphere: 100-600 km (Distinct layers in the thermosphere) The Exosphere: 300+ km (Unbound gasses escape Earth) The Magnetosphere: (Space region inside Earth s magnetic field)

The Basic Rundown Most of the layers in the Earth s atmosphere exist above the regions that we can live in. Parts of the atmosphere are so thin that it is difficult to tell anything is there. For the most part, each region is defined by the rate at which temperatures change with altitude. This also tells us about where the energy is deposited. What is the source of energy?

Solar Heating There is a tremendous amount of energy locked in the interior of the Earth. This is slowly released via volcanic activity and the movement of the continents. Internal energy contributes almost none of the energy that warms the surface of the Earth. Averaging over the entire surface of the Earth, the Sun provides 342 Watts per square Meter. That s about 40 Billion times as much energy as the entire USA produces. How is that energy delivered to the Earth?

Solar Radiation The primary mechanism by which the Sun delivers energy to the Earth (or anywhere else) is LIGHT. Light has properties of both particles and waves. A single photon of light has a packet of energy that is carried by a wave. The amount of energy in a photon defines the spacing of the waves (the wavelength- or color- of light ) and the number of times the wave oscillates each second (the frequency ). All photons travel at the same speed, 300,000 km/sec. It takes 8 minutes for light from the Sun to reach the Earth.

The Solar Spectrum: The amount of energy the Sun produces at a given wavelength is determined by its temperature. This is a general property of any radiating object from a star to the heating element on the stove. The Sun is about 5270 K and produces most light in the visible. The atmosphere is transparent in the visible and so most solar energy reaches the ground. Some gasses absorb certain wavelengths of sunlight, and thus some energy is absorbed directly into the atmosphere.

Depositing Energy Energy from the Sun may be transmitted directly to the ground, absorbed in the atmosphere, or reflected from clouds or the Sun back into space. Some re-radiated heat from the ground is absorbed by the atmosphere, further heating it.

The Troposhphere The troposphere is the region of the atmosphere we live in. The primary source of energy in the troposphere is heat (infrared light ) radiated from the ground. This means that it is warmest at the bottom and coolest at the top. Temperature drops about 11.5 F for each km of altitude. When pressure and temperature (with temp. faster) drop with altitude it triggers convection. Convection makes the troposphere unstable, but in a good way.

Dominant Region The Troposphere contains 80% of the mass in the atmosphere. The Troposphere contains 99% of the water in the atmosphere. Convection (both horizontal and vertical) produces weather in the atmosphere. All storms occur in the troposphere. The top of the troposphere occurs where the temperature change switches to decreasing with altitude to increasing. The top of the troposphere is called the tropopause. It s altitude depends on latitude and season. It s about -60 F at the tropopause.

The Stratosphere: The stratosphere is the atmospheric layer directly above the tropopause. It has much of the remaining mass of the atmosphere and is nearly as important as the troposphere for living things. The primary source of energy in the stratosphere is internal. Incoming ultraviolet light is absorbed by Ozone (O 3 ). O 3 + UV -> O 2 + O Much of the O 3 that is destroyed is at the top of the stratosphere. Temperature rises with altitude. The stratosphere is stable.

The Role of Ozone: Ozone does more than heat the stratosphere. Without ozone absorption, UV radiation would kill most things living on land! Human activity is destroying stratospheric ozone! Chlorofluorocarbons (from spray cans) are transported to the stratosphere in tropospheric convection. They contain chlorine. Cl + O 3 ClO + O 2 ClO + O Cl + O 2 The above is called a catalytic reaction. The O 3 is destroyed, but not Cl! Because the stratosphere is stable, the Cl is trapped. This is a big problem.

The Mesosphere: Above the Stratopause is another, thin atmospheric layer called the Mesosphere. Energy input to the Mesosphere comes from the formation of Ozone, which occurs near the stratopause (the bottom). O 2 + UV O + O O + O 2 O 3 At the same time CO 2 is radiating away energy near the top of the mesosphere, cooling it more rapidly. Like the troposphere, temps in the mesosphere drop rapidly with altitude. -100 32 Temperature (F) The rapid drop in temperature makes this the most turbulent part of the atmosphere.

The Thermosphere & Ionosphere Above the Mesopause the atmosphere begins to heat rapidly as the atmosphere absorbs solar UV and X-Ray radiation. The thermosphere is the hottest and thinnest region of the bound atmosphere. In the thermosphere, the mixing of atmospheric gasses begins to break down. Lighter gasses rise to the top. Some solar photons are so energetic that they break atoms apart, creating charge layers (the ionosphere-several layers). -100 1500 (750) Temperature (F) Temperatures in the thermosphere are very dependent on sunlight. It cools dramatically at night and heats up when the Sun is active.

Ions and Aurora Once ions are formed in the atmosphere it becomes part of a large circuit (like a resistor) that connects via the Earth s magnetic field to the magnetosphere. Ions and electrons in the magnetosphere flow into the atmosphere where they collide with thermospheric gasses and give off light. This is called AURORA

The Exosphere: Unlike the other layers of the atmosphere there is no thermopause. What happens instead is that it simply extends into space getting thinner, hotter, and lighter. Eventually the gasses become so hot and light on average that they are moving fast enough to escape the Earth altogether. We call this region the Exosphere. The exosphere is so thin that it is a better vacuum than we can make in a laboratory on Earth. It s really space, but Earth-Space.

The Magnetosphere: The final layer of the Earth s atmosphere is really not part of the atmosphere per se. This last region is defined by the dimensions of the Earth s magnetic field. This field carves out a volume around the Earth that is controlled by planetary conditions. This is called the Magnetosphere. Beyond the magnetosphere, space is dominated by the Sun. The Earth has no influence.

Atmospheric Layers and Solar Input: Summary