Physics 101 - Astronomy - Spring 2019

Class notes for day 9, Feb. 14, 2019

Ch. 8: Earth as a Planet

The Earth's atmosphere is layered. Starting at the ground, the layers are called Troposphere, Stratosphere, Mesosphere, and Ionosphere. Most of our weather is in the troposphere, near the ground up to about 7 miles above the ground. Above that the stratosphere has very thin air that begins to have ozone in it at about 30 miles altitude. The ozone layer is important, because it absorbs UV light that could cause sunburns and skin cancer. To protect the ozone layer, certain gases known as CFCs (chlorofluorocarbons) have been banned, because they caused the destruction of ozone.

The Greenhouse effect is due to several factors, but the increase of carbon dioxide in the Earth's atmosphere is the most important, since we could do something about it by burning less fossil fuels. I showed the Keeling curve that show how CO₂ has increased in the years since the 1960's. The level of CO₂ has varied over longer spans of time; if you are interested in more detail, see http://cdiac.ornl.gov/trends/co2/ice_core_co2.html for some long-term graphs, and for more recent data, see http://www.esrl.noaa.gov/gmd/ccgg/trends/ The Australians are also monitoring other greenhouse gases in the southern hemisphere; see http://www.csiro.au/greenhouse-gases/  There are also some graphs at http://keelingcurve.ucsd.edu . Global Warming is definitely occurring, but this may be due to several reasons. The influence of carbon dioxide is a topic of some debate, however. Other factors that influence the Greenhouse Effect include:

Clouds - reflect sunlight, prevent the ground from receiving as much light, and keep in the infrared radiation that would cool the ground.
Water vapor - absorbs infrared light
Agriculture - alters the ground and hence how much light is reflected or absorbed
City “heat islands” - dark surfaces like streets and rooftops absorb more sunlight and get hot
Carbon dioxide - absorbs infrared light
Methane - absorbs infrared even better than carbon dioxide, but is less concentrated in the atmosphere
CFC gases - also absorb infrared, and alter the ozone layer
Dust and pollution - reflect or absorb sunlight
Snow cover - reflects sunlight
Desertification - like agriculture, alters the surface of the Earth

The most important gas for the greenhouse effect is water vapor, the next is carbon dioxide. We can't control the water content much, but humans can reduce the use of fossil fuels which are the most important contributor to the excess carbon dioxide in Earth's atmosphere.

I pointed that most of the details about the interior of the Earth are due to seismic studies. Seismic waves pass through the entire Earth whenever there is an earthquake. These waves are really like sound waves traveling through the interior of the Earth. Some of them are not heard on the other side because part of the core of the Earth is liquid and transverse sound waves cannot travel through liquid. This causes a shadow zone (for an animation see the Shadow Zone Flash Animation which is on a web page about the shadow zone). Scientific study of these seismic waves allows us to deduce the structure of the core of the Earth. The Earth's core is believed to be mostly iron and nickel, but could have many of the heavier elements. There is certainly some Uranium and other radioactive elements which produce heat that keeps the core hot and molten. For an alternate description, see this USGS essay on the interior of the Earth. We will compare this structure with other planet’s structures in the next chapters.

The mantle of the Earth is about 80% of the volume of the Earth. Convection in the mantle causes the movement of the tectonic plates that float on the crust of the Earth. I pointed out a few features of plate tectonics. For an online description of plate tectonics, see the short online book "This Dynamic Planet" from the USGS. (This is more than you need for this course, but I hope it is interesting.) There is also a huge map showing these features of the Earth's surface (download the screen-sized versions, they are only 7 MB or so). The Earth is the only planet that we know of that has plate tectonics.