Exam next Tuesday Covers chapters 1-4 in the text + Seasons (p265) and all lecture notes including this week 4 essay questions in which you will be asked to explain concepts Allowed one sheet of notes with writing on one side only

Atmospheres & Climate

Atmospheres Allows existence of liquid surface water Moderates temperature differences Greenhouse gases raise global temperatures

How atmospheres are created 1.Direct capture from solar nebula or solar wind 2.Outgassing 3.Evaporation/sublimation 4.Bombardment

How atmospheric gasses are lost 1.Thermal escape 2.Stripping via solar wind/cosmic rays 3.Atmospheric cratering 4.Condensation 5.Chemical reactions

Thermal escape 1.Escape velocity 2.Temperature 3.The mass of the gas particles

Because some of the atmosphere is constantly escaping into space, atmospheres have no edge. They just get thinner and thinner.

Early atmospheres All of the terrestrial planets started out with atmospheres captured directly from the solar nebula containing primarily hydrogen and helium. But these gases were lost into space by thermal escape.

Each of the terrestrial planets then acquired an atmosphere through a combination of bombardment and volcanic outgassing. In the case of Mercury and the Moon, this atmosphere was also lost into space over time.

Discussion If the current atmospheres of the terrestrial planets comes from volcanic out gassing and bombardment, do you expect there to be a difference in their chemical compositions? Explain.

Discussion Venus and Mars have atmosphere’s of mostly CO 2 while the Earth has very little CO 2 in its atmosphere. Why the difference?

On Earth volcanic eruptions outgas carbon dioxide, water vapor and sulfur dioxide. What happens to the sulfur dioxide? Discussion

Rain water dissolves carbon dioxide, and sulfur dioxide, forming carbonic acid and sulfuric acid. This acid rain erodes rocks that contain calcium- silicate minerals. The carbon-silicate cycle

Carbonic acid reacts with the rock and forms calcium and bicarbonate ions, which are carried into the oceans. There these ions are deposited on the sea floor as limestone or used by plankton and other organisms for calcium carbonate shells, which are deposited on the ocean floor when the organisms die.

Spreading of the sea floor causes these deposits to be cycled into the Earth’s interior where it reacts with quartz to produce new silicate rock and carbon dioxide.

Discussion What would happen if the Earth grew colder so that the oceans completely froze over?

What happens as the Earth heats up? Discussion

The faint Sun problem The enhanced greenhouse gasses kept Earth’s surface warm enough to have liquid water on the surface despite the fact that the Sun was only 70% as bright as it is today. As the Sun brightened, exchange of CO 2 between the atmosphere and crust regulated the surface temperature.

Run-away greenhouse effect Early in Venus’s history, it may have had as much water as Earth. It may have had oceans and possibly even life. Venus must somehow have lost its liquid water.

Water vapor is also a greenhouse gas If water vapor pressure exceeded 20% of Venus’s atmosphere, any oceans which might have existed would evaporate due to the added greenhouse effect. The more water that evaporates, the greater the greenhouse effect becomes.

Discussion Venus and Earth likely started out with the same amount of water. Earth is very wet, while Venus is very dry. How could Venus have lost its water?

Discussion Why are the water molecules broken up in the atmosphere of Venus, but not in the atmosphere of the Earth?

Where did Venus’s water go? Venus does not have an ozone layer to filter out UV light. UV light is energetic enough to dissociate water molecules. The gravity of Venus is not strong enough to hold the hydrogen and it escapes into space. The oxygen helps form sulfuric acid.

How do we know? Sheer bad luck. A drop a H 2 SO 4 got stuck in the intake of the Pioneer Venus atmospheric probe’s mass spectrometer. As the probe descended in the atmosphere this drop slowly evaporated. But, this allowed the measurement of the hydrogen to deuterium ratio.

The results Earth’s oceans contain one deuterium atom per 6000 hydrogen atoms. But the drop that clogged the mass spectrometer’s intake had 120 times as much deuterium.

Discussion How could Venus have ended up with so much more deuterium than the Earth?

The Greenhouse Effect and global warming Human activity is increasing the amount of CO 2 in Earth’s atmosphere.

71% of the Earth is covered with liquid water. As the temperature of the water increases more will become a vapor, which enhances the greenhouse effect and raises the temperature even more. The run away greenhouse effect

Worse than Venus? If Earth’s oceans were to evaporate the atmosphere would be dominated by water vapor and have a pressure 400 times its current value. The added greenhouse gasses would heat the carbonate rocks and cause them to release their CO 2 increasing the greenhouse effect still further and raising the pressure another 70 bar.

Venus is very likely telling us the fate of the Earth. Even without humans, as the Sun builds up helium in its core, the core will contract and heat up. The future Sun will be brighter and hotter. Thus a run away greenhouse effect on Earth is inevitable.

Mars The greenhouse effect on Mars raises the surface temperature only about 6 degrees C. This keeps the average surface temperature of Mars well below the freezing point of water.

The Martian atmosphere, which has a surface pressure 100 times less than Earth’s, is too thin for liquid water to exist on the surface, even where the temperature gets above freezing. Water will boil on the surface of Mars at temperatures above 0 degrees C.

Mars had liquid water in the past The old, heavily cratered southern highlands have channels, valleys, and gullies which appear to have been made by flowing water. The northern lowlands may even have been an ocean. The Martian atmosphere must have been denser in the past.

Layered deposits

Discussion If Mars was so much like Earth in the past, what happened to make it so cold and have such a thin atmosphere today?

Carbon dioxide cycling fails Mars is too small to retain enough heat to the present day to provide enough volcanism to return carbon dioxide to the atmosphere that is now trapped in carbonate rocks buried under the Martian regolith.

Once volcanism shut down on Mars, the atmosphere could escape into space. As Mars became colder, carbon dioxide froze out of the atmosphere at the poles, creating the polar ice caps.

Discussion What is an ice age? How would you explain it to someone else?

Ice ages Snowball Earth episodes are not the same as ice ages

Earth’s rotation

Discussion Why is Kennedy Space Center in Florida and not in Maine?

This extra velocity at the equator pushes the matter out at the equator. Centrifugal force

The Earth is not a perfect sphere The Earth bulges out at its equator. Its diameter measured along its equator is 43 km larger than its diameter from the north to the south pole.

The Earth’s rotation axis remains in a fixed direction as Earth orbits the Sun.

Sun and Moon apply a torque

Precession This off axis force, or torque, on the Earth causes Earth’s rotation axis to vary slightly, or precess, over long periods of time. Thus, the celestial poles trace out a circle against the stars over a period 26,000 years.

Sidereal year vs. tropical year Our calendar year is shorter than the actual orbital period of the Earth by just over 20 minutes. Time of year that Earth is furthest from Sun changes over the years.

Discussion Are the winters as bad as they can get for us northerners? What about the southern hemisphere?

Earth tilt is not constant Due to torques supplied by the other planets, Earth’s tilt changes form 22.5 degrees to 24.5 degrees with a period of 41,000 years.

Discussion Are glaciers more likely when the seasons are extreme or when there is little variation between seasons? Explain your reasoning.

Earth orbital eccentricity is not constant Changes on a time scale of 413,000 years from to 0.058

Discussion Under what conditions would you expect to glaciers covering Cleveland?

Snowball Earth Occurs when even the oceans at the equator or frozen over.

Discussion If all of Earth is covered in ice, most of the Sun light is reflected back into space. With so much less energy being absorbed from the Sun, how is it possible for Earth to ever thaw out again?

Discussion How could life on Earth manage to survive a snowball Earth episode?

Cause of Snowball Earths Formation of a supercontinent may be what triggers snowball Earth episodes.