Terrestrial Planetary Atmospheres

Transcription

1 Terrestrial For the first time in my life, I saw the horizon as a curved line. It was accentuated by a thin seam of dark blue light our atmosphere. Obviously this was not the ocean of air I had been told it was so many times in my life. I was terrified by its fragile appearance. Ulf Merbold (1941 ) German Astronaut

2 Announcements Reading Assignment Chapter 9 (review + read the rest of the chapter) 3 rd Homework is posted on the website (due next Thursday 3/1) Term paper details are posted on the website (due 4/17) Public Lecture next Tuesday (2/27) at 7:30PM in this auditorium Prof. Bob Strom: Global Warming Next week s Study-group session is on Wednesday from 10:30AM- 12:00Noon in room 330.

3 Today Finish discussion of impact cratering Effects of an impact on Earth What are they? How do you get one? Which objects have them? What do they do? What is the basic structure of Earth s atmosphere?

4 The Probability of Impacts with Earth 30-meter sized asteroids come close to Earth about every 2 years They strike Earth every 6000 years or so Recent close call in 6/6/02 (East Mediterranean event) Calculating asteroid trajectories, precisely, can be tricky Need a detailed mapping of the Sun s gravitational field Need a better understanding of the characteristics of the asteroid (rotation, orbit, shape, etc.)

5 Berringer Meteorite Crater aka Meteor Crater northern Arizona Produced ~49,000 years ago 30m-50m diameter iron asteroid Too small to produce global environmental effects, but the regional damage was probably severe The Kinetic Energy of this impact (1/2) x Mass x speed 2 = Joules = 1200 Hiroshima Atomic Bombs

6 Effects of an Impact: Ejecta The impact that created Berringer Meteorite ejected bedrock out to a distance of 1-2 km from the impact site

7 Effects of an Impact: Shock Wave The shock wave would have produced 1000 km/h winds within 3-5 km of the impact strip away grass and flatten trees out to a distance of 20 km. Animals would suffer from both displacement, and internal/external pressure difference (causing internal bleeding) Bikini Atoll atomic bomb test July 1, 1946 Rocks and gravel ejected from the impact would act as shrapnel Thermal effects could cause severe burn damage and possibly forest fires out to a distance of about 20 km

8 The Sum of all Effects destruction of vegetation over an area 800 to 1500 km 2 Animals within 3 to 4 km of the impact site would probably have been killed, with maiming injuries extending out to distances of ~16 to 24 km. While these effects are severe, they are confined to the immediate region and did not cause extinctions.

9 In the period after the impact newly formed bowl shaped depression soon filled with water providing a lake habitat for aquatic plants and animals. Re-colonization of the area was probably accomplished in a few to ~100 years. These types of events, however, are large enough to destroy a modern city. They occur at an average rate of about once in 6000 years.

10 Chicxulub Crater: A somewhat larger impact event! Asteroid roughly 10 km (6 miles) across hit Earth about 65 million years ago. This impact made a huge explosion and a crater about 180 km (roughly 110 miles) across. Debris from the explosion was thrown into the atmosphere, severely altering the climate, and leading to the extinction of roughly 3/4 of species that existed at that time, including the dinosaurs.

11 The KT boundary Fossil records have several breaks when one group of fossilized species gave way to other groups during short intervals The K-T boundary is one of these breaks associated with the disappearance of the dinosaurs and emergence of the mammals

12 Chicxulub: The Evidence Iridium and Soot Found throughout the world Tsunami deposits Found in the clay deposits in the region nearer to the crater All dated at 65 million years old (which coincides with the K-T boundary) coincidence? Quartz grains found in the K-T boundary show lines that are characteristic of high shock. These grains were part of the crater s ejecta blanket (some may have even made it into orbit)

13 The future? Many asteroids of the type that created Chicxulub are now known their orbits pass through the inner solar system and cross Earth's orbit. They hit Earth at a rate of about 1 every 100 million years The question is: when will it happen again?

14 A layer of gas which surrounds a world is called an atmosphere. Need a gas in which the molecules collide with themselves more often than the planet to have an atmosphere! they are usually very thin compared to planet radius

15 Large cool objects more easily can retain an atmosphere Requirements for an atmosphere Appropriate chemical(s) in molecule form (H 2, N 2, CO 2, etc.) Low enough temperature (cool) Enough gravity (big) More or less obvious for the gas giants, but also explains why Titan has an atmosphere, while Mercury and the Moon do not Earth Jupiter Titan

16 Jupiter Saturn Uranus Neptune Venus Earth Mars Mercury Moon Galilean Satellites Titan Triton Pluto

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18 Evolution of Earth s Atmosphere First Atmosphere probably mostly H 2 and He These gases were probably lost to space early in our history because Earth's gravity is not strong enough to hold lighter gases Early Earth was not yet differentiated meaning it had no global magnetic field direct access of the solar wind which can strip away the atmosphere

19 Evolution of Earth s Atmosphere Second Atmosphere Greenhouse gases produced by volcanic outgassing (e.g. H 2 O, CO 2, SO 2 ) No free O 2 at this time (not found in volcanic gases). Ocean Formation - As the Earth cooled, H 2 O produced by out gassing could exist as liquid

20 Evolution of Earth s Atmosphere Oxygen Production Photochemical dissociation (breakup of H 2 0 by UV) Produced O 2 levels approx. 1-2% current levels Life! Photosynthesis

21 How do we detect a Planetary Atmosphere? Spectroscopy! This was how Titan s atmosphere was first detected by G. Kuiper Occultations Observe the dimming of a star s light as it passes behind a planet

22 What does an atmosphere do? creates wind and weather promotes erosion of the planetary surface Can warm the planet through the greenhouse effect We will discuss this more on Tuesday scattering and absorption of light absorbs high-energy radiation from the Sun (ozone absorbs UV) scattering of optical light brightens the daytime sky

23 What else does an atmosphere do? Earth s thick atmosphere protects us from high-energy cosmic rays Cosmic rays are high-energy charged particles When they strike the atmosphere, they produce a cosmic-ray air showers When cosmic rays strike the atmosphere, a chain-reaction of cascading particles is created this is called an air shower These showers can be detected on the ground Cosmic-ray detector in Tibet

24 What else does an atmosphere do? creates pressure can allow water to exist as a liquid (at the right temperature) inhibits evaporation and sublimation! In other words, you need atmospheric pressure to have liquid water! Cassini/Huygens DISR image of Titan

25 Pressure is created by atomic & molecular collisions. heating a gas in a confined space increases pressure, since the number of collisions increase (this is Gay Lussac s Law of gasses) Atmospheric Pressure A change in pressure results in a net force (think of why a balloon filled with helium rises). In an atmosphere this pressuredifference force is balanced by the gravitational force on the air creating an equilibrium known as hydrostatic equilibrium

26 The atmospheric scale height Pressure in an atmosphere decreases with altitude. In fact, it decreases nearly exponentially for several scale heights above the surface The scale height is essentially the thickness of an atmosphere More precisely, the atmospheric pressure decreases by a factor of 2.7 (e 1 ) for every scale height above the surface.