ptys/astr 206planetary atmospheres 2/22/07 ulf merbold (1941 – ) german astronaut terrestrial...

PTYS/ASTR 206 Planetary Atmospheres2/22/07

Ulf Merbold (1941 – )German Astronaut

Terrestrial Planetary Atmospheres

“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.”


Announcements

• Reading Assignment– Chapter 9 (review + read the rest of the chapter)

• 3rd 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.


Today

• Finish discussion of impact cratering– Effects of an impact on Earth

• Planetary Atmospheres– 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?


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.)


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 speed2

= 1017 Joules

= 1200 Hiroshima Atomic Bombs


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


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)

• 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

Bikini Atoll atomic bomb test July 1, 1946


The Sum of all Effects

• destruction of vegetation over an area 800 to 1500 km2

• 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.


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.


• 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.

Chicxulub Crater: A somewhat larger impact event !


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


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)


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?


Planetary Atmospheres

• 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


Large cool objects more easily can retain an atmosphere

• Requirements for an atmosphere

– Appropriate chemical(s) in molecule form (H2, N2, CO2, 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 JupiterTitan


EarthVenus

Mars

Titan

TritonPluto

Mercury

MoonGalileanSatellites

Jupiter

Saturn

UranusNeptune


Evolution of Earth’s Atmosphere

• First Atmosphere– probably mostly H2 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



• Second Atmosphere– Greenhouse gases produced

by volcanic outgassing (e.g. H2O, CO2, SO2)

– No free O2 at this time (not found in volcanic gases).

– Ocean Formation - As the Earth cooled, H2O produced by out gassing could exist as liquid



• Oxygen Production – Photochemical

dissociation (breakup of H20 by UV)

• Produced O2 levels approx. 1-2% current levels

– Life !

Photosynthesis


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


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


• 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

What else does an atmosphere do?

Cosmic-ray detector in Tibet


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


Atmospheric Pressure

• 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)

• A change in pressure results in a net force (think of why a balloon filled with helium rises).

• In an atmosphere – this pressure-difference force is balanced by the gravitational force on the air creating an equilibrium known as “hydrostatic equilibrium”


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 (e1) for every scale height above the surface.

ptys/astr 206planetary atmospheres 2/22/07 ulf merbold (1941 – ) german astronaut terrestrial...

Documents

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