PTYS/ASTR 206Planetary Atmospheres 2/22/07 Ulf Merbold (1941 – ) German Astronaut Terrestrial Planetary Atmospheres “For the first time in my life, I saw.
Post on 16-Jan-2016
212 Views
Preview:
Transcript
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.”
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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.
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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?
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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.)
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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.
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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.
PTYS/ASTR 206 Planetary Atmospheres2/22/07
• 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 !
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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)
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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?
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
EarthVenus
Mars
Titan
TritonPluto
Mercury
MoonGalileanSatellites
Jupiter
Saturn
UranusNeptune
PTYS/ASTR 206 Planetary Atmospheres2/22/07
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
Evolution of Earth’s 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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
Evolution of Earth’s Atmosphere
• Oxygen Production – Photochemical
dissociation (breakup of H20 by UV)
• Produced O2 levels approx. 1-2% current levels
– Life !
Photosynthesis
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
• 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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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”
PTYS/ASTR 206 Planetary Atmospheres2/22/07
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.
top related