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 Atmospheres
2/22/07
Ulf Merbold (1941 – )
German Astronaut
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 10:30AM12:00Noon – in room 330.
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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
PTYS/ASTR 206
Planetary Atmospheres
out to a distance of about 20 km 2/22/07
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.
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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 Atmospheres
2/22/07
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.
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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
PTYS/ASTR 206
Earth
Planetary Atmospheres
2/22/07
Jupiter
Titan
Jupiter
Saturn
Neptune
Uranus
Venus
Earth
Mars
Titan
Mercury
Moon
PTYS/ASTR 206
Galilean
Satellites
Planetary Atmospheres
2/22/07
Triton
Pluto
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
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 Atmospheres
2/22/07
How do we detect a Planetary Atmosphere?
Spectroscopy !
• This was how Titan’s
atmosphere was first detected
by G. Kuiper
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
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 !
PTYS/ASTR 206
Cassini/Huygens DISR image of Titan
Planetary Atmospheres
2/22/07
• 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 pressuredifference force is balanced by the
gravitational force on the air creating
an equilibrium known as “hydrostatic
equilibrium”
PTYS/ASTR 206
Planetary Atmospheres
2/22/07
Atmospheric
Pressure
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 206
Planetary Atmospheres
2/22/07