Transcript 37Temperatures

Where might we find life in the
Solar System?
Temperatures of planets
Recap
• Final assignments
– Class evaluations: counts as 100% for one lab,
must be completed before next Monday!
– Final homework/quiz: post course assessment
– Final exam, next Wednesday
– Lab sections this week
• How the Sun works and evolution of the Sun
– Nuclear reactions
– Stellar evolution driven by depletion of nuclear fuel
• Sun and lower mass stars convert H to He, then He to C,
then end their lives as white dwarfs + planetary nebulae
• More massive stars have longer chain of reactions until
they reach an iron core, then end lives as supernovae,
ejecting heavier elements back into interstellar medium
Where might we find life?
• Without going into details of biology, life as
we’re familiar with it requires the presence of
liquid WATER
• What do we need to find liquid water?
– Hydrogen and oxygen (H20)
– Temperature where water is liquid
• Between 0 and 100 C (32 and 212 F)
• 273 and 373K
• Plenty of H2O throughout the solar system, so
where are temperatures right?
What determines
temperatures in the Solar
System?
• External heat source: the Sun!
– Sun has a surface temperature of 5700 K
• But how hot will it get at different
locations in the Solar System?
Imagine you’re sitting near a fire that is burning
at a temperature of 2000 K and thus emitting
a lot of heat. If you sit there long enough
A. you’ll eventually heat up to 2000 K because
the energy just keeps coming
B. You’ll heat up, but eventually you’ll reach
some stable temperature that is cooler than
2000K
C. You’ll feel warmer, but your body won’t
actually heat up at all
The stable temperature that you eventually
arrive at depends on
A. How far away you are from the fire, but not
on what you’re wearing
B. How far away you are from the fire and on
what you are wearing
C. On what you are wearing but not how far
you are from the fire
D. The temperature of the fire, but not on how
far away you are or what you are wearing
Planetary equilibrium
temperature
• Temperature of planets are determined by a balance between
– Energy coming in: depends on distance from the Sun, size of
planet, and reflectivity
– Energy going out: depends on size of planet, the
temperature, and the material
• This argument applies to average temperatures, there will be
variation around the planet
– Locations where Sun is higher in sky will be warmer than
locations where Sun is lower
– Day side will be warmer than night side
• If planet rotates, then some degree of “equal toasting”
• If same side towards the Sun, more temperature
imbalance
Does this work?
reflectivity “real”
material
Temp
PLANET
BLACKBODY
Temp
Observed temp
Mercury
107 C
0.12
165 C
425 (day)
-175 (night)
Venus
5.3 C
0.75
-39 C
470 C
Earth
-36 C
0.29
-14 C
15 C
Mars
-81 C
0.16
-54 C
-50 C
What else can affect
temperature?
• If you are cold and want to warm up,
what can you do?
If you cover yourself with a blanket, you will
A. be warmer because the blanket absorbs
more heat from outside
B. be warmer because the blanket keeps heat
from leaving you
C. be the same temperature, but just feel
warmer
D. get colder, because the blanket prevents
heat from reaching you
Planetary atmospheres as
blankets
• Some planets have atmospheres that can act
as blankets, preventing heat from getting out
• However, a blanket can also prevent heat
from getting in!
• Planet atmospheres can act as “one-way”
blankets, preventing more heat from getting
out than blocking heat from getting in
– Works because of the nature of
continuous/thermal emission!
The Sun produces an underlying continuous
emission, but not an equal amount at all
wavelengths. What is the dominant type of
light coming from the
surface of the Sun?
A. Gamma rays
B. Ultraviolet light
C. Visible light
D. Infrared light
E. Radio light
The planets are warmed by the Sun to a
temperature of a few hundred degrees K
and thus, also produces thermal continuous
radiation. What is the dominant type of light
(emission, not reflection!) coming from the
surface of the planets?
A. Planets don’t have any emission, they only
reflect
B. Ultraviolet light
C. Visible light
D. Infrared light
E. Radio light
Planetary “Greenhouse” Effect
• Energy from the Sun comes in mostly in
visible light
• Energy from the planets leaves mostly in
infrared light
• If you have a blanket that lets in visible, but
doesn’t let out infrared, the planet will get
warmer!
• Some gases in planetary atmospheres have
exactly this property!
– Carbon dioxide (CO2), methane, several others
– Water vapor to a lesser effect
Planetary Greenhouse Effect
• Effect can be huge! Venus is hundreds of degrees
warmer than expected
– Runaway greenhouse effect: higher temps release more
greenhouse gases, which lead to higher temps, which
release more greenhouse gases, etc.
• Eventutally, process stabilizes at some temperature
that is hotter than the equilibrium temperature
• Venus is the prime example of the greenhouse effect
• However, it exists on other planets
– Earth is about 30 degrees C warmer than expected
– Mars is about 5 degrees C warmer than expected
Greenhouse effect on Earth
• Earth has a natural greenhouse effect that
makes it a bit warmer than expected based
on distance from the Sun
• Primary greenhouse gas is carbon dioxide
– carbon dioxide makes up only a very small fraction
of Earth’s atmosphere!
– Most of atmosphere is nitrogen and oxygen, but
these are not greenhouse gases
• Issue: carbon dioxide content is rising in
Earth’s atmosphere!
Carbon dioxide in Earth’s
atmosphere
• Recent record of carbon dioxide
– http://www.esrl.noaa.gov/gmd/ccgg/trends/co2_data_mio.ht
ml
– Not a huge amount (parts per million) but definitely important
…. And increasing!
• Longer term record
– http://www.ipcc.ch/graphics/ar4-wg1/jpg/ts2.jpg
• Very likely that human activity is causing the increase
– Fossil fuel burning generates greenhouse gases
– Deforestation lowers natural absorption of carbon dioxide
Earth and climate change
• We know that the greenhouse effect is real,
Venus makes this obvious
• Key question: is increased CO2 in Earth’s
atmosphere going to change the climate?
• Temperature data is hard to get because
there are a lot of natural fluctuations
• However, it does seem like the climate is
getting warmer
– Temperature records
– Ocean temperatures
– Glacial ice coverage is getting smaller
Is climate change natural?
• Earth certainly has undergone long term temperature
changes: ice ages, etc
• Recent rise in temperature appears to be much faster
than previous changes
• Difficult to understand as part of a natural cycle
• Observed dramatic increase in CO2 with temperature
change is particularly worrisome!
– Models suggest that it is the increasing CO2 that is
responsible for the increasing temperatures
• Most (but not every!) climate scientists think that
climate change is occurring and is human caused
Climate change: the future
• If temperatures are rising because of increasing CO2, they are
predicted to continue to rise
• Potential very significant impacts
– Ocean levels rise
– Ocean temperatures change
– Significant effect on human habitation
– Dramatic effect on species survival
• What to do?
– Hope this picture is wrong / spend lots of time arguing about
it
– Precautionary principle: if potential effect is severe, even if
you’re not 100% positive it will happen, you take action to
avoid it!
– Individual actions definitely help, but issue has to be tackled
on a larger level as well