Download Life on Other Planets

Searching for Signatures of Life in
Other Planets
Assigned Reading
Units 83 and 84
Final Announcements
Exam # 3: grades are posted in the OWL
Gradebook; solutions will be posted either today
or tomorrow on the course website.
Final exam: Tuesday, Dec 13th, in Hasbrouck Lab
20, at 10:30 am. Duration: 2 hours.
Bring student ID
Bring pencil # 2
Open notes/open book
(no devices with wired/wireless connections)
What is life?
Theory
A Working Definition of Life
•  Something that is alive
–  has both the instructions and the machinery for selfreproduction (either alone or in small groups).
–  metabolizes fuel and produces waste.
–  undergoes evolution through natural selection.
•  Note that this theory is based on what we
have observed. Like all scientific theories it is
open to modification given conflicting
evidence.
•  Like all classification schemes there are some
grey areas.
–  humans, animals, plants, bacteria are alive
–  viruses border on fringes of life
•  require other life-forms to reproduce
•  Have no mechanism for metabolizing energy
•  So far, all known Life on Earth is based on
amino-acid molecules (proteins and DNA)
–  Based on 6 fundamental elements: hydrogen, carbon,
oxygen, nitrogen, sulfur, and phosphorus;
•  Considered irreplaceable until the recent experiment of
replacing phosphorus with arsenic in bacteria dredged from
the bottom of the salty, arsenic-rich Mono Lake
•  Could we have silicon-based life (instead of carbon-based)?
So how did life originate on Earth?
Possibility #1: Life spontaneously generated on its own
–  Building blocks of life: water, ammonia, and methane, can
combine to form amino acids, all present in Earth s early
atmosphere and near volcanic activity.
Possibility #2: Earth could have been seeded by life
from space.
–  Simple amino acids are routinely found in space
–  Earth was not the first planet to form that could support life
… and significant mass exchange occurred between planets
early on.
–  Evidence that simple life forms could survive long trip
buried in meteors.
Possibility #3: ?
The Most Likely Place to Find Extraterrestrial
Life?
Definition: The Habitable Zone
There is a region around each star where the average
temperature of a planet falls between 0 ºC
and 100 ºC. The planets with orbits contained in this region
can support liquid water.
Venus
Earth
Mars
habitable zone
Jupiter
Solar System
Planets
Venus Climate History
•  Although Venus and Earth are believed to
have started with the same amount of
volatile gases, they followed very different
evolutionary paths.
•  The early Venus may have been habitable
with water oceans
•  However, the Sun s luminosity increased
by 30% over the past 4 billion yrs.
Venus water was most probably lost to
space via a runaway greenhouse effect
–  Venus closer proximity to the Sun increased
the amount of water vapor and CO2 in its
atmosphere, which enhanced the greenhouse
effect in a positive feedback loop
–  Over billions of years, Venus may have lost an
ocean of water this way.
Mars Climate History
•  Mars may have had a much warmer climate in its
past
–  Geological evidence from erosion patterns suggest that
liquid water was stable on the surface.
•  Warming was probably due to an enhanced
greenhouse effect.
–  A CO2 atmosphere at 400 times present density would
works for the present Sun
•  Volcanism may have been a source of CO2
–  However, the faint young Sun would require that Mars
had an extra means of warming the surface.
•  Methane [CH4] has been postulated as the missing
greenhouse gas
•  Source of CH4 for early Mars?
Other Possible Locations of Life in the Solar System
•  Mars is our best candidate for having life early on, just after its
formation (Earth has fossil-evidence of life 3.8 billion years ago)
•  Europa (Jupiter moon): water under its surface!
–  Cf Lake Vostok in Antarctica
•  Titan (Saturn moon): thick atmosphere, nitrogen, possible
oceans of methane, but no water and very cold!
Origin of the Terrestrial Atmospheres
•  Terrestrial planets did not capture their
own atmospheres
–  Too small and warm
–  Our atmospheres are considered
secondary
•  Instead, terrestrials were enriched with
impact delivered volatiles.
–  Water, methane, carbon dioxide and other
gases were trapped in the Earth s interior
rock
•  Venus and Earth, forming relatively close
together in the solar nebula, probably
started with a similar inventory of
volatiles.
What Kind of Life Could be Out There?
How Many Planets out there?
There are about 500-600 known planets circling
around other stars (other than the Sun).
Many are likely to be barren (too hot or too cold),
but some may be `habitable .
Habitable = a narrow region of orbits around a
star where water can be in liquid form
Future NASA (and other) missions may establish
whether this is the case, but in order to do that,
we need to understand how we recognize that a
planet may host some life.
What are the Biosignatures of Life?
Different Forms of Life
•  Simple primitive life – single celled organisms
(Prokaryotes)
•  Advanced multi-celled life – animals, plants, etc.
(Eukaryotes)
• 
• 
Intelligent Life – has ability to reason and create.
Advanced Civilization – has ability to organize, create
as a group.
Other Planets
OGLE-2005-BLG-390Lb
A Earth-size planet rotating around a red
dwarf star (low-temperature star), very
cold (-364 F), with an orbital period of
about 10 years.
The planet-star system is located about
6100 lyrs from the Earth, towards the
Galaxy s bulge.
XO-1b
Located 600 lyrs from the Earth, this
planet is orbiting (in 4 days) a sun-like star, in the constellation Corona
Borealis.
What are we likely to `see
•  We will not be able
to see the
extrasolar planet
•  Everything we learn
about the planet will
have to come from
`obvious
characteristics
•  The signs of life
must be a global
phenomenon
Detecting Distant Signs of Life
Life can provide global-scale
modification of (GAIA hypothesis):
–  A planet s atmosphere
–  A planet s surface
–  A planet s appearance over
time
Astronomical Biosignatures
•  Features in spectra indicative of
life.
•  Must be global-scale, and they
need to be easily separated from
the central star.
H 2O
H 2O
Planetary Environmental Characteristics
•  Is it a terrestrial planet? (Mass, brightness,
color)
•  Is it in the Habitable Zone? (global energy
balance?, type of star?)
•  Does it have an atmosphere?
–  clouds, possibly surface
–  Greenhouse gases: CO2,H2O vapor present?
–  UV shield (e.g. Ozone [O3])?
•  What are its surface properties?
–  Presence of liquid water on the surface
–  Land surface cover
?
G. Chin GSFC
The Planet We Know and Love
Atmospheric Greenhouse Effects
σTa4
σTs4
Fo (1-a)
The atmosphere is sufficiently
transparent at solar wavelengths
to allow some sunlight to
penetrate to the surface
–  Ozone and water vapor are
primary absorbers
σTa4
σTs4 - σTa4
The atmosphere is (partially)
opaque at thermal wavelengths,
reducing escape of heat to space
–  Water vapor, CO2 and Ozone are
the primary absorbers
Carbonate-Silicate Cycle
•  Planetary processes can regulate the atmospheric
greenhouse gas concentrations
–  Atmospheric CO2 dissolves in the ocean
–  Rainfall erodes silicate rocks and carries it to the oceans
–  The silicate minerals react with the dissolved CO2 to form carbonate
minerals which fall to the ocean floor
–  The sea floor carbonates are eventually subducted
–  The subducted rocks melt to form CO2 rich magma, which is
released to the atmosphere in volcanic eruptions.
CO2
The Photosynthetic Red Edge
Life Changes a Planet s Surface
Harry Lehto
Harry Lehto
The Effect of Surface Type in the Visible: Got Life?
Crisp, Meadows
In Summary
•  We need to understand the history of our own
Solar System planets, to interpret data from
exoplanets.
•  First and foremost, we need to understand
how the Earth (the only planet we currently
know to host life) `looks from the `outside .
•  There are many known exoplanets, although
we don t know if any is hosting life (unlikely,
as they orbit too close to their stars).
•  Future space missions may give us more
planets, and the answer to `alien life.