Astrobiology 574 -- Planetary Habitability Instructor: James
Kasting Filling in this week: Ravi Kopparapu
Slide 2
What is astrobiology? Astrobiology (also known as exobiology)
is the search for life off the Earth Carl Sagan was a pioneer in
this field, even if the field may not have formally existed when he
began his career Sagan, like many of us, was most interested in the
search for intelligent life Doesnt it make sense, though, to look
for simple life first? Carl Sagan (1934-1996)
Slide 3
Where can we look for life? 1.Within our own solar system
MarsEuropaTitan Question: Are any of these bodies habitable? If so,
why?
Slide 4
Where can we look for life? 2.On planets around other stars
Picture at right shows the Kepler target field Keplers goal is, or
was, to determine the frequency of Earth-like planets around other
stars
Slide 5
What is life? If we are going to search for life on other
planets, we first need to decide what we are looking for One
definition: Life is a self- sustained chemical system capable of
undergoing Darwinian evolution --Jerry Joyce This definition,
however, is better suited to looking for life in a laboratory
experiment than for searching remotely on planets around other
stars Jerry Joyce, Salk Institute
Slide 6
First requirement for life: a liquid or solid surface It is
difficult, or impossible, to imagine how life could get started on
a gas giant planet Need a liquid or solid surface to provide a
stable P/T environment This requirement is arguably universal
Slide 7
Second requirement for life (as we know it) : Liquid water Life
on Earth is carbon- based (DNA, RNA, and proteins) and requires
liquid water So, our first choice is to look for other planets like
Earth Subsurface water is not relevant for remote life detection
because it is unlikely that a subsurface biota could modify a
planetary atmosphere in a way that could be observed (at modest
spectral resolution)
Slide 8
Special properties of H 2 O Strong dipole moment has several
useful consequences Good solvent for polar molecules Hydrogen
bonding in DNA High heat capacity helps moderate climate on planets
(like Earth) with large oceans The solid is less dense than the
liquid, i.e., ice floats! All of this leads to the concept of the
habitable zone around stars
Slide 9
The ZAMS habitable zone
http://www.dlr.de/en/desktopdefault.aspx/tabid-5170/8702_read-15322/8702_page-2/
The liquid water habitable zone, as defined by Kasting et al.
(1993). Figure applies to zero-age-main-sequence stars The
habitable zone is relatively wide because of the negative feedback
provided by the carbonate-silicate cycle
Slide 10
The nine planets of the Solar System Ref.: J. K. Beatty et al.,
The New Solar System (1999), Ch. 2. eight No Pluto!
Slide 11
http://starryskies.com/solar_system/planets.gif Gas giants Ice
giants Terrestrial planets Solar System planet types
Slide 12
Our home planet, Earth Earth is by far the most interesting
planet because it harbors life, including us As already mentioned,
planets with liquid water on or beneath their surfaces are possible
homes for carbon- based life
Slide 13
Question: Why is Earths climate stable? Its not obvious that it
should be, given that solar luminosity increases significantly with
time Possible answers: 1.Stabilizing, largely abiotic feedback
processes (e.g., carbonate-silicate cycle) 2.Stabilizing biotic
feedback processes (the Gaia hypothesis) 3.Because we were lucky
(the Rare Earth hypothesis)
Slide 14
The Gaia Hypothesis James Lovelock 1979 1988 Earths climate is
regulated by (and for?) the biota Lynn Margulis (1938-2011)
Slide 15
GaiaThe Greek goddess Gaia is Mother Earth. She is from whom
everything comes, but she is not quite a divinity, because she is
Earth. She bore the Titans as well as monsters like the hundred
armed men, and some of the Cyclopes - others were sons of Poseidon.
She was the daughter of Chaos, and the mother of all creatures
(according to some). She was the first and the last, and wanted all
of her children, no matter what. She was primarily spoken of as a
Mother of other Gods, rather than having her own myths.
http://www.paleothea.com/Majors.html
Slide 16
The Rare Earth hypothesis Earth is lucky in a number of
respects Life itself may be commonplace, but complex life, i.e.,
animal life, is rare in the universe Do we believe this?
Copernicus/Springer-Verlag (2000)
Slide 17
We will look at these questions. However, well also do a tour
of the Solar System on the way to see what our neighboring planets,
and particularly their atmospheres and climates, are like
Slide 18
Venus UV image (false color) from the Galileo spacecraft Planet
is nearly featureless in the visible 93-bar, CO 2 -rich atmosphere
Surface temperature: 730 K Practically no water Very high D/H ratio
(~150 times Earths value) Image courtesy of NASA
Slide 19
Venus as seen by Magellan Image made using synthetic aperture
radar (SAR) http://www.crystalinks.com/venus703.jpg
Slide 20
http://www.kidsgeo.com/geography-for-kids/0012-is-the-earth-round.php
Earth topography Earths topography shows tectonic features such as
midocean ridges
Slide 21
http://sos.noaa.gov/download/dataset_table.html Earth
topography Earths topography shows tectonic features such as
midocean ridges and linear mountain chains
Slide 22
Mars from HST (Hubble Space Telescope) Mars is small Earths
radius 1/10 th Earths mass Thin CO 2 -rich atmosphere (~6-8 mbar)
Mean surface temperature: 218 K ( 55 o C) Polar caps of frozen H 2
O and CO 2 From: NASA Planetary Photojournal
Slide 23
MARS PATHFINDER Twin peaks view Today, the surface of Mars is a
frozen desert
Slide 24
Courtesy of NASA Nanedi Vallis (from Mars Global Surveyor) ~3
km River channel But there are lots of fluvial features on the
heavily cratered southern highlands Mars was wet early in its
history, and it may have been warm, as well
Slide 25
The famous Martian meteorite This discovery was part of what
jump-started NASAs interest in astrobiology in 1996 Was this
evidence for martian life? Al Gore was convinced, so the discovery
was announced at a White House press conference D. S. McKay et al.,
Science (1996)
Slide 26
SNC Noble Gases vs. martian Atmosphere (from Viking) SNC stands
for Shergotty, Nahkla, and Chassigny, three type-class meteorites
known to originate from Mars The strongest evidence is the plot at
right, which shows the composition of gases trapped within the
meteorites compared with measurements of Mars atmosphere made by
the Viking landers
Slide 27
Martian nanobacteria? The photo at right of a sample from
ALH84001 made headlines in many newspapers The scale is very tiny
(photograph was taken with an SEM, or scanning electron microscope)
Later skeptics speculated that this was just beading up of the gold
film used to prepare the sample 200 nm
Slide 28
Discovery of extrasolar planets The other thing that
jump-started astrobiology in 1996 was the discovery of the first
extrasolar planet, 51 Peg, around a main sequence star Alex
Wolsczan here at Penn State had discovered pulsar planets 5 years
earlier G. Marcy and P. Butler (circa 2000)
Slide 29
Known extrasolar planets Since then, the number of discovered
exoplanets has exploded 4158 probable extrasolar planets identified
as of Aug. 7, 2013 702 by radial velocity 25 by microlensing and
direct imaging 3431 unconfirmed Kepler candidates Info from
www.exoplanets.org http://exoplanets.org/massradiiframe.html
702
Slide 30
Spectroscopy of extrasolar planets The focus now has shifted
towards doing spectroscopy of extrasolar planet atmospheres By
breaking light (or infrared radiation) down into its component
wavelengths, one can look for signatures of different
molecules
Slide 31
Primary transit spectroscopy Primary transit is when the planet
passes in front of the star The planet appears larger or smaller at
different wavelengths depending on how strongly the atmosphere
absorbs Hence, the transit appears deeper at wavelengths that are
strongly absorbed, allowing one to form a crude spectrum Habitable
Planets book, Fig. 12-4
Slide 32
HST observations of HD209458b T. Barman, Ap.J. Lett. (2007)
Key:Green bars STIS data Red curves Baseline model with H 2 O
(solid) and without (dashed) Blue curve No photoionization of Na
and K
Spitzer Space Telescope Transiting extrasolar planets have been
studied in the thermal infrared using the Spitzer Space Telescope,
currently in operation 0.85 m mirror, once cryogenically cooled
with liquid He (now passively cooled), Earth-trailing orbit
http://www.spitzer.caltech.edu/about/ index.shtml
Slide 35
HD 209458b: Evidence for a thermal inversion High fluxes at 4.5
and 5.8 m represent emission by H 2 O, rather than absorption H.A.
Knutson et al., ApJ 673, 526 (2008) Data Model (with H 2 O in
absorption)
Slide 36
Plans for the future TESS (the Transiting Exoplanet Survey
Satellite) has just been selected as a Small Explorer mission TESS
will look for transits around brighter (and closer) stars than
Kepler Planets must have short-period orbits because TESS doesnt
stare long at one spot With luck, it may find some Earth-size
planets that might eventually be characterized spectroscopically
Artists conception of NASAs upcoming TESS mission
Slide 37
Transit spectra from JWST NASAs James Webb Space Telescope is
scheduled for launch in 2018 JWST will be capable of taking
detailed spectra of transiting exoplanets It is thought to be
marginally capable of obtaining a spectrum of an Earth-size planet
orbiting a nearby M star But we would need to find a new nearby
transiting system (maybe with TESS?) NASAs planned James Webb Space
Telescope
Slide 38
What wed really like to do is to build a big TPF (Terrestrial
Planet Finder) telescope and search directly for non-transiting
Earth-like planets around -- This technique is referred to as
direct imaging We can also look for spectroscopic biomarkers (O 2,
O 3, CH 4 ) and try to infer the presence or absence of life on
such planets TPF-I/Darwin TPF-C TPF-O