Slide 1

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

Slide 33

http://www.nasa.gov/mission_pages/spitzer/news/070221/index.html Secondary transit spectroscopy

Slide 34

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