INTRODUCTION TO ASTRONOMY
INTRODUCTION TO ASTRONOMY AST 301
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AST 301ReVieW for exam 5 This exam covers all of stellar evolution described in Chapters 20 21 and 22 except for sections 225228 on black holes which we will cover on the next exam Nearly all the material was covered in class except that I will leave it to you to read the sections describing the phenomena that can occur when stars evolve in binary systems 206 211 although you will be tested on this material and need to understand it in order to read later sections e g 223 Also remember that the section on star clusters at the end of chapter 20 205 should be supplemented with the material we postponed from section 196 on the same subject I strongly recommend that you try all the Review and Discussion and True FalseMultiple Choice questions at the end of each chapter except those on black holes they are nearly all good ones at the level that will be typical on the exam In factI will as usual take a few of the exam questions from the endofchapter and online questions HoweverI do not recommend that you spend most of your study time trying to find the answers to these questions they should be attempted after you have studied as a selftest although a quick look at them might be good to give you an idea of how much you understand A good way to review is to try to tell the story of the evolution of stars of different masses starting with the main sequence phase making sure you can explain all the stages of evolution and the differences between the evolution of lowmass and highmass stars Each time you use some new terminology e g degenerate core try to explain what you mean as if you were explaining this to someone with no background Here are some sample questions to see if you are prepared to take the exam As usual most of these tend to be a little more difficult than the average exam question 1 How was the chemical composition of the sun different 3 billion years ago from what it is now a more hydrogen now b more helium now c more heavy elements now d it won t change until the sun becomes a red giant 2 When a star evolves from the main sequence to the red giant phase a the core gets hotter and the luminosity increases b the core gets cooler and the surface gets hotter c the core gets hotter and the luminosity decreases d the core and the surface both get cooler 3 Which of the following do open clusters of stars tell us a They allow us to estimate the size of our Galaxy b As standard candles they can be used to obtain the distances to other galaxies c That when our galaxy was forming it had a roughly spherical shape d Star formation has been occuning more or less continuously over the past 10 billion years 4 Roughly how long does it take a stellar iron core to collapse a One second b One year c A few million years d Forever 5 Why do the cores of massive stars evolve into iron but not heavier elements a The attempt of the star to fuse iron disrupts the stability of the core by requiring energy b The temperature never gets high enough to allow the fusion of heavier elements c The star goes supernova before the core has a chance to make heavier elements d Iron does not become degenerate 6 If the fusion of lighter elements with iron to form still heavier elements does not occur in a star how is it that stars are able to synthesize many elements heavier than iron a neutron capture in the sprocess b helium capture c photodisintegration d neutronization 7 What is a major reason for believing that nucleosynthesis of some heavy elements occurs in stars by means of neutron capture a Detection of technetium in the atmospheres of some red giant stars b Detection of the neutrino burst from SN 1987A c The fact that there is a peak in the cosmic abundance pattern at iron d The discovery of pulsars which are neutron stars e The statement is not true heavy elements are not made by neutron capture 8 Why did it take until recently to realize that gamma ray bursts are extremely far away a Previous gammaray telescopes were not sensitive enough b Their spectroscopic parallax could not be measured because their luminosity was unknown c Only recently were afterglows observed that allowed the distances to be estimated using spectral lines d Gamma ray stars that correspond to the gamma ray bursters have only recently been discovered 9 What type of object were millisecond pulsars likely to have been before becoming what they are presently a single pulsars b novae c Xray bursters d gamma ray bursters 10 The masses of neutron stars a must be at least 8 solar masses b must be at least 14 solar masses c are only known for those neutron stars in binary systems d are only known for neutron stars that are also white dwarfs e are extremely large compared to the sun Stellar Explosions Ch 21 First a review of low mass stellar evolution by means of an illustration 1 showed in Class You should be able to talk your way through this diagram and it should take at least half an hour Remember that all stars less massive than about 8 Mo go through these phases except not the helium flash above about 2 Mo What is the major reason why the advanced evolution of higher mass stars is so different 7an my 1mm Mm mmnum u39 WWW Mu n39olmmu an nurmlarvuln aquot ASYMPTOVIC SIAM BRANCH STAR Hbummg mu Hahuml g shs l Daganarme c cove HORIZONTAL BRANCH srAR Manning she m39 Hebumlng cum mi HEUUM wquot man 10 m3 5 g m E g 139 nunam y H bnmmg m degeueme core 4 nm emur w BRANCH sun m 5 buming ammpa uegcncms He core w 39 39 MAW SEQUENCE mono mun moan woo 0 STAR SUN Swlnue Hbummg cure gt Before discussing high mass stellar deaths don t forget the material in Sec 211 nova explosions by mass transfer from a RG to a WD companion This is really a continuation of sec 206 on binary star evolution Read this on your own and I will only discuss very briefly in class You will find it extremely useful throughout the rest of the course to understand the idea of an accretion disk that is introduced in this section They will come up again and again Death of a HighMass Star In short Envelope explodes as a core collapse supernova The core implodes and ends up as a neutron star or more massive a black hole Let s see how this occurs Remember this is all theoretical calculations but later you ll see that there is surprising observational confirmation for these calculations Core is layered like an onion with heavier elements closer to center since they are the ashes of a previous fuel He C 0 Ne Mg Si Fe These are the main elements produced up to this phase because they are produced by adding helium He alpha particles to heavier and heavier nuclei Iron Fe is in bold in this list because it is the end of the line for stellar nuclear fusion see below Question to see if you understand nuclear fusion why is so much easier to burn He with say 0 rather than CO or OO Hint why do main sequence stars burn hydrogen rather than something heavier On the next page is a cutaway drawing of what the inside of a massive star might look like as it completes more and more burning stages MASSIVE MAIN SEQUENCE H bums w Ha STAR In cumacme care Nonbummg anvelape Ha bums In C H bums a He Nonbumlng envelone I burnan to Nonbuming envolupe The ash a an quotIch became m mm In m ran 35 I ll WVES m Daganarate Fa care HIGHLY EVOLVED 5 Si bum mg m Fa MASSlVE STA R rala Iron II The deem ms tbwiw 39onlwlika shal lsol mm Wining A u highquotms slarum uu 1 mid up a laymd L c In umwv mm Frogmsilvdy mm admumi Kings 31 slmulm mm hummgj und derpwaud deeper willn u Ihu um First way to get a supernova core collapse Massive stars burn nuclear fuels up to iron Fe But nuclear fusion of iron does not produce energy it uses energy This leads to loss of pressure support gt core collapse Temp is so large 10 billion K that the gamma ray photons Wien s law have huge energies and photodisintegrate the iron into protons and neutrons This absorbs even more thermal heat energy so the core collapses even faster Gravity is having its way The protons combine with electrons to give neutrons and neutrinos which escape immediately later we ll see that they have been accidentally observed in one case The core has been converted into one massive collection of neutrons all collapsing under gravity Now nothing can stop the core collapse except neutron degeneracy pressure which sets in at a density of about 1012 grams per cc Analogous to electron degeneracy pressure which saved lower mass star cores as white dwarfs but at much higher densities But the collapse is so violent that the core overshoots this density and then bounces gt violent shock wave propagating outward All this takes 1 second Remember star took at least millions of years to get to this phase in its evolution The shock wave blasts through all the overlying layers gt star explodes as core collapse supernova We ll see another way to make a supernova below The next two pages show a simulation of the explosion deep in the core and an illustration showing the chain of events that leads to this spectacular death for massive stars 5 milliseconds 10 milliseconds 20 milliseconds The Core of a Supernova These unang from a 6 x10 K blurJ Maucv mat mums of lzn szrlfcncri cure of he slar swcrcompulcr sumat on show the cow of a massive calmed blarki produces a shock wave Inszabulmcs generate turbulent sla during at rst 20 mrlhsu nnds of a supernova cxplos on The cadres that gmw as me shock wave moves olltward K wtm mquot Adam colors m ol C cmpezatms ma urge how 2x 10quot K mu 0 Suvan3 Unwmsvtv cv Mama am chu chi fJASMGSIO The inside clan expluding star 5 shown in his mathematical mmlnl of a supnrnm39a explosion The image ham rcpmsems a region 15 km on u side near lie center of he stm luwer lull Black arrows imlil lm mnlion of the gasl 39l39urbulenl malarial blue has buunund nll39 Hm c s rushing outward 1r meet the rest if Ihe slur which is falling inward red al upper right The innermost parl of 11m slur is forming a neutron star red at lower lc To show the entire star ill lhis scale this gure would have 10110 H km in diameter Juurlusy Adam Burrows ahn Hayes and Bruce FijiXE II in l mu man mwhcn degmmlaf n u m r v 91mm anxwpmmulw M NH m m u my Minimum cave V mm m the m 0 ml l m u39 rln my wlwlu u n r m MW 9 I m H www mm gave at emivg a masslvosl r ways B As mu mmm wi In new mnammo Mm mgu Hulmemmlgmm mymmlws Phawlsmlwmle mquot ammw w 0mm ammunwudmmmnemmrs 0 39 M l aixmulmmrhc neumm mm M Nsulnnn w pmmmamm an mnmwp m m mu 9 w e a am 5 Nulvnn 5w 7 E and Minn MMM ma comm mm 7 IN mm a Mulvmv m E bvwnvg rennin n ypu wemmquot 51 Th Ihvdlt mmmm mmuqn w my nyars a 1 star a TM mandw mm s swamw bum 91mm an m wwm lmwaa mm m In mm Many supernovae SN have been observed in historical times in our own Galaxy visible to the naked eye sometimes in daylight Best example is SN 1054 AD the Crab Nebula Text has great images of what the remains of the explosion look like today nearly 1000 years later A SN produces a billion solar luminosities in just a few hours or less Until discovery of gamma ray bursts ch22 these were gram for gram the most luminous objects in the universe That is why they can be observed even in very distant galaxies eg 100s of Mpc away or even at the edge of the observable universe later we ll see that they play a crucial role as the standard candles that have led to the belief that there is a dark energy that pervades the universe A second way to get a supernova A white dwarf accreting matter from a binary companion This increases the mass of the white dwarf If white dwarf mass gets larger than 14 Mo the Chandrasekhar limit electron degeneracy pressure can t support the white dwarf against gravity and the white dwarf collapses As it collapses it heats up carbon fusion occurs explosively gt carbon detonation supernova Observationally there are two classes of supernovae which differ in their composition and their light curves brightness vs time Type I H poor gt carbon detonation SN Type II H rich gt core collapse SN You don t have to memorize the types just that there are two of them and how they differ See Fig 219 for good illustration of the two mechanisms The SN explosion produces a fastmoving expanding shell of gas observed as supernova remnants SNRs E g Crab Nebula explosion in 1054 AD recorded by Chinese and Native Americans Hundreds of these are known see illustrations in book for the interesting forms that are produced How can you prove that the SNR is from the 1054 AD SN Observed radial velocities of a few thousand kmsec gives age which comes out to be about 950 years as it should See Discovery 212 on p 554 From the observed number of SNe supernovae in our own and other galaxies we expect about 1 SN per 100 years in our Galaxy But the last one was seen 400 years ago Tycho s SN So we are overdue Here is an image of the Cas A SNR in Xrays left and at radio wavelengths right The Xrays are emitted because the gas is so hot million degrees K while the radio emission is from electrons that are gyrating at nearly the speed of light around the strong magnetic field in the remnant this is where it is believed that cosmic rays are accelerated a RIVUG b EIVUXG SN 1987A in the Large Magellanic Cloud LMC Fig 217 and Discovery 2171 on p 550 This was a major event for astronomers since by far it s the nearest SN since we ve been observing them It outshined the entire LMC galaxy at its peak See illustration below RIUXG Supernova 1987A swernuva appeared m a neaer galaxy called lie Large Magellamc Cloud LMC r l987 Thrs photograph taken soon after the illscoverv shows a women nl he LMC hai includes the supernova lSN l987Al and a huge H II reg39or called the Tarantula Neoula SN 1987A was he rst supernova in almost 400 years that was DHng enough to be sum without a lEleSCOpC European Snumem Observatory SN 1987a gives fiarly good confirmation of the core collapse model for Type II SN In particular an unexpected neutrino burst was observed about 20 hours before the explosion was observed visually by underground neutrino detectors in Japan and US The delay is because most of the light from the SN has to diffuse through the thick exploding envelope while the neutrinos pass right through But the progenitor object preiSN was a blue supergiant not red as expected and the light curve was strange so it is something of an anomaly SN as distance indicators If all SN reached the same peak luminosity in their light curve then we could use them as standard candles to get the distances by observing supernova light curves since we know the luminosities of nearby supernovae in galaxies whose distances we know by other means Actually one subclass of supernovae are found to have remarkably similar peak luminosities Later we ll see how these have been used to map the expansion of the most distant parts of the universe and to discover that most of the universe is apparently filled mostly with the most mysterious thing in astronomy quotdark energy quot Formation of the elements This is an especially important subject Where do all the 110 or so known different elements from H He to uranium lead come from Except for H and He they come from stellar nucleosynthesis nuclear fusion in stars Look at the pattern of abundances of t as a function of their atomic weight in Fig 2113 p 556 You can see that the heavier elements are less abundant than the light elements and that there are peaks at C N O and at Fe Most heavier elements up to iron are made by helium capture which we discussed earlier Get C a O 9 Ne 9 Mg 9 9 Fe Their masses are divisible by four do you understand why Proton capture makes the lower abundance elements whose mass is not divisible by four These are the dips between the minor peaks in the figure Neutron capture mainly on iron Two types 1 5 slow process takes place in red giant interiors stops at bismuth 209 2 r rapid process probably takes place in supernova explosions goes out to thorium 232 uranium 238 plutonium 242 Observational evidence for stellar nucleosynthesis 1 The theoretically predicted abundances agree with observations Fig 2113 eg the peaks at C O and Fe and even the patterns in between and the s process patterns This is amazing agreement considering that we are not even sure about the details of stellar explosions 2 Technetium this element is predicted to be produced in the s process but it is radioactive and decays in 200000 years Yet it is observed in the atmospheres of some red giant stars gt must have been produced in the interior and mixed to the surface where the red giant wind will spew it back into the interstellar medium This is direct evidence that sprocess nucleosynthesis occurs in the cores of stars 3 Supernova light curves see Fig 2118 For Type I supernovae the light curves can be fit by the radioactive decay of nickel 56 55 days and cobalt 56 78 days these are predicted to be some of the main nuclear products of nucleosynthesis in carbon detonation supernovae We don t actually see the nickel and cobalt but the timescales for decline match very well And the gamma ray line emitted by cobalt 56 has been observed from a supernova in another galaxy Taken together this is strong evidence that the theory of stellar nucleosynthesis in stars is correct and also supports the explosion models for supernova explosions Life in the Solar System AST 309L Scalo Note Your textbook has excellent discussions of these topics presented independently Our solar system has a large number of objects orbiting the sun terrestrial planets Jovian planets planetary moons comets asteroids We want to narrow down the list for searching for life Mostly search for places where liquid water could exist but also Titan since other biochemistries are possible Good idea to review the general material on our solar system that you encountered in AST 301 here First thing to consider is general requirements and then to see why certain planets in our solar system seem like very low probability choices for life searches Following the textbook there are 3 general requirements we should consider 1 Elements H C N 0 you should know by now why these are important to have these are found everywhere we look so we have the fortunate coincidence that the just the right elements needed for our kind of life are also the very abundant in everything from quasars to comets Since organic compounds made from these could be delivered by asteroids or comets any planet could also contain the basic organic building blocks for life if it can hold onto them andor not destroy them 2 Energy source needed to overcome energy barriers in chemical reactions leading to more complex molecules But there are many sources a Sunlight most important on Earth today and what we used to define habitable zone But for producing organic molecules probably needed to utilize the Sun s higher energy UV radiation b Chemical energy textbook is a little confusing here actually means heat sources to get chemical reactions going in a mixing environment like atmosphere or ocean Liquid water probably crucial here c Could use internal radioactive heat on some planets Venus and Mars but some bodies Moon and Mercury are so small that they have already lost most of their internal heat d Lightning is a possible source on any world with an atmosphere although we don t know what controls the amount of lightning you ll get in a given environment e Tidal heating some moons of the giant planets are heated this way It is the subject of ch 8 to which we ll return 3 Liquid medium for transport of chemicals We have been through this before but remember that the idea is that if molecules were just sitting there on some solid surface their migration would be very slow and they would not react fast enough to produce more complex molecules a liquid medium provides a mixing medium in which the molecules can diffuse and react more rapidly Exploration of the Solar System Read the text on this Notice that imaging observations from Earth require very high resolution in order to make out details that might be signatures of processes related to life This involves some combination of adaptive optics from the ground a space telescope and interferometry which has now advanced into the optical part of the spectrum Also spectroscopy can identify gases in atmosphere minerals and ices on surface obtain the atmospheric temperature pressure and density and even probe the weather and climate Robotic spacecraft are the method of choice for future NASA missions Current or recent projects include the Voyager 2 flybys Mars Orbiter and Lander Cassini Saturn orbiter and lander Titan Sending humans to explore done only for Moon Apollo 1969 1972 has serious problem that duration of trip means a large payload would be required supplies for crew so the fuel requirements and expense become enormous Note Recent 32004 Mars findings may change this Moon and Mercury Small so have lost most of their internal heat 9 no outgassing and weak gravity 9 no atmosphere They are also the least likely to have liquids anywhere Could have ices in craters near poles protected from sunlight by shadow delivered by comets but not liquid Remember with no atmosphere or even a very thin one like Mars heated ice sublimes directly into gas phase not liquid Venus Very thick CO2 atmosphere But at 07 AU from sun temperature so high that water stayed as gas in atmosphere solar UV photons dissociated them and the H then escaped After only a few million years theoretically the water was gone Without liquid water the CO2 couldn t dissolve leaving Venus with a severe runaway greenhouse effect However the time for the water to disappear is extremely uncertain If longer Venus could have had oceans before the greenhouse effect had heated the planet to inhabitability Could life have begun during that interval and then adapted to temperatures as large as current surface We assume not even extremophiles have limits set by the strength of the strongest molecular bonds There is some speculation that life could have adapted to the atmosphere where it is cooler and there might even still be some liquid water see recent book by Grinspoon But it does not seem like a good bet so we are removing it from our list of targets Giant Planets Outer 2iant nlanets Jupiter Saturn Uranus Neptune Important space missions Pioneer 10 Jupiter 11 Jupiter Saturn 1979 Voyager 1 and 2 1989 V2 went on to Uranus and Neptune Galileo reached Jupiter in 1995 continued to explore through 2001 Cassini Huygens will arrive Saturn 2004 probe through atmosphere of Titan Chemical composition Cores probably rockyicy because formed by planetesimal accretion but most of outer layers is gas accreted from the primordial solar nebula gt mostly hydrogen If you let such a gas of cosmic composition 90 H 1 CNO react under temperatures that occur in the outer solar system you get methane CH4 ammonia NH3 hydrogen sulfide H2S water vapor H20 but overwhelmingly a lot of excess molecular hydrogen Jupiter and Saturn are so massive and cold that they will always be able to retain this hydrogen And even though it is very cold at the top of the atmosphere about 100km down the temperature is warm enough for liquid water droplets But if a life form wanted to use the layer whose temperature is right for liquid water it would have to be a floater or else it would sink to the hotter depths Why pessimistic about life on outer planets 1 With so much excess hydrogen the chemical equilibrium favors the formation of Simple molecules like those named above NOT complex molecules that are needed for life no matter what kind This is a weak argurnent even with so much excess hydrogen nonequilibrium chemistry is likely due to sporadic energy sources like lightning 2 Jupiter for example has no solid surface so a no likely microenvironments like Earth s tidal pools or transient ponds where reaction products could become concentrated and undergo more reactions b no opportunity for surfaces to catalyze chemical reactions think about polymerization on clay minerals on Earth 3 Vertical convection mixing circulation takes gas between cool upper layers and deeper layers where temperatures exceed 1000 O C and where complex molecules would be destroyed So the only possibility that has been suggested for life on Jupiter or other giant planets is a buoyant large gasfilled oater that can stay at the height of the water layer by adjusting its density in ating and de ating see below But how could these have originated If they began as simpler complex molecules they would have been destroyed assuming all the atmosphere gets circulated to hot depths But consider Jupiter s Great Red 8120 a vortex in the upper atmosphere that has persisted for at least centuries and whose reddish color still not understood may re ect complex molecules The reds yellows and brown colors on Jupiter have led to the color controversy between those who think they can explain these colors by inorganic compounds eg red from phosphorus compounds and those who think the colors re ect prebiological orgaqnic chemistry something like the goo that formed in the MillerUrey experiment These controversial reddishbrown substances are usually referred to as tholins Another thing to consider is that water clouds probably do form at a layer where the temperature and pressure are like Earth and higher in the atmosphere there are ammoniasulfur clouds Where there are clouds there are usually thunderstorms and lightning observed by Voyager and Galileo spacecraft so there s a good energy source to get some complex molecules In fact lightning UV photons from the sun and heat from Jupiter s interior all probably contribute to forming molecules like hydrogen cyanide HCN acetylene CZHZ ethane CzH and others all which have been observed by their spectral lines The question is how complex can they get in the presence of the upwarddownward convection currents C Sagan amp E Salpeter 1977 worked out the chemistryphysics of speculative lifeforms oaters that might adapt to Jupiter These giant gasbag creatures use warm hydrogen gas to regulate their buoyancy and rise and fall in the atmosphere scavenging food organic molecules and energy lightning along the way Hard to see how life could arise on Jupiter but still can t rule out such creatures on any giant planet until we explore them in detail Upshot Could speculativer see how life might have adapted to environments as strange as Jupiter s but diffith to see how it arose Can you think of a way Mars You will update by researching results of recent rover mission Mass of Mars is only 10 Earth s mass Most of the atmosphere and water has escaped several processes contributed UV photons dissociating water lead to escape of H erosion by solar wind bulk loss during impacts incorporation into carbonate rocks runaway glaciation cold because far from sun so atmosphere freezes snows albedo of planet increases gets even colder can never unmelt Not sure which was most important but certainly currently not much atmosphere 1 as thick as Earth s Composition 95 C02 3 N2 plus traces of others Cold Even at equator the average temp is 60 0C Low T and low pressure means liquid water would either freeze or boil This is equivalent to our statement earlier that at low pressure ice when heated goes directly from the solid form to the gaseous form sublimation But what about the past That s the big question for Mars Note You don t have to memorize the names of any of the scientists mentioned below Geomorphology 1971 Mariner 9 volcanoes canyons and many erosion features such as gullies channels and apparently river valleys and valley networks There is a very good reason to think that the erosion took place long ago Much of Mars surface is gt35Gyr old from number of impact craters in different areas For the oldest regions craters smaller than about 15 km have disappeared while the larger ones have undergone substantial erosion But younger craters have not been significantly altered Your textbook has an excellent discussion of the geomorphology of Mars with many excellent images be sure to read that Viking 1976 3 biology experiments fail to find evidence for current life Details given in text but students aren t responsible for remembering the names of the experiments or the details of their operation You should however understand the ideas behind each experiment But Viking orbiters returned detailed photos that showed outflow channels where underground water burst through the permafrost resulting in floods producing channels But only covered about 10 of planet There could be much more water under the permafrost But most scientists think no oceans just occasional lakes that rapidly freeze Erosion rates inferred from crater rims indicate there could have been a little water erosion but only during the Noachian time of late bombardment ending about 38 Gyr ago period see text for Martian eras Tim Parker searched and found evidence for shorelines in photos Earliest ocean could have covered half the planet according to this Again this is evidence for early liquid water But Nick Hoffman White Mars claims that all the features could have been produced by the flow of CO2 not water Hoffman is most outspoken of anti water interpretations of geological forms March 12 2002 Tanaka et al Use Mars Orbiter Laser Altimeter MOLA to reconstruct the Hellas impact basin 1200 mi wide 6 mi deep Interpret erosional features as flow of liquid CO2 not water during magma eruption Supports Hoffman although most appear not to agree So the surface geomorphology is ambiguous and despite NASA press releases it is not certain whether there was once extensive liquid water on Mars although this is certainly possible More recent results from mineral evidence is listed below Atmosphere Make sure you understand that Mars could have have a much thicker atmosphere in the past and why it doesn t now Could have been warmer in the past due to presence of greenhouse gases like CH4 or recently clouds of dry ice particles So many would agree that there could have been liquid water 38 Gyr ago and some could still exist under the permafrost Global Surveyor images since 1998 some evidence for flooding within the last few million years Was it liquid water from underneath the permafrost Mars Odyssey 2002 gamma ray and neutron detectors Results H in top 1 meter of surface Probably due to water ice But this still doesn t tell us if there is or was liquid water Interpretation of recent Mars rover results are crucial Recent Spectral and Imaging Results Recent analyses of spectral results from orbiters concerning surface minerals suggest that if there ever was a lot of liquid water on Mars it must have been when Mars was very young and that Mars has been cold and dry for 2 3 billion years The products of extensive weathering expected under a humid climate such as w are showing up in unexpectedly tiny quantities if at all using spectrometer TES on orbiting Mars Global Surveyor Much of surface looks like unaltered basalt the volcanic rock of Earth s ocean crust spectrum shown in class The greenish mineral olivine that has recently been recognized mixed in the basalt should have crumbled away in a few thousand years if there was even a tiny bit of moisture Early in 2003 the imaging system THEMIS on Mars Odyssey reported dection of olivine rich basalt at several places that are thought to be at least 03 Gyr old August 2003 Mars Global Surveyor TES spectrometer detects mineral carbonate in TBS spectra spectra shown in class Recall that CO2 dissolves in water to make carbonates e g White Cliffs of Dover Expect about 20 in martian dust if once humid but only 2 3 is observed More Desert varnish a coating that your d expect if even a little humidity as found in desolate Dry Valleys of Antarctica only found in martiam rocks that are older than about a billion years Majority consensus at presen Mars has been cold and dry for a very long time Water does seem to have flowed on the surface brie y early in martian history from geological features and probably gushed to surface in planet s midlife when it seemingly trickled down gullies in the geologically recent past although the lack of weathering products of this makes it questionable Mars may have been nearly always cold and dry The water would be locked up as ice almost all the time except when an exceptional swing of the planet s axis brings extra solar heating to polar regions and brief melting of that snow and flow of resulting water could have shaped the landscape seen today before the tilt went changed again and returned Mars to complete deep freeze Ice ball Mars is the current picture except for possibly in its earliest billion years Was this enough time for life to develop Could it survive in the current frozen dry environment That s why many astrobiologists are exploring the dry valleys of Antarctica Mars Express Europe and Mars Exploration Rovers US 2004 Europe s lander Beagle 2 was to dig 2 meters beneath surface at the site of what some people think may have been an ancient sea floor That should have cleared things up some but Beagle died upon landing US Mars rovers apparently scored a big success I will ask you to research this See Figure 734 in textbook for illustration of all the plans for Mars exploration into the next decade These will probably change due to recent events And what about life in the past on Mars This depends on the climate history of Mars We ll discuss this in class and your textbook has an excellent discussion on pp 185 187 The main question whose likely answers you should understand is did Mars have a thick atmosphere once and where did it go You can find some interesting recent discussion of all these points at some of the links at the course web site Life in a Martian Meteorite Meteorite ALH84001 oldest of 12 rocks discovered in 1984 thought to have come from Mars landed in Antarctica about 13000 years ago Weighs about 4 pounds Why is it believed to be from Mars 1 Abundance ratios of oxygen isotopes are the same in all 12 rocks but different from meteorites from the moon most Earth rocks or asteroids 2 Pockets of gas in the youngest of the 12 have same composition as present Martian atmosphere Graphs shown in class the evidence is very convincing The rock s age is about 45 billion years because made of pure pyroxene not basalt one of the first solids on an initially molten planet so dates from earliest era of solar system Most believe that a 100 km diameter asteroid hitting Mars about 16 million years ago from the number of cosmic ray tracks on the rock that show how long it was exposed in space ejected this rock from the Martian surface Simulations suggest that this occasionally happens to all the inner planets Most of these fall into the sun are kicked out of solar system or get pulverized in asteroid belt Some chance of reaching Earth Older simulations found it would take about 100 million years but more recent simulations including effects of Jupiter and Saturn get about 10 million years So for 16 million years the rock was undergoing a complicated orbit around the sun when by chance its orbit intercepted the Earth Here s the chronology LIKELY HISTORY OF THE MARS METEORITES 0 Rock solidified on Mars 45 billion years ago 1 Pieces of Mars blasted loose by impact of asteroid or comet about 16 Myr ago date from radiation damage 2 A small fraction escape Mars gravity 3 These particles would orbit the sun in relatively stable orbits for most of their lives except for gradual orbit alterations by tug of distant planets 4 Occasionally a close encounter with inner planets abruptly changes the path 5 Many of the objects eventually fall into the sun collide with asteroids or escape the solar system 6 A small fraction of the fragments hits the Earth 13000 years ago in Antarctica Over 10 to 100 million years as much as about 7 percent of the original material could find its way to Earth this way And it goes both ways some life bearing rocks from Earth have probably found their way to Mars and elsewhere This brings up the idea of panspermia again Calculations of the probability that rocks like the Martian meteorites have been ejected from our solar system and made their way to another star system show that it is axtremely improbable as expected but still possible Evidence for life in ALH84001 1 Organic molecules that might be associated with life a Carbonates forms from water and carbon dioxide On earth produced by decay or combustion of plants and other organisms See below for criticism b PAH molecules these are fairly complex organics but are found on the Earth in meteorites and also identified in interstellar dust grains so this doesn t require biology 2 Minerals characteristic of biological activity Iron sulfide and magnetite commonly produced by anaerobic bacteria on Earth Magnetite is especially interesting because it is used by some Earth bacteria to navigate through Earth s magnetic field When bacteria decompose they leave magnetofossils shaped like cubes or teardrops like some those found in ALH84001 3 Tubular and egg shaped structures that resemble fossils of the oldest single celled bacteria found on Earth It is believed that liquid water existed on Mars long ago Mariner 9 1972 found what looked like dry riverbeds and lakebeds and the Viking spacecraft provided even stronger evidence for channels and valley networks Criticisms see your text for additional discussion of pros and cons 1 Possibility of contamination in the last 13000 years But life from Mars proponents point out that the PAH concentration increases from surface to interior opposite from what s expected from contamination Dec 1996 Becker et al UCSD paper published in Geochimica et Cosmochimica Acta claim the PAHs are probably contaminants from Antarctic ice All the PAHs found in the Martian meteorite were found in the ice samples and also in other Antarctic meteorites including several that didn t come from Mars They claim that the PAHs are found deep within fissures because they collect on surfaces of carbonate grains 2 Carbonates and PAH s could have formed in absence of water One proposal suggests asteroid smashing into Mars surface liquifying the carbon dioxide frost 3 Two geochemists at UColo claim that the temperature at which the carbonates formed was higher than NASA scientists suggest possibly hotter than any microorganism could survive This is crucial because the carbonates are central to the lines of evidence They derived a temperature of over 600 OC But another group using a different technique found a formation temperature for the carbonates of only 80 OC So still very uncertain 4 Evidence that the magnetite particles are non biological Dec 1996 paper in GeoCosmoActa by 3 US geoscientists find that the magnetite particles grew like a tightly wound spiral staircasee axial screw dislocation This form is totally absent in any known magnetite produced by living organisms They are formed at fumaroles volcanic vents that release hot gases which then condense need T around 500 800 C for this agreeing with earlier analysis that carbonate globules must have formed at gt 450 C 5 There should be a large round crater on Mars from the impact but none this big have been found But it could have been a low angle impact of a much smaller object creating an elliptical crater In late 1996 Barlow claimed 2 craters out of 42283 Mars craters inspected as possible sites 6 Part of the support for biological interpretation was high enrichment in C12 over C13 But Oxford geologist Martin Brasier claims repeated freezing and thawing could produce a similar result Recall that Martin Brasier is the same person who brought to light the questionable nature of the 35 billion year old fossil evidence for life on Earth 7 The sizes of the purported fossil forms are tiny much smaller than even the smallest prokaryotic cell on Earth There is severe doubt whether such small objects could contain enough genetic material for even the most primitive of organisms Satellites of giant planets Jupiter s four large satellites Io very active volcanoes cover nearly entire surface amazing variety Maybe lakes of liquid sulfur But no evidence for water and extremely severe radiation environment Jupiter s radiation belts Europa long lines many double and triple appear to be fissures in an icy crust that may or may not cover a layer of liquid water Ganymede evidence for plate motions of the crust Callisto lack of large impact craters Io s volcanoes and Europa s possible liquid water are due to the same effect changing tidal forces due to interactions with Jupiter and with other satellites None of these have atmospheres today but could have earlier they are more massive than Mars and further from sun recall that it is almost certain that Mars once had a substantial atmosphere Current interest is Europa Several Galileo close ybys show the complex structures of the lines the spatter cones ice oes and other structures that might be due to subsurface water lack of impact craters shows surface is geologically young Dark reddish coloration of cracksgt bacteria or some kind of organic gunk Galileo spacecraft s near infrared spectrometer indicated salts in this dark material but no signature of organic compounds Under the water analogues of terrestrial hydrothermal vent organisms NASA and astrobiology community is trying to build interest to support a mission sending a probe under the ice But current calculations are indicating that ice layer thickness is much thicker than hoped See text and attend Neal Evans lecture Monday for more detail on this subject Titan Saturn s largest satellite and the object that may be most interesting for astrobiology It is massive like Ganymede and Callisto in the Jupiter system but Titan has an atmosphere Why Apparently the temperature at Jupiter was a little too warm to allow ices that formed the satellites to retain gases like nitrogen methane etc But further away from the sun than Saturn it is so cold that such compounds would be largely frozen like on Triton massive icy moon of Neptune or Pluto Yet Titan is not so massive that it could hold on to the lightest gases like H2 compare with Jupiter So it has an extremely interesting composition with many organic molecules as well as CO2 and H20 observed through spectral lines The hydrocarbons are formed either from methane CH4 or with nitrogen such as HCN Much more complex molecules must be present given the smog layer that totally hides Titan s surface Combination of thick atmosphere and dense clouds suggest significant greenhouse But unlikely that it is enough to allow liquid water Density 2 19 gcm3 compare water1 rock3 iron7 so must be composed largely of ices made from H20 C02 N2 CH4 NH3 all with densities 1 to 15 Atmosphere 97 N2 3 combination CO CH4C2H6 ethane Temperature 94 K 179 C at surface cold The methane would be broken up by sunlight if not resupplied gt probably by liquid methane or ethane see below oceans or lakes This can saturate the atmosphere leading to methane clouds just as H20 clouds form on Earth Calculations show that what really happens is CH4 photon H CH2 CH H QH4 C2H2 and H or H2 which escapes because so light So get irreversible conversion of methane into ethane catalyzed by C2H2 so expect ocean of ethane about a km thick Is it a global ocean NO radar and HST infrared reflectivity show variations suggesting fixed continents Oct 2003 Campbell et al Science report radar observations indicating many lakes of frozen hydrocarbons Liquid water Too cold at surface now but early planetesimal bombardment could have meted the early surface ice But only get 100 to 1000 yr before it refreezes after each impact Life on Titan Some think Titan might be a near ideal site for life s origin others disagree Here are some of the arguments Against a Liquid methane and ethane don t dissolve compounds easily like water Organic compounds formed in the atmosphere probably just settle to the bottom b No clays from silicates without water But on Earth many think that clays were necessary to catalyze polymerization c Probably no silicates at surface anyway it s almost certainly settled to the core along with iron phosphorus So there is a lack of biogenic elements Si Ca Fe P at surface For UV from the sun and hi gh energy charged particles from Saturn break bonds in CH4 and and N2 gt many hydrocarbons including HCN and HC3N all observed by spectra These and more complicated molecules will saturate the atmosphere producing haze observed and some will rain a layer of organic products on the surface gt much of Titan may be covered by an oil slick 1 to 100 meters thick With an energy source lightning could get amino acids and organic polymers in high concentrations One model for the origin of life on Earth is a primordial oil slick very similar But current radar observations favor lakes rather than global oil slick check astrobiology web sites for latest information This is why Titan is considered the best natural exobiology laboratory we have The Cassini Huygens mission will reach Saturn in 2004 and the Huygens probe will descend and either oat on the ocean or land on solid surface Should be interesting
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