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A1199Are We Alone?
The Search for Life in the UniverseSummer 2019
Instructor: Shami Chatterjee
Web Page: http://www.astro.cornell.edu/academics/courses/astro1199/
Think about projects! HW 3 is posted – due Wednesday
Now: Origin and Evolution of Life
Terrestrial Life: Origins and Evolution
Simple molecules ß
organic moleculesß
reaction chainsß
the first cells ß
prokaryotes ß
eukaryotesß
multicellular life
Nitrogen, CO2,methane
atmosphere
ß
trace O2
ß
Nitrogen, O2trace CO2
~ 2-4 Gyr
Cambrianexplosion~ 4 Gyr
Late heavy bombardment
26 MAY 2006 VOL 312 SCIENCE www.sciencemag.org1140
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BOOKS ET AL.
The scientific field devoted to the originof life on Earth is very young, havingtaken its first experimental steps in the
1950s. Though the question has captivatedhuman imagination since the dawn of history,its scientific pursuit has depended on severalcrucial conceptual developments during the20th century. First, the emergence of life had tobe conceived of as an integral part of the gen-eral process of evolution, leading from the geo-chemistry of the barren Earth to the universalcommon ancestor, which later diversified intothe Darwinian tree of life. Following the rise ofmolecular biology in the 1950s and 1960s, theorigin-of-life question could be formulated inbiochemical and genetic terms, making it asubject of experimental investigation.
Early on, most scientists engaged in thisresearch were chemists who attempted to for-mulate plausible scenarios for the prebioticsynthesis of organic building blocks, biologi-cally relevant polymers, and the first metaboli-cally or genetically functional chemical struc-tures. In the late 1970s, however, geologistsalso became increasingly involved in the field.Their participation was associated with the riseof a new paradigm positing that the synthesis oforganic building blocks and the emergence oflife itself took place not in the“primordial soup” of the tradi-tional hypotheses but in thevicinity of undersea hydrother-mal vents, at high temperatureand under extreme pressure.Supporters of this new concep-tion claim that origin-of-lifetheories can now be subjectedto more rigorous constraintsposed by specific primordialphysical settings (1). On theother hand, the “soup people”—in particular,Stanley Miller, renowned pioneer of the 1953prebiotic simulation experiments, and his col-leagues—reject the alternative paradigm asempirically untenable (2).
In Genesis, Robert Hazen tells the story ofthese debates over the origin of life. There is noone better suited to examine recent develop-ments in the experimental study of the topic.Trained in mineralogy and crystallography, hehas been personally involved in the major lines
of research through which Earthscientists have come to shape thefield. Describing these contribu-tions, he vividly portrays numer-ous experiments and observations.Hazen’s academic home, the Geo-physical Laboratory at the Car-negie Institution of Washington,which specializes in investiga-tions of chemical reactions underextreme conditions, serves as anideal setting for his experiments onthe effects of high pressure andtemperature on organic synthesisand particularly on the possiblerole of minerals abundant in hydro-thermal vents in such synthesis.Describing the scientific status ofthis lab, its remarkable members,and their close professional andpersonal relationships, Hazen weavesthe scientific and the personal intoan engaging, sometimes dramatictale. He highlights the excitement involved inresearch, the many setbacks and disappoint-ments, and the inevitable internal politicswithin the origin-of-life community. In addi-tion, his research team’s membership in the
NASA Astrobiology Instituteallows him to comment on therole of geologists in the study ofpossible conditions for life onMars and other extraterrestrialsites within the context of thenew “deep-origin” paradigm.
An underlying theme of thebook is Hazen’s conception ofthe origin of life as part of awider “theory of emergence”(3), a perspective based mainly
on the ideas of theoretical biologist HaroldMorowitz, a colleague of Hazen’s at GeorgeMason University. According to this ambitioustheory, the growth of organization and complex-ity in physical, chemical, biological, and socialsystems follows a general, though as-yet-unknown, principle on a par with the universallaws of nature. Considering the origin of life asa quintessential process of emergence, Hazensuggests that uncovering “the missing law”should advance origin-of-life research. How-ever, although various complex systems doshare common features, the “new science ofemergence” is in danger of downplaying theunique features of living systems as well asthe distinction between physical and chemicalselection on the one hand and natural selection
on the other. Moreover, as Hazen acknowledges,the basic concepts underlying this grand scheme(e.g., complexity) are far from clear. Since theorigin-of-life field itself lacks firm, unequivocalconclusions, it is doubtful whether such additionalconceptual baggage offers much scientific value.
Among the many issues dividing the origin-of-life community, none is more crucial thanthe controversy between “RNA-first” and“metabolism-first” scenarios. This divisionstems from the difficulty of deciding whichemerged earlier, genetic polymers or metaboliccycles. Because nucleic acids and proteinenzymes are tightly interdependent in extantliving cells, an adequate theory must establishhow either could have originally functioned onits own. After describing the rival positionseven-handedly, noting the pros and cons ofboth, Hazen commendably feels that he has toplace his bets on the table. He comes down onthe side of metabolism-first, probably in theform of a molecular layer on a surface of arock. Interestingly, he bases his choice on the“theory of emergence” and the hypothesis thatlife emerged through stages of increasing com-plexity. But wouldn’t a primitive genetic system,made of RNA or a simpler genetic polymer,also have to emerge through such stages?
The chemical requirements for the establish-ment of a self-replicating genetic system underprebiotic conditions are clearly extremely com-plex. Nonetheless, the support for the RNA-firstnotion, despite its difficulties, reflects the dou-ble realization that the emergence of life’s com-
Search for Life’s BeginningsIris Fry
ORIGIN OF LIFE
Genesis
The Scientific Quest forLife’s Origin
by Robert M. Hazen
Joseph Henry Press,Washington, DC, 2005. 359pp. $27.95, C$37.95. ISBN 0-309-09432-1.
Original Eden? The discovery of hydrothermal vent communitiesled to the proposal that hydrothermal systems provided a site for therapid emergence of life through a sequence of abiotic syntheses.
The reviewer is at the Cohn Institute for the History andPhilosophy of Science and Ideas, Tel Aviv University, andthe Department of Humanities and Arts, Technion–IsraelInstitute of Technology, Haifa 32000, Israel. E-mail:[email protected]
Published by AAAS
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Geologic ErasEra Period Epoch Approx. number of years
(millions)Cenozoic Quaternary Holocene (Recent)
Pleistocene
65
225
570
Tertiary PlioceneMioceneOligoceneEocenePaleocene
Mesozoic CretaceousJurassicTriassic
Paleozoic PermianCarboniferous(Pennsylvanian and Mississippian)DevonianSilurianOrdovicianCambrian
Precambrian
• Darwinism (as in “The Origin of Species”)
• Neo-Darwinism
è Evolution by natural selection (survival of and reproduction by the fittest), with possible contribution from inheritance of acquired traits (Lamarckism).
è Modern synthesis:Evolution by natural selection + Genetics + Ecology + Molecular biology – Inheritance of acquired traits.
The Biological Case• Organic building blocks are ubiquitous:
Interstellar clouds, molecules, dust, comets, etc.
• Early Earth: Life formed rapidly after the late heavy bombardment
• RNA world ® DNA world ?– RNA = autocatalytic.– A matter of inevitable chemistry? – If so, expect that bacteria = chemistry. Þ Ubiquitous life.
• Are there alternative chemistries?
Alternative Biochemistries?• Alternative chirality molecules:
Terrestrial: L amino acids, D sugars
• Non-carbon based biochemistry?“Carbon chauvinism”. Silicon biochemistry (‘organosilicon’)?
à Si less versatile than C; cannot form double bonds (so no analog to carbonyl group compounds).
à SiO2 = analog to CO2 but does not dissolve in water.
à Carbon cosmically more abundant 10:1, but less abundant in Earth’s crust. Yet life is carbon based.
à But … c.f. silicate skeletons of diatoms. http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
Silane = analog to methane
No Si analog
Alternative Biochemistries?Other exotic element bases:à Chlorine as an alternative to oxygen (electron
receptor).à Arsenic as an alternative to phosphorus: some
microbes metabolize As.
Non-water solvents?à Ammonia: not as versatile as water in ability to
form both acids and bases (bonding properties); temperature range: 195 to 240 K.
à Methane CH4: Titan? (91 to 112 K.)à Hydrogen flouride (HF).
http://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry
Silane = analog to methane
No Si analog
ASTROBIOLOGYVolume 9, Number 2, 2009© Mary Ann Liebert, Inc.DOI: 10.1089/ast.2008.0251
Hypothesis Article
Signatures of a Shadow Biosphere
Paul C.W. Davies,1 Steven A. Benner,2 Carol E. Cleland,3 Charles H. Lineweaver,4Christopher P. McKay,5 and Felisa Wolfe-Simon6
Abstract
Astrobiologists are aware that extraterrestrial life might differ from known life, and considerable thought hasbeen given to possible signatures associated with weird forms of life on other planets. So far, however, verylittle attention has been paid to the possibility that our own planet might also host communities of weird life.If life arises readily in Earth-like conditions, as many astrobiologists contend, then it may well have formedmany times on Earth itself, which raises the question whether one or more shadow biospheres have existed inthe past or still exist today. In this paper, we discuss possible signatures of weird life and outline some simplestrategies for seeking evidence of a shadow biosphere. Key Words: Weird life—Multiple origins of life—Bio-genesis—Biomarkers—Extremophiles—Alternative biochemistry. Astrobiology 9, 241–249.
241
1. Background
THE HISTORY OF OUR DEVELOPING UNDERSTANDING of life onEarth has been characterized by repeated discovery, dri-
ven largely by improvements in techniques to explore theEarth’s biosphere. The age of enlightenment brought explo-ration technologies that led to the discovery of new biota inthe Americas, Australia, and Africa. The invention of the mi-croscope uncovered an unexpected microbial world. RNAsequencing in the 1960s and 1970s revealed that the prokary-otic biosphere itself consists of two domains that are as dif-ferent from each other as they are from eukaryotes. Together,these discoveries revolutionized our understanding of thehistory of life on Earth over the past three billion years.
Today, it is believed that microbes constitute the vast ma-jority of terrestrial species. Nevertheless, the microbial realmremains poorly explored and characterized. Less than 1% ofmicrobes has been cultured and described (Amann et al.,1995; Pace, 1997; Hugenholtz et al., 2006). Because microbialmorphology is very limited, it is in most cases difficult, if notimpossible, to deduce much about the nature of microbial
life by simply looking at it. Gene sequencing has so farproven to be the only reliable method to determine the re-lationship of a given microbial species to other known life.This extensive ignorance raises the intriguing issue of howsure we can be that all microbial types have been identified.Might it be the case that the exploration of the biosphere isnot complete, and deep additional branches of the tree of lifehave so far been overlooked? Is it even possible that micro-bial life exists that does not share a common descent withfamiliar organisms and, therefore, constitutes a different treealtogether, deriving from an independent genesis?
It is relatively uncontroversial that at least one very dif-ferent kind of life existed on early Earth. It had no encodedproteins but rather used RNA as the sole genetically encodedcomponent of biocatalysts. This conjecture is supported bythe catalytic properties of RNA and the detailed structure ofthe ribosome, a complex structure built from both proteinand RNA, but where the RNA is clearly responsible for theprotein synthesis (Moore and Steitz, 2002). It is not clear thatexisting life-detection strategies, which mainly target the ri-bosomal machinery, would register any surviving RNA or-
1BEYOND: Center for Fundamental Concepts in Science, Arizona State University, Tempe, Arizona.2Foundation for Applied Molecular Evolution, Gainesville, Florida.3Department of Philosophy and the Center for Astrobiology, University of Colorado, Boulder, Colorado.4Planetary Science Institute, Research School of Astronomy and Astrophysics & Research School of Earth Sciences, Australian National
University, Canberra, Australia.5Space Science Division, NASA Ames Research Center, Moffett Field, California.6Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts.
THE ORIGIN OF LIFE“Life? Don’t talk to me about life…”
ContextPrior to 1850:
Spontaneous generation.Pasteur:
Life begets life.20th century view:
Life begets life now but spontaneous generation occurred in the early oceans.Alternative:
“Panspermia:” life was transported to Earth in cosmic dust from comets, etc.(Arrhenius 1859-1927)
What is life?• Life = matter + elan vital (vitalism)• Life = matter + physics + chemistry (mechanistic)• “Life” also includes artificial life?
Is there a sharp transition between living and non-living things?Are the conditions in the early Earth unique, rare or common?
Abiogenesis: the Miller-Urey Experiment• 1952: Stanley Miller and Howard Urey.• Water (H2O), methane (CH4),
ammonia (NH3), and hydrogen (H2).• Heat, sparks.• Over 20 amino acids synthesized!
Note: Racemic mixture (equal parts L- and R-isomers).
Biology is homochiral.
Terrestrial Life• Organic compounds (carbon based).• Proteins based on 20 amino acids.• Enzymes = catalysts.• Common genetic code based on nucleotides:
RNA and DNA.• Chirality: biological amino acids are left-handed.
• Symmetry breaking: why L, not R?
Related question: Why does the universe favor matter over anti-matter? Another case of symmetry breaking.
Central Problem of Origin-of-life Research
• By what series of chemical reactions did the interdependent system of nucleic acids come into being?
• Hypothesis: RNA came first, then DNA.
• So: how did RNA world come about?
Jump starting life on Earth“If twenty-two-year-old humans like Stanley (Miller) could produce amino acids in the laboratory in only a few days, why could not the laboratory of Earth, in an experiment of a thousand or a million years, produce life?”
“…the first life-forms were membrane-bounded self-maintaining cells, like those still alive today.”
Lynn Margulis in Symbiotic Planet
RNA Þ DNA“The RNA molecule, however, is more versatile than its
DNA complement. Given the appropriate chemical milieu, but without any protein, RNA can autocatalytically make more of itself. DNA, on the other hand, requires both RNA and enzyme proteins to complete its work of replication: DNA by itself is dead. The capacity of RNA both to accelerate chemical reactions and to replicate suggests that RNA preceded DNA in the history of life.”
Lynn Margulis in Symbiotic Planet
In summary, prions show the hallmarks of Darwinian evolution: They are subject to mutation, as evidenced by heritable changes of their phenotypic properties, and to selective amplification, as documented by the emergence of distinct populations in different environments. A practical consequence of our findings is the realization that therapeutic approaches aimed at stabilizing PrP orreducing PrP expression are less likely to be thwarted by emergence of drug resistance than those based on targeting PrPSc.
Transport of life to Earth?• Not impossible, but difficult.– Closer sources are easier.– Ultimately only defers the problem to another place.
• Seems worthwhile to investigate whether life can plausibly get going on Earth.– May not be easy.– Will have to make extrapolations and jump over
unknown events.
Panspermia“This idea – called directed panspermia or
‘pangenesis’ and put forth for centuries – that life came in seed form from outer space –seems to me to stem from ignorance of evolution on Earth. To transfer the problem of life’s origin to outer space is intellectually unsatisfactory. Why should it have been easier for life to originate elsewhere than on Earth?”
Lynn Margulis in Symbiotic Planet
All the planets fit between the Earth and the Moon…
è See also: A Tediously Accurate Map of the Solar Systemhttps://joshworth.com/dev/pixelspace/pixelspace_solarsystem.html
ASTROBIOLOGYVolume 5, Number 4, 2005© Mary Ann Liebert, Inc.
Research Paper
Impact Seeding and Reseeding in the Inner Solar System
BRETT GLADMAN,1 LUKE DONES,2 HAROLD F. LEVISON,2 and JOSEPH A. BURNS3
ABSTRACT
Assuming that asteroidal and cometary impacts onto Earth can liberate material containingviable microorganisms, we studied the subsequent distribution of the escaping impact ejectathroughout the inner Solar System on time scales of 30,000 years. Our calculations of the de-livery rates of this terrestrial material to Mars and Venus, as well as back to Earth, indicatethat transport to great heliocentric distances may occur in just a few years and that the de-parture speed is significant. This material would have been efficiently and quickly dispersedthroughout the Solar System. Our study considers the fate of all the ejected mass (not justthe slowly moving material), and tabulates impact rates onto Venus and Mars in addition toEarth itself. Expressed as a fraction of the ejected particles, roughly 0.1% and 0.001% of theejecta particles would have reached Venus and Mars, respectively, in 30,000 years, makingthe biological seeding of those planets viable if the target planet supported a receptive envi-ronment at the time. In terms of possibly safeguarding terrestrial life by allowing its survivalin space while our planet cools after a major killing thermal pulse, we show via our 30,000-year integrations that efficient return to Earth continues for this duration. Our calculationsindicate that roughly 1% of the launched mass returns to Earth after a major impact regard-less of the impactor speed; although a larger mass is ejected following impacts at higherspeeds, a smaller fraction of these ejecta is returned. Early bacterial life on Earth could havebeen safeguarded from any purported impact-induced extinction by temporary refuge inspace. Key Words: Panspermia—Impact—Meteorites. Astrobiology 5, 483–496.
483
INTRODUCTION
THE REALITY OF THE MODERN-DAY impact hazardhas succeeded in infiltrating the conscious-
ness of the general public, with many citizensnow aware that an asteroidal or cometary impactcould produce major damage on a human scaleduring the next century (see, for example, Chap-man, 2004). Via telescopic census, planetary as-tronomers have established that the current near-
Earth object population implies that kilometer-scale celestial bodies in Earth-crossing orbits(whose impacts would have global effects) strikeour planet every few hundred thousand to a mil-lion years (Myr). The notorious Cretaceous-Ter-tiary (K/T) event linked to the extinction of thedinosaurs exemplifies the even rarer and largerimpacts that occur on roughly 100-Myr intervals.However, even these “mass-extinction events”pale by comparison to the largest impacts that
1Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, Canada.2Southwest Research Institute, Boulder, Colorado.3Department of Astronomy, Cornell University, Ithaca, New York.
5765_04_p483-496 7/25/05 11:16 AM Page 483
fering launch conditions (that is, speed distribu-tions), we chose to integrate several groups ofejected particles, each cluster having a differentvalue of v !. To choose these initial conditions, weneed to understand the range of v ! values thatare reasonable.
Launch speeds
Among the terrestrial planets, the Earth has thelargest escape speed compared with its orbital ve-locity (more than one-third), which makes it byfar the most effective of these bodies at disturb-ing the orbits of passing objects. In addition, be-cause Earth’s v esc is large, small increases oflaunch speed above v esc lead to relatively large v !
(see Eq. 3). For example, at v l " 13.0 km s# 1, aspeed only mildly above Earth’s escape speed,the resulting v ! " 6.6 km s# 1 is 22% of the 30 kms# 1 orbital speed of the Earth; this percentage isto be compared with the (!2" # 1) (or 41%) in-crease in circular orbital speed that would yield
an unbound orbit if the particle were projectedtowards the apex of Earth’s motion. Because thevelocity at infinity adds vectorially to the Earth’sheliocentric orbital velocity vector (cf. Gladmanet al., 1995), Table 1 and Fig. 2 show that evenlaunch at 12.25 km s# 1 (only 10% above the es-cape speed), corresponding to v ! " 5 km s# 1,may produce some ejecta with perihelia as low as0.5 AU or as high as 2.1 AU. Such particles thuscross the orbits of Venus, Earth, and Mars withencounter speeds that are large fractions of theescape speeds of those planets. Going even fur-ther, objects sent from Earth’s surface with v ej$ 16.7 km s# 1 (i.e., only 50% larger than Earth’sescape speed, such as could be generated by im-pacts with U $ 33.4 km s# 1 in the basic spall theory) may escape the Solar System entirely iflaunched toward the Earth’s apex of motion.Hence launch speeds just modestly above Earth’sescape speed may produce heliocentric orbits thatare extremely eccentric. The combination of thesetwo aspects means that the early histories of Earth
GLADMAN ET AL.486
FIG. 1. The speed distribution of spall-ejected material for various impactorspeeds under the Melosh (1985) model, ascorrected by Armstrong et al. (2002). For aplanet having a given escape speed v esc,speed distribution curves are shown forthree different impactor speeds U, expressedas multiples of v esc. For reference, the num-bers in parentheses higher up each curveshow the values of U for escape from Earth.The ordinate gives the fraction of all escap-ing mass that is faster than the given speed,which thus is identically 1 at v " v esc anddrops to 0 at U/2 according to the first-orderspall theory. The vertical dotted line indi-cates an ejection speed at Earth’s surface of11.3 km s# 1, corresponding to v ! #1.8 kms# 1, the value taken by Wells et al. (2003); foreach of the three impactor speeds shown,less than 10% of the escaping ejecta leave be-low such slow speeds.
5765_04_p483-496 7/25/05 11:16 AM Page 486
Meteoroid Transfer to Europa and TitanGladman, Dones, Levison, Burns, Gallant 2006 LPS Conference
• “Via extensive numerical simulations, we calculate the delivery efficiency of terrestrial impact ejecta to Europa and Titan. We show that (perhaps surprisingly) in an averaged large-scale impact (KT-level) a few to a hundred terrene meteoroids reach Europa and Titan.”
LIFE FROM MARS?Martian meteorites
Meteorites and Dynamical ChaosMeteorites may follow a chaotic route to Earth.Wisdom (1985, Nature, 315, 731)
© Nature Publishing Group1985
How do you identify a rock from space?
How do you identify a rock from space?
Meteorites from Mars
emphasizes the beneficial effects of the openarchitecture and surface compositional profile ofthe Pt3Ni nanoframes in electrocatalysis.
The open structure of the Pt3Ni nanoframesaddresses some of the major design criteria for ad-vanced nanoscale electrocatalysts, namely, highsurface-to-volume ratio, 3D surface molecular ac-cessibility, and optimal use of precious metals.The approach presented here for the structuralevolution of a bimetallic nanostructure from solidpolyhedra to hollow highly crystalline nanoframeswith controlled size, structure, and compositioncan be readily applied to other multimetallic elec-trocatalysts such as PtCo, PtCu, Pt/Rh-Ni, andPt/Pd-Ni (figs. S26 to S29).
References and Notes1. V. R. Stamenkovic et al., Science 315, 493–497 (2007).2. S. Guo, S. Zhang, S. Sun, Angew. Chem. Int. Ed. 52,
8526–8544 (2013).
3. D. F. van der Vliet et al., Nat. Mater. 11, 1051–1058 (2012).4. D. F. van der Vliet et al., Angew. Chem. Int. Ed. 51,
3139–3142 (2012).5. J. Snyder, K. Livi, J. Erlebacher, Adv. Funct. Mater. 23,
5494–5501 (2013).6. J. Snyder, T. Fujita, M. W. Chen, J. Erlebacher, Nat. Mater.
9 , 904–907 (2010).7. J. Erlebacher, M. J. Aziz, A. Karma, N. Dimitrov, K. Sieradzki,
Nature 410, 450–453 (2001).8. P. J. Ferreira et al., J. Electrochem. Soc. 152,
A2256–A2271 (2005).9. C. Wang et al., J. Am. Chem. Soc. 133, 14396–14403 (2011).10. D. Wang et al., Nat. Mater. 12, 81–87 (2013).11. J. Zhang, H. Yang, J. Fang, S. Zou, Nano Lett. 10,
638–644 (2010).12. C. Cui, L. Gan, M. Heggen, S. Rudi, P. Strasser, Nat. Mater.
12, 765–771 (2013).13. S.-I. Choi et al., Nano Lett. 13, 3420–3425 (2013).14. R. Subbaraman et al., Science 334, 1256–1260 (2011).15. Y. Liu, D. Gokcen, U. Bertocci, T. P. Moffat, Science 338,
1327–1330 (2012).16. Y. Kang et al., J. Am. Chem. Soc. 135, 2741–2747 (2013).17. M. Cargnello et al., Science 341, 771–773 (2013).18. L. Tang et al., J. Am. Chem. Soc. 132, 596–600 (2010).
19. Y. Yin et al., Science 304, 711–714 (2004).20. J. E. Macdonald, M. Bar Sadan, L. Houben, I. Popov,
U. Banin, Nat. Mater. 9 , 810–815 (2010).21. S. E. Skrabalak et al., Acc. Chem. Res. 41, 1587–1595 (2008).22. M. McEachran et al., J. Am. Chem. Soc. 133, 8066–8069
(2011).23. J. X. Wang et al., J. Am. Chem. Soc. 133, 13551–13557
(2011).24. M. E. Davis, Nature 417 , 813–821 (2002).25. S. A. Johnson, P. J. Ollivier, T. E. Mallouk, Science 283,
963–965 (1999).26. M. S. Yavuz et al., Nat. Mater. 8, 935–939 (2009).27. M. A. Mahmoud, W. Qian, M. A. El-Sayed, Nano Lett. 11,
3285–3289 (2011).28. M. H. Oh et al., Science 340, 964–968 (2013).29. D. Wang, Y. Li, Inorg. Chem. 50, 5196–5202 (2011).30. C. E. Dahmani, M. C. Cadeville, J. M. Sanchez,
J. L. Morán-López, Phys. Rev. Lett. 55, 1208–1211 (1985).31. F. Tao et al., Science 322, 932–934 (2008).
Acknowledgments: The research conducted at LawrenceBerkeley National Laboratory (LBNL) and Argonne NationalLaboratory (ANL) was supported by the U.S. Departmentof Energy (DOE), Office of Science, Office of BasicEnergy Sciences (BES), Materials Sciences and EngineeringDivision, under contracts DE-AC02-05CH11231 andDE-AC02-06CH11357, respectively. The portion of work relatedto catalyst mass activity and durability was supported bythe Office of Energy Efficiency and Renewable Energy, FuelCell Technologies Program. Work at the University of Wisconsinwas supported by DOE, Office of Science, BES, undercontract DE-FG02-05ER15731; computations were performedat supercomputing centers located at NERSC, PNNL, andANL, all supported by DOE. We thank King Abdulaziz University forsupport of the Pt-Co bimetallic nanocatalyst work. Microscopystudies were accomplished at the Electron Microscopy Centerat ANL, National Center for Electron Microscopy and MolecularFoundry at LBNL, and the Center for Nanophase MaterialsSciences at Oak Ridge National Laboratory, which is sponsoredby the Scientific User Facilities Division, BES, DOE. XPS studieswere carried out at Advanced Light Source (LBNL). We thankS. Alayoglu for carrying out the site-specific EDX on the alloynanostructures, H. Zheng for her help on TEM work at LBNL,and Z. Liu for help on XPS analysis.
Supplementary Materialswww.sciencemag.org/content/343/6177/1339/suppl/DC1Materials and MethodsFigs. S1 to S30Table S1Movies S1 and S2References (32–38)
27 November 2013; accepted 14 February 2014Published online 27 February 2014;10.1126/science.1249061
The Source Crater of MartianShergottite MeteoritesStephanie C. Werner,1* Anouck Ody,2 François Poulet3
Absolute ages for planetary surfaces are often inferred by crater densities and only indirectly constrained bythe ages of meteorites. We show that the <5 million-year-old and 55-km-wide Mojave Crater on Mars isthe ejection source for the meteorites classified as shergottites. Shergottites and this crater are linked bytheir coinciding meteorite ejection ages and the crater formation age and by mineralogical constraints.Because Mojave formed on 4.3 billion–year-old terrain, the original crystallization ages of shergottitesare old, as inferred by Pb-Pb isotope ratios, and the much-quoted shergottite ages of <600 million yearsare due to resetting. Thus, the cratering-based age determination method for Mars is now calibrated in situ,and it shifts the absolute age of the oldest terrains on Mars backward by 200 million years.
The martian rock collection contains near-ly 150 meteorite specimens: shergottites,nakhlites, and chassignites, as well as one
orthopyroxenite (ALH84001) (1, 2). Currently,the oldest rock linked to Mars appears to beALH84001, crystallized about 4.1 billion years
ago (Ga) (3, 4) and probably ejected from Mars20 million years ago (Ma) (5). The original rockunit from which chassignites and nakhlites wereejected about 11 Ma (5) probably crystallizedaround 1.3 Ga (1, 2, 6). The apparent crystalliza-tion ages of shergottites are more diverse andrange between 150 and 596 Ma (1, 2, 6, 7). Thegeochemically depleted [in incompatible ele-ments such as light rare earth elements (LREE)relative to the martian mantle (8)] shergottitescluster at the old age range, whereas the enriched
Fig. 4. Electrochemical durability of Pt3Ni nanoframes. (A) ORR polarization curves and (inset)corresponding Tafel plots of Pt3Ni frames before and after 10,000 potential cycles between 0.6 and 1.0 V.(B and C) Bright-field STEM image (B) and dark-field STEM image (C) of Pt3Ni nanoframes/C after cycles.
1The Centre for Earth Evolution and Dynamics, University ofOslo, Sem Sælandsvei 24, 0371 Oslo, Norway. 2Laboratoirede Géologie de Lyon: Terre, Planètes, Environnement, Uni-versité de Lyon 1 (CNRS, ENS-Lyon, Université de Lyon),rue Raphaël Dubois 2, 69622 Villeurbanne, France. 3Institutd'Astrophysique Spatiale, Université Paris Sud 11, Bâtiment121, 91405 Orsay, France.
*Corresponding author. E-mail: [email protected]
www.sciencemag.org SCIENCE VOL 343 21 MARCH 2014 1343
REPORTS
Martian meteorites:Shergottites, Nakhlites, Chassignites
SNC Group MeteroritesShergottites, Nakhlites, Chassignites
• S: ¾ of all SNCs, igneous rock, millions of years old, Mojave crater on Mars
• N: igneous, 1.3 Gyr old, water 0.62 Myr ago, ejected ~11 Myr ago, reached Earth < 10 kyr ago; from volcanic region on Mars
• C: only 2 examples, also ~1.3 Gyr old• Named after impact sites of first of type:
- Sherghati, India (1865) - El-Hakhla, Alexandria, Egypt (1911)- Chassigny, Haute-Marne, France (1815)
• Achondritic: stony but without CAIs (chondrules)• 1980s: Different in age, oxygen isotope composition,
similarity to Mars surface rocks analyzed by the Viking landers.
• Trapped-gas analysis à similarity to Mars’ atmosphere as analyzed by Viking
• ALH84001 = Unclassified. http://en.wikipedia.org/wiki/Martian_meteorite
Photo courtesy: Norbert Classen
and intermediate ones form a group with youngages (1, 2) (fig. S1A). Cosmic ray exposure ages andthus the deduced ejection age for the shergottitesoccur in clusters between 1 and 5million years (My)(1, 2, 5), but depletion level and ejection ages ap-pear uncorrelated (fig. S1B).
One indicator of young (<20 Ma) craters isan extended rayed ejecta pattern, and eight suchcraters have been suggested as meteorite sourcecandidates (9). They range from 2.6 to 29 km indiameter and have been dated by cratering sta-tistics to have formed in the past 20 My (10). Wesearched for and have identified additional rayedcraters (fig. S2). The occurrence of these craterswith rayed ejecta patterns is strongly correlatedwith dusty but old terrains, and none are foundon the young volcanoes on the Tharsis plateau.The crater diameters range between 1.5 and 55 km,
and Mojave is the largest crater. The Mojave im-pact site is centered at 7.5˚N and 33.0˚W, whichis at the outflow channel floor of the Simud andTiu Valles confluence. The channels were carvedduring the Hesperian [between about 3.5 and3.75 Ga, derived by our chronology model (11)],into the Noachian (older than 3.75 Gy) XantheTerra plateau, and are about 1500m deep. XantheTerra is one of the oldest units on Mars (12), sug-gesting that the ejected rocky material originallycrystallized in the Early Noachian, or even before.The target has beenmodified by tectonic and shock-induced brecciation, as well as by aqueous altera-tion, fluid migration, and volcanic activity, andthus isotope ages determined for such a target canbe reset or disturbed by these processes.
The Mojave Crater itself has attracted at-tention for its well-preserved fluvial landforms
(13–15), which support a very young formationage. However, because of Mojave’s large size,age speculations range from Late Hesperian (16)to Early Amazonian (15, 17) (younger than3.5 Gy) age. Statistically, a 55-km-sized cratercould form every 35 to 50 My (18). Mojave Craterexposes features similar to the crater class of veryyoung rayed craters (Fig. 1A), and chains of sec-ondary crater clusters coinciding with ray pat-terns observed on THEMIS (Thermal EmissionImaging System on the Mars Odyssey space-craft) nighttime mosaics are present on the pla-teau units and on the floors of Simud and TiuValles at distances as far as 1000 km away (Fig.1B). Mojave’s near-field secondary craters reacha maximum size of less than 1 km in diameter(normally 2 to 5% of primary crater diameter),suggesting a strong brecciation of the target rock.
FRTB585
FRTA34C
FRT5DCD
FRT184C8
HRL13ED1
FRT83A2
50 km
A C
D
10 km
B
10 km10 km
G
G
F
F
Best fits:ShergottyLos AngelesQUE94201ALH84001
E
Rat
ioed
spe
ctra
RMS = 0.00088
ALH84001
RMS = 0.001026
QUE94201
RMS = 0.001017
ShergottyH
wavelengthwavelength wavelength
Fig. 1. Morphology and mineralogy of Mojave Crater. (A) THEMISnighttime image of Mojave Crater showing its well-preserved morphology andray pattern. The direction of the red arrow indicates distant examples ofsecondary crater clusters shown in (B). (B) Examples of Mojave’s secondarycrater clusters at 100, 500, and 840 km distance along the same ray indicatedin (A) (red arrow). (C) Mars Reconnaissance Orbiter Context Camera imagemosaic of Mojave Crater’s interior (D) overlain by OMEGA pyroxene detections(color range blue to green, large pixels) and CRISM pyroxene (blue, smallpixels) and olivine (green, small pixels) detections of the Mojave Crater region.The footprints and names of CRISM observations are indicated. (E) Locations ofbest fits obtained by fitting the four martian meteorite spectra and spectra of
the CRISM FRT83A2 observation (D) of the eastern wall of Mojave Crater areplotted together with the pyroxene detection map. Best fits are indicated withred points for Shergotty, orange points for QUE94201, yellow points for LosAngeles, and blue points for ALH84001. Close-ups of this map (F and G) showthat the best fits for shergottites are associated with rocky outcrops at the hillflanks indicated by arrows on (G). A similar geological context is found forALH84001 in the southern part of the CRISM observation (E). (H) Examples offits for the ALH84001, Shergotty, and QUE94201 martian meteorites ex-tracted from locations marked by colored arrows in (E) and (F). The processedCRISM spectra are shown as black curves and the fitted meteorite spectra asred curves.
21 MARCH 2014 VOL 343 SCIENCE www.sciencemag.org1344
REPORTS
ALH84001: Evidence for Life on Mars?
McKay et al. (1996, Science, 273, 5277)“Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001”
• Nanofossils! 20 – 100 nm. (Earth bacteria are ~1000 nm.)
• Biogenic magnetite.
• Amino acids and polycyclic aromatic hydrocarbons(PAHs).
è Sensation!è Presidential press conference.è Well-timed for two missions arriving at Mars in 1997.
ALH84001: Evidence for Life on Mars?
ALH84001: Evidence for Life on Mars?McKay et al. (1996, Science, 273, 5277)“Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001”
• Nanofossils! 20 – 100 nm. • Biogenic magnetite.• Amino acids and
polycyclic aromatic hydrocarbons(PAHs).
è Sensational claims!è But each item has other,
more mundane, possible explanations: natural features, contamination, etc.
è Does not meet the “Extraordinary Evidence” threshold. Currently, the jury is still out…
A HABITABLE MARS?The Red Planet
Historical map of planet mars from Giovanni Schiaparelli 1888
Mars
Mars is “Earthlike”
The analogy between Mars and the earth is, perhaps, by far the greatest in the whole solar system. The diurnal motion is nearly the same; the obliquity of their respective ecliptics, on which the seasons depend, not very different; of all the superior planets the distance of Mars from the sun is by far the nearest alike to that of the earth ... If, then, we find that the globe we inhabit has its polar regions frozen and covered with mountains of ice and snow, that only partly melt when alternately exposed to the sun, I may well be permitted to surmise that the same causes may probably have the same effect on the globe of Mars ... I have often noticed occasional changes of partial bright belts ... and also once a darkish one, in a pretty high latitude... And these alterations we can hardly ascribe to any other cause than the variable disposition of clouds and vapors floating in the atmosphere of that planet. – William Herschel, 1783
Slides from Ryan Anderson, former Cornell graduate student.
… so it must have life!“Shall we recognize in Mars all that makes our own world so well fitted for our wants – land and water, mountain and valley, cloud and sunshine, rain and ice, and snow, rivers and lakes, ocean currents and wind currents, without believing further in the existence of those forms of life without which all of these things would be wasted? ... it is yet to speculate ten thousand times more rashly to assert ... that Mars is a barren waste, either wholly untenanted by living creatures, or inhabited by beings belonging to the lowest orders of animated existence.”- Richard Proctor, 1870
“Canals”
• ~1877: Giovanni Schiaparelli identifies “canali”– “confirmed” by many other observers
• 1910: Percival Lowell– Intricate drawings– Book: “Mars as the Abode of Life”– Canals were built by a dying civilization!
Historical map of planet mars from Giovanni Schiaparelli 1888
Mars
Martian canals depicted by Percival Lowell1898
Canal Skepticism• Many observers didn’t see the canals!• 1903 - Maunder shows that they may be an
optical illusion• 1907 – Alfred R. Wallace’s book “Is Mars
Habitable?”– It’s too cold!– Pressure is too low!– No detectable water vapor!
• But the idea was too attractive for people to let go…
[T]he present inhabitation of Mars by a race superior to ours is very probable... The considerable variations observed in the network of waterways testify that this planet is the seat of an energetic vitality ... [We] may hope that, because the world of Mars is older than ours, mankind there will be more advanced and wiser. -Camille Flammarion, 1892 Chicago newspaper ad, 1893
…across the gulf of space, minds that are to our minds as ours are to those of the beasts that perish, intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.
- H.G. Wells, The War of the Worlds (1898)
Edgar Rice Burroughs’, “Barsoom” series, 1917-1964
Robert Heinlein’s “Stranger in a Strange Land”, 1961
Ray Bradbury’s “The Martian Chronicles”, 1950
Marvin the Martian, 1948
“On Mars, the crumbling remains of ancient civilizations may be found, mutely testifying to the one-time glory of a dying world.”– P.E.Cleator, 1936, founder of British
Interplanetary Society
Today, Mars is a planet populated entirely by robots.
