Focus: Brave New World The Science of the Undiscovered Astrobiology and the possibility of life in other worlds By Fernando Racimo and specific chemical components to Figure 1. Digital image mosaic of the Mare Boreum pace: the final frontier,” and for area of Mars, where the northern ice cap lies. This “S create anything remotely resembling a scientists, just as much as for Star Trek region could have once harbored microoscopic life cell. 3.8 billion years ago, the conditions capable of withstanding extremely cold tempera- fans, a universe of possibilities for ex- were exactly right on Earth and thus tures. ploration and speculation. The opening the first terrestrial organisms arose. line of the most popular sci-fi show in But how likely is it that life originated history compels us “to seek out new be larger than 10,000. The result of his somewhere else in the universes? In the life and new civilizations, to boldly go equation are optimistic but lack rigor- 1960s, Dr Frank Drake – an astrophysi- where no man has gone before”. But ous empirical evidence to support their cist – set out to calculate how probable before we seek “new life”, we need to mathematical assumptions. it was for humans to make contact with know where to look and how best to do On the other end of the spectrum, extraterrestrial life. Drake quatified the it. That is the task of astrobiology. the paleontologist Peter Ward and the probability of the existence of habit- Can we study and understand ex- astronomer Donald Browniee have able planets in our universe and the traterrestrial life if we have not even proposed the “Rare Earth Hypoth- likelihood that any of those planets determined it exists? The answer is esis”, which states that extraterrestrial harbors life with some level of intelli- both yes and no. We may not be able complex life may in fact be very rare, gence and communication [1]. The so- to analyze living creatures from other because most regions of our universe called “Drake equation” estimates that planets (despite what many a Roswell constitute “dead zones” [2]. The in- the number of life-forms with which enthusiast would make us believe) ner regions of every galaxy possess communication might be possible may but we can make predictions about too much radiation and gravitational the conditions under which life could perturbations, while the outer regions proliferate if given the chance. After “Exoplanetary don’t contain enough metals for Earth- all, more than 300 extrasolar planets organisms need not like planets to form. In fact, Ward and have been detected to date, many of Browniee estimate that 95% of the which strongly resemble our own (and be based on carbon Milky Way is a “dead zone” [3]. The the number grows every year). The compounds and may solar system just happens to sit in the perfect spot for life to occur and it is odds, it seems, are in favor of Star code the information Trek’s omen. unlikely that any other system could for their propagation have been as lucky as ours. But not Is anyone out there? all hope is lost. Ward and Browniee’s in something other / JPL-Caltech credit: NASA Life is a complicated business. It theory says little about the existence takes both high amounts of energy than DNA.” of “simple” life: while other complex 24 Harvard Science Review • fall 2008 Fernando_Racimo_-_Astrobiology2.indd 24 2/9/2009 11:32:31 PM Focus: Brave New World organisms are unlikely to exist, mi- crobes may well be common in harsh conditions. One of the main focuses of astrobiology’s efforts is the search for extraterrestrial biomarkers, chemical traces of microscopic extremophiles— organisms that thrive in habitats where others cannot. In theory, these micro- organisms could, like Earth’s own ex- tremophiles, thrive where nothing else seems able to survive. The great physicist Enrico Fermi once asked his colleagues over lunch: “if extraterrestrial aliens are com- mon, why aren’t they obvious?” The question was later formalized by the astrophysicist Michael Hart and became known as the Fermi-Hart paradox [4]. It appears that the answer to it is that Figure 2. Electron microscope photograph of the ALH84001 meteorite, revealing chain struc- aliens may be simpler and smaller than tures that closely resemble microfossils. what we imagine them to be. After all, terrestrial microbes – the most abun- Living to the extreme in outer space – but not necessarily in dant organisms on our planet – only Extremophiles can exist in an as- liquid form. Microbial acidophiles can became “obvious” to humans just over toundingly wide variety of environ- inhabit the acidic channels of glaciers 330 years ago, with the invention of the ments: from hydrothermal vents, to the [7] and certain bacteria appear to be microscope. polar ice caps, and even in deserts sev- able to live in the stratosphere (41 eral meters underground. They km above us) where trace amounts are found in all three domains of water vapor are all that is left for of life (Bacteria, Archaea and them to use [8]. Most planets in the Eukarya) and are for the most solar system are frozen and it is now part unicellular. Their ability known that there is water ice [9] and to withstand extreme tempera- even snow [10] on Mars. Present ef- tures, radiation, and pH levels forts are thus being directed at trying makes them the prime research to find acidophile-like life forms in the subjects of astrobiologists. The Martian ice caps [11]. cyanobacterium Chroococcidiopsis, Another key element for life is car- for example, has recently been bon. All life-forms on our planet are observed to resist up to 15 kGy based on molecules rich in carbon, like of X-ray radiation [5]. In 2006, glucose and DNA. In fact, in the 1980s bacteria of the genus Trichococcus Mars became an attractive candidate were seen growing and repro- for astrobiology when evidence of ducing at -5 °C in anaerobic carbonate compounds was found in conditions [6]. By understanding the Martian ALH84001 meteorite in how these microbes maintain a Antarctica. The meteorite also contains stable metabolism, astrobiolo- magnetite crystals with a special hexa- gists can make inferences about octahedral shape, which resemble those what extraterrestrial microen- produced by certain bacteria on Earth vironments could potentially [12]. A recent study [13], however, Figure 3. Thin section of an extremophile: the green support extraterrestrial life. has proven that similar crystals can be algal cell Chlamydomonas nivalis, which thrives in freezing water. Organisms like this one are being For instance, water seems to created in the lab simply by simulating studied by astrobiologists seeking to understand how be a general requirement for any meteorite impacts. Though somewhat life can survive in strenous environments that may be such life-supporting microen- discouraging, the finding has not de- common in other planets. Magnified 10,000X. vironments – whether here or terred scientists from their pursuits: credit: NASA fall 2008 • Harvard Science Review 25 Fernando_Racimo_-_Astrobiology2.indd 25 2/9/2009 11:32:31 PM Focus: Brave New World just because our planet is carbon-rich, solar system). By performing spectro- Another approach consists in the extraterrestrial extremophiles need not metric analyses of light reflected from direct study of materials obtained in be carbon-based. How then can we these distant worlds, scientists can infer space missions. The planetary scientist know where life may be hiding? the presence or absence of compounds Christopher McKay has recently come in their atmospheres. This technique up with a way to test whether samples Honing our searching skills enabled scientists to make a striking from Mars, Europa or other astronom- Life alters the environments it con- discovery concerning HD 189733b, ical bodies contain potential biological quers, leaving characteristic signatures an extremely hot planet 63 light-years specimens. His work stresses the fact as it expands and evolves. These sig- away from our own. Its atmospheric that, although life on Earth is made of natures are evident even from afar: infrared signatures match those of distinctive molecular components–like the familiar blue and green colors of water vapor, revealing the presence of acidic nucleotides and nitrogen-rich our planet would not exist without the that gas on the planet’s surface [15]. proteins–the biomolecules of alien life earth’s oxygen-rich atmosphere, itself Moreover, the NASA Virtual Planetary forms may be different in composition. a direct product of photosynthetic cy- Laboratory is pioneering the creation [17]. Exoplanetary organisms need not anobacteria and plants [14]. While some and testing of computer models that be based on carbon compounds and astrobiologists theorize about potential can predict what spectrometric signal might encode information for their conditions for life’s existence, others a habitable planet would produce. The replication in something other than develop methods for actively looking Laboratory has succeeded in creating DNA. McKay does point out, how- for these signatures inside and outside a realistic planetary climate model ever, that a general property of any the solar system. that accounts for thermal flow, cloud kind of living system is the repeated The most promising of these meth- formation, and precipitation in a habit- arrangement of a few specific organic ods relies on the atmospheric composi- able planet that has evolved towards an molecules for the construction of tion of exoplanets (planets outside our Earth-like stable equilibrium [16]. biopolymers. Cellulose is nothing but Figure 4. Data plot from NASA’s Spitzer Space Telescope’s spectrometric analysis of light reflected from the exoplanet HD189733b. Each circle dot represents a different absortion of light for a specific wavelength.