Venus may hold the an­swers about life we’ve been look­ing for

The Washington Post - - WEDNESDAY OPINION - BY JONATHAN LU­NINE The writer is the David C. Dun­can pro­fes­sor in the phys­i­cal sciences and chair of the Depart­ment of Astron­omy at Cor­nell Univer­sity.

Is there life on Venus, Earth’s near­est plan­e­tary neigh­bor? This may seem like an ab­surd ques­tion, given that the sur­face tem­per­a­ture of Venus is above the melt­ing point of lead. But that has not pre­vented 50 years of spec­u­la­tion that life might ex­ist in the cool mid­dle at­mos­phere of the planet, where a thick layer of sul­fu­ric-acid droplets might pro­vide a home. The dis­cov­ery of phos­phine in Venus’s at­mos­phere, an­nounced in a study pub­lished Mon­day, could be telling us those ideas are right.

In 1967, the yet-to-be-fa­mous Carl Sa­gan wrote that life might ex­ist in the Venu­sian clouds. Col­lab­o­rat­ing with bi­ol­o­gist Harold Morowitz, the two posited that life might have evolved to live in the liq­uid sul­fu­ric acid cloud layer of Venus, where tem­per­a­tures are Earth­like and small amounts of wa­ter would dis­solve. That would be the be­gin­ning of many papers on this pos­si­bil­ity, cul­mi­nat­ing in a de­tailed study this year on how mi­crobes might surf waves in the at­mos­phere. MIT pro­fes­sor Sara Sea­ger and col­leagues imag­ined a cy­cle in which mi­crobes live in wa­ter-sul­fu­ric acid droplets that would grow and then sink to the hot­ter lower at­mos­phere, only to evap­o­rate. Freed from the evap­o­rat­ing droplets, the mi­crobes would ride buoy­ancy waves back to the cooler up­per at­mos­phere to bur­row again into the rel­a­tive safety of the sul­fu­ric acid and wa­ter droplets.

How would we know such or­gan­isms might ex­ist? Many chem­i­cal com­pounds that sim­ple mi­crobes pro­duce are also made by non-bi­o­log­i­cal pro­cesses. But one, phos­phine, or PH3, is dif­fi­cult to pro­duce on Earth abi­ot­i­cally (with­out life) and, as ar­gued by Sea­ger and her col­leagues in an­other pa­per, could be a good “biosig­na­ture” or sign of life on plan­ets around other stars. This isn’t al­ways the case: The com­pound is found in the dense, hy­dro­gen-rich at­mos­pheres of Jupiter and Saturn, where it is un­der­stood to be an abi­otic prod­uct of sim­ple chem­istry, and will likely be found on gas gi­ants around other stars us­ing the James Webb Space Te­le­scope, planned for launch next year. But Venus — which has an at­mos­phere in which hy­dro­gen is ex­tremely scarce — is a place where phos­phine is a plau­si­ble biosig­na­ture.

The de­tec­tion of suf­fi­cient quan­ti­ties of phos­phine in Venus’s at­mos­phere would be an in­trigu­ing pointer to the pos­si­bil­ity of life in the sul­fu­ric-acid clouds of our sis­ter planet, but many ques­tions would re­main. Is it pos­si­ble that plan­e­tary chemists have over­looked ways to pro­duce phos­phine on Venus in the ab­sence of life? And if phos­phine is pro­duced by bi­ol­ogy, where did that life orig­i­nate? It is one thing to imag­ine life adapt­ing to and hang­ing out op­por­tunis­ti­cally in the clouds of Venus. It is quite an­other to imag­ine that life could have orig­i­nated there, sand­wiched be­tween the hell of the sur­face and the frozen realms of the thin up­per at­mos­phere.

The high pro­por­tion of heavy (deu­terium-rich) wa­ter in Venus’s at­mos­phere is strong ev­i­dence that the planet once had a large amount of sur­face wa­ter — lit­er­ally an ocean. That ocean was lost by evap­o­ra­tion as the sun bright­ened, driv­ing large amounts of car­bon diox­ide into the at­mos­phere and cre­at­ing the su­per­green­house planet we see to­day. As the ocean was lost, mi­cro­bial marine or­gan­isms might have been able to seek refuge in the clouds. An­other pos­si­bil­ity is that mi­crobes could have been de­liv­ered in rocky ma­te­rial blasted off the sur­face of Earth or Mars (if it had life) in large im­pacts that we know were com­mon dur­ing the early his­tory of our so­lar sys­tem.

A third pos­si­ble source of life would be bad news: con­tam­i­na­tion from space probes. In the 1970s and 1980s, mul­ti­ple probes from the Soviet Union and the United States en­tered Venus’s at­mos­phere and landed on the sur­face. One set of Soviet probes even de­ployed bal­loons that floated in the strato­sphere. If these mis­sions in­jected our own mi­crobes into the clouds, it seems highly un­likely those bugs would sur­vive the acidic con­di­tions in the clouds’ droplets.

The de­tec­tion of phos­phine in Venus is so provoca­tive that it calls for fol­lowup ob­ser­va­tions to con­firm its pres­ence, clever mod­els to ei­ther rule in or rule out abi­otic chem­istry and, ul­ti­mately, new mis­sions if we con­clude life might be present in the clouds of Venus. Sam­pling those clouds would be chal­leng­ing, but that can be bal­anced against the short trip times to Venus, which are mea­sured in months rather than the many years it would take to reach more dis­tant targets.

Venus may hold the an­swers to ques­tions about the long-term liv­abil­ity of our own home planet: How do plan­e­tary cli­mates change from hab­it­able to hellish? What is the fate of Earth’s cli­mate as our sun con­tin­ues to brighten? We surely want to get a hint of how ro­bust or frag­ile our home planet’s cli­mate is from our sis­ter planet next door. But if it turns out that Venus has its own na­tive biota, it would rad­i­cally change our per­spec­tive on what makes a planet hab­it­able and cause us to won­der how many other ex­otic lo­cales in our wildly sur­pris­ing uni­verse might be home to life.

NASA/REUTERS

The planet Venus as it be­gins its tran­sit of the sun in June 2012.

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