Transfer of Life Between Earth and Venus with Planet-Grazing Asteroids
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Transfer of Life Between Earth and Venus with Planet-Grazing Asteroids
Amir Siraj and Abraham Loeb Department of Astronomy, Harvard University, 60 Garden Street, Cambridge, MA 02138, USA
ABSTRACTRecently, phosphine was discovered in the atmosphere of Venus as a potential biosignature. Thisraises the question: if Venusian life exists, could it be related to terrestrial life? Based on the knownrate of meteoroid impacts on Earth, we show that at least ∼ × asteroids have grazed Earth’satmosphere without being significantly heated and later impacted Venus, and a similar number havegrazed Venus’s atmosphere and later impacted the Earth, both within a period of ∼ years duringwhich microbes could survive in space. Although the abundance of terrestrial life in the upper atmo-sphere is unknown, these planet-grazing shepherds could have potentially been capable of transferringmicrobial life between the atmospheres of Earth and Venus. As a result, the origin of possible Venusianlife may be fundamentally indistinguishable from that of terrestrial life. Keywords: astrobiology; planets; asteroids; meteors INTRODUCTIONRecently, the spectral absorption feature of phosphinewas discovered in the temperature cloud deck of Venusas a potential biosignature (Bains et al. 2020; Greaveset al. 2020; Seager et al. 2020; Sousa-Silva et al. 2020).If phosphine is produced through biotic, as opposed toabiotic pathways, the discovery could imply a significantbiomass in the Venusian atmosphere (Lingam & Loeb2020). This raises the following question: if Venusianlife exists, could it be related to terrestrial life?Panspermia is the conjecture that life can propagatefrom one planet to another (for a review, see Wesson2010; Wickramasinghe 2010). Panspermia between theEarth and Venus has been proposed through channels ofrock and dust ejecta (Melosh & Tonks 1993; Gladmanet al. 2005; Wickramasinghe & Wickramasinghe 2008;Joseph 2019). The former channel suffers from signifi-cant heating that occurs during the passage through theatmosphere, and the latter lacks shielding from harmfulradiation during the journey through space. Anotherversion of panspermia alleviates both of these issuesby picking up microbes through an atmospheric graz-ing event (Siraj & Loeb 2020a,b). The grazing objectlater impacts another planet, depositing material thereas it breaks up. If the biota is deposited when Venuswas young and hospitable to life on its surface, then [email protected], [email protected] the deposition could be anywhere. Otherwise, seedingevents are restricted to the cloud deck at elevation of50 −
60 km (Seager et al. 2020) where the temperatureand pressure are similar to the lower atmospheric condi-tions on Earth. Such events could have possibly trans-ferred microbes between the atmospheres of Earth andVenus with radiation shielding and without significantheating.Here, we study the transfer of planet-grazing asteroidsbetween Earth and Venus. In Section 2, we investigatethe abundance and plausibility of grazing asteroids thatwere exchanged between Earth and Venus that couldpossibly transfer life. In Section 3, we discuss furtherdetails regarding the survival of microbial life duringthe asteroid’s atmospheric passages and journey throughspace. Finally, in Section 4, we explore key predictionsand implications of our model. EXCHANGED ASTEROIDSThe recent detection of an Earth-grazing ∼
60 kg as-teroid demonstrated that such objects can pass throughthe atmosphere without significant heating at altitudesof (cid:38)
85 km (Shober et al. 2020). As a result, we considerEarth-grazing asteroids that graze through the scaleheight ( ∼ (cid:38)
85 km, due to the significant drop in density beyond ascale height. Asteroids with mass ∼
60 kg strike Earthat a rate of ∼ . × yr − (Bland & Artemieva 2006),implying that such asteroids pass through a scale heightof the Earth’s atmosphere at a rate of ∼ − . a r X i v : . [ a s t r o - ph . E P ] S e p Siraj & Loeb
An Earth-grazer typically receives a change of or-der 1 km s − in v ∞ through its gravitational interactionwith the Earth (Siraj & Loeb 2020a), which is an in-significant change relative to the total orbital energy ofnear-Earth asteroids. This implies that the fraction ofEarth-crossing objects, which are also Venus-crossing ,remains as observed at ∼ / r V /a V ) ∼ × − , where r V ∼ × kmand a V ∼ . Deinococcus radiodurans are estimated to have an exponential survival proba-bility on a timescale of ∼ yr with minimal radia-tion shielding in space (Mileikowsky et al. 2000; Burchell2004; Ginsburg et al. 2018). Given that Venus-crossingasteroids have typical orbital periods of ∼ ∼ × orbits over ∼ yr during which microbesmay survive, resulting in a ∼ . × − probability ofimpacting Venus over that timescale. The asteroid belthas been in steady-state for ∼ . ∼ × Earth-grazing asteroids of mass ∼
60 kg have impacted Venuswithin ∼ yr of interacting with the Earth. A sim-ilar number of objects grazed a scale height of Venus’atmosphere and later impacted the Earth, since there isonly ∼
5% difference between the radii of Venus and theEarth and the orbital radii are only different by ∼ MICROBIAL SURVIVALLife has been detected in the Earth’s atmosphere up toan altitude of 77 km (Imshenetsky et al. 1978). Furtherwork is needed to investigate the existence and abun-dance of microbial life in the upper atmosphere, partic-ularly at the altitude considered here, ∼
85 km, at which https://ssd.jpl.nasa.gov/ https://ssd.jpl.nasa.gov/ Asteroid mass [kg] N E ( / y r )( / k m ) Figure 1.
Total number of exchanged planet-grazing aster-oids between Earth and Venus, N E , as a function of asteroidmass, per logarithmic bin, scaled to a transfer timescale of τ ∼ yr during which microbes may survive and an at-mospheric altitude cross section of λ ∼ Earth-grazing object would avoid significant heating. Inaddition, if life is discovered by a direct probe sent intothe atmosphere of Venus, it will be necessary to cali-brate the abundance of life as a function of altitude onVenus.If accelerated over a distance of ∼
100 m, the collectedmicrobes will experience accelerations of order ∼ g.Many terrestrial microbe species have been shown tosurvive accelerations of 4 − × g (Mastrapa et al.2001; Deguchi et al. 2011), so we assume that accelera-tion is not an important lethal factor for microbes pickedup by the transporting body.Interiors of objects with radii of (cid:38)
10 cm may not beheated to more than 100 ◦ C during a passage through theEarth’s atmosphere (Napier 2004). This would increasethe grazing cross-sections of Earth and Venus that al-low for microbial survival, and additionally imply thatplanet-grazers would not be heated significantly whentravelling to the surface of Earth or the cloud-decks ofVenus. Many asteroids are known to have significantporosity (Britt & Consolmagno S. J. 2000; Britt et al.2006), making it possible for microbes to become lodgedinside and shielded from exterior heating (Siraj & Loeb2020b). While some polyextremophiles can survive inspace with minimal radiation shielding, asteroids tens ofcm in size may provide sufficient shielding from radiationto protect the survival of a greater diversity microbes(Horneck et al. 2002). Microbes could be potentially bedeposited in clouds when a meteor disintegrates, despitethe heating by friction with the atmosphere, due to theeffects of shielding by the outer layers of the meteor. ransfer of Life Between Earth and Venus DISCUSSIONWe showed that, throughout the history of the so-lar system, at least ∼ × asteroids have grazedEarth’s atmosphere without undergoing significant heat-ing and later impacted Venus within a microbe survivaltimescale of ∼ yr, and that a similar number ofobjects grazed the atmosphere of Venus and later im-pacted the Earth. If these asteroids picked up microbesduring their grazing events, they could have potentiallytransferred life between Earth and Venus.Due to the uncertainties regarding the abundance andnature of microbial life in the Earth’s atmosphere, inaddition to poorly constrained factors relating to micro-bial survival during pick-up, transport, and delivery, itis not at present possible to determine whether any lifewas transferred between Earth and Venus.A future probe that could sample the habitable clouddeck of Venus will potentially enable the direct discoveryof microbial life outside of Earth. Specifically, the ca-pability to either directly analyze microbes in situ or toreturn an atmospheric sample to Earth will be critical in the design of a successful mission. Finding exactly thesame genomic material and helicity on Venus and Earthwould constitute a smoking gun for panspermia.This potentially viable mechanism for transferring lifebetween the two planets implies that if Venusian life ex-ists, its origin may be fundamentally indistinguishablefrom that of terrestrial life, and a second genesis maybe impossible to prove. Our model predicts that puta-tive Venusian life would share chemical and biologicalstructures with life on Earth, such as RNA and DNA.Future space missions to Venus will test this hypothesis,and further investigation of the Earth’s atmosphere willhelp refine models of panspermia. Additionally, tests ofpanspsermia across the rocky planets of the solar systemwill hold important implications for the likelihood of lifeon more than one planet per multiplanetary system.ACKNOWLEDGEMENTSThis work was supported in part by a grant from theBreakthrough Prize Foundation.REFERENCESor toreturn an atmospheric sample to Earth will be critical in the design of a successful mission. Finding exactly thesame genomic material and helicity on Venus and Earthwould constitute a smoking gun for panspermia.This potentially viable mechanism for transferring lifebetween the two planets implies that if Venusian life ex-ists, its origin may be fundamentally indistinguishablefrom that of terrestrial life, and a second genesis maybe impossible to prove. Our model predicts that puta-tive Venusian life would share chemical and biologicalstructures with life on Earth, such as RNA and DNA.Future space missions to Venus will test this hypothesis,and further investigation of the Earth’s atmosphere willhelp refine models of panspermia. Additionally, tests ofpanspsermia across the rocky planets of the solar systemwill hold important implications for the likelihood of lifeon more than one planet per multiplanetary system.ACKNOWLEDGEMENTSThis work was supported in part by a grant from theBreakthrough Prize Foundation.REFERENCES