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On the possibility of microbiota transfer from Venus to Earth


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The possibility of the clouds of Venus providing habitats for extremophilic microorganisms has been discussed for several decades. We show here that the action of the solar wind leads to erosion of parts of the atmosphere laden with aerosols and putative microorganisms, forming a comet-like tail in the antisolar direction. During inferior conjunctions that coincide with transits of the planet Venus this comet-like tail intersects the Earth’s magnetopause and injects aerosol particles. Data from ESA’s Venus Express spacecraft and from SOHO are used to discuss the ingress of bacteria from Venus into the Earth’s atmosphere, which we estimate as ∼1011–1013 cells for each transit event.
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Astrophys Space Sci (2008) 317: 133–137
DOI 10.1007/s10509-008-9851-2
On the possibility of microbiota transfer from Venus to Earth
N.C. Wickramasinghe ·J.T. Wickramasinghe
Received: 22 March 2008 / Accepted: 20 June 2008 / Published online: 22 July 2008
© Springer Science+Business Media B.V. 2008
Abstract The possibility of the clouds of Venus provid-
ing habitats for extremophilic microorganisms has been dis-
cussed for several decades. We show here that the action of
the solar wind leads to erosion of parts of the atmosphere
laden with aerosols and putative microorganisms, forming
a comet-like tail in the antisolar direction. During inferior
conjunctions that coincide with transits of the planet Venus
this comet-like tail intersects the Earth’s magnetopause and
injects aerosol particles. Data from ESA’s Venus Express
spacecraft and from SOHO are used to discuss the ingress
of bacteria from Venus into the Earth’s atmosphere, which
we estimate as 1011–1013 cells for each transit event.
Keywords Panspermia ·Interplanetary dust ·Solar wind ·
Clouds of Venus ·Life on Venus ·Conjunctions of Venus
1 Introduction
The discovery of complex organic molecular structures in
the Martian meteorite ALH84001 (McKay 1996) has led
to a revival of interest in the possibility of microbial life
being transferred between planets (Wallis and Wickramas-
inghe 1995,2004; Napier 2004; Wickramasinghe 2007;
Wickramasinghe and Napier 2008). Most discussions have
focussed on the idea of impact-driven transfers of life-
bearing rocks and dust that can take place sporadically over
long dynamical timescales, >107yr. Transfers over much
shorter timescales between neighbouring planets would in
N.C. Wickramasinghe ()·J.T. Wickramasinghe
Centre for Astrobiology, School of Mathematics, Cardiff
University, 2 North Road, CF10 3DY, Cardiff, UK
principle be possible, but only if dynamical connection
pathways can be identified. In this letter we propose one
such pathway for transfers between Venus and Earth on a
timescale of decades.
2 Clouds of Venus
Before discussing possible transfer pathways it is first nec-
essary to discuss the feasibility of contemporary microbial
life on Venus. Venus is 25% closer to the Sun than the
Earth and has almost the same size, but the two planets dif-
fer in several crucial aspects. Of relevance to the present
discussion is the fact that Venus has a thick convective at-
mosphere dominated by CO2producing a powerful green-
house effect. The average variation of temperature with
height is depicted in Fig. 1, showing a temperature differ-
ence between the surface and the cloud tops of 500 de-
grees. Current estimates of the atmospheric composition of
Venus show CO2making up 96.5%, the rest including N2,
H2O, CO, OH, HCl, H2S, COS and SO2(Oyama et al. 1979;
Svedhem et al. 2007; Piccioni et al. 2008; ESA Venus Ex-
press Data 2008). Venus’s atmosphere has a high opacity to
visible and ultraviolet light, reflecting 80% of the incident
solar radiation. Despite the many spacecraft that have vis-
ited Venus since the 1970’s it is remarkable that the cloud
domain of the planet still harbours so many unsolved mys-
teries. In particular, a complete characterization of the cloud
aerosols that impart an yellow tinge to the clouds still re-
mains uncertain.
Whilst the conditions near the surface of Venus, T>
460°C, rule out microbial life, the temperature and pressure
regime in the altitude range 70–45 km defines a habitable
zone for some types of extremophilic bacteria that have actu-
ally been found on the Earth (see shaded area of Fig. 1). Here
134 Astrophys Space Sci (2008) 317: 133–137
Fig. 1 The average temperature in the atmosphere of Venus as a func-
tion of height (The shaded area represents notional habitable zone for
microorganisms; the pressure ranges from 0,1 to 10 bar)
the ambient temperature varies between 25°C and +75°C,
and the pressure in the range 0.1 to 10 bar. Speculations
relating to a Venus-adapted microbiota have been published
over many years (Morowitz and Sagan 1967; Cockell 1999;
Hoyle and Wickramasinghe 1981; Schulze-Makuch et al.
2004). Water, albeit in small quantities, has been identified
in the atmosphere, adequate for microorganisms to concen-
trate and exploit. Furthermore, with a stable cloud system
primarily circulating between 70 and 45 km, and with a
steady supply of nutrients from sublimating meteorites, a
Venusian aerobiology remains a distinct possibility (Hoyle
and Wickramasinghe 1981,1982).
Data from ESA’s Venus Express probe (2008) has pro-
vided evidence of frequent electrical discharges (lightning)
in the atmosphere. Such energetic events would be expected
to generate large amounts of CO from CO2. The absence of
higher concentrations of CO than is observed, despite the
lightning, might be taken as strongly suggestive of an exotic
microbiology. There is a diverse group of terrestrial bac-
teria and archaea known as hydrogenogens that can grow
anaerobically using CO as the sole carbon source and H2O
as an electron acceptor, producing CO2and H2as waste
products (Wu et al. 2005). The presence of H2S and SO2
in the atmosphere could also point to the presence of ex-
tremophilic “sulphur” bacteria (Cockell 1999); and droplets
of atmospheric sulphuric acid could provide a medium in
which acidophiles can thrive.
As pointed out by Schulze-Makuch and Irwin (2002)the
detection of COS (carbonyl sulfide) may also be taken as a
indicator of biology (Bezard et al. 1993). Finally, the sizes
and refractive indices of aerosol particles found in the up-
per clouds of Venus (55–65 km) (Fig. 2), are consistent
Fig. 2 The distribution of particle sizes in the upper clouds of Venus
(approximately 55–65 km in altitude) from NASA Pioneer Venus data
(Knollenberg and Hunten 1979)
with those appropriate for bacterial spores (Hansen and Ark-
ing 1971; Knollenberg and Hunten 1979; Hoyle and Wick-
ramasinghe 1981,2003). There is no a priori reason for
droplets of sulphuric acid or other atmospheric aerosol to
mimic either the mean refractive index or the distribution of
sizes that is naturally attributable to bacterial spores.
We envisage a situation in Venus analogous to what hap-
pens in terrestrial clouds. Sattler et al. (2001) have demon-
strated bacterial growth in supercooled cloud droplets, argu-
ing that bacteria in tropospheric clouds are actually growing
and reproducing. A stable aerobiology requires processes by
which (a) bacteria nucleate droplets containing water and
nutrients, (b) colonies grow within the droplets, (c) droplets
fall into regions of higher temperature where they evaporate
releasing spores to convect upwards to yield further nucle-
ation. In the case of Venus this cyclical process would be
expected to take place between the tops and bases of clouds.
3 Solar wind erosion of the Venusian atmosphere
The effects of the solar wind in producing erosion and evo-
lution of planetary atmospheres over geological timescales
have been extensively discussed. However, on much shorter
timescales, sputtering losses due to solar wind interactions
have in general tended to be ignored.
Atmospheric erosion due to the solar wind would be ex-
pected to be most efficient for planets such as Venus that
have negligible magnetic field intensities thus providing lit-
tle or no confinement of charged particles. For this reason
Astrophys Space Sci (2008) 317: 133–137 135
Fig. 3 Evidence of solar wind excavating the atmosphere of Venus
(Courtesy ESA;
0.html; ESA Venus Express Data (2008); Svedhem et al. (2007))
there have been early suggestions that Venus and comets
may have similar characteristics in developing wakes or tails
though interaction with the solar wind (Russell et al. 1982;
Wallis and Ip 1982; Russell and Vaisberg 1983). Whilst
comet tails remain coherent over tens of millions of kilo-
meters, the search for ions attributable a Venusian origin be-
yond 10 Venus radii had produced negative results until
very recently (Kar 1996).
The situation has recently changed dramatically, how-
ever. Firstly, Venus Express found evidence that the solar
wind is causing a continual outflow of atmospheric gases in-
cluding O+and H2O+from the clouds of Venus in the form
of a narrow wake or tail in the antisolar direction (Fig. 3)
(Svedhem et al. 2007). In view of the largescale turbulence
in the atmosphere, the wake would include material from the
clouds where molecules and aerosols are present. Secondly,
as part of the ion mass spectrometry program on the SOHO
spacecraft, Grunwaldt et al. (1997) successfully detected a
flux of O+and C+ions that originated in Venus and arrived
near the Earth. The latter observation was made on June 10,
1996 when Venus was in inferior conjunction, and SOHO
was 4.4×107km downstream of Venus at the Earth’s L1
Lagrangian point, 0.01AU sunward of the Earth. Grunwaldt
Fig. 4 Schematic mode of transfer of material from Venus to Earth
during a planetary transit event. The tube of material reaching Earth is
assumed to be 840 km wide, as indicated in the data of Grunwaldt et al.
et al. (1997) estimated the O+flux originating from Venus
to be 3×103ions cm2s1near the Earth. With a Venus
ray (tail) diameter of 820 km Grunwaldt et al. obtained a
total flux of O+
FO+=1.6×1019 s1(1)
with FC+being 0.1FO+. The width of the Venusian tail as
observed by Grunwaldt et al. at a heliocentric distance r=
0.99 AU will not be significantly different from that which
could reach the Earth at r=1.0 AU, if it is appropriately
configured in relation to Venus.
Since the orbit of Venus is inclined at 3.4° to that of the
Earth, material transfer would most effectively take place
during inferior conjunctions that occur along the line of
nodes (transits of the planet Venus). In such situations we
can estimate the mass reaching the Earth, based on the mea-
surements of Grunwaldt et al. (1997). The geometry of the
Venus-Earth transfer process envisaged is schematically de-
picted in Fig. 4.
If ωVand ωEdenote the angular velocities of Venus and
Earth respectively in their orbits, the velocity relative to
Earth of the footprint of the Venusian tail (assumed to be
a 840 km wide cylinder) is
V1)AU/yr (2)
using Kepler’s law. Setting aV=0.75 AU, we obtain a foot-
print speed of the Venusian tail to be
v20 km/s(3)
136 Astrophys Space Sci (2008) 317: 133–137
The effective trapping cross-section for particles with the
Earth will be larger than its geometrical cross-section be-
cause they are charged. It would be reasonable to assume
that the trapping path length is the dimension of the magne-
topause, say 20 RE1.3×105km. Using (3) the inocula-
tion time of Venusian particles into the Earth’s environment
would therefore be 6000 s.
From (1) the total number of O+ions introduced into
the Earth’s magnetosphere during each encounter is 1023 ,
with the number of C+ions being a factor 10 less (Grun-
waldt et al. (1997).
The solar wind at Venus could be assumed to have ex-
cavated not only CO2(which dissociates into C+and O+)
but also dust which forms a substantial fraction of the mid-
dle and upper atmosphere. Dust grains (putative bacteria)
would be charged and coupled to the outflowing plasma tail.
It would be entirely reasonable to speculate that the mass of
grains carried along with the ions makes up 1% of the to-
tal outflow. Thus the mass of bacterial material carried into
the Earth environment along with the O+and C+during a
transit amounts to
Mbact 0.03 g (4)
The dry mass of a typical bacterium being 1013 g, the
total number of bacteria entering the Earth’s magnetosphere
is then 1011 .
A possible transfer of 1011 microbial cells from Venus
to the Earth’s environment in a single injection event, al-
though modest, cannot be ignored. It would require only a
small fraction of this inoculant to find an appropriate terres-
trial niche to effectively link the biosphere of Venus to that
of the Earth. However, it should be noted that the estimate
1011 bacteria, based on a single set of measurements made
by Grunwaldt et al., is on the conservative side, and possi-
bly represents a lower limit. The time in 1997 when Grun-
waldt et al. detected ions in the Venus tail near the Earth
coincided with a deep minimum in the sunspot cycle. It is
well-known that solar flare and solar wind activity increases
near sunspot maximum, and varies erratically from one cy-
cle to the next. Whenever a transit occurs during an active
phase of the sun the bacterial mass from Venus that is added
to the Earth could well exceed 10 g, giving rise to a total
inoculant containing 1014 bacteria. Survival of microbes
(particularly radiation-resistant microbes) in travelling be-
tween Venus and Earth would not pose a problem. The time
of exposure to solar radiation in unshielded interplanetary
space is 105sec, so high survival rates are to be expected
even under the most pessimistic assumptions (Mileikowsky
et al. 2000).
The fate of charged bacteria entering the magnetopause
of the Earth is probably well represented by the known be-
haviour of 0.1–1 µm sized particles from Lunar ejecta that
are known to enter the Earth (Mueller and Kessler 1985;
Zook et al. 1984). Acted upon by the charge-dependent
Lorentz force, convective drag forces, solar radiation pres-
sure and gravity, bacterial dust grains diffuse through the
magnetosphere to reach the atmosphere on timescales of
20–200 hr. However, the details of the entry process in-
volve uncertainties and complications arising from charg-
ing and discharging of grains as well as instabilities in the
field configurations of the magnetosphere. It is reasonable
nevertheless to assume that a large fraction of the parti-
cles find their way ultimately into the troposphere where
they could serve as condensation nuclei of raindrops (Hyde
and Alexander 1989). If the terrestrial clouds can offer a
niche that matches the essential characteristics of the Venu-
sian clouds, an alien aerobiology would become established
on Earth. Otherwise, the iterant Venusian microbes are dis-
charged in the rain to either find a surface niche, or to perish.
Although an inferior conjunction of Venus occurs every
583.9 days a planetary transit event is much less frequent.
The last inferior conjunction associated with a transit oc-
curred in June 2004 and would happen again in June 2012.
Calculations of celestial mechanics show that these events
occur in pairs 8 years apart, and the pairs themselves recur
with time separations of 121.5 and 105.5 years.
In conclusion it should be noted at the idea microbes from
Venus reaching Earth was first proposed by Barber (1963)in
an attempt to explain the periodic “fogging” of photographic
plates at Sidmouth Observatory. At the time a mechanism
for aerosol transfer between the planets was not identified,
and moreover Barber would appear to have been far too opti-
mistic in assuming transfers with every inferior conjunction.
Realistically the two planets would be connected biologi-
cally over timescales of several decades.
Acknowledgements We thank Siegfried Franck for helpful com-
ments that led to improvements to an earlier version of this paper, and
to Hakan Svedhem and David Brain for information and references to
Venus Express results.
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A fundamental question in exobiology remains the degree to which habitats on Venus, past and present, were, or are suitable for life. This has relevance for assessing the exobiological potential of extrasolar Venus-like greenhouse planets. In this paper the parameters of the Venusian surface and atmosphere are considered and the biochemical adaptations required to survive them are explored in the light of new information on microbial adaptations to extreme environments. Neither the pressure (9.5 MPa) nor the high carbon dioxide concentrations (97%) represent a critical constraint to the evolution of life on the surface or in the atmosphere. The most significant constraints to life on the surface are the lack of liquid water and the temperature (464°C). In the lower and middle cloud layers of Venus, temperatures drop and water availability increases, generating a more biologically favorable environment. However, acidity and the problem of osmoregulation in hygroscopic sulfuric acid clouds become extreme and probably life-limiting. If it is assumed that these constraints can be overcome, considerations on the survival of acidophilic sulfate-reducing chemoautotrophs suspended as aerosols in such an environment show that Venus does come close to possessing a habitable niche. Conditions on the surface and in the atmosphere may have been greatly ameliorated on early Venus and may also be ameliorated on extrasolar planets with early Venus-like characteristics where temperatures are less extreme and liquid water is available.
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It is well known that the atmosphere is a conveyor of microorganisms, and that bacteria can act as ice or cloud condensation nuclei, but clouds have not been considered as a site where organisms can live and reproduce. Here we show that bacteria in cloud droplets collected at high altitudes are actively growing and reproducing at temperatures at or below 0°C. Since ~60% of the earth surface is covered by clouds, cloud water should be considered as a microbial habitat.
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In June, 1996, Venus passed through a very close inferior conjunction with the Sun. At that time the CTOF detector of the CELIAS mass spectrometer experiment on the SOHO spacecraft near Earth's L1 Lagrangian point was measuring heavy ions in the solar wind ~4.5×107km downstream of Venus. Close to the time predicted by simple geometric arguments for passage of SOHO through the Venus wake, CTOF made three encounters with unusual fluxes of O+ and C+ ions. Their energy distributions resembled those of tail rays originating in the Venus ionosphere or ionopause region rather than of ions produced in the corona of neutral atoms that surrounds the planet. The C+ abundance was ~10% of O+. The observed O+ speed was very close to the simultaneous solar wind speed and the O+ temperature was a cool 5600 K/amu. The flux densities for the three events were (2.4-4.4)×103cm2s-1.
The possibility and probability of natural transfer of viable microbes from Mars to Earth and Earth to Mars traveling in meteoroids during the first 0.5 Ga and the following 4 Ga are investigated, including:
Orbiting spacecraft about Venus have revealed a partially penetrable boundary sheath with induced fields limiting solar wind effects on the atmosphere structure and evolution. Mars differs structurally, the explanation awaiting an orbiter mission such as the European Space Agency is considering. In weak comets gyro-radii exceed other scale lengths, while in active comets the gas and solar plasma couple over the largest scale, boundary sheaths being unimportant—as occurred during early rapid-outgassing phases of Venus and Mars.
WHILE the surface conditions of Venus make the hypothesis of life there implausible, the clouds of Venus are a different story altogether. As was pointed out some years ago1, water, carbon dioxide and sunlight—the prerequisites for photosynthesis—are plentiful in the vicinity of the clouds. Since then, good additional evidence has been provided that the clouds are composed of ice crystals at their tops2,3, and it seems likely that there are water droplets toward their bottoms4. Independent evidence for water vapour also exists5. The temperature at the cloud tops is about 210° K, and at the cloud bottoms is probably at least 260–280° K (refs. 4 and 6). Atmospheric pressure at this temperature level is about 1 atm.7. The observed planetary albedo falls steeply in the violet and ultra-violet8, which accounts for the pale lemon yellow colour of Venus. The albedo decline would not be expected for pure ice particles, and must therefore be caused by some contaminant. Dust, ozone, C3O2 and other gases may possibly explain these data but, whatever the explanation, the ultra-violet flux below the clouds is likely to be low. If small amounts of minerals are stirred up to the clouds from the surface, it is by no means difficult to imagine an indigenous biology in the clouds of Venus. What follows is one such speculation.