The State of Venusian Astrobiology

Paul F. Wren

Department of Space Studies University of North Dakota Clifford Hall room 512 4149 University Ave Stop 9008 Grand Forks, ND 58202

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Abstract:

The possibility of life on the planet Venus has been speculated upon and investigated

since the middle of the 18th century. Once spacecraft visited the planet and returned data

to Earth, Venus was deemed inhospitable to life (it was, after all, inhospitable to the very

same spacecraft which landed on it). Most astrobiologists turned their gaze elsewhere,

looking to Mars, Europa, Titan, and even other stars for evidence of life. This paper

summarizes the recent views with respect to the search for life on Venus, examines the

open questions, and describes current and future space missions intended to answer them.

Keywords: Venus; Astrobiology; Venus, atmosphere; Venus, surface;

1. Introduction 1

From the moment when Mikhail Lomonosov discovered in 1761 that Venus had an 2

atmosphere, the idea it was an abode for life was a foregone conclusion. Scientists and 3

the public imagined it to have an Earth-like climate, and that it was likely home to a lush 4

world of life very similar to our own (Grinspoon and Bullock 2007). 5

This view of Venus as Earth’s twin persisted for nearly two centuries, until it was met 6

by the harsh reality of new ground-based observational results. Even so, many scientists 7

held tightly to the idea that somehow life had formed and flourished on Venus. A steady 8

diet of new revelations that contradicted this view did not deter them from sharing 9

plausible mechanisms that could explain the observations and still allow for Venusian life 10

(Launius 2012). As the 20th century marched on and our picture of Venus became 11

increasingly clearer, the breathable atmosphere changed to mostly CO2, the water vapor 12

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clouds became sulfuric acid, and the surface changed from vibrant swamps to a waterless 13

environment too hot for life (Cockell 1999). 14

In December of 1962, the first successful visit by a spacecraft hammered one of the 15

last nails into the coffin of extant life on the surface of Venus. Mariner II passed briefly 16

within 35,000 km of Venus’ clouds (NASA 1965), but long enough for its microwave 17

radiometer to measure surface temperatures consistently in excess of 400°K, well above 18

the range for liquid water. The onboard magnetometer detected no evidence of a 19

magnetic field, meaning the solar wind falls directly onto the Venusian atmosphere 20

(Sonnet 1963). 21

The case for life on the surface suffered a final blow when the Venera 7 and 8 landers 22

functioned long enough to transmit the local weather: a balmy 735°K and a surface 23

pressure of 90 bars (Grinspoon and Bullock 2007). Most research points to a runaway 24

greenhouse effect as the cause of these severe surface conditions (e.g. Cockell 1999, 25

Donahue and Russell 1997, Ingersoll 1969, Kasting 1988, Schulze-Makuch and Irwin 26

2002). 27

The reality of the extremely inhospitable conditions severely curtailed inquiry into 28

current or prior life on Venus (Grinspoon and Bullock 2007). Nevertheless, there are 29

people still interested in pursuing the many unanswered questions about Venus being able 30

to harbor life. This paper will present the current state of Venusian astrobiology by 31

discussing ancient and current conditions on the planet, current hypotheses regarding life 32

on Venus, unanswered questions, and proposals for future science. 33

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2. Early Venus 35

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There are researchers who have continued the search for life on Venus, but have taken a 36

different approach. Even if Venus is no longer able to spawn or support life on its 37

surface now (Colin and Kasting 1992), what about its distant past? For Venus to be 38

habitable, certain conditions seem necessary: liquid water, building blocks for organic 39

compounds, and energy (Grinspoon and Bullock 2007). Less address these before 40

examining the current thinking on early Venusian life. 41

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2.1 Water and Oceans 43

Two lines of evidence point toward Venus once having a great deal more water than 44

is found there today. First, models of the early solar system suggest that Earth and Venus 45

would have accumulated similar water inventories as part of the volatiles delivered 46

during the heavy bombardment period (Colin and Kasting 1992; Grinspoon and Bullock 47

2007, Pham et al. 2011). 48

Second, the D/H (deuterium to hydrogen) ratio on Venus has been measured at 2.5 x 49

10-2, a value 150 times higher than for Earth (Donahue and Russell 1997; Svedhem et al. 50

2007). If Earth and Venus started with water from the same source, then their original 51

D/H ratios should have been similar (Cockell 1999; Colin and Kasting 1992; Grinspoon 52

and Bullock 2007). Since Venus has no magnetic field to deflect it, the solar wind comes 53

in direct contact with the upper atmosphere and strips away much more of the lighter 54

hydrogen, leaving deuterium behind. This likely cause of the high D/H ratio indicates 55

much more water was present on early Venus (Grinspoon and Bullock 2007; Svedhem et 56

al. 2007). Undermining the assumption of similar D/H values for the terrestrial planets, 57

recent research notes that several different models for determining the initial D/H ratios 58

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find differing values for each (although none of the models agree on these values), and 59

concludes that “assuming the same initial values for the Venus, the Earth and Mars is 60

dangerous and unfounded” (Horner et al. 2009, p. 1345). 61

It should also be noted that the Venus Express plasma analyzer detected positively 62

charged hydrogen and oxygen escaping the planet in the same proportions as found in 63

water, i.e., 2:1 (Barabash et. al 2007; Konesky 2009). 64

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But how much water was there on early Venus? Amounts range from 0.25 Terrestrial 66

Oceans based on modeling of oxygen ion loss (Kulikov et al. 2006) to 5 TOs based on a 67

different model for hydrogen and oxygen loss (Gillmann et al. 2009). 68

If you accept the assumption of a large water inventory, then you need the surface of 69

early Venus to be cool enough for water to exist in liquid form to conclude it had oceans. 70

David Grinspoon and Mark Bullock (2007) hypothesize that the high albedo of the thick 71

clouds would initially reflect away a good deal of heat, allowing the surface to remain 72

cool and oceans to persist for as long as two billion years, thus providing “a habitable 73

environment for a substantial fraction of solar system history.” 74

James Kasting (1988) designed an atmospheric model for early Venus that predicts a 75

wet and warm surface, with water temperatures nearing 100°C. This “moist greenhouse” 76

model featured a great deal of H2O in the atmosphere, even at very high altitudes, and it 77

predicts that water would be lost fairly rapidly to UV dissociation and solar wind 78

stripping of the released hydrogen. The faster water loss would result in oceans 79

persisting for only a few hundred million years (Colin and Kasting 1992). 80

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Eric Chassefiere’s model (1997) for the escape of water from Venus assumes an 81

increased solar wind and an enhanced solar UV flux from the young Sun that strips 82

Venus of an entire Terrestrial Ocean in as little as 10 million years. 83

Depending on which of these models you choose, substantial liquid water on the 84

surface of Venus was available to support the origin of life for as little as 10 million years 85

or as much as 2 billion years. All could support the beginnings of life if the timing was 86

right. 87

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2.2 Building Blocks of Life 89

Most of the primary materials required for life (as we know it) are present in one form 90

or another on Venus today: carbon, hydrogen, oxygen, nitrogen, and sulfur. Nutrient 91

minerals such as ammonia and phosphorus have also been detected (Landis 2003). 92

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2.3 Energy Sources 94

Two possible energy sources that could trigger the synthesis of amino acids are 95

available on Venus: Ultraviolet radiation (UV) and lightning. Even with its thick cover 96

of sulfuric acid clouds, the surface of present-day Venus receives 70% of the UV that 97

reaches Earth (Cockell 1999). Lightning has long been suspected on Venus, although 98

inconclusive evidence both for and against it has been presented. A team of researchers 99

using data from the magnetometer aboard Venus Express report strong whistler-mode 100

waves with frequencies nearing 100 Hz detected in the ionosphere, which are strong 101

indications of lightning discharges in the clouds of Venus (Russell et al. 2007). 102

103

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2.4 Ancient Life 104

Little can be known about life that may have appeared and grown billions of years 105

ago on Venus (we have enough trouble finding evidence of the earliest life here on 106

Earth). Many still speculate that life could have begun on Venus (or been deposited there 107

by impacting bodies) and continued to thrive there until runaway greenhouse conditions 108

rendered the surface hostile to life forms (e.g. Cockell 1999; Grinspoon and Bullock 109

2007; Morowitz and Sagan 1967; Schulze-Makuch and Irwin 2002). 110

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3. Life on Venus Today 112

Could life still be found to exist on the Venus we see now? The extreme pressures 113

and temperatures at the surface, the relative lack of water, the high acidity of the 114

atmosphere, and exposure to UV radiation all seem to argue against it. And yet, there is 115

still not compelling evidence that rules it out. 116

117

3.1 Extant Life on the Surface 118

A recent paper by a revered senior statesman of Russian planetary science drew a lot 119

of attention from both academics and the popular press when it claimed to present 120

evidence of living organisms on the surface of Venus. In the absence of new data (it has 121

been decades since the last Soviet-built lander transmitted from the Venusian surface), 122

Leonid Ksanfomality (2012) decided to revisit the panoramic images taken by Veneras 9 123

and 13. Using modern image enhancement techniques, Ksanfomality discovered objects 124

ranging in size from 10 to 50 centimeters in length that were observed to move, change 125

shape, or disappear from the view of the cameras. He envisioned these objects as local 126

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fauna escaping from soil inadvertently heaped onto them by the invading landers. A 127

number of responses to this article were also published, praising Ksanfomality for his 128

diligent work while thoroughly discounting his conclusions. These responses declared 129

the objects to be abiotic in nature, and proposed that any apparent movement was caused 130

by artifacts created during image processing, changing shadows cast by clouds, or a 131

combination of the two (Avenesov 2012; Basilevsky 2012; Mitchell 2012). 132

Even if the Venera landers did not discover indigenous life forms on the surface (and 133

future missions are not likely to), others believe there is a possibility of life on Venus 134

below the surface. Dirk Schulze-Makuch and Louis Irwin (2002) consider the possibility 135

that liquid water may still exist in subsurface environments where the pressure combined 136

with somewhat lower temperatures (still well above 100°C) would allow it. Extreme as 137

this might seem, it might serve as a niche for organisms similar to microbes on Earth that 138

live off volcanic exhalations. 139

140

3.2 Extant Life in the Clouds 141

Within a few years of the damage dealt by Mariner II to the possibility of life on the 142

surface of Venus, scientists were speculating that life might still survive on Venus in the 143

clouds. Harold Morowitz and Carl Sagan (1967) published a brief article in Nature 144

containing a great deal of speculation about the nature of a life form they imagined could 145

survive in such an environment: an organism constructed as a float bladder filled with 146

molecular hydrogen for buoyancy. This macroorganism would collect water from rain or 147

by contact with droplets in the clouds, acquire nutrients from minerals picked up from the 148

surface by the powerful winds, and produce its own lifting gas as a by-product of 149

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photosynthesis. Given what they knew of the Venusian atmosphere at the time, they 150

claim such life in the Venus clouds “can be envisaged which operates entirely on known 151

terrestrial principles.” 152

More realistic hypotheses involving cloud-borne microorganisms have followed that 153

are compatible with our current knowledge of the Venusian atmosphere. These 154

hypotheses should be taken seriously in light of bacteria found actively growing and 155

reproducing—at temperatures below 0° C—in cloud droplets collected at high altitude on 156

Earth (Sattler et al. 2001). 157

158

3.2.1 Conditions in the Clouds 159

Venus may be a terribly inhospitable place on or near its surface, but the conditions at 160

altitudes between 50 and 60 km are remarkably Earth-like. The pressure is close to 1 bar, 161

the temperature is in a range where water is liquid (0-100° C), there is abundant solar 162

energy, and the atmosphere contains the primary materials required for life: carbon, 163

oxygen, nitrogen, and hydrogen (Landis 2003). Also present: sulfur, phosphorus, 164

chlorine, fluorine, and iron (Grinspoon and Bullock 2007). 165

166

3.2.1.1 Attributes that Favor Life 167

In addition to the general conditions above, the following attributes are favorable for 168

supporting life in the clouds: 169

• Aqueous environment: It is certainly not abundant, but water vapor 170

concentrations approach a few hundred parts per million in the cloud layers 171

(Ingersoll 2007) 172

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• Continuous clouds: the clouds on Venus are much larger, more continuous, and 173

more stable than those of Earth, which provides an ongoing habitat for 174

microorganisms (Schulze-Makuch et al. 2004). 175

• Superrotation: The clouds of Venus make a complete rotation about the planet 176

once every 4-6 days (van den Berg et al. 2006), providing a day-night cycle for 177

life in the clouds that is much shorter than the 117-day cycle experienced at the 178

planet’s surface (Ingersoll 2007). This enhances the potential for photosynthetic 179

reactions by reducing the duration of “night” (Grinspoon and Bullock 2007). 180

• Atmosphere in disequilibrium: O2, H2, H2S, and SO2 coexist, providing the 181

basis for energy-yielding redox reactions that could be harvested by microbial life 182

(Schulze-Makuch and Irwin 2002) 183

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3.2.1.2 Challenges for cloud-hosted life 185

Ultraviolet (UV) radiation from the Sun presents a challenge for life in the clouds of 186

Venus. UV is damaging to biological macromolecules, and any surviving organisms 187

must adapt to it in some fashion. Using Earth-based organisms for reference, several 188

examples are available: there are organisms that use pigments such carotenoids and 189

scytonemin for protection, others grow beneath the safety of soil or water, and some 190

make a shield from organic compounds derived from dead cells. A more elaborate 191

example are microbes such as cyanobacteria that possess internal mechanisms for 192

repairing DNA and resynthesize UV-sensitive proteins (Schulze-Makuch et al. 2004). 193

Charles Cockell (1999) points out that the UV flux in the upper clouds of Venus is 194

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comparable to the surface flux on the Archean Earth, the time when life is believed to 195

have appeared. 196

The acidity of the clouds of Venus (pH=0) has been raised as a possible obstacle to 197

life (Cockell 1999). Nevertheless, acidophile organisms have been found on Earth, such 198

as Ferroplasma acidarmanus which thrives at pH 0 (Schulze-Makuch et al. 2004), 199

Picrophilus oshimae, which showed optimal growth at pH 0.7, but still grew at pH 0 200

(Schleper et al. 1995), and the green alga Dunaliella acidophila which can survive at Ph 201

0, but prefers pH 1 for maximum growth (Grinspoon and Bullock 2007). 202

203

3.2.2 Speculations on potential life forms 204

Venus researchers have proposed feasible forms that life might take to survive in the 205

clouds. Wickramasinghe and Wickramasinghe (2008) suggest that hydrogenogens, a 206

group of terrestrial bacteria and archaea that can grow anaerobically using CO as their 207

sole carbon source, are good analogs for cloud-borne organisms on Venus. They note 208

that the lightning present on Venus (mentioned in section 2.3) could generate large 209

amounts of CO from the predominantly CO2 atmosphere. They imagine a scenario 210

occurring within the three cloud layers of Venus where “(a) bacteria nucleate droplets 211

containing water and nutrients, (b) colonies grow within the droplets, (c) droplets fall into 212

regions of higher temperature where they evaporate releasing spores to convect upwards 213

to yield further nucleation.” 214

Dirk Schulze-Makuch, David Grinspoon, and colleagues (2004) propose that 215

microbial life forms, in response to the high doses of ultraviolet radiation received in the 216

upper atmosphere, could shroud themselves in elemental sulfur, possibly a layer of 217

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cycloocta-sulfer (S8). It is a strong UV absorber, and Venusian organisms could produce 218

elemental sulfur via a simple photochemical reaction combining H2S and CO2, just as 219

some organisms on Earth do. 220

In another paper co-authored by Schulze-Makuch and Louis Irwin (2006), they 221

proposed phototrophic organisms in the Venusian atmosphere that could employ a 222

photosystem based on the oxidation of sulfur, as many terrestrial organisms thriving in 223

warm seas and hot springs do. 224

225

3.2.3 Possible evidence for life in the clouds 226

Is there any current evidence that could suggest the existence of cloud-borne 227

organisms on Venus? There is more than one might think. Of particular interest are the 228

larger droplets or particles (referred to as “mode 3” particles) found only in the lowest of 229

Venus’ three cloud layers (Grinspoon and Bullock 2007). They are non-spherical 230

(indicative of a solid core), and comparable in size to Earth bacteria. Their composition 231

is currently unknown, but they could represent even small bacteria colonies. 232

The dark regions plainly visible on UV images of Venus are caused by an unknown 233

UV absorber. The Venus Monitoring Camera aboard the Venus Express spacecraft took 234

wide-angle images at the characteristic wavelength of the UV absorber, and determined 235

that the brightness variation is the result of compositional differences, not elevation 236

differences (Titov et al. 2008). Elemental sulfur in the form S8 is a strong UV absorber, 237

and could be the cause of the dark regions. It has been proposed that the potential S8 in 238

the Venusian clouds could be a byproduct of microbiological processes (Schulze-Makuch 239

and Irwin 2006). 240

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Compounds positively identified in the Venusian atmosphere could also indicate the 241

presence of organisms. The presence of oxygenated gases such as O2 and SO2, observed 242

at the same time with reduced gases such as H2S and H2, indicates the atmosphere is in a 243

state of disequilibrium. Some active process is working to maintain this situation, and it 244

may be biological (Landis 2003). The second-most common sulfur gas in the Venusian 245

atmosphere, Carbonyl sulfide (COS), is considered a possible indicator for life since its 246

sources on Earth are almost entirely biological (Landis 2003; Schulze-Makuch and Irwin 247

2002). 248

249

4. Future Research 250

There are many unanswered questions regarding the possibility of life on Venus, and 251

much of the recent literature devotes some space to making the case for additional 252

research, both in-situ and here on Earth. Some broad, basic questions are: 253

• Could life have existed on Venus in an earlier, pre-greenhouse-effect phase? 254

• Did Venus once have an ocean? 255

• What caused the geological resurfacing of the planet? 256

• Is Venus still geologically active? 257

• What is the “snow” on Venus’ mountaintops? 258

• Is the current atmosphere of Venus suitable for life? 259

• Assuming any life in the clouds originated on the surface when it was more 260

hospitable to life (i.e., billions of years ago), have the clouds persisted all that 261

time? 262

• Are there living organisms in the clouds? 263

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264

4.1 Earth-based Research 265

Earth-based investigations could attempt to better understand extremophiles on Earth 266

with an eye toward those that could exist in the Venusian environment, particularly those 267

that can tolerate multiple stressor environments simultaneously. In addition, further 268

study of cloud-borne organisms in the clouds of Earth could also help to better 269

understand how similar life could survive on Venus. It would also be worthwhile to 270

attempt to locate meteorites that originated from Venus (Cockell 1999). Attempting to 271

grow Earth organisms in a lab environment simulating Venus cloud droplets would lead 272

to a greater understanding of what is possible, and assist in the design of in-situ 273

experiments to be flown on future missions (Cockell 1999, Grinspoon and Bullock 2007). 274

275

4.2 In-situ Observations 276

There is a long list of investigations/observations that will be of great interest to 277

astrobiologists: 278

1. Detailed measurement of noble gases (Chassefiere et al. 2012). 279

2. Further observations of the escape fluxes of isotopes and their interaction with the 280

solar wind to test current assumptions about early water inventories and hydrogen 281

escape to space (Grinspoon and Bullock 2007). 282

3. Making known the composition of the unknown UV absorber (Grinspoon and 283

Bullock 2007). 284

4. A better understanding of the global atmospheric circulation and its effect on 285

cloud particle lifetimes (Grinspoon and Bullock 2007). 286

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5. Measurements of trace elements and compounds in the atmosphere and the clouds 287

as a function of altitude (Baines et al. 2007; Cockell 1999; Landis et al. 2005). 288

6. Geochemical and geochronological observations to better characterize the 289

resurfacing of Venus and the sulfur cycle. This could help to confirm whether the 290

cloud cover has been continuous, and could also reveal the process that underlies 291

the atmospheric disequilibrium (Grinspoon and Bullock 2007). 292

7. Characterization of surface rocks(Grinspoon and Bullock 2007). 293

8. Determination of the ages of major surface units (Grinspoon and Bullock 2007). 294

9. Seismic measurement at the Venusian surface (Landis et al. 2005). 295

10. Measure IR absorption, reflection, and emission in the atmosphere to gain a better 296

understanding of the greenhouse effect mechanisms (Landis et al. 2005). 297

11. Sampling sulfur and correlating the location of such measurements with surface 298

features would help to answer whether the surface is producing the sulphur 299

(Landis et al. 2005). 300

It should be noted that these observations are not only of use to astrobiologists—they 301

are essential to acquiring a more general understanding of the evolution of Venus 302

from a planetary science viewpoint. 303

304

4.3 Current and Pending Missions 305

At this time, only one spacecraft is actively studying Venus: The European Space 306

Agency’s Venus Express orbiter. Very similar to the Mars Express orbiter, Venus 307

Express launched in 2005 and entered orbit around Venus five months later (Ingersoll 308

2007). It carries three instruments (the Planetary Fourier Spectrometer, the Venus 309

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Monitoring Camera, and the Visible and Infrared Thermal Imaging Spectrometer) that 310

sense the visible and near-infrared spectrum, along with an ultraviolet spectrometer 311

(SPICAV), all intended to create a detailed and comprehensive picture of the Venusian 312

atmosphere (Baines et al. 2006). It also carries a plasma detector and a magnetometer. 313

Venus Express is now in its extended mission, which is scheduled to end on 31 314

December, 2014. 315

One other spacecraft is enroute to Venus, and (unfortunately) has been for some time. 316

Akatsuki, formerly known as the Venus Climate Orbiter and also as Planet-C, was 317

launched by JAXA (the Japanese space agency). On December 6th of 2010, its orbital 318

insertion burn ended prematurely. Akatsuki failed to enter Venus orbit, and instead 319

continued on in a heliocentric orbit. JAXA officials hope they will be able to modify the 320

spacecraft’s trajectory and enter Venus orbit when it approaches the planet again in 2015 321

(Cyranoski 2010). Akatsuki is designed to study the circulation of the Venusian 322

atmosphere. It carries four cameras for mapping clouds at both UV and IR wavelengths, 323

and to search for evidence of volcanic activity. It also has a high-speed imager it will use 324

to detect lightning, and a radio science package for characterizing the vertical structure of 325

the atmosphere (Nakamura et al. 2007). 326

327

5. Conclusions 328

It is clear there is still much to learn about Venus and its habitability, both now and in its 329

distant past. As inhospitable as it seems at first, the clouds provide a tantalizing 330

environment that could harbor life, and there is at this time no conclusive proof to the 331

contrary. Only future missions to the Earth’s twin will settle these questions. Mars is 332

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currently receiving a great deal of attention (three active orbiters and two active rovers on 333

the surface), with several more being planned with committed funds. Sending spacecraft 334

to Venus (and even returning them to Earth) is a much easier prospect (Schulze-Makuch 335

and Irwin 2002), and from an astrobiological viewpoint, perhaps a bit more interesting. 336

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