Could there be life in the clouds of Venus?

[note: this post is an excerpt from a literature review paper on Venusian Astrobiology that I wrote last year.]

3. Extant Life in the Clouds

Data returned by Mariner II in 1962 drastically changed the picture of conditions on Venus, causing most to lose interest in it as a harbor for life.  Nevertheless, within just a few years scientists were speculating that life might still survive on Venus in the clouds.  Harold Morowitz and Carl Sagan (1967) published a brief article in Nature containing a great deal of speculation about the nature of a life form they imagined could survive in such an environment: an organism constructed as a float bladder filled with molecular hydrogen for buoyancy.  This macroorganism would collect water from rain or by contact with droplets in the clouds, acquire nutrients from minerals picked up from the surface by the powerful winds, and produce its own lifting gas as a by-product of photosynthesis.  Given what they knew at the time, they claimed such life in the Venus clouds “can be envisaged which operates entirely on known terrestrial principles.”

    More realistic hypotheses involving cloud-borne microorganisms have followed that are compatible with our current knowledge of the Venusian atmosphere.  These hypotheses should be taken seriously in light of bacteria found actively growing and reproducing—at temperatures below 0° C—in cloud droplets collected at high altitude on Earth (Sattler et al. 2001).

3.1  Conditions in the Clouds

Venus may be a terribly inhospitable place on or near its surface, but the conditions at altitudes between 50 and 60 km are remarkably Earth-like.  The pressure is close to 1 bar, the temperature is in a range where water is liquid (0-100° C), there is abundant solar energy, and the atmosphere contains the primary materials required for life: carbon, oxygen, nitrogen, and hydrogen (Landis 2003).  Also present: sulfur, phosphorus, chlorine, fluorine, and iron (Grinspoon and Bullock 2007).

3.1.1  Attributes that Favor Life

In addition to the general conditions above, the following attributes are favorable for supporting life in the clouds:

• Aqueous environment:  It is certainly not abundant, but water vapor concentrations approach a few hundred parts per million in the cloud layers (Ingersoll 2007)
• Continuous clouds: the clouds on Venus are much larger, more continuous, and more stable than those of Earth, which provides an ongoing habitat for microorganisms (Schulze-Makuch et al. 2004).
• Superrotation: The clouds of Venus make a complete rotation about the planet once every 4-6 days (van den Berg et al. 2006), providing a day-night cycle for life in the clouds that is much shorter than the 117-day cycle experienced at the planet’s surface (Ingersoll 2007).  This enhances the potential for photosynthetic reactions by reducing the duration of “night” (Grinspoon and Bullock 2007).
• Atmosphere in disequilibrium: O₂, H₂, H₂S, and SO₂ coexist, providing the basis for energy-yielding redox reactions that could be harvested by microbial life  (Schulze-Makuch and Irwin 2002)

3.1.2  Challenges for cloud-hosted life

Ultraviolet (UV) radiation from the Sun presents a challenge for life in the clouds of Venus.  UV is damaging to biological macromolecules, and any surviving organisms must adapt to it in some fashion.  Using Earth-based organisms for reference, several examples are available: there are organisms that use pigments such carotenoids and scytonemin for protection, others grow beneath the safety of soil or water, and some make a shield from organic compounds derived from dead cells.  A more elaborate example are microbes such as cyanobacteria that possess internal mechanisms for repairing DNA and resynthesize UV-sensitive proteins (Schulze-Makuch et al. 2004).  Charles Cockell (1999) points out that the UV flux in the upper clouds of Venus is comparable to the surface flux on the Archean Earth, the time when life is believed to have appeared.

The acidity of the clouds of Venus (pH=0) has been raised as a possible obstacle to life (Cockell 1999).  Nevertheless, acidophile organisms have been found on Earth, such as Ferroplasma acidarmanus which thrives at pH 0 (Schulze-Makuch et al. 2004),  Picrophilus oshimae, which showed optimal growth at pH 0.7, but still grew at pH 0 (Schleper et al. 1995), and the green alga Dunaliella acidophila which can survive at Ph 0, but prefers pH 1 for maximum growth (Grinspoon and Bullock 2007).

3.2        Speculations on potential life forms

Venus researchers have proposed feasible forms that life might take to survive in the clouds.  Wickramasinghe and Wickramasinghe (2008) suggest that hydrogenogens, a group of terrestrial bacteria and archaea that can grow anaerobically using CO as their sole carbon source, are good analogs for cloud-borne organisms on Venus.  They note that the lightning present on Venus (mentioned in section 2.3) could generate large amounts of CO from the predominantly CO₂ atmosphere.  They imagine a scenario occurring within the three cloud layers of Venus where “(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 nucleation.”

Dirk Schulze-Makuch, David Grinspoon, and colleagues (2004) propose that microbial life forms, in response to the high doses of ultraviolet radiation received in the upper atmosphere, could shroud themselves in elemental sulfur, possibly a layer of cycloocta-sulfer (S₈).  It is a strong UV absorber, and Venusian organisms could produce elemental sulfur via a simple photochemical reaction combining H₂S and CO, just as some organisms on Earth do.

In another paper co-authored by Schulze-Makuch and Louis Irwin (2006), they proposed phototrophic organisms in the Venusian atmosphere that could employ a photosystem based on the oxidation of sulfur, as many terrestrial organisms thriving in warm seas and hot springs do.

3.3        Possible evidence for life in the clouds

Is there any current evidence that could suggest the existence of cloud-borne organisms on Venus?  There is more than one might think.  Of particular interest are the larger droplets or particles (referred to as “mode 3” particles) found only in the lowest of Venus’ three cloud layers (Grinspoon and Bullock 2007).  They are non-spherical (indicative of a solid core), and comparable in size to Earth bacteria.  Their composition is currently unknown, but they could represent even small bacteria colonies.

The dark regions plainly visible on UV images of Venus are caused by an unknown UV absorber.  The Venus Monitoring Camera aboard the Venus Express spacecraft took wide-angle images at the characteristic wavelength of the UV absorber, and determined that the brightness variation is the result of compositional differences, not elevation differences (Titov et al. 2008).  Elemental sulfur in the form S8 is a strong UV absorber, and could be the cause of the dark regions.  It has been proposed that the potential S8 in the Venusian clouds could be a byproduct of microbiological processes (Schulze-Makuch and Irwin 2006).

Compounds positively identified in the Venusian atmosphere could also indicate the presence of organisms.  The presence of oxygenated gases such as O2 and SO2, observed at the same time with reduced gases such as H2S and H2, indicates the atmosphere is in a state of disequilibrium.  Some active process is working to maintain this situation, and it may be biological (Landis 2003).  The second-most common sulfur gas in the Venusian atmosphere,  Carbonyl sulfide (COS), is considered a possible indicator for life since its sources on Earth are almost entirely biological (Landis 2003; Schulze-Makuch and Irwin 2002).

References:

Cockell, C. S. (1999). Life on venus. Planetary and Space Science, 47(12), 1487-1501. 

Grinspoon, D. H., & Bullock, M. A. (2007). Astrobiology and venus exploration. Exploring Venus as a Terrestrial Planet, (176), 191. 

Ingersoll, A. P. (2007). Venus: Express dispatches. Nature, 450(7170), 617-618. 

Landis, G. A. (2003). Astrobiology-the case for venus. Journal of the British Interplanetary Society, 56, 250-254. 

Morowitz, H. (1967). Life in the clouds of venus? Nature, 215, 1259-1260. 

Sattler, B., Puxbaum, H., & Psenner, R. (2001). Bacterial growth in supercooled cloud droplets. Geophysical Research Letters, 28(2), 239-242. 

Schleper, C., Puehler, G., Holz, I., Gambacorta, A., Janekovic, D., Santarius, U., . . . Zillig, W. (1995). Picrophilus gen. nov., fam. nov.: A novel aerobic, heterotrophic, thermoacidophilic genus and family comprising archaea capable of growth around pH 0. Journal of Bacteriology, 177(24), 7050-7059. 

Schulze-Makuch D, Grinspoon DH, Abbas O, Irwin LN, & Bullock MA (2004). A sulfur-based survival strategy for putative phototrophic life in the venusian atmosphere. Astrobiology, 4 (1), 11-8 PMID: 15104900

Schulze-Makuch, D., & Irwin, L. N. (2002). Reassessing the possibility of life on venus: Proposal for an astrobiology mission. Astrobiology, 2(2), 197-202. 

Schulze-Makuch, D., & Irwin, L. N. (2006). The prospect of alien life in exotic forms on other worlds. Naturwissenschaften, 93(4), 155-172. 

Titov, D. V., Taylor, F. W., Svedhem, H., Ignatiev, N. I., Markiewicz, W. J., Piccioni, G., & Drossart, P. (2008). Atmospheric structure and dynamics as the cause of ultraviolet markings in the clouds of venus. Nature, 456(7222), 620-623. 

van den Berg, M., Falkner, P., Atzei, A., Phipps, A., Underwood, J., Lingard, J., . . . Peacock, A. (2006). Venus entry probe technology reference study. Advances in Space Research, 38(11), 2626-2632. 

Wickramasinghe, N., & Wickramasinghe, J. (2008). On the possibility of microbiota transfer from venus to earth. Astrophysics and Space Science, 317(1), 133-137. 

Venus has an Ozone Layer, too

An atmospheric study using the SPICAV-UV instrument recently came to my attention where researchers (Montmessin, et al. 2011) used the data archive to identify (for the first time) a layer of ozone in the upper atmosphere of Venus (previously, ozone had only been identified in the atmospheres of Mars and Earth).

The team analyzed the complete SPICAV dataset, and determined that UV absorption by O₃ was observed during a stellar occultation run on the night side of Venus during orbit #348.  They confirmed ozone detection in 28 additional orbits, and isolated the ozone to a discrete layer no more than 10 km thick near a mean altitude of 100km.

The observed concentrations of 10⁷ – 10⁸ molecules per cubic centimeter are consistent with expected values if the upper atmosphere were dominated by the same chlorine-catalyzed destruction cycles present in Earth’s stratosphere.

Even if the same mechanisms are at work in he Venusian atmosphere, the authors state that the observed ozone layer seems too tenuous to filter out UV radiation and provide protection to organisms that could have existed on Venus.

Referenced:

Montmessin, F., Bertaux, J., Lefèvre, F., Marcq, E., Belyaev, D., Gérard, J., Korablev, O., Fedorova, A., Sarago, V., & Vandaele, A. (2011). A layer of ozone detected in the nightside upper atmosphere of Venus Icarus, 216 (1), 82-85 DOI: 10.1016/j.icarus.2011.08.010

The SPICAV-UV Instrument Aboard Venus Express

Introduction

The European Space Agency’s Venus Express (VEX) is the only active spacecraft mission at the planet Venus.  It carries a number of instruments: A magnetometer, a wide-angle CCD camera, a space plasma detector, a Fourier spectrometer, a thermal spectrometer, a radio science package, and a cluster of spectrometers specifically designed to study the Venusian atmosphere: SPICAV (Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus).  This package contains three spectrometers, one of which operates in the ultraviolet and is the subject of this article.

The Instrument

The UV instrument in SPICAV (described in great detail in Bertaux et al. 2007) is a refurbished flight spare from the Mars Express spacecraft.  Sensitive to wavelengths from 118 to 320 nm, it was designed to measure different aspects of the atmosphere in different modes.  In nadir orientation, SPICAV-UV measures SO2 and the distribution of the mysterious UV absorber in the clouds.  On the night side, it observes the γ and δ bands of NO, and it can measure vertical profiles of CO2, SO2, clouds, and aerosols in stellar occultation mode.

The UV spectrometer collects light with a 40 mm off-axis parabolic mirror that reflects light toward the spectrometer entrance.  There is a configurable slit mechanism in the focal plane that, when left in place, is used for extended source viewing.  The slit can be removed from the focal plane entirely for stellar occultation observations.  A concave UV grating causes the spectrum to fall onto an image intensifier that is blind to wavelengths greater than 320 nm.  The image created on the phosphor output screen of the intensifier is transferred to a 288x384 pixel CCD which can optionally be cooled to 270K using a Peltier cooling unit to reduce dark current.  The focal length of the telescope (120mm) results in each CCD pixel having a FOV of 0.01 x 0.01°.

The slit of the spectrometer is divided into two parts with different widths to allow differing spectral resolutions during observing of an extended source.  The narrow part of the slit is 50 μm wide, giving resolving power between 120 and 300 with lower flux, while the wider part (500 μm) provides more sensitivity at the expense of a lower resolving power of  ≈20.

Public Datasets

SPICAV-UV data (in fact, all ESA data more than six months old) are available to the public as part of the ESA Planetary Science Archive (PSA), and are found at http://www.rssd.esa.int/index.php?project=PSA&page=vex.  NASA’s Planetary Data System (PDS) standard was adopted by the PSA as a baseline for defining the structure and format of datasets. The PSA allows members of the public to browse the entire available archive via an FTP (File Transfer Protocol) interface, and also provides a Java-based Advanced Search Interface tool that lets a user specify dozens of search parameters, select datasets from search results, and it then “delivers” the requested datasets to a special folder on their server and sends the user an email containing a hypertext link to the data.

Figure 1: Quick look analysis of SPICAV-UV data from VEX orbit 520.

Datasets for the SPICAV instrument cluster are stored by spacecraft orbit number.  Each individual dataset comes with human-readable descriptor files that define the format, encoding, and semantic meaning of all data provided.  In addition to the spectral data themselves, the dataset also include all of the contextual data about the conditions under which the observations were made, such as spacecraft altitude and orientation, instrument temperature, the time and date, exposure times, instrument configuration settings, and many more.

A SPICAV-UV dataset file includes one or more Collection objects, each containing a header descriptor and data table.  A data table consists of five rows of 408 columns, which represents five rows of the CCD with each row containing 408 pixels.  Each 16-bit pixel value is a digital representation of the charge collected by that pixel during integration.

The PSA also provides a data browsing area on the FTP server where summary images of each dataset are available for a “quick look” at the data (see Figure 1).

Science with SPICAV

One example of an investigation which used data from SPICAV-UV is a recent study hoping to find evidence of extant volcanic activity.  Six years of ultraviolet spectrometer data were used to examine the density of sulphur dioxide above the clouds of Venus (Marcq et al. 2012).  They found that SO2 column densities increased prior to 2007, and then decreased by a factor of 5 over the next five years.  Read the rest of my summary of this research here.

Another study a year earlier (Montmessin, et al. 2011) used SPICAV-UV data to identify, for the first time, a layer of ozone in the upper atmosphere of Venus (previously, ozone had only been identified in the atmospheres of Mars and Earth).  Read more details here.

References

Bertaux, J., Nevejans, D., Korablev, O., Villard, E., Quémerais, E., Neefs, E., Montmessin, F., Leblanc, F., Dubois, J., Dimarellis, E., Hauchecorne, A., Lefèvre, F., Rannou, P., Chaufray, J., Cabane, M., Cernogora, G., Souchon, G., Semelin, F., Reberac, A., Van Ransbeek, E., Berkenbosch, S., Clairquin, R., Muller, C., Forget, F., Hourdin, F., Talagrand, O., Rodin, A., Fedorova, A., Stepanov, A., Vinogradov, I., Kiselev, A., Kalinnikov, Y., Durry, G., Sandel, B., Stern, A., & Gérard, J. (2007). SPICAV on Venus Express: Three spectrometers to study the global structure and composition of the Venus atmosphere Planetary and Space Science, 55 (12), 1673-1700 DOI: 10.1016/j.pss.2007.01.016

Esposito, L. W. (1984). Sulfur dioxide: Episodic injection shows evidence for active venus volcanism. Science (New York, N.Y.), 223(4640), 1072-1074.

Marcq, E., Bertaux, J. L., Montmessin, F., & Belyaev, D. (2012). Variations of sulphur dioxide at the cloud top of Venus's dynamic atmosphere. Nature geoscience, 6(1), 25-28.

Montmessin, F., Bertaux, J. L., Lefèvre, F., Marcq, E., Belyaev, D., Gérard, J. C., ... & Vandaele, A. C. (2011). A layer of ozone detected in the nightside upper atmosphere of Venus. Icarus, 216(1), 82-85.

VEXAG Update at NASA's Planetary Science Subcommittee meeting

Yesterday and today, NASA's Planetary Science Subcommittee met at NASA Headquarters in Washington D.C. Here is the agenda for the meeting, which was available for remote participants via teleconference.

During today's session, Dr. Lori Glaze (current VEXAG chair) presented the VEXAG report.  She outlined

• Recent activities,
• Results of the annual VEXAG meeting last November,
• VEXAG Endorsements (including my favorite, the use of stratospheric balloon observatories), and
• Plans for updating the Goals, Objectives, and Investigations document by the end of the year.

Here is a link to her presentation.

Scorpion-like Life Discovered on Surface of Venus?

In a recent paper published in Solar System Research (Ksanfomality, 2012), revered senior statesman of Russian planetary science Leonid Ksanfomality reported the detection of possible life forms on Venus.

Using modern image enhancement techniques to re-analyze the panoramic images captured by Veneras 9 and 13 (in 1975 and 1982, respectively), Ksanfomality discovered objects that he observed moving,

"Scorpion" appeared in image V-13-1-6 BW at the
90th minute after landing of Venera 13.  It is absent in
subsequent images.

changing shape, and disappearing from view of the cameras (the most interesting one he labeled as the "Scorpion").  He speculated that these indigenous fauna were disturbed from their hiding places in the soil when the spacecraft touched down, and made their way back to safety.

A number of scientists responded to this report in the same issue, heaping equal doses of skepticism and respect on the senior scientist and his work.  They conclude that the objects identified are abiotic, and that any apparent movement was the result of image processing artifacts and/or changing shadows cast by clouds.

Even though this is a false alarm for current life on Venus, the jury is still out on whether life may still exist below the surface, or in the thick cloud cover.  More to come.  Happy April 1st!


REFERENCES:

Ksanfomality, L. (2012). Venus as a natural laboratory for search of life in high temperature conditions: Events on the planet on March 1, 1982 Solar System Research, 46 (1), 41-53 DOI: 10.1134/S0038094612010042