A false-colour image of the planet Venus, enveloped in clouds, taken in 2017.

A mystery on Venus

Scientists have reported that they have found abnormal amounts of a toxic compound called phosphine in Venus’s atmosphere, at 55-80 km altitude. This story is currently all over my Twitter feed because one way to explain this unexpected abundance is that microbes could be producing this gas – as we know them to do on Earth – in oxygen-starved conditions. Nonetheless, we shouldn’t lose sight of the fact that the real proposition here is that there is too much phosphine, not that there is a potential sign of life.

While some scientists have been issuing words of caution along similar lines, others have cut to the other end, writing that making sense of this discovery doesn’t require “alien microbes” at all because chemistry offers possibilities that are much more likely to be the case – and verging on the argument that this possibly can’t be aliens. Between them is the option to keep an open mind, so difficult these days – between an Avi Loeb-esque conception of the universe in which the role of creativity is overemphasised to dream up plausible (but improbable) theories and a hyper-conservative reality that refuses to admit new possibilities because we haven’t plumbed the depths of what we already know to be true enough.

Nonetheless, this is where it is best to stand today – considering we simply don’t know enough about the Venusian atmosphere to refute one argument or support the other. At the same time, I would like to make a finer point. In November 2014, I had published a post explaining the contents of a scientific paper published around then, describing how an exotic form of carbon dioxide could host life. As I wrote:

At about 305 kelvin and 73-times Earth’s atmospheric pressure, carbon dioxide becomes supercritical, a form of matter that exhibits the physical properties of both liquids and gases. … As the study’s authors found, some enzymes were more stable in supercritical carbon dioxide because it contains no water. The anhydrous property also enables a “molecular memory” in the enzymes, when they ‘remember’ their acidity from previous reactions to guide the future construction of organic molecules more easily. The easiest way – no matter that it’s still difficult – to check if life could exist in supercritical carbon dioxide naturally is to … investigate shallow depths below the surface of Venus. Carbon dioxide is abundant on Venus and the planet has the hottest surface in the Solar System. Its subsurface pressures could then harbour supercritical carbon dioxide.

When we do muster as much caution as we can when reporting on recently published papers presenting evidence of new mysteries, we evoke the possibility of ‘unknown unknowns’ – things that we don’t know we don’t know, as perfectly illustrated in the case of carbon monoxide on Titan. At the same time, are we aware that ‘unknown unknowns’ also make way for the possibility of alien life-forms with biological foundations we may never conceive of until we encounter a real, live example? I am not saying that there is life on Venus or elsewhere. I am saying that the knowledge-based defences we employ to protect ourselves from hype and reckless speculation in this case could just as easily work against our favour, and close us off to new possibilities. And since such caution is often considered a virtue, it is quite important that we don’t indulge it.

There is a wonderful paragraph in a paper from 2004 that I’m reminded of from time to time, when considering the possibility of aliens for a science article or a game of Dungeons & Dragons:

The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.

Cassini's last shot of Titan, taken by the probe's narrow-angle camera on September 13, 2017. Credit: NASA

A new map of Titan

It’s been a long time since I’ve obsessed over Titan, primarily because after the Cassini mission ended, the pace of updates about Titan died down, and because other moons of the Solar System (Europa, Io, Enceladus, Ganymede and our own) became more important. There have been three or four notable updates since my last post about Titan but this post that you’re reading has been warranted by the fact that scientists recently released the first global map of the Saturnian moon.

(This Nature article offers a better view but it’s copyrighted. The image above is a preview offered by Nature Astronomythe paper itself is behind a paywall and I couldn’t find a corresponding copy on Sci-Hub or arXiv nor have I written to the corresponding author – yet.)

It’s fitting that Titan be accorded this privilege – of a map of all locations on the planetary body – because it is by far the most interesting of the Solar System’s natural satellites (although Europa and Triton come very close) and were it not orbiting the ringed giant, it could well be a planet of its own accord. I can think of a lot of people who’d agree with this assessment but most of them tend to focus on Titan’s potential for harbouring life, especially since NASA’s going to launch the Dragonfly mission to the moon in 2026. I think they’ve got it backwards: there are a lot of factors that need to come together just right for any astronomical body to host life, and fixating on habitability combines these factors and flattens them to a single consideration. But Titan is amazing because it’s got all these things going on, together with many other features that habitability may not be directly concerned with.

While this is the first such map of Titan, and has received substantial coverage in the popular press, it isn’t the first global assessment of its kind. Most recently, in December 2017, scientists (including many authors of the new paper) published two papers of the moon’s topographical outlay (this and this), based on which they were able to note – among other things – that Titan’s three seas have a common sea level; many lakes have surfaces hundreds of meters above this level (suggesting they’re elevated and land-locked); many lakes are connected under the surface and drain into each other; polar lakes (the majority) are bordered by “sharp-edged depressions”; and Titan’s crust has uneven thickness as evidenced by its oblateness.

According to the paper’s abstract, the new map brings two new kinds of information to the table. First, the December 2017 papers were based on hi- and low-res images of about 40% of Titan’s surface whereas, for the new map, the authors write: “Correlations between datasets enabled us to produce a global map even where datasets were incomplete.” More specifically, areas for which authors didn’t have data from Cassini’s Synthetic Aperture Radar instrument for were mapped at 1:2,000,000 scale whereas areas with data enabled a map at 1:8,000,000 scale. Second is the following inferences of the moon’s geomorphology (from the abstract the authors presented to a meeting of the American Astronomical Society in October 2018):

We have used all available datasets to extend the mapping initially done by Lopes et al. We now have a global map of Titan at 1:800,000 scale in all areas covered by Synthetic Aperture Radar (SAR). We have defined six broad classes of terrains following Malaska et al., largely based on prior mapping. These broad classes are: craters, hummocky/mountainous, labyrinth, plains, lakes, and dunes [see image below]. We have found that the hummocky/mountainous terrains are the oldest units on the surface and appear radiometrically cold, indicating icy materials. Dunes are the youngest units and appear radiometrically warm, indicating organic sediments.

SAR images of the six morphological classes (in the order specified in the abstract)

More notes once I’ve gone through the paper more thoroughly. And if you’d like to read more about Titan, here’s a good place to begin.

Why Titan is awesome #11

Titaaaaan!

Here we go again. 😄 As has been reported, NASA has been interested in sending a robotic submarine to Saturn’s moon Titan to explore the hydrocarbon lakes near its north pole. Various dates have been mentioned and in all it seems likely the mission will be able to take off around 2040. In the 22 years we have left, we’ve got to build the submarine and make sure it can run autonomously on Titan, where the sea-surface temperature is about 95 K, whose waterbodies liquid-hydrocarbon-bodies are made of methane, ethane and nitrogen, and with density variations of up to 30%.

So researchers at Washington State University (WSU) tried to recreate the conditions of benthic Titan – specifically as they would be inside Kraken and Ligeia Mare – by working with the values of four variables: pressure, temperature, density and composition. Their apparatus consisted of a small, cylindrical cartridge heater submerged inside a cell containing methane, ethane and nitrogen, with controls to measure the values of the variables as well as modify conditions if needed. The scientists took a dozen readings as they varied the concentration of methane, ethane and nitrogen, the pressure, sea temperature, the heater surface temperature and the heat flux at bubble incipience.

The experimental setup used by WSU researchers to recreate the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA
The experimental setup used by WSU researchers to recreate the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: WSU/NASA

The data logged by WSU researchers pertaining to the conditions inside one of Titan's liquid-hydrocarbon lakes. Source: WSU/NASA
The data logged by WSU researchers pertaining to the conditions inside one of Titan’s liquid-hydrocarbon lakes. Source: Hartwig and Leachman, 2017/WSU

Based on them, they were able to conclude:

  • The moon’s lakes don’t freeze over even though their surface temperature is proximate to the freezing temperature of methane and ethane because of the dissolved nitrogen. The gas lowers the mixture’s freezing point (by about 16 K below the triple point), thus preventing the formation of icebergs that the robotic submarine would then have had to be designed to avoid (there’s a Titanic joke in here somewhere).
  • However, more nitrogen isn’t necessarily a good thing. It dissolves better in its liquid-hydrocarbon surroundings as the pressure increases and the temperature decreases – both of which will happen at lower depths. And the more nitrogen there is, the more the liquids surrounding the submarine are going to effervesce (i.e. release gas).

What issues would this pose to the vehicle? According to a conference paper authored among others by Jason Hartwig, a member of the WSU team, and presented earlier this year,

Effervescence of nitrogen gas may cause issues in two operational scenarios for any submersible on Titan. In the quiescent case, bubbles that form may interfere with sensitive science measurements, such as composition measurements, in acoustic transmission for depth sounding, and sidescan sonar imaging. In the moving case, bubbles that form along the submarine may coalesce at the aft end of the craft and cause cavitation in the propellers, impacting propulsive performance.

  • The quantity of effervescence and the number of sites on the submarine’s surface along which bubbles formed was observed to increase the warmer the machine’s outer surface got.

The planned design of the submarine NASA plans to use to explore Titan's cold hydrocarbon lakes. Source: Hartwig and Leachman, 2017/WSU
The planned design of the submarine NASA plans to use to explore Titan’s cold hydrocarbon lakes. Source: Hartwig and Leachman, 2017/WSU

If NASA engineers get all these details right, then their submarine will work. But making sure the instruments onboard will be able to make the observations they’ll need to make and the log the data they’ll need to log presents its own challenges. When one of the members of the WSU team decided to look into the experimental cell using a borescope (which is what an endoscope is called outside a hospital) and a video recorder, this is what he got:

(Source)

Oh, Titan.

(Obligatory crib: the university press release‘s headline goes ‘WSU researchers build -300ºF alien ocean to test NASA outer space submarine’. But in the diagram of the apparatus above, note that the cartridge heater standing in for the submarine is 5 cm long. So the researchers haven’t built an alien ocean; they’ve simply reconstructed a few thimblefuls.)

  1. Why Titan is awesome #1
  2. Why Titan is awesome #2
  3. Why Titan is awesome #3
  4. Why Titan is awesome #4
  5. Why Titan is awesome #5
  6. Why Titan is awesome #6
  7. Why Titan is awesome #7
  8. Why Titan is awesome #8
  9. Why Titan is awesome #9
  10. Why Titan is awesome #10

Featured image: A radar image obtained by Cassini during a near-polar flyby on February 22, 2007, showing a big island in the middle of Kraken Mare on Saturn’s moon Titan. Caption and credit: NASA.

Note: This post was republished from late February 15 to the morning of February 16 because it was published too late in the night and received little traffic.

Why Titan is awesome #10

Titaaaaan!

How much I’ve missed writing these posts since Cassini passed away. Unsurprisingly, it’s after the probe’s demise that we’ve really begun to realise how much of Cassini’s images and data we were consuming on a daily basis, all of which is gone. There’s no more the steady stream of visuals of Saturn’s rings, bands, storms and panoply of moons – in fact all of which have been replaced by Jupiter’s rings, bands, storms and panoply of moons thanks to Juno. Nonetheless, one entire area of the Solar System has been darkened in my imagination. Until the next full mission to the Saturnian system (although nothing of the kind is in the works), we’ll have to make do with what Cassini data trickles down through NASA’s and ESA’s data-processing sieves.

One such is a new study about the temperature of the air high above Titan’s poles. Before Cassini’s death-dive into Saturn, the probe spent some time studying the moon’s polar atmosphere. Researchers from the University of Bristol who obtained this data noticed something odd: the part of the atmosphere over Titan’s poles began to develop a warm spot over late 2009 but that by 2012, it had become a ‘cold spot’. By 2015, the temperature at about 550 km above had dropped to 120 K (that’s a little below the temperature at which supercooled water turns into a glass).

On Earth, a warm spot forms over the poles because of two principle reasons: how Earth’s wind circulates around the planet and because of the presence of carbon dioxide. During winter, air over the corresponding hemispheric pole sinks down, becomes compressed and heats up. Moreover, the carbon dioxide present in the air also emits the heat it has trapped in its chemical bonds.

In 2012, astronomers using Cassini data had found that Titan also exhibits a wind circulation process that is moon-wide. It can be understood as Titan having two atmospheres, or layers, one on top of the other. In the lower atmosphere, there are three Hadley cells; each cell represents a distinct air circulation system wherein air rises up for 10 km or so from near the equator, moves up/down towards subtropical regions, sinks back down and returns to the equator along the surface. In the second, upper atmosphere, air moves between the two poles directly in a unified, global Hadley cell.

Titan_south polar vortex

Now, remember that Titan’s distance from the Sun means that one Titan-year is 29.5 Earth-years, that each Titanic season lasts over seven Earth-years and that seasonal shifts are much slower on the moon as a result. However, in 2012, scientists studying Cassini data found that the rate at which the air over one of Titan’s poles was sinking into the pole – like the air does on Earth – was happening really quickly: according to Nick Teanby, a researcher at the University of Bristol and also the lead author of the latest study, the rate of subsidence increased from 0.5 mm/s in January 2010 to 1.5 mm/s in June 2010. In other words, it was a shift that, unlike the moon’s seasons, happened rapidly (in just 12 Titanic days).

The same study concluded that Titan’s atmosphere was thicker than previously thought because trace gases like ethane, hydrogen cyanide, acetylene and cyanoacetylene were found to be produced at an altitude of over 500 km over the poles thanks to photochemical reactions induced by ultraviolet radiation and high-energy electrons streaming in from the Sun. These gases would then subside into the lower atmosphere over the polar region – which brings us to the latest study. It says that, unlike what carbon dioxide warming Earth’s atmosphere, the (once) trace gases actually cool the atmosphere, resulting in the dreadfully cold spot over Titan’s poles. They also participate in the upper Hadley cell circulation.

This is similar to a unique phenomenon observed over Saturn’s south pole in 2005.

Changes in trace gas abundances over Titan's south pole. Credit: ESA
Changes in trace gas abundances over Titan’s south pole. Credit: ESA

What a beauty you are, Titan. And I miss you, Cassini, more than I miss many other things in life.

I couldn’t find a link to the paper of the latest study; here’s the press release. Update: link to paper.

Links to previous editions:

  1. Why Titan is awesome #1
  2. Why Titan is awesome #2
  3. Why Titan is awesome #3
  4. Why Titan is awesome #4
  5. Why Titan is awesome #5
  6. Why Titan is awesome #6
  7. Why Titan is awesome #7
  8. Why Titan is awesome #8
  9. Why Titan is awesome #9

Featured image: Cassini’s last shot of Titan, taken with the probe’s narrow-angle camera on September 13, 2017. Credit: NASA.

A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA

Titan's lakes might be fizzing with nitrogen bubbles

Featured image: A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA.

TITAAAAAAAAAAN!

One more study reporting cool things about my favourite moon this week. Researchers from Mexico and France have found that the conditions exist in which the lakes of nitrogen, ethane and methane around Titan’s poles could be fizzy with nitrogen bubbles. In technical terms, that’s nitrogen exsolution: when one component of a solution of multiple substances separates out. In this case, the nitrogen forms bubbles and floats to the surface of the lakes, becoming spottable by the Cassini probe. The results were published in the journal Nature Astronomy on April 18.

The Cassini probe has been studying Saturn and its moons since 2004. In 2013, its RADAR instrument – which makes observations using radio-waves – found small, bright features on some of Titan’s lakes that winked out over time. These features have been whimsically called ‘magic islands’ and there has been speculation that they could be bubbles. The Mexican-French study provides one scientific form for this speculation.

The researchers used a numerical model to determine how and why the nitrogen could be degassing out of the lakes. Specifically, they extracted estimates of the temperature and pressure on the surface and interiors of the Ligeia Mare lake from past studies and then plugged them into simulations used to predict the properties of Earth’s oil and gas fields. They found that the bubbles could form if the solution of methane, ethane and nitrogen was forced to split up at certain temperatures and pressures. So, the researchers had to figure out the simplest way in which this could happen and then the likelihood of finding it happening in a Titanic lake.

When the lake’s innards are not forced to split up, they’re thought to exist in a liquid-liquid-vapour equilibrium (LLVE). In an LLVE, two liquids and a vapour can coexist without shifting phases (i.e. from liquid to vapour, vapour to liquid, etc.). The researchers write in their paper, “In the laboratory, LLVEs have been observed under cryogenic conditions for systems comparable to Titan’s liquid phases: nitrogen + methane + (ethane, propane or n-butane).” While cryogenic conditions may be hard to create on Earth’s surface, they’re the natural state of affairs on Titan because the latter is so far from the Sun. The surfaces of its lakes are thought to be at 80-90 K (-190º to -180º C), with the lower reaches being a few degrees colder.

For an LLVE-like condition to be disrupted, the researchers figured the lake itself couldn’t be homogenous. The reasons: “A sea with a homogeneous composition that matches that required for the occurrence of an LLVE at a specific depth is an improbable scenario. In addition, such a case would imply nitrogen degassing through the whole extent of the system.” So in a simple workaround, they suggested that the lake’s upper layers could be rich in methane and the lower layers, in ethane. This way, there’s more nitrogen available near the surface because the gas dissolves better in methane – and also because it could be dissolving into the top more from the moon’s nitrogen-rich atmosphere.

Over time, the lake’s top layers could be forced to move downward by weather conditions prevailing above the lake, and push the material at the bottom to the top. But during the downward journey, the rising pressure breaks the LLVE and forces the nitrogen to split off as bubbles. Given the size and depth of Ligeia Mare, the researchers have estimated that nitrogen exsolution can occur at depths of 100-200 m. The bubbles that rise to the top can be a few centimetres wide – not too small for Cassini’s RADAR instrument to spot them, as well as in keeping with what previous studies have recorded.

Of course, this isn’t the only way nitrogen bubbles could be forming on Ligeia Mare. According to another study published in March, when an ultra-cold slush of ethane settling at the bottom of the lake freezes, its crystals release the nitrogen trapped between their atoms. Michael Malaska, of NASA’s Jet Propulsion Lab, California, had said at the time:

In effect, it’s as though the lakes of Titan breathe nitrogen. As they cool, they can absorb more of the gas, ‘inhaling’. And as they warm, the liquid’s capacity is reduced, so they ‘exhale’.

The Mexican-French researchers are careful to note that their analysis can’t say anything about the quantities of nitrogen involved or how exactly it might be moving around Ligeia Mare – but only that it pinpoints the conditions in which the bubbles might be able to form. NASA has been tentative about sending a submarine to plumb the depths of another Titanic lake, Kraken Mare, in the 2040s. If it does undertake the mission, it could speak the final word on the ‘magic islands’. Ironically, however, NASA scientists will have to design the sub keeping in mind the formation of LLVEs and nitrogen exsolution.

But won’t the issue be settled by then? Maybe, maybe not. Come April 22, Cassini will fly by Titan’s surface at a distance of 980 km, at 21,000 km/hr. It will be the probe’s last close encounter with the moon, as mission scientists have planned to take a look at some of the smaller lakes. After this, the probe will fly a path that will take it successively through Saturn’s inner rings. Finally, on September 15, NASA will perform the probe’s ‘Grand Finale’ manoeuvre, sending it plunging into Saturn’s gassy atmosphere and unto its death, bringing the curtains down on a glorious 13-year mission that has changed the way we think about the ringed planet and its neighbourhood.

Published in The Wire on April 20, 2017.

 

A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA

Titan’s lakes might be fizzing with nitrogen bubbles

Featured image: A shot by Cassini of the lakes Kraken Mare and Ligeia Mare near Titan’s north pole. Credit: NASA.

TITAAAAAAAAAAN!

One more study reporting cool things about my favourite moon this week. Researchers from Mexico and France have found that the conditions exist in which the lakes of nitrogen, ethane and methane around Titan’s poles could be fizzy with nitrogen bubbles. In technical terms, that’s nitrogen exsolution: when one component of a solution of multiple substances separates out. In this case, the nitrogen forms bubbles and floats to the surface of the lakes, becoming spottable by the Cassini probe. The results were published in the journal Nature Astronomy on April 18.

The Cassini probe has been studying Saturn and its moons since 2004. In 2013, its RADAR instrument – which makes observations using radio-waves – found small, bright features on some of Titan’s lakes that winked out over time. These features have been whimsically called ‘magic islands’ and there has been speculation that they could be bubbles. The Mexican-French study provides one scientific form for this speculation.

The researchers used a numerical model to determine how and why the nitrogen could be degassing out of the lakes. Specifically, they extracted estimates of the temperature and pressure on the surface and interiors of the Ligeia Mare lake from past studies and then plugged them into simulations used to predict the properties of Earth’s oil and gas fields. They found that the bubbles could form if the solution of methane, ethane and nitrogen was forced to split up at certain temperatures and pressures. So, the researchers had to figure out the simplest way in which this could happen and then the likelihood of finding it happening in a Titanic lake.

When the lake’s innards are not forced to split up, they’re thought to exist in a liquid-liquid-vapour equilibrium (LLVE). In an LLVE, two liquids and a vapour can coexist without shifting phases (i.e. from liquid to vapour, vapour to liquid, etc.). The researchers write in their paper, “In the laboratory, LLVEs have been observed under cryogenic conditions for systems comparable to Titan’s liquid phases: nitrogen + methane + (ethane, propane or n-butane).” While cryogenic conditions may be hard to create on Earth’s surface, they’re the natural state of affairs on Titan because the latter is so far from the Sun. The surfaces of its lakes are thought to be at 80-90 K (-190º to -180º C), with the lower reaches being a few degrees colder.

For an LLVE-like condition to be disrupted, the researchers figured the lake itself couldn’t be homogenous. The reasons: “A sea with a homogeneous composition that matches that required for the occurrence of an LLVE at a specific depth is an improbable scenario. In addition, such a case would imply nitrogen degassing through the whole extent of the system.” So in a simple workaround, they suggested that the lake’s upper layers could be rich in methane and the lower layers, in ethane. This way, there’s more nitrogen available near the surface because the gas dissolves better in methane – and also because it could be dissolving into the top more from the moon’s nitrogen-rich atmosphere.

Over time, the lake’s top layers could be forced to move downward by weather conditions prevailing above the lake, and push the material at the bottom to the top. But during the downward journey, the rising pressure breaks the LLVE and forces the nitrogen to split off as bubbles. Given the size and depth of Ligeia Mare, the researchers have estimated that nitrogen exsolution can occur at depths of 100-200 m. The bubbles that rise to the top can be a few centimetres wide – not too small for Cassini’s RADAR instrument to spot them, as well as in keeping with what previous studies have recorded.

Of course, this isn’t the only way nitrogen bubbles could be forming on Ligeia Mare. According to another study published in March, when an ultra-cold slush of ethane settling at the bottom of the lake freezes, its crystals release the nitrogen trapped between their atoms. Michael Malaska, of NASA’s Jet Propulsion Lab, California, had said at the time:

In effect, it’s as though the lakes of Titan breathe nitrogen. As they cool, they can absorb more of the gas, ‘inhaling’. And as they warm, the liquid’s capacity is reduced, so they ‘exhale’.

The Mexican-French researchers are careful to note that their analysis can’t say anything about the quantities of nitrogen involved or how exactly it might be moving around Ligeia Mare – but only that it pinpoints the conditions in which the bubbles might be able to form. NASA has been tentative about sending a submarine to plumb the depths of another Titanic lake, Kraken Mare, in the 2040s. If it does undertake the mission, it could speak the final word on the ‘magic islands’. Ironically, however, NASA scientists will have to design the sub keeping in mind the formation of LLVEs and nitrogen exsolution.

But won’t the issue be settled by then? Maybe, maybe not. Come April 22, Cassini will fly by Titan’s surface at a distance of 980 km, at 21,000 km/hr. It will be the probe’s last close encounter with the moon, as mission scientists have planned to take a look at some of the smaller lakes. After this, the probe will fly a path that will take it successively through Saturn’s inner rings. Finally, on September 15, NASA will perform the probe’s ‘Grand Finale’ manoeuvre, sending it plunging into Saturn’s gassy atmosphere and unto its death, bringing the curtains down on a glorious 13-year mission that has changed the way we think about the ringed planet and its neighbourhood.

Published in The Wire on April 20, 2017.

 

Saturn in the background of Titan, its largest moon. Credit: gsfc/Flickr, CC BY 2.0

Titan's chemical orgies

Titan probably smells weird. It looks like a ball of dirt. It has ponds and streams of liquid ethane and methane and lakes of the two ethanes, with nitrogen bubbling up in large patches, near its poles. It has clouds of hydrocarbons raining down more methane. And like the water cycle on Earth, Titan has a methane cycle. Its atmosphere is a stifling billow of (mostly) nitrogen. Its surface temperature often dips below -180º C, and the Sun is as bright in its sky as our moon is in ours. In all, Titan is a dank orgy of organic chemistries playing out at the size of a small planet. And it smells weird – like gasoline. All the time.

But it is also beautiful. Titan is the only other object in the Solar System known to have bodies of liquid something flowing on its surface. It has a thick atmosphere and seasons. Its methane cycle signifies a mature and stable resource recycling system, just the way a functional household allows you to have routines. Yes, it’s cold and apparently desolate, but Titan can’t help these things. Water would freeze on its surface but the Saturnian moon has made do with what wouldn’t, and it has a singularly fascinating surface chemistry to show for it. Titan has been one of the more unique moons ever found.

And new observations and studies of the moon only make it more unique. This week, scientists from the Georgia Institute of Technology reported Titan possibly has dunes of tar that, once formed, stay in formation because their ionised particles cling together. The scientists stuck naphthalene and biphenyl – two organic compounds thought to exist on Titan’s surface – into a tumbler, tumbled it around for about 20 minutes in a nitrogen chamber and then emptied it. According to a Georgia Tech press release, 2-5% of the mixture lumped up.

The idea of tarry sands is not new. The Cassini probe studying the Saturn system found strange, parallel dunes near Titan’s equator in 2006, over a hundred metres tall. Soon after, scientists were thinking about ‘sediment cohesiveness’, the tendency of certain particles to stick together because of weak but persistent static charges, to explain the dunes. These charges are much weaker among sand particles and volcanic ash on Earth. Then again, in a 2009 paper in Nature Geoscience – the same journal the Georgia Tech study was published in – planetary geologists showed that longitudinal dunes, as they were called, were known to form in the Qaidam Basin in China. A note accompanying the paper explained:

More recent models for linear dune formation are centred on two main scenarios for formation and perpetuation. Winds from two alternating directions, separated by a wide angle, result in the formation of dunes whose long axis falls somewhere between the two wind directions. Alternatively, winds blowing from a single direction along a dune surface that has been stabilized in some way, for example by vegetation, an obstacle or sediment cohesiveness, can produce the same dune form.

That the Georgia Tech study affirmed the latter possibility doesn’t mean the former has been ruled out. Scientists have shown that bi-directional winds are possible on Titan, where wind blows in one direction over a desert and then shifts by 120º and blows over the same patch, forming a longitudinal dune. One of the Georgia Tech study’s novelties is in finding a way for the dune’s particles to stick together. Previous studies couldn’t confirm this was possible because the dunes mostly occur near Titan’s equator, where the weather is relatively much drier than at the poles, where mud-like clumps can form and hold their shape.

The other novelty is in using their naphthalene-biphenyl model to explain why the longitudinal dunes are also facing away from the wind. As one of the study’s authors told New Scientist, “The winds are moving one way and the sediments are moving the other way.” This is because the longitudinal dunes accrue on existing dunes and elongate themselves backwards. And once they do form, more naphthalene and biphenyl grains stick on them thanks to the static produced by them rubbing against each other. Only storms can budge them then.

The Georgia Tech group also writes in its paper that infrared and microwave observations suggest the dune’s constituent particles don’t become available through the erosion of nearby features. Instead, the particles become available out of Titan’s atmosphere, in the form of ‘haze particles’. They write: “[Frictional] charging provides an efficient process for the aggregation of simple aromatic hydrocarbons, and may serve as a mechanism for the formation of dune grains with diameters of several hundred micrometers from micrometer-sized haze particles.”

A big-picture implication is that Titan’s surface features are shaped by agents that are almost powerless on Earth. In other words, Titan doesn’t just smell weird; it’s also sticky. Despite the moon’s being similar to Earth in many ways, there are still drastic differences arising from small mismatches, mismatches we’d think wouldn’t make a difference. They remind us of the conditions we take for granted at home that are friendly to life – and of the conditions in which we can still dream of the possibility of life. Again, studies (described here and here) have shown this is possible. One has even warned us that Titanic lifeforms, if they exist, would smell nowhere as good as their name at all.

Understanding the dunes is a way to understand Titan’s winds. This is important because future missions to the moon envisage wind-blown balloons and cruising gliders.

Featured image: Saturn in the background of Titan, its largest moon. Credit: gsfc/Flickr, CC BY 2.0.

I’d written this post originally for Gaplogs but it got published in The Wire first.

Saturn in the background of Titan, its largest moon. Credit: gsfc/Flickr, CC BY 2.0

Titan’s chemical orgies

Titan probably smells weird. It looks like a ball of dirt. It has ponds and streams of liquid ethane and methane and lakes of the two ethanes, with nitrogen bubbling up in large patches, near its poles. It has clouds of hydrocarbons raining down more methane. And like the water cycle on Earth, Titan has a methane cycle. Its atmosphere is a stifling billow of (mostly) nitrogen. Its surface temperature often dips below -180º C, and the Sun is as bright in its sky as our moon is in ours. In all, Titan is a dank orgy of organic chemistries playing out at the size of a small planet. And it smells weird – like gasoline. All the time.

But it is also beautiful. Titan is the only other object in the Solar System known to have bodies of liquid something flowing on its surface. It has a thick atmosphere and seasons. Its methane cycle signifies a mature and stable resource recycling system, just the way a functional household allows you to have routines. Yes, it’s cold and apparently desolate, but Titan can’t help these things. Water would freeze on its surface but the Saturnian moon has made do with what wouldn’t, and it has a singularly fascinating surface chemistry to show for it. Titan has been one of the more unique moons ever found.

And new observations and studies of the moon only make it more unique. This week, scientists from the Georgia Institute of Technology reported Titan possibly has dunes of tar that, once formed, stay in formation because their ionised particles cling together. The scientists stuck naphthalene and biphenyl – two organic compounds thought to exist on Titan’s surface – into a tumbler, tumbled it around for about 20 minutes in a nitrogen chamber and then emptied it. According to a Georgia Tech press release, 2-5% of the mixture lumped up.

The idea of tarry sands is not new. The Cassini probe studying the Saturn system found strange, parallel dunes near Titan’s equator in 2006, over a hundred metres tall. Soon after, scientists were thinking about ‘sediment cohesiveness’, the tendency of certain particles to stick together because of weak but persistent static charges, to explain the dunes. These charges are much weaker among sand particles and volcanic ash on Earth. Then again, in a 2009 paper in Nature Geoscience – the same journal the Georgia Tech study was published in – planetary geologists showed that longitudinal dunes, as they were called, were known to form in the Qaidam Basin in China. A note accompanying the paper explained:

More recent models for linear dune formation are centred on two main scenarios for formation and perpetuation. Winds from two alternating directions, separated by a wide angle, result in the formation of dunes whose long axis falls somewhere between the two wind directions. Alternatively, winds blowing from a single direction along a dune surface that has been stabilized in some way, for example by vegetation, an obstacle or sediment cohesiveness, can produce the same dune form.

That the Georgia Tech study affirmed the latter possibility doesn’t mean the former has been ruled out. Scientists have shown that bi-directional winds are possible on Titan, where wind blows in one direction over a desert and then shifts by 120º and blows over the same patch, forming a longitudinal dune. One of the Georgia Tech study’s novelties is in finding a way for the dune’s particles to stick together. Previous studies couldn’t confirm this was possible because the dunes mostly occur near Titan’s equator, where the weather is relatively much drier than at the poles, where mud-like clumps can form and hold their shape.

The other novelty is in using their naphthalene-biphenyl model to explain why the longitudinal dunes are also facing away from the wind. As one of the study’s authors told New Scientist, “The winds are moving one way and the sediments are moving the other way.” This is because the longitudinal dunes accrue on existing dunes and elongate themselves backwards. And once they do form, more naphthalene and biphenyl grains stick on them thanks to the static produced by them rubbing against each other. Only storms can budge them then.

The Georgia Tech group also writes in its paper that infrared and microwave observations suggest the dune’s constituent particles don’t become available through the erosion of nearby features. Instead, the particles become available out of Titan’s atmosphere, in the form of ‘haze particles’. They write: “[Frictional] charging provides an efficient process for the aggregation of simple aromatic hydrocarbons, and may serve as a mechanism for the formation of dune grains with diameters of several hundred micrometers from micrometer-sized haze particles.”

A big-picture implication is that Titan’s surface features are shaped by agents that are almost powerless on Earth. In other words, Titan doesn’t just smell weird; it’s also sticky. Despite the moon’s being similar to Earth in many ways, there are still drastic differences arising from small mismatches, mismatches we’d think wouldn’t make a difference. They remind us of the conditions we take for granted at home that are friendly to life – and of the conditions in which we can still dream of the possibility of life. Again, studies (described here and here) have shown this is possible. One has even warned us that Titanic lifeforms, if they exist, would smell nowhere as good as their name at all.

Understanding the dunes is a way to understand Titan’s winds. This is important because future missions to the moon envisage wind-blown balloons and cruising gliders.

Featured image: Saturn in the background of Titan, its largest moon. Credit: gsfc/Flickr, CC BY 2.0.

I’d written this post originally for Gaplogs but it got published in The Wire first.

A falling want of opportunity for life to grip Titan

There is a new possibility for life on Titan. Scientists affiliated with Cornell University have created a blueprint for a cellular lifeform that wouldn’t need water to survive.

Water on Earth has been the principal ingredient of, as well as the catalyst for, the formation of life. The Cornell scientists believe water could be replaced by methane on Saturn’s moon Titan, where there are seas full of liquid methane.

The scientists’ work essentially lies in finding a suitable alternative for the phospholipid bilayer, a double-layer of fatty acids that constitutes every cell’s membrane on Earth. Because Titan’s atmosphere is rich in nitrogen and methane, their analysis suggested that a combination of the two molecules could create acrylonitrile azotosome, which when stacked together tends to assemble in membrane-like structures.

Acrylonitrile, it turns out, is already present in Titan’s atmosphere. What’s more is that the molecule could be reactive in the moon’s dreadfully cold environment (at -292 to -180 degrees Celsius). The next step is to see if cells with azotosome membranes can reproduce and metabolize in the methane- and nitrogen-rich environment. Their findings were published in Science Advances on February 27.

Incidentally, the formation of azotosomes also requires some hydrogen. This is interesting because astrobiologists have recently shown that the surface of liquid methane lakes on Titan could be host to microbes that metabolize acetylene, ethane and some other organic compounds, along with hydrogen.

Astrobiologists from the NASA Ames Research Center, who did this research, presumed that the microbes would need to consume hydrogen to make their metabolic reactions work. And because there is no other known process on Titan that could reduce the concentration of atmospheric hydrogen in the moon’s lower atmosphere, their calculations gave other astronomers an intriguing way to interpret anomalous deficiencies of hydrogen. Such deficiencies were recorded on Titan in 2010 by the NASA’s Cassini space probe.

There is an alternative explanation as well. Hydrogen may also be involved in chemical reactions in the atmosphere spurred by the bombardment of cosmic rays. Only continued observation will tell what is actually eating Titan’s hydrogen.

Yet another possibility for life on the moon was conceived in August 2014, when Dirk Schulze-Makuch, an astrobiologist from Washington State University, reported in Science that methane-digesting bacteria had been found in a lake of asphalt in Trinidad. The water content of the lake was only 13.5%. Schulze-Makuch suggested that, even if very little water was present on the moon, it would be enough to encourage the formation of these bacteria. What he couldn’t account for was the substantially lower temperature at which these reactions would need to occur on, say, Titan.

Slowly there has been a mounting number of possibilities, which suggest that life on Titan needn’t have to be fine-tuned or capitalize on one or two narrow windows of existential opportunity. Instead, there exist a variety of routes through which enterprising molecules could aspire for self-organization and awareness, even in an oxygen-deficient, methane-rich environment.

A close encounter with the mid-sized, icy kind

Cassini looks over the heavily cratered surface of Rhea during the spacecraft's flyby of the moon on March 10, 2012, from a distance of about 43,000 km.
Cassini looks over the heavily cratered surface of Rhea during the spacecraft’s flyby of the moon on March 10, 2012, from a distance of about 43,000 km. Image: NASA/JPL-Caltech/Space Science Institute

In three days, NASA’s Cassini mission will fly by Saturn’s second-largest moon Rhea. While interest in the Saturnian moons has been hogged by the largest – Titan – Cassini‘s images of Rhea could provide important new information about a class of natural satellites that it exemplifies: the so-called ‘mid-sized’ moons. While Titan is big enough to be a planet, it is also an exception. Only 13 of Saturn’s 62 confirmed moons are bigger than 50 km in diameter. Among them, Rhea is the largest, and its diameter is still less than a third of Titan’s.

As the artist Michael Carroll wrote,

With the exception of planet-sized Titan, these moons make up a mid-sized family of bodies that are more poorly understood than the larger Galilean satellites.

Cassini will take two sets of images of Rhea during its fly-by scheduled for February 10. The first set, an 11-frame mosaic of its northern hemisphere facing away from Saturn, will be taken from a distance of 46,943 km. The second set will be a 16-frame mosaic of the southern hemisphere shot from a distance of 53,700 km. The images will have a peak resolution of 305 m/pixel. On both occasions, the probe will use the Imaging Science Subsystem (ISS), whose cameras are specifically designed to be able to study the moons’ surfaces, as well as the planet’s glorious rings.

Including Rhea, Saturn has seven known sizeable moons with icy surfaces; the other six are Dione, Enceladus, Hyperion, Iapetus, Phoebe and Tethys). Of them, Dione and Enceladus are thought to be significant contributors to the halo of charged particles that also orbits Saturn, around its giant magnetosphere. Enceladus is especially known to eject fountains of water vapor from near its south pole, whose constituent droplets become ionized by the magnetosphere. On the other hand, Rhea does the opposite of contribute to the halo: it absorbs charged particles, as a result of which its surface is charged. In fact, its surface has the strongest negative potential among the mid-sized icy moons.

Extensive studies of Earth’s moon have revealed that such an electrostatic potential (caused due to charged particles streaming in from the Sun) has accelerated dust particles into space. Astronomers have reason to believe a similar mechanism at play on some of Saturn’s moons – but with more intensity – could be moving dust particles between the moons. And Rhea could be at the center of this dusty relay.

Moreover, its tendency to absorb charged particles promotes a feeble radiolysis of its surface ice, feeding a thin atmosphere of ozone, hydrogen peroxide and molecular oxygen around it. Similar mechanisms have been used to explain molecular oxygen in the atmospheres of Dione and Jupiter’s moons Europa and Ganymede, and possibly on other Saturnian and Jovians moons, and on exoplanets, as well.

These finds could be further augmented by continued observations of Rhea. The last of them happened in 2013, during Cassini‘s last targeted mission for the moon when it came within 1,000 km. Then, it had found a surface studded with more craters than were seen on the other icy moons. Astronomers think this implies Rhea was battered with comets that could have been the source of a whiff of carbon dioxide that has been detected emanating from it. At the same time, Rhea alone might not have been battered. It is farther outward than Dione, Enceladus and Tethys, whose innards are warmed by tidal forces generated by Saturn’s gravitational pull. The warmth melts some of the surface ice and could cover up possibly hidden craters.

These features – mid-range mass, a highly charged surface with an unusual number of craters, oxygen in its atmosphere, and a depravity of tidal warming – all together keep Rhea in the spotlight, if only a fluctuating one. Even now, as Cassini embarks on the first of its 20 flybys of Saturn’s icy satellites in 2015, the ISS’s cameras will be turned toward Rhea only on February 10. On February 12, it will image Titan during the second of seven flybys, all slated for this year. In fact, at this moment, Europa remains the most talked-about icy moon in the Solar System.

Life on Titan’s world of goo

In the August 8 issue of Science, an international team of scientists has a paper that submits evidence of life in an asphalt lake in Trinidad. Despite having a low water content of 13.5%, it still possesses methane-digesting microbes huddled up in tiny water droplets. One of the authors, Dirk Schulze-Makuch, speculates in an Air & Space Magazine article that the find could have important implications for Saturn’s moon Titan, which is wrapped in chemistries similar to what was found in the lake minus the presence of liquid water.

In fact, its atmosphere is mainly nitrogen, with lakes of liquid methane and ethane on its surface. So, that there are extremophiles living in a world of goo means not all hope is lost for alien life to form on Titan, no matter that such hopes are still too far beyond the ambit of scientific conservatism inspired by how little we know about life’s origins. Nevertheless, the Science paper isn’t the first to demonstrate that life can exist in such extreme conditions similar to those spotted on planetary bodies in the Solar System; in fact, going by previous reports, it isn’t likely to be the last either.

In a 2011 study published in Microbial Biotechnology, South American researchers reported the presence of a fungus, Neosartorya fischeri, that could metabolize asphaltene, “which is considered the most recalcitrant petroleum fraction”. Their work in turn draws from a 1993 study that proved asphalt is susceptible to reacting with certain extracellular enzymes.

However, Schulze-Makuch’s article makes many assumptions. For example, Titan is much colder than Trinidad’s Pitch Lake, a tropical deposition of oil rising up from a tectonic fault at its bed. For another, it is not known if Titan harbors liquid water, which – at least on Earth – is known to decisively encourage the formation of life, just as it did in the lake.

Two pairs of moons make a rare joint appearance. The F ring's shepherd moons, Prometheus and Pandora, appear just inside and outside of the F ring. Meanwhile, farther from Saturn the co-orbital moons Janus (near the bottom) and Epimetheus (near the top) also are captured. This view looks toward the sunlit side of the rings from about 47 degrees above the ringplane. Credit: NASA/JPL-Caltech/Space Science Institute
Two pairs of moons make a rare joint appearance. The F ring’s shepherd moons, Prometheus and Pandora, appear just inside and outside of the F ring. Meanwhile, farther from Saturn the co-orbital moons Janus (near the bottom) and Epimetheus (near the top) also are captured. This view looks toward the sunlit side of the rings from about 47 degrees above the ringplane. Credit: NASA/JPL-Caltech/Space Science Institute

 

Fortunately – rather, optimistically – astrobiologists have been able to rationalize how life could form on Titan. In 2005, Chris McKay and Heather Smith, both astrobiologists at NASA Ames Research Center, were able to come up with a mechanism by which methanogenic microbes in Titan’s troposphere could be metabolizing acetylene, ethane and some other organic compounds – of which the moon has plenty – to release 54-334 kJ/mol, an amount of energy that similar extremophile critters on Earth have been known to get by on.

They also think it’s possible that the microbes could be catalyzing biochemical reactions despite the low temperature, around -180 degrees Celsius. In either case, their calculations are dependent on the microbes consuming hydrocarbons along with atmospheric hydrogen – an adjustment for convenience. Being a gas with no other sources or sinks in Titan’s atmosphere, any dip in its concentration could be a sign of life, albeit a distant one. McKay had said in a NASA press release in 2010 that “We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth.”

His and Smith’s hypothesis found some validation in that year – 2010 – when the Cassini space probe found anomalous deficiencies of hydrogen and acetylene, which should be evenly distributed around the moon but weren’t, meaning they were disappearing into somewhere or something, like being consumed. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth,” McKay had said.

Just as well, some other astrobiolgists think the cosmic rays bombarding Titan’s atmosphere could be transforming acetylene into more complex hydrocarbons, constituting the non-biological explanation that scientists would like to have out of the way first. Even today, this attitude hasn’t changed because the basis of methanogenic life is still very theoretical, a possibility hinged on chemical reactions worked out by supercomputers. Yet, it’s a tantalizing possibility.

In a 2004 paper by Steven Benner, University of Florida, et al, the authors discuss how life could form without liquid water if only a few other conditions are met: a thermodynamic disequilibrium (a natural mechanism to maintain periodically varying temperatures), “temperatures consistent with chemical bonding” and the presence of a solvent system. The paper itself begins by questioning not how life originated but, in deference to its great adaptability, why life on Earth is what it is.

I reproduce a paragraph from it that I find provides a fitting explanation to why the search for life on Titan (and perhaps also Io and Enceladus) is worth keeping up:

The universe of chemical possibilities is huge. For example, the number of different proteins 100 amino acids long, built from combinations of the natural 20 amino acids, is larger than the number of atoms in the cosmos. Life on Earth certainly did not have time to sample all possible sequences to find the best. What exists in modern Terran [i.e. Earth-bound] life must therefore reflect some contingencies, chance events in history that led to one choice over another, whether or not the choice was optimal.

It’s also after 2004 – in 2013, actually – that we also discovered that Titan might be running out of methane soon. Studies conducted since around 2005 showed that the moon’s source of methane could be less from photochemical reactions in its atmosphere and more from subsurface pockets where the gas could have been trapped. According to a NASA statement, “the current load of methane at Titan may have come from some kind of gigantic outburst from the interior eons ago possibly after a huge impact,” and could run out in tens of millions of years, a short span on the geological timescale.

If so, then, if methanogenic life hasn’t already formed but is likely to, it better do so quickly. If our models have an as yet undetected or undetectable flaw, then, as always, time will tell. If, ultimately, life is already present on Solar System’s second-largest moon, then one can only hope it’s as versatile as the world that hosts it.

~

References

  1. Scientists Find Life in a Lake of Oil, Air & Space Magazine. Accessed August 10, 2014.
  2. Meckenstock, R.U. et al, Water droplets in oil are microhabitats for microbial life. Science, 8 August 2014: 345 (6197), 673-676. doi: 10.1126/science.1252215
  3. Uribe-Alvarez, C., Ayala, M., Perezgasga, L., Naranjo, L., Urbina, H. and Vazquez-Duhalt, R. (2011), First evidence of mineralization of petroleum asphaltenes by a strain of Neosartorya fischeri. Microbial Biotechnology, 4: 663–672. doi: 10.1111/j.1751-7915.2011.00269.x
  4. Fedorak, P.M, Semple, K.M., Vazquez-Duhalt, R., Westlake, D.W.S., Chloroperoxidase-mediated modifications of petroporphyrins and asphaltenes. Enzyme and Microbial Technology, Volume 15, Issue 5, May 1993, Pages 429–437. doi: 10.1016/0141-0229(93)90131-K
  5. Tobie, G., Lunine, J.I. and Sotin, C., Episodic outgassing as the origin of atmospheric methane on Titan. 28 November 2005, Nature 440, 61-64. doi:10.1038/nature04497
  6. Benner, S.A., Alonso Ricardo, A. and Carrigan, M.A., Is there a common chemical model for life in the universe?. Current Opinion in Chemical Biology, 2004, 8:672–689. doi: 10.1016/j.cbpa.2004.10.003

What life on Earth tells us about life ‘elsewhere’

Plumes of water seen erupting form the surface of Saturn's moon Enceladus. NASA/JPL-Caltech and Space Science Institute
Plumes of water seen erupting form the surface of Saturn’s moon Enceladus. NASA/JPL-Caltech and Space Science Institute

In 1950, the physicist Enrico Fermi asked a question not many could forget for a long time: “Where is everybody?” He was referring to the notion that, given the age and size of the universe, advanced civilizations ought to have arisen in many parts of it. But if they had, then where are their space probes and radio signals? In the 60 years since, we haven’t come any closer to answering Fermi, although many interesting explanations have cropped up. In this time, the the search for “Where” has encouraged with it a search for “What” as well.

What is life?

Humankind’s search for extra-terrestrial life is centered on the assumption – rather hope – that life can exist in a variety of conditions, and displays a justified humility in acknowledging we really have no idea what those conditions could be or where. Based on what we’ve found on Earth, water seems pretty important. As @UrbanAstroNYC tweeted,

And apart from water, pretty much everything else can vary. Temperatures could drop below the freezing point or cross to beyond the boiling point of water, the environment can be doused in ionizing radiation, the amount of light could dip to quasi-absolute darkness levels, acids and bases can run amok, and the concentration of gases may vary. We have reason to afford such existential glibness: consider this Wikipedia list of extremophiles, the living things that have adapted to extreme environments.

Nonetheless, we can’t help but wonder if the qualities of life on Earth can tell us something about what life anywhere else needs to take root- even if that means extrapolating based on the assumption that we’re looking for something carbon-based, and dependent on liquid water, some light, and oxygen and nitrogen in the atmosphere. Interestingly, even such a leashed approach can throw open a variety of possibilities.

“If liquid water and biologically available nitrogen are present, then phosphorus, potassium, sodium, sulfur and calcium might come next on a requirements list, as these are the next most abundant elements in bacteria,” writes Christopher McKay of the NASA Ames Research Center, California, in his new paper ‘Requirements and limits for life in the context of exoplanets’. It was published in Proceedings of the National Academy of Sciences on June 9.

Stuff of stars

McKay, an astro-geophysicist, takes a stepped approach to understanding the conditions life needs to exist. He bases his argument on one inescapable fact: that we know little to nothing about how life originated, but a lot about how, once it exists, it can or can’t thrive on Earth. Starting from that, the first step he devotes to understanding the requirements for life. In the second step, he analyzes the various extreme conditions life can then adapt to. Finally, he extrapolates his findings to arrive at some guidelines.

It’s undeniable that these guidelines will be insular or play a limited role in our search for extraterrestrial life. But such criticism can be partly ablated if you consider Carl Sagan’s famous words from his 1980 book Cosmos: “The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

In 1991, RH Koch and RE Davies published a paper (titled ‘All the observed universe has contributed to life’) presenting evidence that “a standard 70 kg human  is always making about 7 3He, 600 40Ca, and 3,000 14N nuclei every second by radioactive decay of 3H, 40K, and 14C, respectively”. In other words, we’re not just made of starstuff, we’re also releasing starstuff! So it’s entirely plausible other forms of life out there – if they exist – could boast some if not many similarities to life on Earth.

To this end, McKay postulates a ‘checklist for habitability’on an exoplanet based on what we’ve found back home.

  • Temperature and state of water – Between -15° C and 122° C (at pressure greater than 0.01 atm)
  • Water availability – Few days per year of rain, fog or snow, or relative humidity more than 80%
  • Light and chemical energy sources
  • Ionizing radiation – As much as the bacterium Deinococcus radiodurans can withstand (this microbe is the world’s toughest extremophile according to the Guinness Book of World Records)
  • Nitrogen – Enough for fixation
  • Oxygen (as the molecule O2) – Over 0.01 atm needed to support complex life

McKay calls this list “a reasonable starting point in the search for life”. Its items show that together they make possible environmental conditions that sustain some forms of chemical bonding – and such a conclusion could inform our search for ‘exo-life’. Because we’re pretty clueless about the origins of life, it doesn’t mean we’ve to look for just these items on exoplanets but the sort of environment that these items’ counterparts could make possible. For example, despite the abundance of life-friendly ecosystems on Earth today, one way life could have originated in the first place is by meteorites having seeded the crust with the first microbes. And once seeded, the items on the checklist could have taken care of the rest.

Are you sure water is life?

Such otherworldly influences present yet more possibilities; all you need is another interstellar smuggler of life to crash into a conducive laboratory. Consider the saturnine moon Titan. While hydrocarbons – the principal constituents of terran life – on Earth are thought to have gassed up and out from the mantle since its formative years, Titan already boasts entire lakes of methane (CH4), a simple hydrocarbon. A 2004 paper by Steven Benner et al discusses the implications of this in detail, arguing that liquid methane could actually be a better medium than water for certain simple chemical reactions that are the precursors of life to occur in.

Another Solar System candidate that shows signs of habitability is Titan’s peer Enceladus. In April this year, teams of scientists studying data from the Cassini space probe said there was evidence that Enceladus hosts a giant reservoir of liquid water 10 km deep under an extensive ice shell some 30-40 km thick. Moreover, Cassini flybys since 2005 had shown that the moon had an atmosphere of 91% water vapor, 3-4% each of nitrogen and carbon dioxide, and the rest of methane.

These examples in our Solar System reveal how the conditions necessary for life are possible not just in the Goldilocks zone because life can occur in a variety of environments as long some simpler conditions are met. The abstract of the paper by Benner et al sums this up nicely:

A review of organic chemistry suggests that life, a chemical system capable of Darwinian evolution, may exist in a wide range of environments. These include non-aqueous solvent systems at low temperatures, or even supercritical dihydrogen– helium mixtures. The only absolute requirements may be a thermodynamic disequilibrium and temperatures consistent with chemical bonding.

As humans, we enjoy the benefits of some or many of these conditions – although we know what we do only on the basis of what we’ve observed in nature, not because some theory or formula tells us what’s possible or not. Such is the amount of diversity of life on Earth, and that should tell us something about how far from clued-in we are to understanding what other forms of life could be out there. In the meantime, as the search for extra-terrestrial life and intelligence goes on, let’s not fixate on the pessimism of Fermi’s words and instead remember the hope in Sagan’s (and keep an eye on McKay’s checklist).