Saturn – Close Read

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 Astronomy; the 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.

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.)

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:

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

The significance of Cassini’s end

Many generations of physicists, astronomers and astrobiologists are going to be fascinated by Saturn because of Cassini.

I wrote this on The Wire on September 15. I lied. Truth is, I don’t care about Saturn. In fact, I’m fascinated with Cassini because of Saturn. We all are. Without Cassini, Saturn wouldn’t have been what it is in our shared imagination of the planet as well as the part of the Solar System it inhabits. At the same time, without Saturn, Cassini wouldn’t have been what it is in our shared imagination of what a space probe is and how much they mean to us. This is significant.

The aspects of Cassini’s end that are relevant in this context are:

The abruptness
The spectacle

Both together have made Cassini unforgettable (at least for a year or so) and its end a notable part of our thoughts on Saturn. We usually don’t remember probes, their instruments and interplanetary manoeuvres during ongoing missions because we are appreciably more captivated by the images and other data the probe is beaming back to Earth. In other words, the human experience of space is mediated by machines, but when a mission is underway, we don’t engage with information about the machine and/or what it’s doing as much as we do with what it has discovered/rediscovered, together with the terms of humankind’s engagement with that information.

This is particularly true of the Hubble Space Telescope, whose images constantly expand our vision of the cosmos while very few of us know how the telescope actually achieves what it does.

From a piece I wrote on The Wire in July 2015:

[Hubble’s] impressive suite of five instruments, highly polished mirrors and advanced housing all enable it to see the universe in visible-to-ultraviolet light in exquisite detail. Its opaque engineering is inaccessible to most but this gap in public knowledge has been compensated many times over by the richness of its observations. In a sense, we no longer concern ourselves with how the telescope works because we have drunk our fill with what it has seen of the universe for us…

Cassini broke this mould by – in its finish – reminding us that it exists. And the abruptness of the mission’s end contributed to this. In contrast, consider the story of the Mars Phoenix lander. NASA launched Phoenix (August 2007 to May 2010) in August 2007. It helped us understand Mars’s north polar region and the distribution of water ice on the planet. Its landing manoeuvre also helped NASA scientists validate the landing gear and techniques for future missions. However, the mission’s last date has a bit of uncertainty. Phoenix’s last proper signal was sent in November 2, 2008. It was declared not on the same day but a week later, when attempts reestablish contact with Phoenix failed. But the official declaration of ‘mission end’ came only in May 2010, when a NASA satellite’s attempts to reestablish contact failed.

Is it easier to deal with the death of someone because their death came suddenly? Does it matter if their body was found or not? For Phoenix, we have a ‘body’ (a hunk of metal lying dormant near the Martian north pole); for Cassini, we don’t have a ‘body’. On the other hand, we don’t have a fixed date of ‘mission end’ for Phoenix but we do for Cassini, down to the last centisecond and which will be memorialised at NASA one way or another.

Spectacle exacerbates this tendency to memorialise by providing a vivid representation of ‘mission end’ that has been shared by millions of people. Axiomatically, a memorial for Cassini – wherever one emerges – will likely evoke the same memories and emotions in a larger number of people, and all of those people will be living existences made congruent by the shared cognisance and interpretation of the ‘Cassini event’.

However, Phoenix’s ‘mission end’ wasn’t spectacular. The lander – sitting in one place, immobile – slowly faded to nothing. Cassini burnt up over Saturn. Interestingly, both probes experienced similar ‘deaths’ (though I am loth to use that word) in one sense: neither probe knew the way an I/AI could that they were going to their deaths but both their instrument suites fought against failing systems all guns blazing. Cassini only got the memorial upper hand because it could actively reorient itself in space (akin to the arms on our bodies) and because it was in an environment it was not designed for at all.

The ultimate effect is for humans to remember Cassini more vividly than they would Phoenix, as well as associate a temporality with that remembrance. Phoenix was a sensor, the nicotine patch for a chain-smoking planet (‘smoking’ being the semantic variable here). Cassini moved around – 2 billion km’s worth – and also completed a complicated sequence of orbits around Saturn in three dimensions in 13 years. Cassini represents more agency, more risk, more of a life – and what better way to realise this anthropomorphisation than as a time-wise progression of events with a common purpose?

We remember Cassini by recalling not one moment in space or time but a sequence of them. That’s what establishes the perfect context for the probe’s identity as a quasi-person. That’s also what shatters the glaze of ignorance crenellated around the object, bringing it unto fixation from transience, unto visibility from the same invisibility that Hubble is currently languishing in.

Featured image credit: nasahqphoto/Flickr, CC BY-NC-ND 2.0.

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.

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.

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?

@AstroKatie @1amnerd @BBCOS everywhere we find a smidgen of liquid H20 on earth, regardless of conditions, we find life

— rettiwT delliK nolE (@UrbanAstroNYC) April 18, 2014

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,

@AstroKatie @1amnerd @BBCOS there are parts of the Atacama Desert – driest place on earth – that are so arid NO LIFE survives.

— rettiwT delliK nolE (@UrbanAstroNYC) April 18, 2014

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.

Yes. We only have 1 example. MT @1amnerd: So does this mean we don't know under what other conditions life could form?

— Katie Mack (@AstroKatie) April 18, 2014

@1amnerd @AstroKatie @BBCOS yes, even more unknowns; life as we know it is carbon/DNA based, we don't even know what alternates there are…

— Paul Salanitri (@Paul_Salanitri) April 18, 2014

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?

@1amnerd @AstroKatie @BBCOS that's why we always care so deeply about the subsurface oceans of moons like Europa/Enceladus as well 🙂

— rettiwT delliK nolE (@UrbanAstroNYC) April 18, 2014

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).