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

Why Titan is awesome #11


Here we go again.¬†ūüėĄ¬†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 ‚Äď specifically as they would be inside Kraken and Ligeia Mare ‚Äď 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 ‚Äď 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:


Oh, Titan.

(Obligatory crib: the university press release‘s headline goes ‘WSU researchers build -300¬ļF 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.

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¬†-180¬ļ 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 ‚Äď 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 ‚Äď two organic compounds thought to exist on Titan’s surface ‚Äď 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 ‚Äď the same journal the Georgia Tech study was published in ‚Äď 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 120¬ļ 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 ‚Äď 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 submarine on Titan in 2040

An artist's conception of the proposed Titan Submarine, which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon's largest hydrocarbon lake.
An artist’s conception of the proposed Titan Submarine (conceived before the latest design was released), which NASA could land on Titan around 2040 to explore the depths of Kraken Mare, the moon’s largest hydrocarbon lake. Image: NASA

Nothing bespeaks humankind’s potential more than the following statement: Around 2040, NASA¬†plans to splash down a submarine to explore¬†a liquid hydrocarbon lake on Titan.

Fore¬†more than a decade now, Titan has captivated astronomers¬†not simply by being Saturn’s largest moon by far but also with its vast seas of liquid methane and ethane. NASA has its eyes on the largest such lake, called Kraken Mare, located near the moon’s north pole. The Cassini mission helped map the lake in great detail since it reached the Saturnian system in 2004, accompanied by the Huygens probe that landed on the moon’s surface in 2005. Thanks to them, we know Kraken Mare has an intricate shoreline and deposits of water-soluble minerals around it. According to the scientists who authored the article describing the submarine, these features “hint at a rich chemistry and climate history”.

They continue: “The proposed ~1-tonne vehicle, with a radioisotope Stirling generator power source, would be delivered to splashdown circa 2040, to make a ~90-day, ~2,000 km voyage of exploration around the perimeter, and across the central depths of Kraken.” While its design is by no means final (it’s described as a “first cut”), that NASA is considering exploring Titan in great detail belies its interest in the moon as well as continued commitment to studying the Saturnian system in general. Note that the agency cancelled the development of the proposed Titan Mare Explorer – a nautical surface probe¬†– soon after 2013 to channel the funds into developing Stirling radioisotope generators, which we now find could¬†be used to power the submarine.

Notwithstanding future budgetary cuts, delivering such a vehicle to the surface of a faraway moon might just signify the next leap in astronautical engineering. As the scientists remark,

Even with its planetary application aside, this exercise has forced us to look at submarine vehicle design drivers in a whole new way.

The current design has been developed by scientists from the JHU Applied Physics Laboratory, the NASA Glenn Research Center, and the Penn State Applied Research Lab. It will be presented at the 46th Lunar and Planetary Science Conference in Texas, during March 16-20.

1970s Space Shuttle ditching tests at Langley show lifting bodies can make safe landing on liquid.
1970s Space Shuttle ditching tests at Langley show lifting bodies can make safe landing on liquid. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

Around 2040, they expect to be able to deliver it to Titan on board a ‘spaceplane carrier’, essentially a repurposed US Air Force DARPA X-37. According to them, Titan’s thick atmosphere could allow the carrier to descend to the surface at hypersonic speeds, following which attempt a soft-landing on the Kraken Mare. Finally, “the backshell covering the submarine would be jettisoned and the lifting body would¬†sink, leaving the submarine floating to begin operations.¬†(Alternatively, the submersible could be extracted¬†in low level flight by parachute).”

Once inside, it will explore tidal currents in Kraken Mare, use a camera mounted on the mast to explore the shoreline landscape, make meteorological observations, analyze sediments from the seabed, and study trace organic compounds to learn how they evolved.

The slender low-drag hull has propulsors at rear, and a large dorsal antenna at the front of which is a surface camera is mounted in a streamlined cowl. A sidescan sonar, seafloor camera, and seafloor sampling system are visible on ventral surfaces.
The slender low-drag hull has propulsors at¬†rear, and a large dorsal antenna at the¬†front of which is a surface camera is mounted in a¬†streamlined cowl. A sidescan sonar, seafloor camera,¬†and seafloor sampling system are visible on ventral¬†surfaces. Image: ‘Titan Submarine: Vehicle Design and Operations Concept for the Exploration of the Hydrocarbon Seas of Saturn’s Giant Moon’ by Lorenz et al

The submarine itself looks conventional apart from a large dorsal antenna and two cylindrical buoyancy tanks that jut out of the upper surface. According to its designers, the antenna was shaped so to be able to send data across billions of kilometers to Earth. And such large buoyancy tanks are necessary because the lake the submarine will explore is composed of methane and ethane, whose densities range from 450 kg/m3 to 670 kg/m3, as well as to counter the unique drag effects arising due to the dorsal antenna.

Another complication is thermodynamics. Titan has a frigid surface, cold enough to keep methane, whose boiling point is -161.5 degrees Celsius, in its liquid form. As a result, extra heat rejected from the submarine’s radioisotope power source could cause the surrounding methane and ethane to bubble. As the scientists explain, this results in “heat transfer uncertainties” as well as the potential to interfere with sonar observations. At the same time, the vessel must also be heavily insulated to allow the power source to warm its¬†insides.

NASA first announced its intention to explore Kraken Mare with a submarine in June 2014, elaborating that the mission would help scientists learn more about the history and evolution of organic compounds in the Solar System, in turn a “critical step” along the path to understanding the formation of life. Earth and Titan are the only two objects in the System to host liquid lakes on their surfaces, albeit of different compositions.

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¬į C and 122¬į 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 Рthe principal constituents of terran life Рon 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‚Ästhelium 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).