Why Titan is awesome #9

Featured image: Four of the 66 antennae of the ALMA array. Credit: Carlos Padilla – AUI/NRAO.

Editions onetwothreefourfivesixseven and eight

In edition #8 of ‘why Titan is awesome’, we visited a lake near Titan’s north pole, named Ligeia Mare, and how it could sport long lines of tiny – ~2 cm – waves. In a new study, scientists have uncovered evidence that Ligeia Mare also has enough vinyl cyanide dissolved in its waters liquid methane to be able to form 30 million biological cells per millilitre.

Does this mean we know that there’s life on Titan?+ No. We only know that if there is life on Titan in the form of microbes, then there could be 30 million cells per millilitre of Ligeia Mare with membranes composed of vinyl cyanide (with the additional assumption that each of these cells is 10 micrometers wide). But in spite of the particularity of this statement, it’s an exciting discovery nonetheless. To make it, scientists did not use the Cassini probe, although they were inspired enough to undertake follow-up studies by Cassini data published in 2007 that showed signs of vinyl cyanide in Titan’s atmosphere.

Much of what we think we know about Titan comes from computational models generated by labs on Earth. Some of the data for these models comes from space-borne observatories like Cassini and powerful telescopes on the ground. The remaining data is predicted by scientists based on what they know about how a a large body like Titan evolves over time. Even in the new study, published in the journal Science Advances on July 28, scientists used telescopes – technically called antennae – part of the Atacama Large Millimetre/submillimetre Array (ALMA), Chile, to infer that there is a certain quantity of vinyl cyanide in Titan’s atmosphere. But as the abstract of their paper continues:

Radiative transfer modeling suggests that most of the C2H3CN emission originates at altitudes of ≳200 km, in agreement with recent photochemical models. The vertical column densities implied by our best-fitting models lie in the range of 3.7 × 1013 to 1.4 × 1014 cm−2. The corresponding production rate of vinyl cyanide and its saturation mole fraction imply the availability of sufficient dissolved material to form ~107 cell membranes/cm3 in Titan’s sea Ligeia Mare.

The presence of vinyl cyanide itself does not indicate that cell membranes will be formed with it. Instead, this thesis is based on a study published in 2015, which said that the integrity of cell membranes forming in cold methane lakes – like the ones on Titan – would depend on certain electrochemical properties of molecules of nitrogen. The study’s authors referred to such membranes as ‘azotosomes’ – akin to the liposomevesicles that make up all cell membranes on Earth. And the new study’s authors surmise that vinyl cyanide is one such molecule of nitrogen that could lead to the formation of azotosomes (‘azote’ is the French name for nitrogen).

Image: What ALMA saw. The blue ring is Titan; the vertical dot-dashed blue line is the polar axis and the dashed white line represents Titan’s equator. The colour intensity denotes the flux density of the vinyl cyanide emission. Source: Palmer et al., Sci. Adv. 2017;3: e1700022

Another feature of the study that provides wonderful insight into how Titan ‘works’ is where the vinyl cyanide forms. The scientists used a photochemical model to determine this using data obtained by Cassini and ALMA, and figured the substance was forming at least 200 km above Titan’s surface, in the reaction:

CN + C2H4 → C2H3CN + H
(cyanide ion + ethylene → vinyl cyanide + hydrogen)

At a lower altitude (~100 km), their models also predicted that vinyl cyanide was forming in much lower quantities by the reaction:

HCN + C2H3 → C2H3CN + H
(Hydrogen cyanide + vinyl radical → vinyl cyanide + hydrogen)

From there, it is carried in a steady stream by “a rain of haze particles” to the surface. This isn’t new: Titan is the only body in the Solar System, apart from Earth, known to have a fully functional counterpart of our water cycle; there, it is a methane cycle. To obtain all this data, scientists used ALMA to record 11 observations of Titan as the moon transited the antennae’s gaze between February and May 2014, when it was between 8.9 and 9.7 AU (1.3 and 1.4 billion km) away.

Ah, Titan.

+This assessment excludes questions arising from the possibility that there is an ocean of liquid water and ammonia some 55-80 km below Titan’s surface.