Bubbles in Titan’s Seas: Nucleation, Growth, and RADAR Signature

Abstract

In the polar regions of Titan, the main satellite of Saturn, hydrocarbon seas have been discovered by the Cassini-Huygens mission. RADAR observations have revealed surprising and transient bright areas over Ligeia Mare surface. As suggested by recent research, bubbles could explain these strange features. However, the nucleation and growth of such bubbles, together with their RADAR reflectivity, have never been investigated. All of these aspects are critical to an actual observation. We have thus applied the classical nucleation theory to our context, and we developed a specific radiative transfer model that is appropriate for bubbles streams in cryogenic liquids. According to our results, the sea bed appears to be the most plausible place for the generation of bubbles, leading to a signal comparable to observations. This conclusion is supported by thermodynamic arguments and by RADAR properties of a bubbly column. The latter are also valid in the case of bubble plumes, due to gas leaking from the sea floor.

Nitrogen Exsolution and Bubble Formation in Titan's Lakes

“…Our experimental population of bubbles (average density of 10 bubbles/cm 2 , diameter of 1.3 mm (Figure S1), and liquid depth of 4.4 mm) has an equivalent of 2.6 × 10 4 cm 3 of nitrogen gas per m 3 of liquid, > 60 times above the volume estimated by Cordier & Liger‐Belair (). This indicates that the conditions identified by Cordier & Liger‐Belair, () are easily achievable under Titan conditions.…”

“…Bubbles may produce the “Magic Islands” (RADAR‐bright transient features) observed on Titan's Ligea Mare and Kraken Mare (Hofgartner et al, , ). Cordier & Liger‐Belair, () modeled a potential bubble event within a 100‐m column (approximate depth of Ligeia Mare; Mastrogiuseppe et al, ), and found that 100 bubbles/m 3 with radii of ~1 cm are required to match the Magic Island's tenfold RADAR brightening (equivalent to 420 cm 3 of nitrogen gas per m 3 of liquid). Our experimental population of bubbles (average density of 10 bubbles/cm 2 , diameter of 1.3 mm (Figure S1), and liquid depth of 4.4 mm) has an equivalent of 2.6 × 10 4 cm 3 of nitrogen gas per m 3 of liquid, > 60 times above the volume estimated by Cordier & Liger‐Belair ().…”

“…Recent laboratory studies (Luspay‐Kuti et al, ; Malaska et al, ) confirmed predictions (Battino et al, ; Hartwig et al, ) that nitrogen is significantly more soluble in methane than ethane and that solubility increases at lower temperatures. This led to the hypothesis that dissolved nitrogen may form bubbles upon exsolution (Malaska et al, ; Cordier et al, ; Cordier & Liger‐Belair, ). Recent modeling suggests that nitrogen bubbles are thermodynamically plausible at depth in Titan's methane–ethane lakes, at temperatures of 80–85 K (Cordier et al, ; Cordier & Liger‐Belair, ).…”

“…This led to the hypothesis that dissolved nitrogen may form bubbles upon exsolution (Malaska et al, ; Cordier et al, ; Cordier & Liger‐Belair, ). Recent modeling suggests that nitrogen bubbles are thermodynamically plausible at depth in Titan's methane–ethane lakes, at temperatures of 80–85 K (Cordier et al, ; Cordier & Liger‐Belair, ). Additionally, a laboratory study revealed that the enormous waste heat from plutonium decay (the energy source of a hypothetical Titan submarine) can trigger bubble formation (Richardson et al, ).…”

The floatability of aerosols and wave damping on Titan’s seas

“…Recently, a mechanism has been proposed to explain the occurrence of efficient RADAR reflectors at Ligeia Mare, one of the main Titan's seas 59,60 . These, so-called, "Magic Islands" could be produced by streams of nitrogen bubbles rising from the sea depths 39,50 . This scenario is not in conflict with the existence of a thin film at the sea surface: bubbles arriving at the surface could locally break the layer, and, in the same time, the RADAR-waves transparency of that slick could not prevent the observation of bubbles still in the volume of the sea liquid, as it is proposed.…”