Titan: Lakes created by explosions?

The sudden evaporation of nitrogen could explain unusual lakes on Saturn's moon

Titan lakes with steep banks and ridges are puzzles - have they been caused by gas explosions? © NASA / JPL-Caltech
Read out

Explosive Origin: The enigmatic lakes on Titan's Saturn Moon could have been created entirely differently than thought - through explosive gas eruptions. Similar to the earthly maaren, the sudden evaporation of a liquid provided the necessary explosion pressure. In the case of Titan, however, it was not water but nitrogen that experienced this explosive phase change, as researchers report in the journal Nature Geoscience.

Saturn's moon Titan is the only celestial body in the solar system besides the earth, which has lakes and rivers on its surface. They are not filled with water, but predominantly with liquid methane and ethane hydrocarbons, which occur on the Saturn moon as ice, liquid and gas. In this way landscapes and processes on the Titan, in spite of this unusual chemistry, are strikingly similar to those of the earth.

A whole side of the Titan-Nordpolargebiets is littered by the small lakes with marginal ridges. © NASA / JPL-Caltech / Space Science Institute

Karst burglaries - or not?

But a landscape form gives riddles to researchers: hundreds of lakes as stamped in the northern polar area. Its steep banks, deep basins, and high ridges are difficult to explain with common geological processes. In terms of their shape and depth, these lakes are similar to earthly karst lakes and sinkholes. But although there seems to be some kind of karst on Titan, it's not the taller ones that are not fitting, as Guiseppe Mitri of the University of Annunzio in Italy and his colleagues explain.

The problem: The nearly crater-like uplifted edge of the shores of these lakes sometimes rises more than a hundred meters above the sea level of Titan. "Dolines are formed by collapse, solution or subsidence of the terrain these processes do not produce elevated edges, " the researchers say. But then what could have created these lakes surrounded by marginal forests?

Striking similarity to maaren

In search of an explanation, Mitri and his team come upon an earthly landscape that is surprisingly similar to the Titanic lakes: Maare. These roundish lakes, which occur mainly in the Eifel, were created by the explosive reaction of hot magma with groundwater. The contact with the glowing rock caused the water to evaporate abruptly and the resulting explosion blew the Maars into the landscape. display

"Many Maars are surrounded by ejected deposits, called tephra rings, " say Mitri and his team. Similar to the Titan lakes, these tephra rings form a high-edged margin around the sunken maars. Could the warlords around the Titan lakes also be such ejecta rings? Further analysis of NASA spacecraft Cassini data confirms this: At least some of these lakes not only have the rippled shores, but also seem to be surrounded by a junk spit, researchers said discovered.

This means that even the "stamped" lakes of Titan could have been caused by an explosion.

Nitrogen explosion as a cause?

However, there is no water or hot magma on Saturn's moon so what could have exploded there? In search of an explanation, Mitri and his team looked more closely at the chemical composition of the liquid in the Titan Lakes. They found that in addition to the liquid methane, a small proportion of liquid nitrogen is contained. There is also plenty of nitrogen in the Titan atmosphere.

It is also known that the Saturn moon has gone through much shorter periods - phases in which it was cold enough to rain gaseous nitrogen as rain on the Titan landscape. "On Titan's surface, this could have caused lakes of liquid nitrogen and also reservoirs of liquid nitrogen below the surface, " the researchers said. However, as it gradually got warmer, this nitrogen made a phase transition and turned into gas.

This is exactly where the researchers' scenario comes in: Because nitrogen gas occupies more space than liquid nitrogen, the overburden in the subterranean reservoirs is overpressurized, as they explain. When a critical threshold was reached, this pressure blew off the ceiling of the reservoir an explosion occurred.

Enough pressure for a maar

"We interpret the morphological features of these Titan lakes with raised edges as evidence of their origin in an explosive eruption, " say Mitri and his colleagues. "They were formed when liquid nitrogen underwent a phase transition near the subsurface." According to their calculations, the sudden pressure increase from such a change to gaseous nitrogen would be strong enough to have formed the titanium lakes.

The necessary conditions - first a cold phase, then a warming - could even have happened several times during the history of Saturn's moon, according to the researchers. Should their scenario be confirmed, then the "stamped" titanium lakes would be the evidence for such nitrogen-dominated cold periods. (Nature Geoscience, 2019; doi: 10.1038 / s41561-019-0429-0)

Source: NASA JPL

- Nadja Podbregar