The geysers of Enceladus, this icy moon of Saturn, continue to reveal their secrets long after the Cassini probe's passage. Twenty years after the first samples were collected, a meticulous analysis of archive data has led to a remarkable discovery.
Deep within the data from the Cosmic Dust Analyzer, the German instrument aboard Cassini, scientists have identified complex organic molecules that had escaped initial analyses. These carbon compounds, essential to biological processes, are found in ice grains ejected directly from the plumes before they reach Saturn's E ring. The high impact velocity of the particles on the detector, reaching 11 miles per second (18 kilometers per second), revealed chemical signatures that slower collisions had previously masked.
Artist's representation of plumes emanating from the tiger stripe fractures on Enceladus.
Credit: NASA/JPL-Caltech/Space Science Institute/Lunar and Planetary Institute The oceanic origin of these molecules represents a decisive argument for researchers. Unlike E ring particles that undergo the influence of radiation from Saturn's magnetosphere, the grains analyzed directly in the plumes come from Enceladus's subsurface ocean without spatial alteration. This discovery dispels doubts about the formation of these compounds by irradiation and confirms their origin from the moon's depths. Nozair Khawaja's team was thus able to establish a direct link between the chemistry of the hidden ocean and the samples collected in space.
Among the newly identified molecules are aliphatic compounds, cyclic esters, ethers, and substances containing nitrogen and oxygen. On Earth, these molecules participate in chemical reactions that lead to the formation of amino acids and other fundamental building blocks of life. Their presence in Enceladus's ocean suggests the existence of sophisticated chemical processes capable of generating significant molecular diversity in this extraterrestrial environment.
However, some recent research adds important nuances. A study led by Grace Richards of the Istituto Nazionale di Astrofisica e Planetologia Spaziale indicates that radiation could also create organic molecules on Enceladus's surface, particularly at the tiger stripe fractures. This possibility introduces additional complexity in interpreting the data, as it would become difficult to distinguish the exact origin of compounds detected in the plumes.
Saturn's E ring with Enceladus (black dot) and light reflected by ice grains in a plume.
Credit: NASA/JPL/Space Science Institute
The definitive solution to this scientific enigma might come from a future space mission. The European Space Agency is currently considering a project combining an orbiter and a lander that could reach Enceladus around 2054. Only a direct analysis of fresh ice on the moon's surface would unambiguously confirm the nature and origin of the organic chemistry detected remotely by Cassini.
Enceladus's Subsurface Ocean
Beneath Enceladus's icy crust lies a vast ocean of liquid water that maintains its state thanks to energy generated by tidal forces exerted by Saturn. These gravitational interactions cause internal deformations and friction that warm the moon's interior, preventing the water from freezing completely.
The presence of this ocean was deduced from measurements of Enceladus's libration, which reveal that the outer icy layer is not firmly attached to the rocky core. Mathematical models suggest that the ice layer thickness varies between 12 and 16 miles (20-25 kilometers), while the ocean depth could reach several tens of kilometers.
The chemical composition of this ocean remains poorly understood, but Cassini data indicate the presence of dissolved salts, notably sodium chloride, similar to the composition of Earth's oceans. The detection of nanometric silicas also suggests hydrothermal activity at the ocean floor, where hot water interacts with the rocky core.
Water circulation in this environment could create conditions favorable to the development of complex chemical processes, with temperature and composition variations creating micro-environments conducive to organic reactions.
Organic Molecules in Space
Organic molecules, defined by the presence of carbon atoms, are much more widespread in the Universe than initially thought. They form in interstellar molecular clouds, around nascent stars, and in protoplanetary disks, thanks to chemical reactions occurring on cosmic dust grains.
In the Solar System, these compounds have been detected in comets, asteroids, and some planetary atmospheres. Carbonaceous meteorites, like the Murchison meteorite, contain more than 70 different amino acids, demonstrating that prebiotic chemistry is a universal process not limited to Earth.
The complexity of organic molecules varies considerably, ranging from simple hydrocarbons like methane to polycyclic aromatic structures and carboxylic acids. The detection of molecules as complex as those found on Enceladus indicates that elaborate chemical processes can occur in extraterrestrial environments.
The persistence of these molecules in space depends on their protection from ultraviolet and cosmic radiation. Ice and dust grains act as natural shields, preserving fragile organic compounds and allowing their accumulation over long periods.