Silicon-rich dust particles have been detected in Enceladus's plume, and now an analysis of the conditions that formed them has boosted the moon's status as one of the best places in the solar system to look for life by providing the first evidence for hydrothermal vents outside the Earth.
Enceladus has been attracting attention ever since the Voyager mission identified it as the likely source of Saturn's E Ring. In 2005, Cassini provided evidence of geysers. Last year came confirmation that these geysers originate from a liquid ocean beneath the surface, kept warm by tidal flexing.
While liquid water is considered essential for life, more is needed. If the water is interacting with rocks from which it might draw minerals, we are one step closer to an environment conducive to life.
In Nature, an international team have announced evidence for “rock-water interactions,” which they presume are occurring at the bottom of the internal ocean, where deep waters meet the moon's core.
The evidence comes in the form of dust particles 4-16 nanometers across in the plume from Enceladus' geysers. The authors say, “We interpret these grains as nanometre-sized SiO2 (silica) particles, initially embedded in icy grains emitted from Enceladus’ subsurface waters.” They attempted to replicate the production of particles of this size and chemistry in conditions that might exist in the Enceladian ocean. The only ones that worked were alkaline environments with temperatures greater than 90°C (194°F).
The only plausible sites for such conditions is around hydrothermal vents at the bottom of the 10 kilometer deep ocean beneath Enceladus' south pole. Given the rich ecology around similar vents on Earth, and theories that these may have been the original sites of life on our planet, such vents have been proposed as possible locations for life in the outer solar system. Enceladus is thought to have a porus rocky core, which would provide plenty of opportunity for water to erode rocky materials away.
"It's very exciting that we can use these tiny grains of rock, spewed into space by geysers, to tell us about conditions on—and beneath—the ocean floor of an icy moon," says first author Dr. Sean Hsu of Colorado University, Boulder.
It is also significant that materials formed at the vents do not stay there, but instead reach the surface, presumably through a process of convection, and are flung into space. Their small size means this process can happen quite fast. Circulation between the ocean floor and space is thought to increase the chances of life existing, and would certainly help us find it.
The particles, however, were found to be surprisingly poor in metals, which may limit the opportunities for life, or at least the complexity it can achieve.
Enceladus' weak gravity cannot hold onto much of the material its geysers hurl into space, and both the dust particles and ice grains become part of Saturn's E ring. The dust particles erode over a period of tens of thousands of years and are swept into interplanetary space, requiring replacements to sustain the ring.