The watery plumes observed spewing from Saturn鈥檚 moon Enceladus owe their existence to a flash of radioactivity that melted the icy moon right after it formed, a new study says.
快猫短视频s were astonished when the Cassini spacecraft revealed plumes of water vapour jetting from Saturn鈥檚 moon Enceladus in 2005 (see Giant water plume spews from Saturn鈥檚 moon). The observation suggested liquid water lay beneath the surface, providing conditions favourable for life.
But the source of the heat required to power the plumes has not been clear. Previously, researchers suggested it was a combination of two factors 鈥� the decay of radioactive isotopes in the moon鈥檚 rocky core, and friction as the moon is stretched and squeezed by Saturn鈥檚 gravity 鈥� a mechanism called tidal heating.
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But those two processes alone could not explain how Enceladus developed a rocky core to begin with, given that it should have started as a uniform mixture of rock and ice.
A new study proposes that a brief period of intense radioactive decay melted the moon鈥檚 ice, allowing rocks to fall to its centre and form a molten core. Julie Castillo-Rogez of NASA鈥檚 Jet Propulsion Laboratory (JPL) in Pasadena, California, US, led a team that studied this idea.
Precocious moon
Enceladus could have accumulated a high proportion of the short-lived isotopes aluminium-26 and iron-60 by sweeping up tiny rocky objects rich in the two isotopes. Called calcium aluminium-rich inclusions (CAIs), these centimetre-sized objects are found embedded in some meteorites. They were among the first solids to form in the solar system about 4.6 billion years ago, suggesting Enceladus also coalesced early on.
For a few million years after Enceladus鈥檚 formation, these isotopes would decay rapidly and heat the moon up. 鈥淭here is a full melting of ice inside the satellite,鈥� says Castillo-Rogez. 鈥淎s a result of this, the rock is going to separate from the ice and sink to the centre.鈥�
At the moon鈥檚 centre, the rock would form a molten core, then tidal heating of this core would keep it warm until today. Heat transported from the core, which should still contain some molten pockets, would provide enough heat to power the plumes, the researchers say.
Short half-life
The overall picture is that the short-lived isotopes 鈥� aluminium-26 has a half-life of just 700,000 years, for example 鈥� are ultimately responsible for the plumes. 鈥淭he decay of short-lived radioisotopes doesn鈥檛 do the job itself, but it can jumpstart it, so that other process can take over,鈥� says Dennis Matson of JPL, a member of the study team.
This scenario could also explain the observation of molecular nitrogen in Enceladus鈥檚 plumes by Cassini, Matson says.
The nitrogen could come from the breakdown of ammonia where the moon鈥檚 core meets the water and ammonia above it. For this reaction to occur, the temperature needs to be at least 300掳 Celsius. 鈥淚t eventually makes its way to the surface and it escapes to space where we observe it,鈥� he says.
But David Stevenson of the Caltech in Pasadena, California, US, who is not a member of the team, is doubtful that short-lived isotopes are the key to explaining Enceladus鈥檚 plumes, since the timing of their activity and decay might not have been right.
鈥淭here is no good reason to suppose that aluminium-26 was even alive when Enceladus 鈥� or any satellite 鈥� formed,鈥� he told 快猫短视频, though he acknowledges that the possibility is not completely ruled out.
He also thinks the various substances, such as molecular nitrogen, that the team suggests come from very high temperature reactions could instead have simply been part of the original material that formed Enceladus. 鈥淭hat doesn鈥檛 mean that the proposed model is wrong, merely that it is not mandatory,鈥� he says.
The results were announced on Monday at the Lunar and Planetary Science Conference in Houston, Texas, US.
Cassini: Mission to Saturn 鈥� Learn more in our continually updated .
