Pointing the James Webb Space Telescope at Neptune<\/p>\n
Triton’s odd retrograde orbit has led researchers in the past to theorise that the moon originally came from the Kuiper Belt, a ring of asteroids beyond Neptune.<\/p>\n
As Triton wandered in towards Neptune it was “captured” by the planet’s gravity.<\/p>\n
In the process, the cosmic gate-crasher may have destroyed Neptune’s original moon system, if it had one, according to Ryleigh Davis, a planetary scientist at the University of California San Diego, and lead author on the paper.<\/p>\n
“We don’t actually know whether Neptune ever had such a system, and the small inner moons and rings are one of the best places to look for clues,” Dr Davis said.<\/p>\n
“The problem is that these moons are tiny, faint, and so close to Neptune that the planet’s light overwhelms them.”<\/p>\n
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Neptune is named after the Roman god of the sea, and its biggest moon, Triton, is named after Neptune’s Greek equivalent, while the smaller moons are lesser Greek sea gods. (Supplied: NASA\/\u00a0Space Telescope Science Institute)<\/p>\n
To look for hints of Neptune’s original moon system, the researchers used the James Webb Space Telescope (JWST).<\/p>\n
They analysed light signals coming from three of Neptune’s inner moons, Larissa, Galatea, and Proteus, as well as its inner rings, to see what minerals they contained.<\/p>\n
Dr Davis said the JWST was sensitive enough to look closely at these moons for the first time.<\/p>\n
Moon minerals hint at a destructive origin<\/p>\n
The researchers found signals from a type of clay, called a phyllosilicate, on two of the moons (Larissa and Galatea), as well as in the rings.<\/p>\n
“That was not on our list of things we expected to find,” Dr Davis said.<\/p>\n
That’s because these clay minerals can’t form in cold environments: they need liquid water.<\/p>\n
“We spent a lot of time convincing ourselves it was real before we let ourselves get excited,” Dr Davis said.<\/p>\n
The only place around Neptune that can get warm enough for liquid water is the centre of a large object.<\/p>\n
“The moons we’re looking at, Larissa and Galatea, are far too small and cold for that chemistry to have ever occurred in place,” Dr Davis said.<\/p>\n
“So the clay had to come from somewhere else: the deep interior of a much larger ancient world.<\/p>\n
“The most natural explanation is that Neptune once had a system of large moons, similar to what Uranus has today, that were shattered when Triton was captured billions of years ago.”<\/p>\n
Neptune’s closest neighbour, Uranus, has 28 known moons, five of which are large like Triton.<\/p>\n
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Neptune’s brightness makes it difficult to study its inner moons and rings. (Supplied: NASA\/\u00a0Space Telescope Science Institute)<\/p>\n
The researchers believe Neptune’s remaining rings and moons are mostly the innards of those ancient moons.<\/p>\n
Helen Maynard-Casely, a planetary scientist at the Australian Nuclear Science and Technology Organisation, who wasn’t involved in the research, said the findings supported a generally accepted theory for Triton’s origin.<\/p>\n
“Triton turned up and caused havoc,” Dr Maynard-Casely said.<\/p>\n
“This gives another piece to the story of how Neptune’s moons came to be how they are.”<\/p>\n
Mysterious mineral spotted<\/p>\n
The team also found signals from a mineral they couldn’t identify on all three of the moons and the rings.<\/p>\n
Dr Maynard-Casely said this was exciting, because it could potentially be a previously unknown material.<\/p>\n