December 10, 2018
Day 18: Onboard communicator, Bethany Green
Every good voyage has its mysteries; here are two particularly colourful ones.
Mystery 1. The elusive green flash
On Investigator, astronomic and atmospheric observations are a frequent topic of conversation: second only to whale sightings. Recent conversations have centred around the elusive green flash.
The green flash refers to a brief moment at either sunset or sunrise – but more commonly sunset – when the light emitted from the sun suddenly appears distinctly green. Observation requires clear skies and an unobstructed view of the horizon.
The atmospheric phenomenon, prominent in sea-faring folklore, is caused by the way the light from the sun is scattered and refracted through different layers of the Earth’s atmosphere, separating out into different colours. As the sun dips below the horizon the colours of the spectrum disappear gradually, longest wavelength to shortest, with green towards the shorter end of the spectrum.
After several weeks of overcast skies and rough seas, the weather had significantly improved and people became more optimistic and enthusiastic about a possible sighting. On Sunday afternoon, the conditions finally aligned. The waters were calm, the temperature was pleasant, and, despite a few clouds, the sun was setting over a clear horizon.
At about 8:20 pm, people started emerging on the stern, vying for an unobstructed view. We stared as the sun descended slowly below the horizon, determined to discover for ourselves whether this flash was legend, or reality. And then, just as we thought the sun had set and we had missed it, there it was: an almost unnatural, yet undeniably green, glow. The sight brought loud cheers of excitement and high fives, as the green flash delighted even the most skeptical among us. It was a truly remarkable experience.
Mystery 2. The (most-probably) misidentified mineral
Twelve years ago, scientists aboard Investigator discovered a bright orang, needle-like, crystalline structure on top of a basaltic seamount, approximately 740 metres below the sea surface, at the boundary of the Huon Marine Park. Images from the deep-tow camera showed the mineral in incredible detail; the structure and colour proving a convincing likeness to the Tasmania’s state mineral, crocoite.
In the years following, speculation circulated regarding a possible deep-sea crocoite discovery: the first of its kind. Crocoite, a relatively rare mineral is usually found in a number of Tasmanian mines. Some of you may have seen the shining specimens at the Zeehan West Coast Heritage Centre.
So when the mystery mineral was spotted for the second time on Mongrel seamount during a deep camera tow on this survey, biologists onboard sought advice from the mineralogists. They consulted the UTAS School of Natural Sciences, the Metallic Minerals and Geochemistry Mineral Resources Tasmania, and Geoscience Australia. The feedback was that the mystery mineral is unlikely to be crocoite due to the setting. Crocoite contains the elements oxygen chromium and lead, and these elements need to be in abundance under specialised circumstances for crocoite to form. Basaltic seamounts such as Mongrel typically contain prohibitively small quantities. Rather, it is more likely to be the ‘lookalike’ zeolite, Thomsonite.
Thomsonite can form in a variety of ways and places, including on basaltic seamounts in the Southern Ocean. It contains the elements sodium, calcium, aluminium, and silicon, all of which are plentiful in basaltic seamounts, and requires water to form: an abundant substance under the sea. Until a sample is collected from the seamount, however, which is not within the science objectives of the seamount survey, we can’t be absolutely certain. Maybe we can keep that mystery alive?