There could be oceans鈥� worth of water hiding deep within giant planets. Minerals with water bound up in their molecular structure can remain stable at extremely high pressures, so they could act as reservoirs for water even inside planets much larger than Earth.
Earth has a reservoir of water like this. It is bound up in a mineral called ringwoodite, deep underground in our planet鈥檚 mantle, the layer between the surface and the core. But at higher pressures than those in the mantle, we are unsure how these sorts of water-bearing, or hydrous, minerals behave.
Masayuki Nishi at Ehime University in Japan and his colleagues explored this problem using aluminium hydroxide, a mineral composed of aluminium, oxygen and hydrogen. 鈥淲e succeeded in observing the hydrous mineral under pressures far higher than those in previous studies,鈥� says Nishi. He says they reached pressures more than double those used to observe hydrous minerals in earlier studies.
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The researchers did this by squeezing samples of the powdered mineral between two small diamonds and heating them with laser beams. This was done to mimic the hot and pressurised environments at the centres of large planets. Then the crystal structure of the squeezed and heated samples was examined using X-rays. Armed with that structure, the researchers calculated the stability of the mineral at those high temperatures and pressures.
They found that the aluminium hydroxide shifted to a new phase with a sturdier structure at high pressures. The water bound up in the mineral remained there even under incredibly high pressures and at temperatures well over 2000掳C.
鈥淚 consider that there are many hydrous minerals which can be stabilised under much higher pressures, although we do not know yet due to the difficulty of the validation,鈥� says Nishi.
These minerals could act as an underground reservoir for surface water on large terrestrial exoplanets called super-Earths and help them to maintain the liquid oceans necessary for life as we know it.
Icarus
