Rubidium overturned the hypothesis of a comet-asteroid origin of terrestrial and lunar water

(ORDO NEWS) — In addition, the new work shows that the Moon did not lose water and other light elements during its formation. This adds doubt to Thei’s hypothesis.

The question of the origin of water on Earth and the Moon has long and seriously worried scientists. The fact is that the surface of both of these celestial bodies in the first tens (or even hundreds) of millions of years of their existence could be very hot, and to a level at which water should have evaporated.

In the case of the Earth, the water would then have entered the upper atmosphere, where ultraviolet light would have split it into hydrogen and oxygen.

The first, as a very light gas, would leave the planet, leaving it waterless. As for the Moon, with its six times weaker gravity, water vapor itself could leave the Earth’s satellite, even before splitting into hydrogen and oxygen.

Nevertheless, the surface of the Earth is clearly rich in water, and even on the Moon, although it is less, it is not at all as small as it was thought in the 20th century. Only the discovered reserves of polar ice are one hundred billion tons.

In addition, water is contained in lunar basalts, that is, in the internal rocks of the satellite. A natural question arises: where did it come from? It was often answered as follows: at first the Earth and the Moon were indeed waterless, but then asteroids and comets rich in water brought it to these celestial bodies, making earthly life possible.

Rubidium overturned the hypothesis of a comet asteroid origin of terrestrial and lunar water 2Before that, the hypothesis about the delivery of water with impacts of small celestial bodies on the lunar surface was popular / © Wikimedia Commons

To test such ideas, American and German researchers decided to track how the amount of other relatively volatile elements on Earth and the Moon changed throughout their history.

To do this, in their work at PNAS , they turned to rubidium, an alkali metal with a boiling point of 688 °C. This is an element of medium volatility, but over time (half-life – 49.23 billion years), the nucleus of its atom undergoes decay, as a result of which strontium-87 is formed – an almost completely non-volatile element.

In addition, the amount of rubidium in a particular rock directly correlates with the total indicator of volatile elements in it.

Comparing the amount of strontium-87 in various ancient rocks, you can determine how much rubidium was in them – and how much they have lost in their history. For analysis, the most ancient samples of the rocks of the Earth and the Moon were selected, including those older than four billion years.

As a result, it turned out that the amount of rubidium-87 in the rocks of both the Moon and the Earth was relatively small from the very beginning – that is, these bodies from the moment of formation were relatively poor in volatile elements and compounds of all types, including water.

Especially important: the rubidium-strontium ratio shows that throughout history, both the Earth and its satellite did not receive large amounts of volatile elements, including water.

In theory, it could be assumed that the water came with some falling celestial bodies, where there was ice, but there was no rubidium. However, the available samples of meteorite-asteroid materials and data on the composition of comets do not show among them bodies rich in water but poor in rubidium.

In other words, neither the Earth nor the Moon could get large amounts of their water from either comets or asteroids: almost all the water in the Earth-Moon system has been there since their formation.

The authors of the work proceed from the existence of Theia – a certain planet, whose impact on the Earth formed the Moon.

This hypothesis has long been justifiably criticized by a number of Russian and Israeli researchers, but in the United States, where the Teija hypothesis originated, it is still popular. The new article concludes that Theia, in composition, should have been close to the Earth and the Moon in terms of poverty in volatile elements.

It should be noted that this work organically complements a recent study by Chinese scientists who managed to show that the water on the surface of our planet appeared from the now “extinct” compound that held water in the core of the ancient Earth, but then gradually disintegrated, after which it contained water rose up with volcanic gases and made the main contribution to the hydrosphere.

Such an approach organically explains the origin of terrestrial water without a massive comet-asteroid delivery of extraterrestrial water ice, the impossibility of which has now been shown by researchers from the USA and Germany.

Also, the hypothesis creates new difficulties for Thei’s hypothesis. The fact is that if lunar water, as the authors showed, was mainly part of the satellite from its formation, then it would be extremely difficult for the earth satellite to form as a result of the megacollision of the Earth and Theia.

Such a collision would have tremendous energy, inevitably heating up all the material knocked out by Theia from the Earth (and the Earth from Theia).

However, the red-hot “drops” of rocks, from which the Moon was later formed, would inevitably lose the water that was in them. Nevertheless, there is water in the lunar rocks. It is almost impossible to explain this in the scenario of a mega-impact with Theia.

About how the Moon actually arose and why this process did without Theia and the “dehydration” of the rocks of Selene.


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