The possibility of life, the moon and a satellite that is just right came to the earth at the same time, says S.Ananthanarayanan.

It is the presence of liquid water, apart from many other conditions being just right, that enabled life to arise on the earth. The presence of the moon, which is the largest major satellite, as a proportion of the mother planet, in the Solar system, made for stability of the axis of the earth’s rotation, in its path around the sun, and this has ensured regular seasonal cycles. Water, in quantity, also stabilized the atmosphere, regulated the temperature, brought winds and rain and created the conditions for life to flourish

Gerrit Budde, Christoph Burkhardt and Thorsten Kleine, from the Institute for Planetology, University of Munster, Germany, write in the journal, Nature Astronomy, that they have identified the extraterrestrial sources from which the earth got its water. And the event that brought most of the water to the earth, they say, may be the same as what gave rise to the moon.

The origin of the planets is considered to be the clumping together of nebulous matter that surrounded the sun after its formation. A pattern in how components of this matter were distributed, the authors say, could be an indicator of the course the earth followed in its formation. They hence rely on markers that identify the inner part, closer to the sun, and the outer, more distant part, to see where the matter that built up the earth came from. There are “two genetically distinct reservoirs of meteoric matter,” they say, and if we knew which reservoir the matter on the earth came from, this would help us determine which part was the source for the formation of earth.

The markers they use are the isotopes, or different forms, of the element, molybdenum, whose atoms are chemically equivalent although there differences in the mass of their nuclei. While the clouds from which stars formed were initially mainly the simplest, hydrogen atoms, more complex atoms were formed, by the fusion of nuclei of simpler atoms, in the exceedingly high energy at the time of formation of stars. And depending on different conditions, different forms of the same atoms got formed.

The paper explains that the band in the hotter region nearer the sun had more of non-carbonaceous (NC) meteorites, the kind that is drier and rich in metals. The band further away, beyond the orbit of Jupiter, had carbonaceous chondrite (CC) meteorites, the kind that formed in cooler conditions and are rich in volatiles and water. As the two kinds of meteorites occupy different regions, the radiation that they were exposed was not the same. The matter in the outer reaches, the CC, is richer in molybdenum nuclei formed by rapid neutron capture (r-process) and proton capture (p-process). Nuclei formed by slow neutron capture (s-process), however, are found in both regions.

slow neutron capture (s-process), however, are found in both regions. As molybdenum is an element that combines readily with iron, a siderophile, or iron-loving element, all the molybdenum on the earth in the early stages of its formation got drawn in with the heavy, iron content, to the earth’s core. Most of the molybdenum that is found in the mantle, which is the region that comes after the core, closer to the surface, must hence have come during later stages of accretion of material to form the earth, the paper says. Measurements of the ratios of the isotopes of molybdenum in the mantle and on the surface of the earth is found on the fall between the levels for the CC and the NC meteorites, which points to the origin of the material of the earth being the outer Solar System. And also that this material come to the earth in the later stages of formation. This accretion of CC material, from the outer Solar System was hence the probable source of the quantity of water on the earth.

Mars size impact

Apart from having the largest amount of water among the planets of the Solar system, the earth is believed to have suffered a massive impact, by an object the size of Mars, or half the size of the earth, some 4.5 billion years ago. This is the object, called Theia, which is believed to have struck Gaia, the early earth, and set free the mass that we see as the moon. The current understanding has been that Theia was a planet in the inner Solar System, whose path was perturbed by the gravitational influence of Jupiter and perhaps Venus, which set Theia on a collision course with the Earth. But considering the quantity of water on the earth, and the level of CC material in the mantle, the paper in Nature Astronomy raises a question about Theia being from the inner Solar System.

Most of the molybdenum in the mantle, called the Bulk Silicate Earth, the papers says, has come during the last 10-20% of the formation of the earth. This content, the paper says, should hence have been strongly affected by this last major impact. The paper hence considers the measured level of CC material, and an assumed level in the mantle just before the impact, to make an estimate of what the size of the object and what its composition may have been.

The different levels of CC content in the impactor, and the possible level of integration of the impactor’s material into the core, are found to indicate that the impactor could not have been of NC composition but must have had mixed or pure CC composition. The immediate implication is that the impactor could not have originated in the inner Solar System, as has been believed, but must have come the further reaches, with the higher CC content.

One factor that does not seem to fit in is that the moon itself has very little CC content. This implies, the paper says, that the moon was formed from the material of the earth before then impact. The paper cites work that suggest that the impact was even more energetic than believed, and the moon formed out of vaporized earth material that was launched as an orbiting disk, in those violent days 4.5 billion years ago.

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