PlanetS: Better to Dry a Rocky Planet before Use

[bystander]

Better to Dry a Rocky Planet before Use
National Center for Competence in Research (NCCR) | PlanetS | 2019 Feb 11

Earth’s solid surface and clement climate may be in part due to a massive star in the birth environment of the Sun. Without its radioactive elements injected into the early solar system, our home planet could be a hostile ocean world covered in global ice sheets. ...

Planetary systems born in dense and massive star-forming regions inherit substantial
amounts of Aluminium-26, which dries out their building blocks before accretion (left).
Planets formed in low-mass star-forming regions accrete many water-rich bodies and
emerge as ocean worlds (right). Credit: Thibaut Roger

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Water covers more than two thirds of the surface of the Earth, but in astronomical terms the inner terrestrial planets of our solar system appear very dry – fortunately, because too much of a good thing can do more harm than good. If the water content of a rocky planet is significantly greater than on Earth, the silicate mantle is covered by a deep, global ocean and an impenetrable layer of ice on the ocean floor. This prevents geochemical processes, such as the carbon cycle on Earth, which stabilize the climate and create surface conditions conducive to life as we know it. “So, it seems we were just extraordinarily lucky. Were we? Or are there systematic effects at play that distinguish solar system-like planetary systems from others?” Tim Lichtenberg asked when he started working on his doctoral thesis at the institutes of Astronomy and Geophysics at ETH Zurich.

Together with colleagues from the universities of Bayreuth, Bern, and Michigan, he developed computer models to simulate the formation of planets from their building blocks, the so-called planetesimals – rocky-icy bodies of probably dozens of kilometers in size. During the birth of a planetary system the planetesimals form in a disk of dust and gas around the young star and grow into planetary embryos. “Current thinking goes that Earth inherited most of its water from such partly water-rich planetesimals,” explains Lichtenberg, who now works as a postdoctoral fellow at the University of Oxford. “But if a terrestrial planet accretes lots of material from beyond the so-called snowline, it receives way too much water.” As it turns out, however, if these planetesimals are heated from the inside, part of the initial water ice content evaporates and escapes to space before it can be delivered to the planet itself. ...

A Water Budget Dichotomy of Rocky Protoplanets from ²⁶Al-Heating ~ Tim Lichtenberg et al

• Nature Astronomy (online 11 Feb 2019) DOI: 10.1038/s41550-018-0688-5
  arXiv.org > astro-ph > arXiv:1902.04026 > 11 Feb 2019

[BDanielMayfield]

This is very interesting. They show how vitality important RADIOACTIVE ALUMINUM is in the formation of Earth-like planets

Part of their paper's abstract:

Here we employ numerical models of planet formation, evolution, and interior structure, to show that a planet's bulk water fraction and radius are anti-correlated with initial ²⁶Al levels in the planetesimal-based accretion framework. The heat generated by this short-lived radionuclide rapidly dehydrates planetesimals prior to accretion onto larger protoplanets and yields a system-wide correlation of planet bulk abundances, which, for instance, can explain the lack of a clear orbital trend in the water budgets of the TRAPPIST-1 planets. Qualitatively, our models suggest two main scenarios of planetary systems' formation: high-²⁶Al systems, like our solar system, form small, water-depleted planets, whereas those devoid of ²⁶Al predominantly form ocean worlds, where the mean planet radii between both scenarios deviate by up to about 10%.

[neufer]

<<Aluminium-26, ²⁶Al, is a radioactive isotope of the chemical element aluminium, decaying by either of the modes beta-plus or electron capture, both resulting in the stable nuclide magnesium-26. The half-life of ²⁶Al is 717,000 years. This is far too short for the isotope to survive to the present, but a small amount of the nuclide is produced by collisions of argon atoms with cosmic ray protons.

Aluminium-26 also emits gamma rays and X-rays. The x-rays and Auger electrons are emitted by the excited atomic shell of the daughter ²⁶Mg after the electron capture which typically leaves a hole in one of the lower sub-shells. Because it is radioactive it should be stored behind at least 5 cm of lead and special tools should be used for transfer, use, and storage.

Aluminium-26 can be used to calculate the terrestrial age of meteorites. After the breakup of the meteorite parent body, it will be bombarded by cosmic rays, which will saturate it in aluminium-26. After falling to earth, ²⁶Al production ceases, which means that the amount of ²⁶Al in the sample can be used to calculate the date the meteorite fell to earth.

The distribution of Aluminium-26 in Milky Way

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The gamma emission at 1809 keV was the first observed gamma emission from the galactic center. The observation was made by the HEAO-3 satellite in 1984. The isotope is mainly produced in supernovas ejecting many radioactive nuclides in the interstellar medium. The isotope is believed to provide enough heat to small planetary bodies so as to differentiate their interiors, such as has been the case in the early history of the asteroids 1 Ceres and 4 Vesta. This isotope also features in hypotheses regarding the equatorial bulge of Saturn's moon Iapetus.

That ²⁶Al was present in the interstellar medium as a major gamma ray source was not explored until the development of the high-energy astronomical observatory program. The HEAO-3 spacecraft with cooled Ge detectors allowed the clear detection of 1.808 Mev gamma lines from the central part of the galaxy from a distributed of ²⁶Al source. This discovery was greatly expanded on by observations from the Compton Gamma Ray Observatory using the COMPTEL telescope in the galaxy. Subsequently, the ⁶⁰Fe lines (1.173 & 1.333 Mev) were also detected showing the relative rates of decays from ⁶⁰Fe to ²⁶Al to be ⁶⁰Fe/²⁶Al~0.11.>>