A synestia is a hypothesized rapidly spinning

doughnut A doughnut or donut () is a type of pastry made from leavened fried dough. It is popular in many countries and is prepared in various forms as a sweet snack that can be homemade or purchased in bakeries, supermarkets, food stalls, and fran ...

-shaped mass of vaporized rock. The term was coined in 2017 by

Sarah T. Stewart-Mukhopadhyay Sarah T. Stewart-Mukhopadhyay is an American Planetary science, planetary scientist known for studying Nebular hypothesis, planet formation, planetary geology, and materials science. She is a professor at the University of California, Davis in th ...

, taken from

Hestia In ancient Greek religion and mythology, Hestia (; ) is the virgin goddess of the hearth and the home. In myth, she is the firstborn child of the Titans Cronus and Rhea, and one of the Twelve Olympians. In Greek mythology, newborn Hestia, alo ...

, goddess of the hearth, combined with ''syn-'' meaning together. In computer simulations of giant impacts of rotating objects, a synestia can form if the total

angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...

is greater than the co-rotational limit. Beyond the co-rotational limit, the velocity at the equator of a body would exceed the orbital velocity. In the case of a ''synestia'', the result is an inner region rotating at a single rate with a loosely connected

torus In geometry, a torus (: tori or toruses) is a surface of revolution generated by revolving a circle in three-dimensional space one full revolution about an axis that is coplanarity, coplanar with the circle. The main types of toruses inclu ...

orbiting beyond it. Synestias also have differences in the mantles, both thermally and in their composition, from previous terrestrial evolution models due partially to a lower interior pressure.

Composition

A synestia is composed of three primary components: the innermost area called the corotating region, a middle area called the transition region, and the area farthest out, known as the disk-like region. The corotating region rotates as a solid body. It is characterized by hot vapor and high

entropy Entropy is a scientific concept, most commonly associated with states of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynamics, where it was first recognized, to the micros ...

levels, as well as higher angular velocities. The transition region is generally a continuous change between the corotating region and ring-like region. Here, in most simulations, the angular velocity and temperature follow a smooth gradient, both decreasing with radius. The temperature gradient is created by the mixture of hot vapor from the inner regions with colder condensed material from farther out. Given time this equilibrates into solely a vapor. This transitions into the disk-like region whose appearance can vary dramatically with different initial conditions for angular momentum, mass, and entropy.

Giant-impact hypothesis

According to studies, synestia was an early-stage process for the formation of the

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

within the

giant-impact hypothesis The giant-impact hypothesis, sometimes called the Theia Impact, is an astrogeology hypothesis for the formation of the Moon first proposed in 1946 by Canadian geologist Reginald Daly. The hypothesis suggests that the Early Earth collided wi ...

. In that model, a synestia formed following a collision with an object of high energy and high angular momentum. The synestia's surface temperatures are constrained by the boiling point of rock, around ). As the resulting synestia cooled by radiating heat to space, magma droplets formed in its outer layers and then rained inward over a period of tens of years, causing the synestia to contract. Mass remaining outside the

Roche limit In celestial mechanics, the Roche limit, also called Roche radius, is the distance from a celestial body within which a second celestial body, held together only by its own force of gravity, will disintegrate because the first body's tidal force ...

of the inner region accreted to form moonlets, and subsequently combined to form the Moon. The Earth re-formed later, once the synestia had cooled sufficiently to fall within the co-rotational limit. By this model, the Moon's having formed within a cloud of vapor that originated from the Earth is why its isotopic ratios are similar to those of the Earth. The later formation of the Earth (after the synestia cooled) accounts for its having accreted more volatile elements than the Moon.

Notes and references

External links

* {{cite AV media , title=TED talk about synestia , people=Planetary scientist Dr. Sarah T. Stewart (presenter) , date=February 2019 , medium=video , quote=duration 11 minutes , publisher= Technology, Entertainment, Design , place=TED Salon, U.S. Air Force, 2019 , url=https://www.ted.com/talks/sarah_t_stewart_where_did_the_moon_come_from_a_new_theory , access-date=12 July 2019 Rotation Types of planet Hypothetical bodies of the Solar System Hypothetical astronomical objects