A synestia is a hypothesized rapidly spinning

doughnut A doughnut or donut () is a type of food 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 franc ...

-shaped mass of vaporized rock. It was named by Sarah T. Stewart-Mukhopadhyay, taken from Hestia, 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 In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed syst ...

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 (plural tori, colloquially donut or doughnut) is a surface of revolution generated by revolving a circle in three-dimensional space about an axis that is coplanar with the circle. If the axis of revolution does not tou ...

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, as well as a measurable physical property, that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept are used in diverse fields, from classical thermodynam ...

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 harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

and

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

within the

giant-impact hypothesis The giant-impact hypothesis, sometimes called the Big Splash, or the Theia Impact, suggests that the Moon formed from the ejecta of a collision between the proto-Earth and a Mars-sized planet, approximately 4.5 billion years ago, in the Hadean ...

. 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 forc ...

of the inner region accreted to form moonlets, and subsequently combined to form our 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