Synestia
Hypothesized doughnut-shaped mass of vaporized rock from planetary collisions.
A synestia is a theoretical structure that could result from a planetary collision: a rapidly spinning, doughnut-shaped mass of vaporized rock. The name, introduced in 2017 by Sarah T. Stewart-Mukhopadhyay, combines *syn-* (together) with Hestia, the goddess of the hearth. In computer simulations of giant impacts involving rotating bodies, a synestia may arise when the total angular momentum exceeds the co-rotational limit—the point at which a body’s equatorial velocity would surpass orbital velocity.
Inside a synestia, an inner region rotates uniformly, while a loosely connected torus orbits beyond it. Compared to earlier models of terrestrial evolution, synestias show differences in mantle composition and thermal properties, partly due to lower internal pressure.
The structure has three main parts. The innermost is the corotating region, which spins as a solid body and consists of hot, high-entropy vapor with higher angular velocities. Next is the transition region, where angular velocity and temperature gradually decrease with radius. This gradient results from hot vapor mixing with colder condensed material from farther out; over time, the mixture becomes entirely vapor. The outermost disk-like region varies in appearance depending on initial conditions of angular momentum, mass, and entropy.
Within the giant-impact hypothesis, a synestia is considered an early stage in the formation of Earth and the Moon. In this model, a high-energy, high-angular-momentum collision produced a synestia, whose surface temperature was limited by rock’s boiling point—around 2,300 K (about 2,000 °C; 3,700 °F). As the synestia cooled by radiating heat into space, magma droplets formed in its outer layers and rained inward over tens of years, causing the structure to contract. Material remaining outside the inner region’s Roche limit coalesced into moonlets, which later combined to form the Moon. Earth re-formed later, once the synestia cooled enough to fall within the co-rotational limit. This model explains the Moon’s similar isotopic ratios to Earth’s by its formation within a vapor cloud originating from Earth, and Earth’s later accretion of more volatile elements accounts for its greater abundance of them.
- field
- Planetary science
- known_for
- Hypothesized structure for planetary collision debris
- coined_by
- Sarah T. Stewart-Mukhopadhyay
- year_coined
- 2017
- composition
- Corotating region, transition region, disk-like region
Lore & Background
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, characterized by hot vapor and high entropy levels, as well as higher angular velocities. The transition region is generally a continuous change between the corotating region and ring-like region, where 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. This transitions into the disk-like region whose appearance can vary dramatically with different initial conditions for angular momentum, mass, and entropy.
Reader's Guide
According to studies, synestia was an early-stage process for the formation of the Earth and Moon within the giant-impact hypothesis. 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 2,300 K. 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 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 accounts for its having accreted more volatile elements than the Moon.
Did You Know?
- The term synestia combines Hestia, goddess of the hearth, with syn- meaning together.
- A synestia can form if the total angular momentum is greater than the co-rotational limit.
- The synestia's surface temperatures are constrained by the boiling point of rock, around 2,300 K.
- In the giant-impact hypothesis, the Moon formed from mass outside the Roche limit of the inner region.
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