# Planets/Histories

The history of planets begins with the origin of the idea of a wanderer in the sky.
The word "planet" comes from the Greek planetes, a wanderer.

## Wanderers

A wanderer may move in a leisurely, casual, or aimless way.

## Theoretical planets

Def. one who wanders, who travels aimlessly is called a wanderer.
Def. one who moves slowly away from a fixed point or place is called a wanderer.
Def. one who moves or travels slowly through or over a place or area is called a wanderer.

## Meteorites

"Since the distribution of radioactive materials is uncertain, size estimates for bodies with a radius exceeding 100 km can only be narrowed to a range of values bounded by the uniform and the fractionated case. This range of body sizes is still compatible with an origin of iron and stony-iron meteorites in asteroidal bodies."
"The present calculations are not consistent with the development of iron meteorites by surface heating of parent bodies."
The "variable cooling rates for iron meteorite groups IIIa and IVa are explained by a model where small iron bodies are distributed throughout a large region of the parent body from the deep interior out to a fractional radius of 0.90 to 0.96. It is proposed that such a distribution of iron meteorites could be produced by a zone melting process."

## Craters

The most intense cometary "showers" may have serious implications [...] for the impact history of planets and satellite systems."

## Terrestrial planets

Hypotheses:
* "the terrestrial planets were formed by agglomeration of solid particles in the gas-dust solar nebula."
* "The radioactivity of the terrestrial planets is usually [...] equal to that of chondritic meteorites."
* "The potassium content for chondrites is essentially constant, viz. 8 x 10$^{-4}$ g/g, whereas the uranium content varies from 1 x 10$^{-8}$ g/g to 3 x 10$^{-8}$ g/g for different groups of chondrites."
* "Carbonaceous chondrites, which [...] represent the primary type of meteorites, contain about 2 x 10$^{-8}$ g/g of uranium."
"The thermal history of planets can be studied only theoretically, namely by calculations based on the heat conductivity equation."
"The thermal history of planets is determined by (a) heating by impact of accreting matter; (b) generation of radioactive heat during the formation period; (c) adiabatic compression of the interior by growing exterior layers."
"The initial temperature is determined by (a) heating by impact of accreting matter; (b) generation of radioactive heat during the formation period; (c) adiabatic compression of the interior by growing exterior layers."

## Mercury

Hypotheses:
* "A homogeneous [composition is] a mixture of 58 per cent metallic iron and 42 per cent of silicate".
* "A constant temperature T$_c$ = 1000 °K [...] for the central regions, with the temperature decreasing to 700 °K on the surface."
"According to recent data, the rotation period of Mercury does not coincide with its orbital period. Neglecting the decrease of surface temperature towards the poles, it is possible to get an idea of its thermal history by solving the heat conductivity equation for a body with spherical symmetry."
"As the metallic phase is extremely poor in radioactive elements, their mean content in Mercury [is] 2.5 times smaller than in chondrites."
Because "of the large content of metallic iron, Mercury [possesses] a high heat conductivity."
The "central temperature of Mercury never exceeded 2 300 °K, even with maximum content of radioactive elements".
"Mercury's interior did not undergo melting. As Mercury's temperature is less than the melting temperature of both silicates and metallic iron, Mercury [has] neither a crust nor an iron core. The maximum heating of Mercury took place 2.0-3.0 aeons ago; at the present time the interior of Mercury is cooling."

## Venus

Hypotheses:
* Venus has "a core of metallized silicates".
* Venus has "an iron core".
* "A chondritic composition of the whole planet" with hypothesis 1.
* "a chondritic mantle" composition with hypothesis 2.
* "a uniform distribution of radioactivity" is a stage of the thermal history of Venus.
* "radioactive elements from the upper 1 000 km were gradually carried out into the crust." is a stage of the thermal history of Venus.
* "This transport [of radioactive elements begins] at the start of the melting of the mantle".
* "The mantle of Venus, as that of the Earth [is] a mixture of different minerals and [melts] in some interval of temperatures [...] of the order of 200°."
* "In the core the melting [occurs] at constant temperature for each given depth."
* "The core [has] a metallic conductivity independent of the temperature."
"Preliminary estimates by Safronov (1965, 1969) of the initial temperature of the Earth were used to choose initial temperatures of Venus and Mercury."
The "molecular conductivity of amorphous matter does not decrease with temperature (as it does in crystalline bodies)."
The "molecular conductivity ceases to depend on temperature with the onset of melting."
For hypothesis 1, the "core of metallized silicates is liquid at the present moment."
For hypothesis 2, "the core is liquid with temperature 12 400 °K."

## Earth

"In calculations of the thermal history of the Earth, the observed value of surface heat flow and seismological evidence for the absence of extensive completely molten regions in the mantle were used as criteria of a correct model (Majeva 1967)."

## Mars

"As there are no similar criteria for other planets, calculations of thermal histories were made for models analogous to the best models for the Earth."

## Binary stars

The "most massive short-period planets are all found in multiple star systems. We show here that the planets orbiting in multiple star systems also tend to have a very low eccentricity when their period is shorter than about 40 days."
"These observations seem to indicate that some kind of migration has been at work in the history of these systems."
"There is thus no obvious correlation between the properties of these [exoplanets] planets and the known orbital characteristics of the binaries or the star masses."

## Classical history

The classical history period dates from around 2,000 to 1,000 b2k.
Usually the wanderers are the seven classical planets, Saturn, Jupiter, the Moon, the Sun, Mercury, Mars, and Venus. Additional wanderers may also have existed in ancient times, such as the Earth's pole stars.

## Ancient history

The ancient history period dates from around 8,000 to 3,000 b2k.
Def. "any of the seven heavenly bodies sun, moon, Venus, Jupiter, Mars, Mercury, and Saturn that in ancient belief have motions of their own among the fixed stars" is called a planet.

## Temperatures

"Constraints are reported on the thermal history of the constituents of the Abee enstatite chondrite. From thermal experiments on laboratory-prepared alloys, and on actual samples of the meteorite, it is concluded that the metal phase of Abee cooled from above 700°C to room temperature in less than ten hours."
"The early thermal history of planets [is evidenced] from meteorites."
The "early thermal history of chondritic asteroids [can be] derived by $^{244}$Pu fission track thermometry."

## Hypotheses

* A planet may not, or no longer, have an orbit but it should have a history.