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Age of inner core (rewrite the section of the page "Inner core": History)
The age of the Earth’s solid inner core has been a long debated topic and is still under discussion at the present time. It is widely believed that the Earth’s solid inner core grows from an initially liquid core as the Earth cool down. However, the onset time of this process is still unresolved. Unlike studies of the mantle’s age, almost no sample of the Earth’s core is available for direct measurement. Two types of indirect constraints have been used to infer the age of the inner core, that are thermal and paleomagnetic evidences. The two types of methods are equally important in the current scientific community but their estimations of the age of inner core vary over a large range: from 0.5 to 2 billion years old. Estimations from various studies are listed in Table 1. The following two subsections summarize the studies supported by thermal and paleomagnetic evidences.

Thermodynamic evidences
Due to the lack of direct samples from the core, one of the solutions is to model the Earth’s thermodynamic behavior. The main idea is to model the cooling process of the Earth using the constraint of the heat flux at the core-mantle boundary(CMB) and the current size of Earth’s inner core. The radius of the current inner core is determined by the travel time of the seismic waves reflected at the inner core-outer core boundary, and the heat flux at the CMB is related to the heat flux at Earth’s surface and mantle convection. By assuming no radioactive element in the core, Labrosse et al.(2001) gave an estimation of 1±0.5 billion years old. They also reported that with reasonable amount of radioactive elements, the history of Earth’s inner core could be up to 3 billion years old. In a later work, Labrosse(2003), however, present a different view, suggesting that if we consider Ohmic dissipation, the age of Earth’s inner core should be only a few hundred million years older with the presence of radioactive elements. In his study in 2015, He further lowered the inner-core age estimation to less than 700 million years old based on a high thermal conductivity value in the cores. He claim that to drive the same dynamo, higher CMB heat flow is required with a higher thermal conductivity value and thus lead to a younger inner core. Ohta et al.(2016) confirmed the upward adjustment of thermal conductivity in Earth’s core. They measured the electrical resistivity of iron in the laboratory at the Earth’s core condition and observed a lower value than expected, which suggest higher thermal conductivity. Based on their measurements, Ohta et al.(2016) agree on a young inner core age of less than 700 million years old. Konôpková et al.(2016), however, support an older inner core that could even exist at the beginning of Earth’s dynamo. They put upper bound of 4.2 billion years old considering uncertainties. They directly measured the thermal conductivity of solid iron at the Earth’s core condition and observed low thermal conductivity as oppose to what proposed by Ohta et al.(2016) and Gomi et al.(2013).

Paleomagnetic evidences
Paleomagnetic evidences are also helpful to constrain the Earth’s core evolution. With or without the solid inner core, the core dynamic process could be different which could lead to a different magnetic field in the Earth’s history. Previously, this change in the paleomagnetic field is not recognized due to the lack of enough robust measurements. Until 2015, Biggin et al.(2015) observed long-term intensity variations of the paleomagnetic field by examining an extended set of Precambrian samples. They observed a prominent increase in the paleomagnetic field strength and variance around 1 to 1.5 billion years ago and related the change to the birth of Earth’s solid inner core. Based on their estimation of the age of inner core, Biggin et al.(2015) calculated the corresponding core thermal conductivity and found that the value matches an intermediate value of previously defined range. The corresponding core thermal conductivity value is thought to fit a simple thermal evolution model of the Earth. Smirnov et al.(2011) examined the latitudinal dependence of paleosecular variation data and showed variations in dipolar component. The change is linked to core dynamo and inner core formation stages. They propose three stages of the Earth’s core evolution: before 3.5 billion years ago, the core was completely liquid; between 3.5 and 2 billion years ago, the growth of inner core resulted in highly dipolar magnetic field; after 2 billion years ago, the CMB evolved with heterogeneity which lead to a less dipolar magnetic field. Bono et al.(2019) studied the young rock samples from the Ediacaran which falls in the lower end of the age estimation of Earth’s inner core. They observed unusually low paleomagnetic field intensity and high reversal frequency during that time. Bono et al. related those anomalies to the initial point of inner core formation and estimated the age to be 0.5 billion years old. They suggest that their estimation agrees with the geodynamo model with high core thermal conductivity.