The three principal internal sources of heat for the terrestrial planets are;
The first two mainly occur during the planets formation. Accretion, as gas, dust and smaller bodies collide to form the planet, generating heat on impact and differentiation. Differentiation is the process where the most dense outer material falls to the centre of the planet heating the core through friction during accretion. The source of the thermal energy is the gravitational potential energy which the extended mass had before accretion. The planet holds this heat for billions of years.
Radioactive material was present in the original accretion disk. The long lived radioactive elements decay for billions of years. (On Earth, these are potassium-40, uranium-238, uranium-235, and thorium-232.) This also contributes significantly to internal heating, as unstable radioactive nuclei split and release energy (fission).
the sun- all the plants need it or photosynthesis and animals need it for oxygen.
pressure
The two principal sources of heat are the Sun and the Earth itself via geothermal heat. The sun's heat will vary more than geothermal, and therefore contribute more to flux.
The sun
The Earth system is powered by energy from two major sources: the Sun and the planet's internal heat.
the sun- all the plants need it or photosynthesis and animals need it for oxygen.
Because gases exist even at great distances from the sun. Also the large gas planets have internal heating sources emitting from their core.
What is internal and external sources?
Main sources of internal heat are Magmatism and Radioactivity.
external or internal sources
Planets Reflect light, they do not contain any light apart from internal heat sources.
Natural background radiation
Internal sources of information could be a database management system that is used by the company. Employees and management are also examples of internal sources of information. External sources are outside of the organization and harder and could include studies and market research.
internal sources are personnel, colleagues and the library whereas external sources can be consultants andservice providers and catalougues.
entourage
pressure
The principal sources for Genesis were the Yahwist, the Elohist and the Priestly Source.The principal sources for Exodus were the Yahwist, the Elohist and the Priestly Source.The principal source for Leviticus was the Priestly Source.The principal source for Numbers was the Priestly Source, with some material by the Yahwist.The principal sources for Deuteronomy was the Deuteronomist, who was also responsible for the Deuteronomic History.The JE Redactor combined early material by the Yahwist and the Elohist and may have made minor changes to content. The final Redactor redacted the books into much the form we know today and made minor changes to content.