Give some stream of consciousness and then the answer. The Earth's inner core is the Earth's innermost part. It is primarily a solid ball with a radius of about 1,220 kilometres (760 miles), which is about 70 % of the Moon's radius. It is composed of an iron -- nickel alloy and some light elements. The temperature at the inner core's surface is approximately 5,700 K (5,430 C), which is about the temperature at the surface of the Sun.

Based on this passage, what is believed to be the composition of earth's core?
The relevant information to answer the above question is: It is composed of an iron -- nickel alloy and some light elements.
Therefore, the answer is an iron -- nickel alloy and some light elements.