Science Questions. Anyone Know?

Question

Could anyone answer these two questions for me please? Thanks.

1. How can astronomers use the age of a lunar rock to estimate the age of the surface of a planet such as Mercury?

2. Charon stays in the same place in Pluto's sky, but the moon moves across Earth's sky. What causes this difference?

Thanks!

Answer 1

1. Both the moon and Mercury are dead worlds, so their rocks are just about as pristine as when they were formed (earth's tectonics and erosional forces churn the rocks up here, and new rock is formed from geological processes).

2. Pluto's rotation is the same as the orbital period of Charon.

Our moon orbits in about 28 days, so every day the moon at the same time as the previous day, has shifted about 1/28th of the total sky, or about 1/14th of horizon to horizon.

Answer 2

Since the planets and moons in the entire solar system would have formed at pretty much the same time, if we have the age of rocks on the moon (which wouldn't have been "recycled" due to geological forces as they were on Earth) then we have the age of the solar system and its parts.

Charon and Pluto revolve about each other every 6.387 days. The two objects are gravitationally locked, so each keeps the same face towards the other (think of a dumbell with Pluto on one end and Charon on the other).
Our moon orbits the Earth in 28 days or so and rotates in the same time, so it always faces one way to the Earth. But Earth is a lot larger than the moon (compared to Pluto and Charon) and Earth's rotation hasn't slowed down to meet the moon's orbital speed.

Answer 3

Though radioactive isotopes that give us approximate ages of the lunar surface , has led to assumptions of the cratering of other planets surfaces such as mercury. There is evidence of wide spread cratering thorough out the solar system from a population of asteroids and meteors about 4.5 billion years ago.

http://www.astro.washington.edu/larson/Astro150b/Lectures/Craters/lcraters.html


The duo's gravity has locked them into a mutually synchronous orbit, which keeps each one facing the other with the same side. Many moons - including our own - keep the same hemisphere facing their planet.
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Plu_Charon