Astronomers have realized that the surface of Ida is older than the surface of Gaspra. How do they measure this?
A) we have radioactive rock samples from each asteroid to analyze
B) the surface composition (elements and compounds) can easily tell us the ages
C) from counting craters on each asteroid's surface
D) from the magnetic field of each body, which weakens with age
E) it's purely a guess; astronomers have no way of really measuring the ages of these asteroids

Question

Quizplus · Accepted Answer

Astronomers can measure the age of a planetary body's surface by counting the number of impact craters. The more craters a surface has, the older it is, since more time has passed for objects to come crashing into it. Ida has more craters on its surface than Gaspra, indicating that it is older.

Astronomers Have Realized That the Surface of Ida Is Older