Crater counts are used to estimate the ages of planetary surfaces. Which of the following is the correct implication?

• A. Ages of planetary surfaces.
• B. Magnetic field strength.
• C. Planetary albedo.
• D. Chemical composition

Answer: (A)

Crater counts test how long a surface has been exposed in space, because the number of impact craters builds up over time. A surface that formed recently will have few craters, while one that’s been around longer accumulates more, assuming the impact environment is reasonably steady. By counting craters and comparing with the known rate of impacts, scientists can estimate how long the surface has been exposed, and with calibration from samples that have radiometric ages, they can convert crater density into an absolute age. It’s also important to note that resurfacing events like lava flows or tectonic activity can erase older craters and reset the record, so crater counts reflect ages after the last such event.

Crater counts don’t directly tell us about magnetic field strength, planetary albedo, or chemical composition, which require other measurements.