# Comets do not give off their own light but reflect light from other sources, such as the Sun. Scientists estimate the...

Marianne-Xiong on June 6, 2019

I don't quite understand question 16

I chose D. Not sure why B is correct

Replies

Ravi on June 7, 2019

@Marianne-Xiong,

Happy to help. Let's take a look at (D) and (B).

(D) says, "The reflective properties of the material of which comets
are composed vary considerably from comet to comet."

The problem with (D) is that we don't have any sort of idea of how
reflective the other comets are. We could have simply just been
incorrect about all comets, so this is why (D) is out.

(B) says, "Previous estimates of the mass of Halleyâ€™s comet which were
based on its brightness were too low."

(B) looks great. Halley's comet has greater mass than we would have
anticipated based on its brightness, so the former estimates of
Halley's mass had to have been too low, making (B) the correct answer.

Hope this helps. Let us know if you have any other questions!

dace on October 29, 2019

Why not C?

dace on October 29, 2019

Shouldn't answer (B) indicate the opposite?

avif on June 4, 2020

@ Danielle. I too originally thought C. But I see why B is correct. There is no indication that they got the brightness wrong. Rather the brightness which they saw in their mind reflected a certain mass which they then discovered that its mass reflects much less than they thought. Which would mean that for = brightness there would need to be greater mass than originally thought which is what answer B says, that original estimates of mass were too low.