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

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 Halley’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.

Emil-Kunkin on August 18 at 07:53PM

Dace its pretty tough to keep track of all the effects, but I like to walk my way through the story. If a comet is brighter, then we think it has more mass. However, if a comet is artificially dark, as is the case here, this means that we have underestimated its mass. Since it appears less bright than it should, we would thus think that it is less massive than it really is.