#16 - Comets do not give off their own light but reflect

[nusheenaparvizi]

Think about it this way: let's say Halley's comet has a reflectivity of "100." In the past, scientists estimated the mass of Halley's comet based on this reflectivity of 100. They supposed that a certain amount of mass, say 10 kg, would indicate a reflectivity of 100. However, now we've discovered that based on the composition of Halley's comet, it would take far more mass to have this same reflectivity of 100, say 600 kg instead (since the material that makes up Halley's comet is less reflective than predicted). Thus, Halley's comet must be larger than expected to have this same reflectivity of 100. Instead of taking only 10 kg to have a reflectivity of 100, based on the materials in Halley's comet, it would take 600 kg to have this reflectivity.

I hope this helps!

I have thoroughly read this thread and still have a few questions that I am not quite following. For one, you mention that to still have that reflectivity of 100, instead of it taking 10 kg of mass it would take 600 kg of mass to have this reflectivity... but we don't want the same reflectivity because Halley's comet reflects 60x less light per unit of mass? I am confused how you were able to extrapolate that we still want Halley's comet to have the same reflectivity. Knowing that is really helpful to understanding the stimulus but I don't see where you got that from.

Really appreciate all the clarification!

[Jeremy Press]

Hi Nusheena,

That's a great question, and it's not entirely obvious from the surface of this stimulus. The reason Jonathan is keeping the "reflectivity" (which I think of as the "brightness") number the same is simply that the stimulus doesn't indicate it's changed. Is it theoretically possible that our observations of the comet's brightness (its reflectivity) have changed? I suppose so, though without any evidence to that effect, we are reasonably justified in assuming it hasn't. Answer choice B is therefore "most support[ed]" by the stimulus (and notice it's not a hard-and-fast "Must Be True" question stem here, which makes a lot of sense given some of these slight uncertainties).

I hope this helps!

Jeremy

[Nicholas Noyes]

I think you may have the ratio reversed, lanereuden. If we previously thought it was 1 unit of mass to 1 unit of light, we now have discovered that it is 60 units of mass to 1 unit of light. In other words, to produce the same amount of reflected light, the comet has to be 60 times the mass that we originally believed it was. It's not 1:1 changing to 6 - it's 1:1 changing to 60:1.

If the ratio of mass to light is 60 times larger than we thought, and if the amount of light remains constant (we still see what we see, after all), then there must be more mass.

Again, going back to your earlier post, and correcting your numbers:

Believed to be: 1:1 ratio (1 mass units: 1 light units)

Actuality it is: 60:1 ratio

Here's an analogy, because I love those so much:

I hired a painter to paint my barn. The job was completed in one hour. I thought a single painter could do the whole job in an hour, so I concluded that he worked alone. Later, I discovered that an individual painter is actually 60 times slower than I thought. What can I conclude? I know the barn was painted in an hour, but now I know one guy could not have done it, not even close. There must have been more people doing the painting. Looks like it required a crew of 60 painters to finish my barn in that one hour.

I hope this clears things up!

Thank You Adam the ratios help clarify what is going on here in this question.

-Nicholas