Self-study
Support Spectroscopic analysis has revealed the existence of frozen nitrogen, methane, and carbon monoxide on the surface of Pluto. ████ ████ ████ █ ████████ ██ █████████ █████████ ██ ███████████ █████ ███ ██████████ ██ ███ ███ ██ ████ ██ ██████████ ███████ ████████ ██ ███ ███████ ███ █████████████ ███ ██████████ ███████████ ████ █████████ ████ ███ ██████████ ██ █████████ ██████████ ███ █████████ ██████ █████████ ███ ████████ ██ █████ ██ ██████████ ██████████
Argument Summary
The argument runs like this. Spectroscopy detected three frozen substances on Pluto's surface (nitrogen, methane, and CO). Ices vaporize, producing an atmosphere. The proportion of any gas in that atmosphere depends on how readily the corresponding ice vaporizes. The astronomers conclude that the atmosphere consists of nitrogen, CO, and methane, in decreasing abundance.
The argument makes at least two assumptions. First, the conclusion specifies a particular order (nitrogen most, then CO, then methane), but the premises don't actually say which of the three ices vaporizes most or least readily. The conclusion is relying on a specific vaporization-rate ordering of the three named substances. Second, the conclusion lists three components and only three, but the premises don't say those are the only three substances detected by spectroscopy or the only three substances in Pluto's atmosphere. So the argument is also relying on those three being the only substances on Pluto's surface that vaporize. Any additional vaporizing substance would join the atmosphere and either extend the list or rearrange the ranking.
Anticipation
For a Necessary Assumption question, we want a claim the argument needs to be true. This argument has at least two assumptions: the assumed ordering of vaporization rates among nitrogen, CO, and methane, and the absence of any additional substance that would interfere with that ordering.
19.
The astronomers’ argument relies on █████ ███ ██ ███ █████████ ████████████
a
There is no ████ ██████ ████████ ██ ███ ███████ ██ █████ ████ █████ ██ ██████ ██████ ██████ ████████ ██ ████████
The argument explicitly says abundance in the atmosphere depends on how readily the ice vaporizes, not on how much ice is on the surface. So how the amounts of nitrogen, CO, and methane ice compare to each other doesn't affect the reasoning. Negate (A) and assume there is more nitrogen ice than CO or methane ice. The argument still works, because vaporization rate is what determines atmospheric proportion.
b
Until space probes █████ ██████ ██████ ████████ ██ ███ ██████████ ██ ███████████
Irrelevant. The argument is making an indirect inference about Pluto's atmosphere using spectroscopy of the surface plus what we know about how ices vaporize. Whether direct analysis is currently possible doesn't bear on whether that indirect inference is sound.
c
There is no ██████ █████████ ██ ███ ███████ ██ █████ ████ █████████ ████ ███████ ████ ███████ ███ ████ ███████ ████ ██████ █████████
This is necessary. If a frozen substance on Pluto's surface vaporized at a rate between methane's and CO's, that substance would show up in the atmosphere with abundance between CO's and methane's. The conclusion's "nitrogen, CO, and methane in decreasing abundance" would no longer be the right list of components, since a fourth would slot in between CO and methane.
Negation test: there IS such a substance. The atmosphere has at least four components, not three, and the listed ordering misses one. The conclusion fails. Since negating (C) breaks the argument, (C) is necessary.
Conclusion's claim
nitrogen ◼◼◼◼◼◼◼◼
CO ◼◼◼◼
methane ◼
If (C) is false
nitrogen ◼◼◼◼◼◼◼◼
CO ◼◼◼◼
X ◼◼ ← slots in here
methane ◼
d
Nitrogen is found ██ ███ ██████████ ██ █ ██████ ████ ██ ████████ ███ ██ █████ ██ ███ ███████ ██ ████ ███████
This goes in the wrong direction. (D) says nitrogen in an atmosphere requires nitrogen ice on the surface. So (D) tells us how to infer surface composition from atmospheric composition. The argument needs the opposite direction. It starts from known surface composition (nitrogen ice was detected on Pluto) and predicts atmospheric composition (nitrogen will vaporize into the atmosphere). Negation test: nitrogen could exist in some atmosphere without surface nitrogen ice. The argument is unaffected, because we already know Pluto has surface nitrogen ice; we're using that to predict nitrogen in the atmosphere. We're not using nitrogen in the atmosphere to conclude something about nitrogen on the surface.
e
A mixture of █████████ ██████ █████████ ███ ███████ ██ ██████████████ ██ ███ ██████████ ████ █████ ███ █████ ██████ ███████
Irrelevant. The argument doesn't depend on what substances were typical of Solar System formation. Whether nitrogen, CO, and methane are characteristic of formation materials has no bearing on whether the spectroscopy + vaporization reasoning holds for Pluto specifically.