PrepTest 155, Section 1, Question 23
On Earth, biological activity leads to a change(45 more words not shown)
Each of the following, if(7 more words not shown)
- ALife forms that(22 more words not shown)
- BThe effects of(25 more words not shown)
- CThe ratio of(27 more words not shown)
- DRelatively few terrestrial(16 more words not shown)
- EThe current ratio(20 more words not shown)
Reading the argument
Would life on Mars leave the same chemical sign that life leaves on Earth?
The passage tells us what biological activity does to an isotope ratio on Earth. The Martian meteorite has a ratio resembling Earth's minerals from before life began. That is interesting evidence, but using it to judge whether life ever occurred anywhere on Mars requires several further connections.
Maybe Martian life would affect the ratio differently. Maybe the conditions on Mars change how that process works. And how much can one piece of material, which left Mars at some earlier time, tell us about the whole planet's history?
The conclusion is cautious enough to say life is unlikely, but those possibilities can still weaken it. We don't have to prove Martian life existed. We need reasons this particular evidence might be a poor guide.
What we’re looking for
This is an EXCEPT question: four choices weaken, and one does not. The exception could even strengthen the argument. Keep asking whether a choice makes the meteorite's ratio less useful as evidence against life.
Working the answers
- A
This weakens the comparison between planets. If other life forms could affect the isotope ratio differently, Martian life might exist without leaving the Earth-style sign we're looking for. Failing to find that sign would then be less telling.
- B
This also undermines applying Earth's pattern directly to Mars. The effects of life depend on environmental and climatic factors, and the planets differ on those factors. Even similar biological activity could therefore produce a different ratio under Martian conditions. The missing Earth-style ratio becomes less decisive.
- CCorrect
This makes the meteorite more representative, not less. Its ratio matches Mars as a whole at the time the material left. One concern was that a single fragment might give an unusual picture of the planet; C reduces that concern. It doesn't settle every question about timing or Martian life, but it helps the argument rather than weakening it.
- D
Even on a planet where life exists, relatively few mineral samples show the ratio associated with biological activity. Imagine, just to illustrate, that only 1 percent did. A random sample from Earth would usually lack the sign despite all the life on Earth. That makes the absence of the sign in one Martian sample weak evidence against life on the planet. We don't need to claim the other 99 percent all have the exact same ratio as this meteorite; the point is that a missing marker needn't mean missing life.
- E
The meteorite gives us an older picture, and Mars's current ratio is different. That weakens using the old material to reach a conclusion about whether life occurred on Mars at any time. The change doesn't itself prove life caused it. But it shows why the meteorite's ratio may leave out a later part of the history we're trying to assess.
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