Passage Summary: After a comet crashed into Jupiter, scientists didn't see any water at the crash sites. Because of this lack of water, they concluded the comet was just rock and didn't go very deep into the planet's atmosphere.

Conclusion: The comet fragments were composed of rock rather than ice and did not reach Jupiter's lower atmosphere.

Reasoning: Observations of the impact sites following the collision of the comet fragments with Jupiter showed no evidence of water.

Analysis: The author is making a huge leap from 'we didn't see water' to 'there was no water.' This argument only works if we assume that if water had been present in the comet or the lower atmosphere, it would have necessarily been visible in the resulting formations. If the water could have been there but remained hidden or was destroyed in a way that left no trace, the conclusion fails. We are looking for a necessary assumption that bridges the gap between the lack of observation and the actual absence of the substance.