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Difficulty: MediumIdentifying Points of Disagreement

### Murchison Meteorite Organics

The Murchison meteorite, which fell in Australia in 1969, contains a rich variety of organic compounds, including amino acids. Two scientists discuss the origin of these organic molecules.

Scientist 1
The organic molecules in the Murchison meteorite were synthesized within the meteorite's parent asteroid *after* its accretion. The heat generated by the radioactive decay of short-lived isotopes, specifically aluminum-26 (26Al^{26}\text{Al}), melted interior water ice. This liquid water facilitated aqueous alteration, driving chemical reactions that combined simpler precursor molecules (such as aldehydes, hydrogen cyanide, and ammonia) into complex amino acids. Therefore, liquid water and active internal heating within the parent body were necessary for the formation of these organic compounds.

Scientist 2
The organic molecules were synthesized in the interstellar medium (ISM) *before* the accretion of the solar nebula and the parent asteroid. These compounds formed on the surfaces of interstellar ice grains through ultraviolet (UV) irradiation of simple ice mixtures (containing carbon monoxide, methane, and ammonia) at temperatures below 30 K30\text{ K}. These pre-formed organic molecules were subsequently incorporated into the parent asteroid as it accreted. No liquid water or internal heating within the asteroid was involved in the synthesis of these compounds; in fact, subsequent aqueous alteration would have degraded these fragile organic structures.

Scientist 1 and Scientist 2 disagree on which of the following aspects of the organic compounds found in the Murchison meteorite?

  1. A
    Whether the rate of amino acid synthesis determines the amount of radioactive decay occurring within the parent asteroid.
  2. Whether liquid water was required for the synthesis of the organic compounds.Answer
  3. C
    Whether the organic molecules were synthesized on the surfaces of interstellar ice grains, as proposed by Scientist 1.
  4. D
    Whether ammonia was involved as a precursor molecule in the formation of the organic compounds.

Answer

Whether liquid water was required for the synthesis of the organic compounds.
The correct answer is the option regarding whether liquid water was required for synthesis. Scientist 1 argues that liquid water was necessary to facilitate chemical reactions on the parent asteroid, whereas Scientist 2 argues that no liquid water was involved and that liquid water would actually degrade the organic compounds.

Step-by-Step Solution

1
Analyze Scientist 1's claim regarding the necessity of liquid water.
Scientist 1 explicitly states that liquid water was necessary to facilitate the chemical reactions that formed the amino acids on the accreted parent asteroid.
To determine how Scientist 1 views the role of liquid water in synthesis.
2
Analyze Scientist 2's claim regarding the necessity of liquid water.
Scientist 2 states that no liquid water was involved in the low-temperature interstellar synthesis and that subsequent exposure to liquid water would degrade the compounds.
To determine how Scientist 2 views the role of liquid water in synthesis.
3
Compare the viewpoints of both scientists to identify the point of direct disagreement.
Scientist 1 asserts liquid water was required, while Scientist 2 asserts liquid water was not involved and is destructive, demonstrating a clear disagreement on this point.
To identify the correct option based on the conflicting viewpoints.

Key Concept

Identifying Points of Disagreement
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Identifying Points of Disagreement

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