Worked Solutions
1. Chemical residence time
A decade-scale residence time is far shorter than the star-planet system's age (Gyr-scale), so Earth's observed methane abundance requires an ongoing source (biological and anthropogenic) to replenish what reacts away; a hypothetical billion-year residence time would instead be consistent with methane persisting from an ancient, possibly abiotic origin (e.g., primordial outgassing) with no ongoing source required -- the same molecule, very different evidentiary strength depending on the residence time.
| Criterion | Points |
|---|
| Correct short-residence-time-implies-active-source reasoning | 8 |
| Correct contrast with the long-residence-time case | 7 |
2. A specific false-positive scenario
This pattern (strong O2, no water vapor or reduced context gases, close-in planet around an active M-dwarf with a historically inflated early HZ) matches the published abiotic-oxygen-buildup pathway (Luger & Barnes 2015) closely and is therefore more consistent with an abiotic origin than a confident biological claim; a genuinely strong biosignature case would require co-detected reduced gases providing disequilibrium context, which are explicitly absent here.
| Criterion | Points |
|---|
| Correct identification of the abiotic pathway as the better-supported explanation | 8 |
| Correct justification citing the missing context gases | 7 |
3. Growth versus survival
Tardigrade pressure survival is documented in a dormant, largely metabolically inactive cryptobiotic state; it says nothing about whether any organism can actively grow, metabolize, and reproduce at 600 MPa. A biosignature claim for a high-pressure subsurface environment requires evidence of active biological processes (e.g., a maintained chemical disequilibrium), not merely evidence that dormant survival at that pressure is possible for some Earth organism.
| Criterion | Points |
|---|
| Correct growth-versus-survival distinction applied to this specific case | 8 |
| Correct connection to biosignature-claim standards | 7 |
4. Polyextremophile relevance
A real Mars-surface or icy-moon-surface environment presents multiple simultaneous extremes (desiccation, radiation, temperature swings) rather than one extreme in isolation; an organism documented to tolerate only one of these in a controlled lab setting has not been shown to tolerate their simultaneous combination, so a polyextremophile like Chroococcidiopsis is a more representative real-world analog for assessing plausible habitability of such environments.
| Criterion | Points |
|---|
| Correct statement that real environments present combined, not isolated, extremes | 8 |
| Correct conclusion favoring polyextremophile analogs | 7 |
Grading Notes
Award full marks for correct method, units, and a numeric result consistent with the arithmetic shown (allow reasonable rounding). Verify that the student re-derives the number for this problem's specific inputs rather than quoting a lecture example without adaptation. Watch specifically for unit-conversion errors (AU/m, days/seconds, Jupiter/Earth mass-radius units), for comparative-magnitude statements given in the wrong direction, and for level-1/level-2/level-3 data-provenance conflation (e.g., presenting an illustrative Miller-Urey bar height as a precise measured yield) -- these are the most common sources of mistakes in this course.