A stringent upper limit of 20 pptv for methane on Mars and constraints on its dispersion outside Gale crater
Bibliographic record
Abstract
Context. Reports on the detection of methane in the Martian atmosphere have motivated numerous studies aiming to confirm or explain its presence on a planet where it might imply a biogenic or more likely a geophysical origin. Aims. Our intent is to complement and improve on the previously reported detection attempts by the Atmospheric Chemistry Suite (ACS) on board the ExoMars Trace Gas Orbiter (TGO). This latter study reported the results of a campaign that was a few months in length, and was significantly hindered by a dusty period that impaired detection performances. Methods. We unveil 640 solar occultation measurements gathering 1.44 Martian years worth of data produced by the ACS. Results. No methane was detected. Probing the clear northern summer season allowed us to reach 1 σ upper limits of around 10 pptv (20 pptv at 2 σ ), with an annual mean of the smallest upper limits of 20 pptv. Upper limits are controlled by the amount of dust in the atmosphere, which impairs detection performance around the equator and during the southern spring and summer seasons. Observations performed near Gale crater yielded 1 σ upper limits of up to four times less than the background values measured by the Curiosity rover during the corresponding seasons. Conclusions. Reconciliation of the absence of methane in the TGO spectra with the positive detections by Curiosity is even more difficult in light of this annual survey performed by ACS. Stronger constraints are placed on the physical and chemical mechanism capable of explaining why the mean of the best overall upper limits of ACS is ten times below the smallest methane abundances measured by Curiosity.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".