{"id":"6939160f-893a-4895-8c87-728a307722f0","arxiv_id":"2501.08892","paper_version":2,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":7.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Dimethyl sulfide has been detected for the first time in the interstellar medium toward the Galactic Center cloud G+0.693-0.027.","lead":"Astronomers report the first detection of dimethyl sulfide (DMS) in the interstellar medium, in the Galactic Center cloud G+0.693-0.027. The detection supports abiotic routes to a molecule proposed as a potential biosignature, complicating its use in exoplanet searches.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"The LTE assumption is the load-bearing weakness: with no DMS collisional rate coefficients and a measured Tex=13 K below Tkin=70-140 K, the quoted N(DMS) and the DMS/CH3OH comet comparison carry unquantified systematic uncertainty, so the 'conclusive' abundance claim overreaches.","rationale":"The paper's central claim has two layers: the detection ('DMS exists in the ISM') and the quantitative interpretation (abundance, efficient abiotic formation, cometary resemblance). The detection is well supported by roughly 50 observed lines including ten clean transitions with consistent LTE fits, so I do not question it. The load-bearing condition for the second layer is the LTE/excitation assumption and the extended-source assumption; the paper itself admits there are no DMS collisional rates and that the densities are low enough for non-LTE effects. The measured Tex=13±3 K versus Tkin=70-140 K is direct evidence of sub-thermal excitation, so the single-Tex LTE fit is not justified by the data alone. Because the DMS/CH3OH ratio and the 'efficient abiotic production' statement are built on this column density, the systematic uncertainty is understated by quoting only the statistical 0.3×10^13 error. This is exactly the reader's weakest assumption, and I agree with that identification. A non-LTE calculation with approximate rates, or an internal fit excluding the lowest-frequency lines, would quantify the bias. Until then the verdict should remain conditional: the detection is solid, but the abundance and biomarker implications should be presented with the LTE caveat as a first-order uncertainty, not a footnote.","tokens_in":18106,"tokens_out":6751,"duration_ms":51243,"concrete_test":"Run a non-LTE radiative-transfer calculation (e.g., RADEX or MADCUBA) for DMS with approximate collisional rate coefficients scaled from a structurally similar molecule such as dimethyl ether or methyl mercaptan, for n(H2)=10^4-10^5 cm^-3 and Tkin=70-140 K, and compare the predicted intensities of the ten clean lines with the observed values. If the best-fit N(DMS) differs from the LTE value by more than about 30%, the quoted abundance and DMS/CH3OH ratio are not robust; if it agrees, the LTE-derived column density is empirically adequate despite the absence of DMS-specific rates.","verdict_should_be":"UNCHANGED","load_bearing_attack":"Section 3 derives N(DMS)=(2.6±0.3)×10^13 cm^-2 from a single-Tex LTE fit to ten transitions with Eup=8.6-34.2 K, while the same section states that the H2 density of G+0.693 (few×10^4 cm^-3) is low enough to produce non-LTE effects and that no collisional rate coefficients for DMS exist. The derived Tex=13±3 K is far below the kinetic temperature (70-140 K), so the observed populations are demonstrably sub-thermal. A single rotational temperature is then an empirical fitting device, not a physical guarantee; the true N(DMS) can be biased by non-LTE population effects, especially for the lowest-frequency Q-branch lines (41.97 and 47.78 GHz) closest to the <30 GHz regime where the authors note weak-maser anomalies can occur. The extended-source assumption, adopted without a dedicated map, adds another unquantified systematic: if the emission is partly compact, N is higher. The quoted ±0.3×10^13 error is statistical only and does not include these effects. This does not weaken the multi-line detection itself, but it undermines the quantitative abundance, the DMS/CH3OH ratio of 1.7×10^-3 used in the comet-67P comparison, and the word 'conclusive' attached to 'efficient abiotic production.'","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","summary":"The manuscript reports the first detection of dimethyl sulfide (CH3SCH3, DMS) in the interstellar medium, toward the Galactic Center cloud G+0.693-0.027, using ultradeep Yebes 40 m and IRAM 30 m spectral surveys. The authors identify ten clean b-type lines with integrated S/N >= 6, supplemented by roughly fifty additional blended or lower-S/N transitions, and find no missing lines in the survey coverage. From an LTE fit and a rotational diagram they derive N(DMS) = (2.6 +/- 0.3) x 10^13 cm^-2 and Tex = 13 +/- 3 K, corresponding to a fractional abundance of ~1.9 x 10^-10 relative to H2. They compare DMS abundances with related species in the same cloud and with comet 67P, finding DMS/CH3OH ~ 1.7 x 10^-3, and argue that the detection provides conclusive evidence for efficient abiotic DMS production in the ISM, with implications for the use of DMS as an exoplanet biomarker. The paper also provides a new spectroscopic catalogue of DMS, including partition function values at several temperatures, and reports all observed transitions in the appendices.","tokens_in":18421,"tokens_out":6382,"duration_ms":69944,"significance":"If the detection holds, this is the first identification of dimethyl sulfide in the interstellar medium, a significant addition to the inventory of interstellar organosulfur molecules and directly relevant to the ongoing debate about DMS as a biosignature. The observational case is strong: ten unblended transitions with S/N >= 6, a consistent rotational diagram, additional blended but consistent lines, and no missing lines across the survey. The paper also ships a new catalogue built from the most recent laboratory spectroscopy, with partition functions at multiple temperatures, which is a reproducible and useful resource. The quantitative abundance and the comparison with comet 67P rest on LTE assumptions and an extended-source filling factor that are acknowledged in the text but not quantified; these caveats matter for the claimed 'conclusive' wording but do not undermine the detection itself.","major_comments":[{"comment":"The LTE-derived column density and excitation temperature are the basis for the fractional abundance, the DMS/CH3OH ratio, and the comparison with comet 67P, yet the same section states that the H2 density of G+0.693 is low enough to produce non-LTE effects and that no collisional rate coefficients for DMS exist. The fitted Tex = 13 +/- 3 K is far below the kinetic temperature of 70-140 K, so the populations are demonstrably sub-thermal; the single rotational temperature is an empirical fitting device rather than a physical guarantee. The quoted uncertainty of +/- 0.3 x 10^13 cm^-2 is statistical only and does not include the systematic bias from the LTE assumption, which could shift N(DMS) and hence the DMS/CH3OH ratio used for the comet comparison. Please quantify this bias if possible (for example, by rescaling collisional rates from a similar molecule such as DME), or at minimum state explicitly in the abstract and Section 4 that the quantitative abundance ratios are LTE-based values with potentially large unquantified systematics, and soften the word 'conclusive' accordingly.","section":"Section 3, Table 1, Figure 2"},{"comment":"The spectra are presented in antenna temperature units under the assumption that the molecular emission toward G+0.693 is extended compared to the telescope beam, but no dedicated map of DMS is shown to justify this filling factor for this specific molecule. If the DMS emission is partly compact, the derived column density would increase, and the comparison with CH3OH (which may have a different spatial distribution) could shift. The extended-source assumption is therefore load-bearing for the absolute abundance and for the comet ratio. Please provide a brief justification that DMS follows the extended emission pattern of the cloud, or state explicitly how a compact component would change the derived quantities.","section":"Section 2, Section 3"}],"minor_comments":[{"comment":"The text refers to 'the presence in the atmosphere of K2-12b'; this appears to be a typo for K2-18b, the exoplanet discussed in the introduction and in the cited Madhusudhan et al. (2023) work.","section":"Section 4, first paragraph"},{"comment":"The phrase 'O-protonated cabonyl sulfide' contains a typo; it should read 'O-protonated carbonyl sulfide'.","section":"Section 1, introduction"},{"comment":"The word 'stablish' should be 'establish' in the sentence 'aiming to stablish general trends'.","section":"Section 4, paragraph 2"},{"comment":"The text says 'a total of ten clean b-type lines' but Table 1 includes one line labeled 'slightly blended' (at 97.022 GHz). Please clarify the wording: for example, 'ten lines used in the fit, of which nine are unblended and one is slightly blended according to the criterion defined in the text.'","section":"Section 3, Table 1"},{"comment":"The sentence 'we derived a Tex = 13 +/- 3 K, in agreement with the sub-thermal excitation conditions found for many other molecules toward G+0.693' could be misread as agreement with thermal equilibrium; consider rephrasing to clarify that the agreement is with the low excitation temperatures found for other molecules under the same sub-thermal conditions.","section":"Section 3, paragraph after Eq. for Tex"},{"comment":"In the expression for the line intensity, the constant 4.16231 x 10^-5 should be verified for consistency with the stated units (nm^2 MHz) and the adopted dipole moment; adding a reference for the numerical conversion factor would help reproducibility.","section":"Appendix A"}],"recommendation":"major_revision","confidential_remarks":"This is a strong detection paper with a secure multi-line identification of DMS in the ISM; the main issue is that the abstract and discussion attach 'conclusive' and quantitative abundance claims to an LTE analysis that the authors themselves acknowledge is the only viable method but is not calibrated by collisional rate coefficients. I think a major revision is appropriate: the authors should either add a quantitative estimate of the non-LTE systematic uncertainty (even a rough one based on scaling from analogous molecules) or explicitly and prominently soften the quantitative claims. The detection itself, which is the core new result, does not require this additional work, but the 'efficient abiotic production' and 'biomarker' conclusions depend on the abundance ratios. The paper fits the scope of the journal and I see no citation or novelty concerns."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"First, the take: this paper delivers a genuine first detection of DMS in the ISM, and the line evidence is strong. Ten clean transitions with S/N ≥ 6, a consistent rotational diagram, no missing lines across the survey, and a sensible set of additional blended lines that match the LTE model. The new catalogue they assembled from Ilyushin et al. is a practical contribution. On the observational side, this is careful work.\n\nThe soft spots are in the interpretation, not the detection. The column density and the comet comparison rest on a single-Tex LTE fit, and the paper itself tells you the conditions are sub-thermal: Tex ≈ 13 K while Tkin is 70–140 K, H2 densities a few × 10^4 cm^-3, and no collisional rate coefficients for DMS. The quoted ±0.3×10^13 error is statistical only. The extended-source assumption is adopted without a map. So the abundance ratio DMS/CH3OH ~ 1.7×10^-3, used for the 67P comparison, carries real unquantified systematic uncertainty. The line identification does not depend on these assumptions, but the abundances do.\n\nThat is why the abstract's 'conclusive observational evidence on its efficient abiotic production' overreaches. A detection shows DMS exists and forms in space; it does not pin down the rate or efficiency, and the quantitative abundance is model-dependent. The authors are transparent about the LTE caveat in the text, which is good, but the abstract and the summary bullet points go beyond what the data can conclusively say.\n\nThe comparison with comet 67P is interesting but inherently tentative because the ROSINA m/z=62 signal cannot distinguish DMS from its isomer, as they note. The 'remarkable resemblance' is a ratio of two model-dependent numbers.\n\nWho is this for? Astrochemists working on sulfur chemistry and anyone debating DMS as an exoplanet biosignature. It is a useful new data point, not a paradigm shift. The detection itself deserves to be published; the claims about efficacy and biomarker refutation need tempering.\n\nRecommendation: send it to peer review. A serious referee should push on the error budget and the word 'conclusive' but the core detection will stand. My own verdict would be accept after minor revision.","headline":"Solid first detection of interstellar DMS, but the LTE-based abundance and the 'conclusive abiotic' framing are softer than the abstract claims.","tokens_in":19042,"tokens_out":2989,"would_cite":true,"duration_ms":29297,"reading_group":"yes","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"Dimethyl sulfide, a molecule proposed as an exoplanet biosignature, has been detected for the first time in the interstellar medium, with an abundance that matches purely chemical formation.","keywords":["dimethyl sulfide","interstellar medium","astrochemistry","Galactic center","sulfur chemistry","biomarker","exoplanet atmospheres","rotational spectroscopy"],"falsifier":"Measure DMS-H₂ collisional rate coefficients and rerun the excitation modeling of the same ten lines; if a non-LTE calculation drives the column density well below the LTE value, or shifts the derived DMS/CH₃OH ratio far from the cometary value, the abundance claim and the comet-connection argument would need revision. A simpler check is a high-angular-resolution observation of one of the brightest lines, which would show whether the emission is truly extended on the scale assumed here.","tokens_in":17922,"feed_emoji":"🌌","tokens_out":5573,"duration_ms":51954,"temperature":0.7,"pith_summary":"The paper reports the first detection of dimethyl sulfide (CH₃SCH₃, DMS), the molecule once proposed as a sign of life on habitable exoplanets, in the interstellar medium, toward the Galactic Center cloud G+0.693-0.027. Using ten clean rotational lines from an ultradeep survey with the Yebes 40 m and IRAM 30 m telescopes, the authors derive a column density of $(2.6 \\pm 0.3)\\times10^{13}$ cm$^{-2}$, an abundance of ~$1.9\\times10^{-10}$ relative to H₂, and an excitation temperature near 13 K. The central claim is that this detection proves DMS can be produced efficiently by abiotic interstellar chemistry, so its presence alone is not evidence for biology on an exoplanet. A sympathetic reader would care because the same molecule is a suggested biosignature for worlds like K2-18b, and its tentative detection there now needs an abiotic explanation.","feed_headline":"Dimethyl sulfide, a proposed life sign, found in space","feed_subtitle":"A proposed exoplanet life marker turns up in a starless cloud, with a comet-matching abundance ratio.","key_machinery":"The load-bearing tool is a new astronomical line catalogue for DMS built from the most complete laboratory data (Ilyushin et al. 2020), converted to a standard catalog format with partition functions summed over all four internal-rotation substates (AA, EE, EA, AE). DMS has two equivalent methyl rotors, so every rotational level splits into four symmetry substates whose closely spaced transitions coalesce into enhanced line clusters at the broad (~20 km s$^{-1}$) linewidths of this cloud; this auto-blending strengthens the combined signal. The astronomical analysis compares the observed spectra against synthetic LTE spectra of DMS plus all ~130 previously identified molecules in the survey using the SLIM/Autofit tools, with column density and excitation temperature as free parameters and a rotational-diagram analysis as an independent check. The LTE assumption is defended by the cloud's sub-thermal but well-behaved excitation, and is acknowledged as the only viable method because no collisional rate coefficients for DMS exist.","core_discovery":"Dimethyl sulfide, the C₂H₆S isomer whose m/z signature was attributed to DMS in comet 67P, is present in the interstellar medium at an abundance of about $(1.9 \\pm 0.4) \\times 10^{-10}$ relative to H₂ in G+0.693-0.027. The detection rests on ten unblended R- and Q-branch lines consistent with the laboratory spectrum across 31–173 GHz, with about 50 further lines consistent with the same LTE model. The paper further establishes that DMS is ~1.6 times less abundant than its structural isomer ethanethiol and ~30 times less abundant than its oxygen analogue dimethyl ether, matching the O/S trends of related pairs; and that the DMS/CH₃OH ratio in the cloud, $(1.7 \\pm 0.4) \\times 10^{-3}$, matches the cometary C₂H₆S/CH₃OH ratio of $(1.3 \\pm 0.4) \\times 10^{-3}$. The authors conclude that DMS is efficiently formed abiotically in space, even before star formation begins, and that its use as a unique biomarker for exoplanet life needs caution.","pith_inferences":["The same LTE caveat means the ~30-fold DME/DMS ratio is more robust than the absolute DMS abundance, because both molecules would suffer similar beam and excitation biases; the ratio, not the column density, is the safer quantity to compare across sources.","A testable prediction follows from the proposed formation routes: if DMS forms on grains via CH₃ + CH₃S, its abundance should track CH₃SH and methanol desorption, whereas gas-phase routes would predict a transient early enhancement like that of dimethyl ether.","The tentative DMS signal in K2-18b's atmosphere becomes a test of delivery rate: interstellar DMS that survives cloud-to-comet inheritance could be delivered to planets and then destroyed or recycled by photochemistry, so atmospheric DMS would need replenishment at a rate that future time-series observations could bound."],"forward_implications":["DMS is part of the interstellar sulfur inventory: it exists in a Galactic Center cloud before any star has formed, so its synthesis does not require a planet or a biosphere.","The near-agreement of the DMS/CH₃OH ratio between G+0.693 and comet 67P adds a data point to the case that comets inherit their organic sulfur inventory from the parental cloud.","Proposals to treat DMS as an unambiguous biosignature in exoplanet atmospheres must be revised, because abiotic production in the interstellar medium is now an observationally supported alternative channel.","The O/S abundance trends predict that other sulfur analogues of common oxygen-bearing molecules should be detectable in G+0.693 and similar shocked clouds, extending the pattern this detection fits."],"supporting_citations":[{"why":"supplies the complete laboratory rotational spectrum of DMS from which the astronomical search catalogue is built","marker":"Ilyushin et al. 2020"},{"why":"reports the DMS (C₂H₆S) signatures in comet 67P that motivate the ISM search and provide the cometary ratio for comparison","marker":"Hänni et al. 2024"},{"why":"provides the G+0.693 abundances of CH₃CH₂SH, CH₃SH, and CH₃OH used for the abundance ratios and the comet comparison","marker":"Rodríguez-Almeida et al. 2021"},{"why":"gives the H₂ column density adopted to convert the DMS column density into a fractional abundance","marker":"Martín et al. 2008"},{"why":"establishes the extended emission and kinematics of G+0.693 underlying the beam-filling assumption","marker":"Requena-Torres et al. 2006"},{"why":"provides the O/S abundance-ratio framework across interstellar sources and comet 67P that places the DME/DMS ratio in context","marker":"Sanz-Novo et al. 2024a"},{"why":"grounds the argument that non-LTE effects for large molecules are expected mainly below ~30 GHz, supporting the LTE treatment of the detected lines","marker":"Faure et al. 2018"}],"fun_headline_variants":["First interstellar DMS detection: a proposed life marker goes abiotic","Dimethyl sulfide found in space, but its origin is likely not biological","Abiotic DMS in a starless cloud challenges exoplanet biosignature","DMS, a biomarker candidate, detected in ISM with comet-matching ratio","Space chemistry: DMS forms abiotically, so rethink that exoplanet signal"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"The derived abundance and the comet comparison assume the DMS lines can be modeled in local thermodynamic equilibrium with the emitting gas filling the telescope beams; the authors have no collisional rate coefficients to test the first part, and non-LTE effects are plausible for a large molecule in the low-density gas of this cloud.","fun_headline_variants_meta":{"raw":{"variants":["First interstellar DMS detection: a proposed life marker goes abiotic","Dimethyl sulfide found in space, but its origin is likely not biological","Abiotic DMS in a starless cloud challenges exoplanet biosignature","DMS, a biomarker candidate, detected in ISM with comet-matching ratio","Space chemistry: DMS forms abiotically, so rethink that exoplanet signal"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.001473,"raw_usage":{"total_tokens":6011,"prompt_tokens":1122,"completion_tokens":4889,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":738,"completion_tokens_details":{"reasoning_tokens":4788}},"tokens_in":738,"tokens_out":4889,"duration_ms":37075,"temperature":1.0,"reasoning_tokens":4788,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-10T20:15:01.454040+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"Measure DMS-H₂ collisional rate coefficients and rerun the excitation modeling of the same ten lines; if a non-LTE calculation drives the column density well below the LTE value, or shifts the derived DMS/CH₃OH ratio far from the cometary value, the abundance claim and the comet-connection argument would need revision. A simpler check is a high-angular-resolution observation of one of the brightest lines, which would show whether the emission is truly extended on the scale assumed here.","supporting_citations":[{"cited_title":"2020, Journal of Molecular Structure, 1200, 127114, https://doi.org/10.1016/j.molstruc.2019.127114","cited_arxiv_id":null,"evidence_quote":"supplies the complete laboratory rotational spectrum of DMS from which the astronomical search catalogue is built"},{"cited_title":null,"cited_arxiv_id":null,"evidence_quote":"grounds the argument that non-LTE effects for large molecules are expected mainly below ~30 GHz, supporting the LTE treatment of the detected lines"}],"review_version":1}