The detection of phosphine (PH₃) has been recently reported in the atmosphere of Venus employing mm-wave radio observations (Greaves et at. 2020). We here demonstrate that the observed PH₃ feature with JCMT can be fully explained employing plausible mesospheric SO₂ abundances (~100 ppbv as per the SO₂ profile given in their figure 9), while the identification of PH₃ in the ALMA data should be considered invalid due to severe baseline calibration issues. We demonstrate this by independently calibrating and analyzing the ALMA data using different interferometric analysis tools, in which we observe no PH₃ in all cases. Furthermore, for any PH₃ signature to be produced in either ALMA or JCMT spectra, PH₃ needs to present at altitudes above 70 km, in stark disagreement with their photochemical network. We ultimately conclude that this detection of PH₃ in the atmosphere of Venus is not supported by our analysis of the data

Arney, G. N.

Biver, N.

Butler, B.

Charnley, S.

Cordiner, M.

Cosentino, R.

de Kleer, K. R.

de Pater, I.

Faggi, S.

Fauchez, T.

Gurwell, M.

Hartogh, P.

Irwin, P.

Kofman, V.

Kopparapu, R.

Lippi, M.

Liuzzi, G.

Luszcz-Cook, S. H.

Mandell, A. M.

Milam, S. N.

Molter, E. M.

Moullet, A.

Nixon, C. A.

Thelen, A. E.

Vandaele, A. C.

Villanueva, G. L.

Wilson, C.

English

Caltech Authors - Main

No phosphine in the atmosphere of Venus 
 
Villanueva G.L., Cordiner, M., Irwin P., de Pater I., Butler B., Gurwell M., Milam S.N., Nixon 
C. A., Luszcz-Cook S. H., Wilson C., Kofman V., Liuzzi G., Faggi S., Fauchez T., Lippi M., 
Cosentino R., Thelen A. E., Moullet A., Hartogh P., Molter E.M., Charnley S., Arney G.N., 
Mandell A.M, Biver N., Vandaele A.C., de Kleer K. R., Kopparapu R. 
 
Submitted on October 26th 2020 to Nature Astronomy as a “Matters Arising” article 
Submitted on October 21st 2020 to Jane Greaves and Luca Maltagliati (Nature Astronomy editor) 
 
The detection of phosphine (PH3) in the atmosphere of Venus has been recently reported 
employing mm-wave radio observations1, hereafter G2020. In this report, we identify several 
fundamental issues in the analysis and interpretation of the spectroscopic data, which would 
mean that the detection PH3 is not supported by our analysis of the data. 
 
The measurements target the fundamental first rotational transition of PH3 (J=1-0) at 266.944513 
GHz, which was observed with the James Maxwell Clark Telescope (JCMT) in June 2017 and 
with the Atacama Large Millimeter/submillimeter Array (ALMA) in March 2019. This line is 
very close to the SO2 (J=309,21-318,24) transition at 266.943329 GHz (only 1.3 km/s away from 
the PH3 line) and therefore this SO2 line represents a serious source of contamination. The 
presented JCMT and ALMA datasets as presented in G2020 are at spectral resolutions 
comparable to the velocity separation of the two lines, and the line cores are several km/s in 
width, which does not permit spectroscopic separation of these two species.  
 
To explore the hypothesis that the observed 267 GHz feature is SO2, and not PH3, we employed 
the same VIRA45 temperature/pressure (T/P) profile used by G2020 (“extended Data Figure 8”) 
and the G2020 SO2 profile (“extended Data Figure 9”). See further information about plausible 
Venus SO2 abundances and known variability in S1. As shown in Figure 1, we can fully explain 
the claimed “PH3” feature with their SO2 profile. For these simulations, we employed three 
independent radiative transfer analyses: the Planetary Spectrum Generator (PSG, 
https://psg.gsfc.nasa.gov)2, the Non-linear optimal Estimator for MultivariatE spectral analySIS 
(NEMESIS)3 and the CfA planetary modeling tool4. The PSG radiative transfer analysis included 
the latest HITRAN SO2 line parameters for a CO2 atmosphere5, a layer-by-layer line-by-line 
study, and a full disk sampling scheme with 10 concentric rings. The NEMESIS analysis was 
also performed in line-by-line mode, and used the same line data with a 5-point Gauss-Lobatto 
disc-integration scheme. As described in G2020, there is some uncertainty in the line-shape 
parameters for the PH3 line in a CO2 atmosphere. HITRAN reports an air linewidth of 0.067   
cm-1/atm for this line, which could correspond to 0.12 cm-1/atm for CO2 if we scale by the typical 
1.8 scaling ratio observed for the SO2 lines6. G2020 employed the line shape for the NH3(J=1-0) 
line at 572.498160 GHz, which has a much greater linewidth of 0.2862 cm-1/atm. This 
uncertainty in the linewidth has a dramatic impact on the inferred PH3 abundance, and our upper 
limit for the abundance of PH3 from these data (after removing SO2) ranges from < 5 ppb (3s) 
for 0.12 cm-1/atm to < 12 ppb (3s) for 0.2862 cm-1/atm (Figure 1). This upper-limit is consistent 
with the recent reports of non-detection of PH3 (< 5 ppb) at infrared wavelengths with 
TEXES/NASA-IRTF7. 
The analysis of interferometric data is relatively complex, in particular for such a bright and 
extended source as Venus (15.4 arcsecs angular diameter for the data ALMA data in G2020). 
The completeness of the different baselines (short and long) defines the accuracy in capturing the 
disk fluxes8, while the bandpass calibration is a crucial factor in the ultimate quality of the 
resulting spectra9,10. How these parameters are treated and defined in the calibration scripts can 
have a dramatic impact on the quality and validity of the resulting ALMA interferometric data. 
The extracted spectra in “extended Figure 4” and interferometric map in “extended Figure 3” of 
G2020 show strong fringing patterns within the PH3 region. Particularly problematic is the fact 
that this fringing has a pattern/width comparable to their defined ± 5 km/s PH3 line core region;  
as we present in S2, artificial features can easily be produced when removing high-order 
polynomials from these residual data11. 
 
Many of these extraneous features are introduced by the selection of baselines and the 
parameters considered in the calibration scripts. For instance, we noticed notable differences in 
the residual spectra and the fringing quality/pattern when we enable the “usescratch=True” 
setting in CASA’s setJy when compared to the G2020 calibration script without this feature. 
This issue has been officially flagged by the ALMA NASSC, and the JAO has taken a decision 
to re-evaluate these data in QA3. Independent processing of the ALMA data by several teams 
(e.g., NASA/GSFC, Berkeley, NRAO) leads to residual spectra with less notable fringing. We 
analyzed the data employing two separate analysis packages: the ALMA/CASA pipeline (with 
corrected scripts), and the Astronomical Image Processing System (AIPS). For the analysis, we 
took the data from the ALMA calibration pipeline, recalibrated the bandpass, self-calibrated 
Venus, subtracted the continuum, made an image cube, and formed a disk-averaged spectrum, 
shown in Figure 2. When employing AIPS, we found no need to exclude short baselines, and no 
need to fit a high-order “continuum baseline” in the disk-averaged spectrum – the resulting 
spectrum is flat, to the noise level (see Figure FS3). Ultimately, our analysis of the data using 
several different approaches reveals no signature of PH3, leading to an upper-limit of < 1 ppb 
(3s) for 0.12 cm-1 to < 2 ppb (3s) for 0.2862 cm-1/atm (Figure 2). We further validate our 
analysis of the ALMA data by probing the abundances of SO2 and HDO employing nearby lines, 
see S3. 
 
Additionally, there is a fundamental mismatch between the inferred vertical profile of PH3 from 
the data done by G2020 with their predictions of the photochemical model employed to interpret 
them. The narrow features of PH3 and SO2 at these frequencies sample the middle/upper 
atmosphere of Venus. Considering a typical linewidth of ~0.2 cm-1/atm for a CO2 atmosphere, 
this would correspond to ~6 GHz of linewidth at 1 bar, and the narrow linewidths of ± 5 km/s 
claimed in G2020 can only be ascribed to altitudes above 70 km (< 2 x 10-2 bar, < 110 km/s 
width). Therefore, any molecular abundance below this altitude range would only contribute to 
the continuum, which is removed by their polynomial fitting scheme. This indicates a major 
inconsistency between the photochemical model results for PH3 presented in “extended Data 
Figure 9” (<65 km) and the data (> 70 km). The photochemical results in G2020 indicate < 0.001 
ppb of PH3 at > 70 km, much lower than the 20 ppbv suggested to be present for the assumed 
deeper altitudes. Thus, the photochemical modeling predictions do not match the PH3 
abundances at the correct altitudes, and significantly higher production rates would be required 
to produce PH3 at the claimed abundances of G2020 for > 70 km, calling into question the 
claimed photochemical network for these results.  
In summary, we demonstrate that the observed JCMT feature can be fully modeled employing 
plausible mesospheric SO2 abundances (~100 ppbv as per the SO2 profile given in G2020’s 
extended figure 9), while the identification of PH3 in the ALMA data should be considered 
invalid due to severe baseline calibration issues. Furthermore, for any PH3 signature to be 
produced in either ALMA or JCMT spectra, PH3 needs to present at altitudes above 70 km, in 
stark disagreement with the G2020 photochemical network. We ultimately conclude that this 
detection of PH3 in the atmosphere of Venus is not supported by our analysis of the data. 
 
  
 
Figure 1: Comparison between the residual JCMT data as presented in Figure 1 of G2020 and 
models of SO2 and PH3. Left: The JCMT data (Figure 1 of G2020) for their most conservative 
polynomial solution is shown with a dashed blue trace. “SO2” is a model spectrum synthesized 
using their T/P (G2020, Figure 8) and their SO2 profile (G2020, Figure 9, ~100 ppbv in the 70-
90 km region), while PH3 is a model spectrum using their PH3 profile (G2020, Figure 9). The 
residual of the data minus the SO2 model is shown as a black trace, leading to no significant 
remnant signal. Right: Comparison of the residual JCMT spectrum (after removing SO2) to 
three models of PH3. The green dotted line shows a model assuming their PH3 profile, while 
the orange and blue lines assume a constant 20 ppbv abundance ratio across all altitudes, 
where the orange line model uses their assumed linewidth (0.2862 cm-1/atm) and blue model 
assumes the HITRAN air-broadened value scaled by 1.8 (0.12 cm-1/atm). 
 
 
  
 
Figure 2: Comparison between ALMA data (G2020 and our independently processed) with 
models of SO2 and PH3. Left: The ALMA data as presented in Figure 2 of G2020 for the 
whole planet, while “SO2 Greaves” is a synthetic spectrum as modeled using their T/P (G2020 
Figure 8) and their SO2 profile (G2020 Figure 9, ~100 ppbv in the 70-90 km region). “SO2 
reduced” is a model with a reduced mesospheric SO2 (~10 ppbv in the 70-90 km region) 
matching the nearby (J=133,11-132,12) line at 267.537458 GHz (see Figure 4) – abundances 
comparable to those reported by Encrenaz et al. (2015). The PH3 model shown employs a 
constant 20 ppbv abundance ratio across all altitudes, with a linewidth of 0.12 cm-1/atm. 
Right: Same as the left panel but showing the newly processed data employing the default 
ALMA pipeline. 
 
 
 
  
Supplementary material 
 
S1: Vertical profiles 
 
The temperature profile as presented in “extended Data Figure 8” of G2020 is consistent with the 
Venus International Reference Atmosphere (VIRA)13 for mid-latitudes (45 degrees latitude), and 
also consistent with later observations14. Their SO2 profile, in particular at altitudes above 70 km 
relevant to this investigation, in “extended Data Figure 9” is consistent with previous 
observations15–17. Yet, SO2 is known to vary greatly temporally and spatially12,16–18, ranging from 
<10 to >200 ppbv in the 70-100 km altitude region. Large variations have been observed on 
timescales of hours to months12, so mesospheric SO2 abundances at the time of the JCMT 
observations in June 2017 cannot be constrained by SO2 abundances measured using ALMA in 
March 2019. The reported SO2 profile in “extended Data Figure 9” of ~100 ppbv in the 70-90 
km altitude range can then be assumed to be a plausible and average value for SO2. 
 
S2: Analysis of the ALMA using G2020 calibration scripts 
 
We investigated their data present on the archive, and analyzed it using the original pipeline 
calibration (G2020 Supplementary Software 2, 3). We mapped Venus including all baselines, 
subtracted the continuum emission using the uvcontsub task in CASA, and produced the 
spectrum in Figure FS1(left) integrated over Venus, including its limb. We then overlaid two 12th 
order polynomial fits: the blue one a fit to the entire spectrum, and the red one excluding the 
center 10 km/s (i.e., from -5 to +5 km/s), as done in G2020. The right panel shows the spectra 
after subtracting the polynomial fits (blue for the blue polynomial, red for the red one). As 
expected, a feature shows up in the red spectrum, i.e., part of the original dip in the bandpass. 
This process clearly shows that with harmonics in the bandpass as observed, subtracting 
polynomial fits to part of the data may create line profiles that might be confused with real lines. 
 
S3: Validation of our ALMA analysis by interpreting other nearby lines 
 
We independently analyzed the ALMA data using our calibration scripts for the region near the 
SO2 line at 267.537458 GHz and the HDO (J=22,0-31,3) line at 266.16107 GHz (Figure FS2). 
From these data, we obtain a mesospheric SO2 abundance value of  ~10 ppb, lower than that 
specified in the G2020 “extended Data Figure 9” yet well within the range previously reported 
SO2 abundances from ALMA observations at 346.652167 GHz12 and comparable to the value 
retrieved from this line by G2020. This level of variability observed between JCMT, ALMA, and 
previous ALMA measurements is also consistent with previous reports of the SO2 
temporal/spatial variability12,16–18. Similarly for HDO, we also observe a consistent set of data 
and model results when employing a plausible water profile and assuming a D/H of 20019. 
 
 
  
 
Figure FS1: Left: Disk-integrated spectrum of Venus (including its limb) as constructed from 
the data used in G2020 (but without deleting short spacings). Polynomial fits (12th order) are 
superposed: “blue” trace is a fit to the entire spectrum, while “red” is a fit excluding the center 
-5 to + 5 km/s. Right: Spectra after subtracting the polynomial fits (red: subtracting red 
polynomial; blue: subtracting blue polynomial).  
 
 
Figure FS2: Comparison between models and ALMA data, as presented in Figure 4 of 
G2020, for the SO2 (J=133,11-132,12) transition at 267.537458 GHz and for the HDO (J=22,0-
31,3) transition at 266.161070 GHz. Left: Our independently processed ALMA data for the 
SO2 line, while “SO2 Greaves” is a synthetic spectrum modeled using their T/P (G2020 
Figure 8) and their SO2 profile (G2020 Figure 9), “SO2 reduced” is a model with reduced 
mesospheric SO2 levels (~10 ppb in the 70-90 km range) and comparable to those reported 
by 12. Right: Our independently processed ALMA data for a nearby HDO line presented, 
while “HDO model” is a synthetic spectrum as modeled using their T/P (their figure 8), a 
D/H of 20019, and plausible H2O abundances of ~60 ppbv in the mesosphere (70-100 km). 
 
 
Figure FS3: Disk-averaged spectrum in the PH3 narrow spectral window, after re-reduction in 
AIPS.  No PH3 line is detected. 
 
 
 
 
 
 
  
References 
 
1. Greaves, J. S. et al. Phosphine gas in the cloud decks of Venus. Nat. Astron. 1–10 (2020) 
doi:10.1038/s41550-020-1174-4. 
2. Villanueva, G. L., Smith, M. D., Protopapa, S., Faggi, S. & Mandell, A. M. Planetary 
Spectrum Generator: An accurate online radiative transfer suite for atmospheres, comets, 
small bodies and exoplanets. J. Quant. Spectrosc. Radiat. Transf. 217, 86–104 (2018). 
3. Irwin, P. G. J. et al. The NEMESIS planetary atmosphere radiative transfer and retrieval tool. 
J. Quant. Spectrosc. Radiat. Transf. 109, 1136–1150 (2008). 
4. Gurwell, M. A., Bergin, E. A., Melnick, G. J. & Tolls, V. Mars surface and atmospheric 
temperature during the 2001 global dust storm. Icarus 175, 23–31 (2005). 
5. Gordon, I. E. et al. The HITRAN2016 molecular spectroscopic database. J. Quant. 
Spectrosc. Radiat. Transf. 203, 3–69 (2017). 
6. Wilzewski, J. S., Gordon, I. E., Kochanov, R. V., Hill, C. & Rothman, L. S. H2, He, and 
CO2 line-broadening coefficients, pressure shifts and temperature-dependence exponents for 
the HITRAN database. Part 1: SO2, NH3, HF, HCl, OCS and C2H2. J. Quant. Spectrosc. 
Radiat. Transf. 168, 193–206 (2016). 
7. Encrenaz, T. et al. A stringent upper limit of the PH$_3$ abundance at the cloud top of 
Venus. ArXiv201007817 Astro-Ph (2020). 
8. Pater, I. de et al. First ALMA Millimeter-wavelength Maps of Jupiter, with a 
Multiwavelength Study of Convection. Astron. J. 158, 139 (2019). 
9. Thelen, A. E. et al. Detection of CH$_3$C$_3$N in Titan’s Atmosphere. (2020). 
10. Yamaki, H., Kameno, S., Beppu, H., Mizuno, I. & Imai, H. Optimization by Smoothed 
Bandpass Calibration in Radio Spectroscopy. Publ. Astron. Soc. Jpn. 64, (2012). 
11. Snellen, I. A. G., Guzman-Ramirez, L., Hogerheijde, M. R., Hygate, A. P. S. & van der Tak, 
F. F. S. Re-analysis of the 267-GHz ALMA observations of Venus: No statistically 
significant detection of phosphine. ArXiv201009761 Astro-Ph (2020). 
12. Encrenaz, T., Moreno, R., Moullet, A., Lellouch, E. & Fouchet, T. Submillimeter mapping of 
mesospheric minor species on Venus with ALMA. Planet. Space Sci. 113–114, 275–291 
(2015). 
13. Seiff, A. et al. Models of the structure of the atmosphere of Venus from the surface to 100 
kilometers altitude. Adv. Space Res. 5, 3–58 (1985). 
14. Limaye, S. S. et al. Venus Atmospheric Thermal Structure and Radiative Balance. Space Sci. 
Rev. 214, 102 (2018). 
15. Vandaele, A. C. Composition and Chemistry of the Neutral Atmosphere of Venus. Oxford 
Research Encyclopedia of Planetary Science 
https://oxfordre.com/planetaryscience/view/10.1093/acrefore/9780190647926.001.0001/acre
fore-9780190647926-e-4 (2020) doi:10.1093/acrefore/9780190647926.013.4. 
16. Vandaele, A. C. et al. Sulfur dioxide in the Venus atmosphere: I. Vertical distribution and 
variability. Icarus 295, 16–33 (2017). 
17. Vandaele, A. C. et al. Sulfur dioxide in the Venus Atmosphere: II. Spatial and temporal 
variability. Icarus 295, 1–15 (2017). 
18. Marcq, E. et al. Climatology of SO2 and UV absorber at Venus’ cloud top from SPICAV-
UV nadir dataset. Icarus 335, 113368 (2020). 
19. Piccialli, A. et al. Mapping the thermal structure and minor species of Venus mesosphere 
with ALMA submillimeter observations. Astron. Astrophys. 606, A53 (2017). 


No phosphine in the atmosphere of Venus

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No phosphine in the atmosphere of Venus

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