Future Venus Missions Options

[stevesliva]

Aha. This layman had been wondering about SOFIA, and this reddit thread (where you are free to go argue about whether other people are more or less excited than they should be), his this tidbit:

Nicely pullquoted from a NG article that I didn't read because I didn't want their spam:
https://www.nationalgeographic.com/science/...-phosphine-gas/

Sousa-Silva agrees that the team needs to confirm the phosphine detection by finding additional fingerprints at other wavelengths. She and her colleagues had planned such observations using the Stratospheric Observatory for Infrared Astronomy, a plane-mounted telescope, and with NASA’s Infrared Telescope Facility in Hawaii. But COVID-19 got in the way, and the team’s attempts have been put on hold.

[Bjorn Jonsson]

The UMSF moderating team has been discussing the recent Venus news in the context of rule 1.3 (which I assume everyone here is familiar with ;-).

The result is that it has been decided to relax rule 1.3 a bit in this thread, at least temporarily. This means that discussion of instrumentation and methods is entirely appropriate, and of course the purpose of some of the instruments would be to detect biosignatures (this means that it is perfectly OK to mention biosignatures in the discussion).

We need to emphasize the following, though:

1. Anybody claiming the discovery of life will be booted (no "unsung basement genius" bullshit).

2. Organic chemistry in the context of signatures is okay, but not extrapolation based on findings since that can get stupid quickly. This includes linking to the inevitable torrent of imaginative 'life in the clouds of Venus' nonsense articles that will shortly pop up all over.

3. Discussions that veer into sheer speculation (or even crackpot theories) will be shut down immediately.

Now let's have fun discussing possible future Venus missions etc. which suddenly have become a whole lot more exciting! (and even before this I found many of them interesting, e.g. DAVINCI+, VERITAS and EnVision).

[Phil Stooke]

How about this?

Phil

[dtolman]

Wondering about whether VEX could have picked up signatures, I noticed this in an article in Forbes - could VEX's VIRTIS have collected evidence of Phosphine's presence already?

Phosphine has an absorption band in the infrared spectrum at roughly 3.05 microns, Julie Castillo, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, told me. So, one should check the observations of the VIRTIS instrument on Venus Express for the possible presence of that phosphine in its VIRTIS observations, she says.

[JRehling]

It has been well noted that the SO2 composition of Venus' atmosphere has varied dramatically over the duration of observations. It remains possible that the phosphine detected on Venus in 2017 wasn't present when Venus Express ended in 2014. (I'm not sure when VIRTIS last took measurements, so the EOM in 2014 may not be the relevant start of the window between last Venus Express measurement and the phosphine detection.

The Sousa-Silver paper notes that "phosphite can disproportionate to phosphine at T > 323K and acidic pH." If a volcanic event on Venus liberated phosphorus-bearing minerals in the last few years, the detection of phosphine could record a very temporary event.

In fact, I wonder if Venus provides a loophole here that hasn't been discussed (Sousa-Silver, et al were talking about exoplanets in general, not Venus), that a non-volcanic mass movement event on Venus (earthquakes, landslides) could introduce subsurface minerals to surface conditions and begin chemical reactions at the time of the event. The "acidic pH" condition might be met at the surface, but not in the subsurface. No volcano required. No inherent paradox that a gas that should be destroyed quickly in Venus' atmosphere is present now.

From my armchair (I am literally sitting in an armchair right now), VERITAS would likely provide better insight as to the rate and recency of mass movement events on Venus, while DAVINCI+ obviously provides different opportunities to investigate the phosphine result.

[Gerald]

The Sousa-Silver paper notes that "phosphite can disproportionate to phosphine at T > 323K and acidic pH."...

H₃PO₃ is thermally decomposed ("disproportionated") into PH₃ and H₃PO₄ near 200°C.
If there would exist some catalytic process in the presence of H₂SO₄ and CO₂ to recover the H₃PO₃ from H₃PO₄, we would get a closed cycle, depending a bit on the oxidation products of PH₃, and less supply from exogenic sources would be required.
Just to get rid of that one easy-looking O is probably the point where some "new chemistry" is needed. At least I wasn't able to find a paper describing or providing at least a hint how to track this way back.

[mcaplinger]

Wondering about whether VEX could have picked up signatures...

To detect something in absorption, you have to 1) know where the absorption is, and then 2) you have to have a light source that would normally have a lot of energy at that wavelength (either reflected sunlight or thermal emission, depending on wavelength) and 3) it can't be absorbed or scattered by the other stuff between the instrument and the target, and 4) the instrument has to have enough SNR and spectral/spatial resolution to detect the absorption.

1 depends on the behavior of the substance under venusian conditions (if that's known; phosphine is horrifically toxic so people may not have enthusiastically studied it in the lab, I sure wouldn't); 2 and 3 depend on the complex properties of the atmosphere above (and for emission, below) the clouds; 4 is usually something the instrument designers know.

As I said upthread, I'm sure this will be gone over carefully for every dataset there is, and we'll see what happens.

[Explorer1]

We might not need to wait that long for new atmospheric data: Bepicolombo has two flybys coming up (this October and next August), and the MERTIS instrument does have the spectral range to attempt observations. The main hurdle is resolution; the August flyby is more likely to make any phosphine detection as it is only going to be 500 km away vs 10,000 for the first flyby (see this article)

[JRehling]

It occurred to me yesterday that the phosphine is the second mystery of Venus that (may) relate to potassium, or at least to elemental composition in the same part of the periodic table.

Venus has an extremely high ratio of 36Ar/40Ar compared to that on Earth, reflecting a somewhat lower amount of 40Ar but a much higher amount of 36Ar. 40Ar is produced by the radioactive decay of 40K, so this seems to indicate that the Venusian atmosphere has had less interchange with any reservoir containing 40K.

That said, this doesn't point in the same direction as any excess of potassium, and even if it did, 40K is a very small fraction of terrestrial potassium anyway. This is also a comment on nuclear chemistry, not the chemistry that produces/sustains phosphine. What it does say is that the elemental composition of the Venusian atmosphere and crust is not like Earth's in some striking ways. If 36Ar can be superabundant on Venus, could 39K (related in no causal way that I see) also be superabundant? And if so, that could help boost the signal that we're seeing here.

Venera 8 and Venera 13 both reported high levels of K in the surface, while the other five sites tested reported low K. It seems to me like we need more and better understanding of venusian crustal composition and landing on one or two new sites isn't going to resolve the issue. Perhaps emissivity maps from Veritas could use the Soviet data to ground truth maps of surface composition. Failing that, we would need more surface probes or, as I suggested before, one with great mobility.

[Gerald]

Venus has an extremely high ratio of 36Ar/40Ar compared to that on Earth, reflecting a somewhat lower amount of 40Ar but a much higher amount of 36Ar. 40Ar is produced by the radioactive decay of 40K, so this seems to indicate that the Venusian atmosphere has had less interchange with any reservoir containing 40K.

Isn't this a straightforward consequence of the much higher mass of the atmosphere (including Ar-36) of Venus compared to that of Earth, with composition (especially Ar-36 ratio of the atmosphere and bulk K-40 ratio in the planet), and planet mass and density otherwise assumed to be initially almost the same? Both planets would then produce the same amount of Ar-40 and release it to atmospheres of very different mass, especially of Ar-36.

[JRehling]

There's no consensus on why Venus shows the isotopic balances that it does, and some explanations suppose that Venus has simply never outgassed all of the Ar-40 it has inside. With rarer noble gases, we don't even know the isotope ratios, and determining those is a major motivation for the DAVINCI+ probe, which will let us check the validity of hypotheses regarding argon.

[Phil Stooke]

https://arxiv.org/abs/2009.12758


Maybe Phosphine in Pioneer Venus Probe data! (still of unknown origin)

Phil

[Gerald]

... obtained with the entirely different method of mass spectroscopy.
Plus hints towards traces of reduced species like molecular hydrogen, hydrocarbons, e.g. methane, the not fully oxidized species NO, and molecular oxygen O₂.
And if it's "only" unknown chemistry, we could learn something from accurate und unabiguous in-situ measurements.

[vjkane]

Another team observing Venus in the thermal infrared has failed to detect phosphine, suggesting an upper limit on concentrations several times lower than that suggested by observations at millimeter wavelengths.

Press release link on SpaceRef

Following the announcement of the detection of phosphine (PH3) in the cloud deck of Venus at millimeter wavelengths, we have searched for other possible signatures of this molecule in the infrared range.

Since 2012, we have been observing Venus in the thermal infrared at various wavelengths to monitor the behavior of SO2 and H2O at the cloud top. We have identified a spectral interval recorded in March 2015 around 950 cm−1 where a PH3 transition is present.

From the absence of any feature at this frequency, we derive, on the disk-integrated spectrum, a 3-σ upper limit of 5 ppbv for the PH3 mixing ratio, assumed to be constant throughout the atmosphere. This limit is 4 times lower than the disk-integrated mixing ratio derived at millimeter wavelengths.

Our result brings a strong constraint on the maximum PH3 abundance at the cloud top and in the lower mesosphere of Venus.

T. Encrenaz (1), T. K. Greathouse (2), E. Marcq (3), T. Widemann (1), B. Bézard (1), T. Fouchet (1), R. Giles (2), H. Sagawa (4), J. Greaves (5), C. Sousa-Silva (6) ((1) LESIA, Observatoire de Paris, PSL Université, CNRS, Sorbonne Université, Université de Paris, (2) SwRI, (3) LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, (4) Kyoto Sanyo University, (5) School of Physics and Astronomy, Cardiff University, (6) Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology)

Comments: Astronomy & Astrophysics, in press
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2010.07817 [astro-ph.EP] (or arXiv:2010.07817v1 [astro-ph.EP] for this version)
Submission history
From: Bruno Bézard
[v1] Thu, 15 Oct 2020 15:11:37 UTC (805 KB)
https://arxiv.org/abs/2010.07817
Astrobiology, Astrochemistry,
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[Xerxes]

The original paper attempted to pre-rebut this argument:

We caution that mid-infrared and far-infrared observations might not detect any phosphine in absorption, depending on the height where the quasi-continuum signal is generated in these wavebands; e.g. at high altitudes, the molecules are expected to be photo-destroyed.

This paper makes some arguments as to why the original signal would actually have to be at high altitude:

the core of millimeter line is very narrow (less than about 20 MHz). If the PH3 millimeter line was formed within the clouds, at a pressure level of 100 mbars or more, its HWHM would be at least 0.01 cm−1, i.e. 300 MHz. Such a broad line would not be observable by heterodyne spectroscopy. This implies that the millimeter line observed by Greaves et. al. (2020) must be formed relatively high in the mesosphere

I suppose one would need a great deal more expertise to know whose argument is stronger.