Did they ever figure out why Parker's WISPR cameras were able to see the surface of Venus? Mischaracterized filter, or unexpected atmospheric window?

Question

Credit: NASA/Johns Hopkins APL/Naval Research Laboratory/Guillermo Stenborg and Brendan Gallagher

Also refer to the technical material cited in the Space Exploration SE question

• How brightly does Venus's hot surface glow at night? Could you see it? Could you see well enough to walk around?

and see

• At high temperatures, do planets glow like blackbodies?

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Phys.org's Parker Solar Probe offers stunning view of Venus includes the image above taken by the Parker Solar Probe.

My previous question Has Hubble photographed Venus in near IR? If so how does it compare to the new and exciting Parker Solar Probe image? provides extensive specifications and details and several links that I won't copy/paste here. Basically Parker Solar Probe's WISPR cameras have long wavelength cutoffs of 740 and 725 nm implemented by filters.

There was "surprise" that surface features on the dark side of Venus were imaged by one of these cameras during a Parker Solar Probe flyby (gravitational assist) of Venus.

With what was known about the camera filters' band passes, Venus' atmospheric optical transmission windows, and the available light, it was not expected to see anything originating from the surface.

At the time it was suspected that either the filters or Venus' atmosphere had some unexpected transmission at wavelengths that had not been characterized, and so thermal infrared light radiated by Venus' surface was reaching the camera's imaging chip. (With a bandgap of about 1.1 eV, silicon can detect out to about 1100 nm or so depending on doping)

Of course anomalous emission of Venus' surface due to unexpected temperature might also be part of the explanation.

So I'd like to ask:

Question: Did they ever figure out why Parker's WISPR cameras were able to see the surface of Venus? Mischaracterized filter, or unexpected atmospheric window? Or maybe something else?

Answer 1

There's actually a paper by Wood et al. (2022) that came out focused on this. It discusses the expected detection given the expected emission (using a model that combines the surface emission and the emission and radiative transfer through the Venusian atmosphere). They do mention checking to see if the detector might have been more sensitive than assumed, but the answer seems to be no:

... one concern is whether a red leak might exist for the WISPR optics that could account for the detected emission. Additional laboratory measurements were made using analogs of those optics in order to look for any indication of throughput above 0.8 μm, but no evidence for such a leak was found.

The discussion in the paper clearly indicates that the observations are consistent with what was known about Venus and the imager. E.g.,

Further evidence that we are seeing thermal surface-emission within WISPR's optical bandpass comes from demonstrating that the WISPR count rates are consistent with model predictions.

and

... the model spectrum exhibits a broadband attenuation of about a factor of 4 due to absorption from the sulfuric acid clouds. The opacity of the thick atmosphere leads to significant blurring. This effect is presumably why the WISPR images do not show a finer scale structure.

Part of Figure 3 from the paper, showing "A model spectrum of the surface thermal emission from Venus, assuming a temperature of 735 K (left axis). Also shown are the effective area curves of Wide-Field Imager for Parker Solar Probe (WISPR) WISPR-I and WISPR-O (right axis)."

and

Integrating over wavelength, the count rates predicted for WISPR-I and -O are 40.0 DN/s and 33.5 DN/s, respectively. The 33.5 DN/s WISPR-O prediction is in very good agreement with the zero-altitude value displayed in Figure 2

(The zero-altitude value in their Figure 2 is between 30 and 35 DN/s.)

There is also, towards the end, some interesting speculation about whether any of this could be visible to the human eye:

... there have actually been many reports of faint emission from the Venusian night side from credible amateur and professional astronomers, dating back to the 1600s (Baum, 2000; Benton, 2018; McKim, 2019). This “ashen light” phenomenon, as it has come to be called, has never been successfully imaged, however, leading to the suspicion that the phenomenon may be an optical illusion...

(They note that seeing this from Earth with the human eye would be difficult due to the glare from the sunlit crescent of Venus, and because what the eye sees might be dominated by the upper-atmospheric O$_{2}$ emission, since that's at wavelengths where human eye is much more sensitive.)

My impression is that there turned out to be nothing really unusual going on: what the imager(s) saw is more or less what you would expect, given what was known about Venus and the characteristics of the (very) broadband filter and the detector.

So I suspect the "wow, we weren't expecting this!" communicated by the original press release and quotes was more due to the scientists not carefully checking things in advance, and reacting somewhat naively to the results. Remember, this spacecraft and its instruments are for solar science; it was never intended to study Venus at all, and so it wouldn't surprise me if no one had tried to seriously predict what would be seen.