Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids

arXiv:2608.11779 · astro-ph.EP · Submitted 2026-08-12 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. The week's best astrophysics papers, unpacked for curious ears.

Vera: Next we'll be talking about the paper "Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids".

Jocelyn: The paper was written by T. J. Dyer, J. Beccarelli, M. Delbo, C. Avdellidou, M. Pajola et al. from Université Côte d’Azur and Centre national d’études spatiales and Istituto Nazionale di Astrofisica and University of Leicester.

Vera: Stay tuned as we take you through the paper and discuss its implications.

Paper summary: Vera: Welcome back to the discussion. We're continuing with the paper 'Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids'. Jocelyn, when you look at their Table one what's the most striking thing?

Jocelyn: The clean progression across the three spectral units — Blue, Transitional, Red — with infrared-to-visible ratios of roughly one point six to one point eight five, then two point three two to two point three five, then two point nine zero to three point one two. Those are clearly separated from each other. And when the authors compare them to the asteroid distributions, the Red and Transitional units land right in the high-ratio regime of D/T-type asteroids, while the Blue unit pulls down toward the dark X/P-types. So Phobos isn't just one kind of primitive object; its surface spans a range of primitive behaviour.

Vera: So even the Blue unit is redder than the C-complex average?

Jocelyn: Yes, that's the important point — the C-complex mean is only one point one six with a small uncertainty, so all three Phobos units sit well above the core C-complex distribution. The Blue unit’s points are still compatible with some C-complex outliers, but the bulk of that population is much lower. [Subrahmany

Page 1 of the paper: Vera: So this first page really sets the stage with the abstract and the opening of the introduction. What stands out to me is that the authors are proposing a very simple, almost blunt tool: the ratio of infrared to visible albedo, pIR over pV, as a way to gauge the overall slope of a surface from visible wavelengths out to a few microns. It's not a substitute for full spectra, but it's something you can compute consistently for thousands of asteroids from WISE and ground-based surveys.

Jocelyn: So it's like taking the color of the surface, but with a much wider baseline than your usual color index?

Subrahmanyan: Exactly, and that's the clever part. A broadband ratio like that is crude, but it's remarkably powerful for population comparisons because you can apply the same recipe to Phobos and to tens of thousands of asteroids. The introduction explains why that matters: red-sloped, primitive classes like D- and T-types and Jupiter Trojans tend to have high pIR over pV, while C-complex objects sit lower, so the ratio carries information about composition and even weathering state.

Vera: And they argue that this is especially valuable for Phobos, because its spectrum is notoriously bland. The page reminds us that decades of visible and infrared observations have failed to pin down its mineralogy due to the lack of strong absorption features.

Jocelyn: Right, so you can't just look for a diagnostic band, you have to lean on things like the overall spectral slope and how it varies across the surface. And that's where the three units come in — Blue, Transitional, Red — which I gather are defined by how steeply their reflectance rises toward longer wavelengths.

Subrahmanyan: Yes, and the key novelty on this page is that they're not treating Phobos as a single point. They're isolating spatially resolved regions from Mars Express OMEGA data and measuring that infrared-to-visible ratio separately for each unit. That lets them ask whether the different terrains on Phobos fall into the same parameter space as primitive asteroid populations, unit by unit, rather than just comparing the whole moon.

Vera: The introduction also frames this within the big open question: is Phobos a captured primitive body from the outer solar system, or debris from a giant impact on Mars? The authors are careful to say that the ratio isn't a magic bullet, but it's a fresh way to test the capture scenario because it directly connects Phobos' surface to asteroid taxonomy.

Jocelyn: So the abstract is almost like a promise, and the introduction is the reasoning for why that ratio is worth computing in the first place. It's a good setup, but I'm curious how the actual numbers come out, especially for the Red unit.

Vera: We'll get there — the results are just a few pages on. But before that, I want to ask about how they dealt with thermal emission and phase reddening, because I suspect that's where the real care had to go.

Page 2 of the paper: Vera: So moving on to page two, this is where the paper really sets up the puzzle and the tool they’re using to crack it.

Jocelyn: Yes, and the first thing that struck me is that they admit spectroscopy alone hasn’t settled the big question about Phobos, because its spectrum is basically featureless, just slightly red, and that can be explained in too many different ways.

Subrahmanyan: Exactly, so instead of hunting for a unique mineral fingerprint, they pivot toward the relative spectral variations across Phobos, the famous Blue, Transitional, and Red units, and ask whether those variations behave like primitive asteroid surfaces do.

Vera: And that’s where the method comes in: they use the OMEGA spectrometer on Mars Express, which gives them spatially resolved spectra at a much finer scale than ground-based telescopes ever could.

Jocelyn: Right, the paper points out that OMEGA covers roughly zero point three five to five point one microns, so it catches both the visible and the near-infrared, which is exactly the wavelength range where primitive materials show their diagnostic slopes.

Subrahmanyan: They also emphasize that a single well-characterized instrument avoids the systematic headaches of stitching together different telescopes, and the particular observation they use gave them about one hundred seventy-one meters per pixel on Phobos, which is remarkable for a moon that’s only about twenty-two kilometers across.

Vera: So on the asteroid side, they pull from the MP3C catalogue, taking infrared albedos from NEOWISE and visible albedos from a bunch of literature sources, and then they filter out anything with a visible albedo above zero point one two.

Jocelyn: Why that cutoff? Because Phobos itself is very dark, its brightest terrain has a visible albedo of only about zero point zero eight, so comparing it to bright asteroids would be apples and oranges.

Subrahmanyan: Precisely, and then they sort the remaining asteroids into the relevant primitive classes: C-complex, dark X and P types, the D and T types, and also a separate Z-type class from a newer taxonomy, because those are expected to be the reddest of the red.

Vera: So this page is essentially saying: we can’t rely on spectral features alone, so we’ll use a broadband visible-to-infrared ratio as a proxy for the spectral slope, and we’ll compare Phobos’s individual terrain units against a large, carefully filtered population of low-albedo asteroids.

Jocelyn: And the neat part is that this sets up the actual results that come on the next pages, where each Phobos unit lands at a different place in that parameter space, which is exactly what you’d want if you’re trying to test whether Phobos is a captured asteroid or something else.

Subrahmanyan: It also sets a nice precedent, because the paper is careful to say that a single ratio like this can’t uniquely identify composition, so the comparison is really about population-level behavior, not a one-to-one match.

Page 3 of the paper: Vera: So we've seen how they built the asteroid comparison catalog from MP3C, and we've got the big picture of what they're measuring. But page three is where they get down to the actual Phobos data. Subrahmanyan, what should we make of the OMEGA observations?

Subrahmanyan: A great deal of care goes into even getting a usable spectrum out of Phobos. The OMEGA instrument on Mars Express took these images from about one hundred fifty kilometers away, which sounds close, but at that range, thermal emission from the moon itself drowns out reflected sunlight beyond about two point five microns. So the team has to model and subtract that thermal component before they can measure anything like an albedo ratio, and they do that correction per pixel, then repeat the whole process a hundred times to see how stable it is.

Jocelyn: So the raw data is essentially a mix of reflected light and heat, and you have to peel away the heat to see the actual surface reflection? That sounds like a tricky subtraction.

Subrahmanyan: Exactly, and it gets trickier because they're not just averaging whole images. They carefully choose regions of interest within each of the three spectral units, and since the red unit is much more extensive, they can sample three separate spots there, while the transitional and blue units each get just two representative areas. That spatial sampling matters for the uncertainties, which they track using three separate components: instrumental noise, the thermal correction itself, and how much the spectra vary across the region.

Vera: I noticed they also spend a whole section on phase reddening. Why is that so important on this particular page?

Subrahmanyan: Because the OMEGA observation was taken at a high phase angle of over sixty degrees, meaning the Sun, Phobos, and the spacecraft were at a wide angle, which can make a surface appear redder than it really is. The WISE asteroid observations, by contrast, were made at much lower phase angles, so if they hadn't corrected Phobos, they would be comparing apples and oranges. They use phase-reddening coefficients from the CaSSIS camera to apply a linear tilt to each spectrum, essentially predicting what Phobos would look like at low phase angle, and the correction turns out to be small, which is reassuring.

Jocelyn: And that small correction is consistent with what we'd expect for D-type asteroids, right? They mention that as a check on the whole approach.

Subrahmanyan: Exactly, it's a good sanity check. Also on this page they explain why they only use the WISE W1 band for the infrared side and not the longer W2 band: the OMEGA uncertainties grow rapidly beyond four microns, making any W2 comparison unreliable, but since the Phobos spectra are basically linear in the near-infrared, sticking with W1 doesn't lose them any real information.

Vera: There's also that nice caution about the red unit's curvature beyond two point five microns. They say it might be intrinsic or might be a residual thermal artifact.

Subrahmanyan: Yes, they're very honest about that ambiguity. The thermal correction itself has a concave shape in that wavelength range, so the curvature they see in the red unit could easily be a leftover from the subtraction rather than a real surface property. That's why they don't build any interpretation on it, and why the numbers that come out of this page, the synthetic V and W1 reflectance ratios with properly propagated uncertainties, are the anchors for all the asteroid comparisons that follow.

Jocelyn: So the bottom line from page three is that the data reduction is solid enough to trust those ratios, and the phase correction doesn't change the story dramatically. Good, now we can actually look at the results.

Page 4 of the paper: Vera: So Subrahmanyan, after all that careful correction for the thermal emission and phase reddening, what did they actually do with the Phobos spectra to get a number you can compare directly to the asteroid catalogue?

Subrahmanyan: They built synthetic photometry. Instead of just taking a simple average of the spectrum over the visible and infrared ranges, they folded the OMEGA spectra through the actual passbands of the Bessell V filter and the WISE W1 filter, weighted by the solar spectrum and the photon-counting response of the detector. That gives them an effective V-band and an effective three point four-micron reflectance for each region of Phobos, and then they take the ratio of those two.

Jocelyn: So it’s like mimicking what a telescope would have seen if you pointed it directly at the surface, rather than using the spectrometer values raw.

Subrahmanyan: Exactly. And that ratio they call RP, and it comes out in Table one with all the uncertainties propagated from the spectra. The three Red unit regions give ratios of two point nine zero, three point zero zero, and three point one two. The two Transitional regions sit lower, at two point three five and two point three two. And the Blue regions are much lower, one point eight five and one point six four. So there’s a clean progression from blue to transitional to red, which tells you the red surfaces really are much brighter at three point four microns relative to visible light than the blue surfaces.

Vera: And that progression matches the old spectral slope story, but now it’s in a specific band ratio that can be compared to asteroids.

Subrahmanyan: Yes. And the comparison is on the histograms in Figure two plus the mean values for each asteroid class. The C-complex asteroids have a mean ratio of one point one six, dark X and P types one point four seven, Z-types two point one two, and D/T-types two point two nine. So the Phobos Blue unit lands on the high tail of the C-complex and the lower part of the D/T distribution, while the Transitional and Red units sit right in the D/T range, in some cases actually pushing above the mean.

Jocelyn: So the reddest parts of Phobos are behaving like D-type or T-type asteroids, the kinds of primitive bodies that are common in the outer belt and Jupiter Trojans?

Subrahmanyan: That’s the main takeaway, but with a caveat. The Z-types formally span the whole range of Phobos values, but their distribution is irregular and only thirty-two objects survive the quality cut. So even though they overlap the Transitional and Red regions, they don’t really form a strong peak there, so you can’t claim they’re the best match on this parameter alone.

Vera: And I suppose that’s also why the authors are careful to say the ratio is a broad-band proxy, not a unique fingerprint of composition.

Subrahmanyan: Right, because several different primitive classes can produce similar visible-to-infrared ratios, and reddening from space weathering can mimic composition. So this page establishes that Phobos' units are compatible with primitive asteroids, especially D/T-types, but it doesn’t yet pin down a single parent body. The real discriminating power will have to come from the detailed spectroscopy over a wider range, like what MMX will send back.

Page 5 of the paper: Vera: Alright, we're at page five now, and this is where the paper actually delivers the numbers we've been waiting for. Jocelyn, what do we see in that table?

Jocelyn: So Table one gives the infrared-to-visible reflectance ratio for each of the seven regions they studied. The Red unit sits between two point nine zero and three point one two, the Transitional unit is right around two point three two to two point three five, and the Blue unit drops to between one point six four and one point eight five. That's a clear progression — the reddest terrain is nearly twice as steep in its visible-to-infrared slope as the bluest.

Vera: And that's exactly what we'd expect from the spectral units we've been discussing.

Subrahmanyan: Yes, but the new part is the comparison to asteroid populations. They've computed mean ratios for different classes, and the D/T-type asteroids average two point two nine, which matches the Transitional unit almost perfectly. The Red unit falls even higher, into the tail of that same distribution, while the C-complex average is only one point one six, well below even the Blue unit.

Jocelyn: So the reddest parts of Phobos are really pointing toward the D/T-types.

Subrahmanyan: That's the headline, but it's not a clean verdict. They also show that the Blue unit overlaps the high tail of the dark X/P-types and even the C-complex to some degree. And the Z-types, though only thirty-two objects, span the entire range of all three Phobos units. So the authors are careful to say that no single asteroid class is uniquely associated with Phobos.

Vera: So all three units live in the same broad parameter space as primitive asteroids, but the correspondence is a matter of degree rather than a fingerprint.

Jocelyn: Exactly. And to strengthen the comparison, they add a second parameter on the same page — the visible geometric albedo. When you plot that against the ratio, the Red unit measurements cluster around a visible albedo of about zero point zero six six, which puts them near the D/T-types again. But the Blue unit sits somewhat outside the dense core of every population shown in figure three.

Vera: So the Blue unit is the odd one out?

Subrahmanyan: It's more that the Blue unit is harder to assign. Its ratio and albedo place it partly with D/T-types and partly with dark X/P-types, but it's not central to any of them. The authors conclude that this combined comparison does not uniquely associate either unit with a single class, though they do note that the Red unit measurements mostly favour D/T-types and Z-types.

Jocelyn: And that's a key point for the origin debate. If the Red unit is the most ancient, heavily space-weathered terrain, then its resemblance to D/T-types supports a primitive captured body. But the Blue unit's ambiguity leaves room for compositional mixing or exogenous material delivered by later impacts.

Vera: So page five effectively gives us the evidence, but also the caveats. The ratio alone can't settle whether Phobos is a captured asteroid or a pile of Martian debris.

Subrahmanyan: Precisely. They're very explicit that this parameter is an integrated slope proxy, not a unique compositional indicator. You need the full spectroscopy, which they point to as the next step.

Vera: Good. Let's hold that thought and come back after the break to talk about what the future instruments might resolve.

Page 6 of the paper: Jocelyn: Now we're getting to the heart of the comparison, on page six, where the authors put Phobos side by side with actual asteroid populations. We've already seen the measured ratios, but this page is where they ask which asteroid types those numbers actually resemble.

Subrahmanyan: Exactly, and the key result is that the different terrain units on Phobos do not all match the same kind of asteroid. The histograms show that the C-complex asteroids, the dark carbon-rich ones, are mostly concentrated at lower infrared-to-visible ratios, so they barely reach the Blue Unit values and miss the Red and Transitional Units entirely. In contrast, the D and T-type asteroids spread across the full range, and all seven Phobos regions fall somewhere inside their distribution. That is a meaningful statement, because it means the surface of Phobos behaves like these very red, primitive objects rather than like the more common dark asteroids.

Vera: So the Red Unit, which we thought was the most weathered, sits in the tail of the D/T distribution, while the Blue Unit is more toward the lower end. But what about those mysterious Z-types that were brought in separately?

Subrahmanyan: That is a really important caveat, because the Z-types actually span the entire range of Phobos values, which sounds like a perfect match at first glance. However, the paper points out that most Z-type asteroids cluster at moderate ratios, and the Transitional and Red Units only overlap with their sparse, flat high-ratio tail. So the authors are careful to say that a range overlap is not the same as a strong population-level match, especially with only thirty-two objects in the sample. They even add that Z-types can have concave near-infrared spectra, meaning a low ratio might hide a steep slope that turns over at longer wavelengths.

Jocelyn: That makes sense, but it feels like there is a tension with earlier work that suggested the Blue Unit might be P-type, is that what they are addressing here? The paper mentions Wargnier and others who found a similarity with P-type asteroids.

Subrahmanyan: Yes, and the authors do not dismiss that, they just point out that their own observable is different. Wargnier compared spectral slopes up to about two and a half microns using CRISM data, while this study uses a broad-band ratio between the visible and the W1 band near three point four microns. With that broader window, the Blue Unit also matches D and T-type asteroids, and to a lesser extent some dark X or P-types, so the P-type connection is not necessarily wrong, it is just not the only possibility.

Vera: So page six is essentially saying that Phobos is compatible with several primitive asteroid classes, but nothing is a unique fingerprint. Is that why they bring up the disk-integrated spectra from Takir as well?

Subrahmanyan: Precisely, because Takir and colleagues found that the overall spectrum of Phobos looked D-type, and that is consistent with what this page shows, but only when you look at the population as a whole. The authors stress that the D/T distribution is the best match, yet they also keep the door open for a mixture of materials and even Martian impact debris as contributing to the redder terrain. So the new contribution here is really the population-level view, showing that the visible-to-infrared albedo ratio places Phobos squarely among primitive outer-belt and Trojan-like objects, without being able to pin down one exclusive origin story.

Jocelyn: And that sets up the final pages nicely, because they then discuss what could cause those spectral differences, whether it is weathering or composition. For now, though, page six gives us the main takeaway that all three Phobos units sit within the D/T-type parameter space, with the Z-types being a possible but weaker match.

Page 7 of the paper: Vera: On page seven, the authors stop presenting numbers and start arguing about what those numbers mean. They've shown that all three Phobos units overlap with primitive asteroid populations, but they now stress that this does not settle the question of whether Phobos is a captured asteroid or debris from a Mars impact. The overlap with D/T-types is real, but they write that it does not uniquely support a captured-asteroid origin, because Martian material could be mixed in as well.

Jocelyn: So the spectral match isn't decisive at all?

Subrahmanyan: Exactly, and they make that point explicitly. They say the stronger correspondence with the D/T-type population does not uniquely support a captured-asteroid origin, since Martian material may contribute to the observed properties, particularly for the Red Unit, either directly or through mixtures produced in an impact-generated formation scenario. That means the broad-band ratios alone can't tell us whether we're seeing a pristine captured body or a pile of impact rubble.

Vera: They also bring up an interesting twist about redness. You might think the reddest terrain is the oldest, most space-weathered surface, but they point to observations of Bennu where redder material is actually interpreted as relatively fresh exposure. And on the T-type asteroid Scheila, impact-exposed material also shows red spectral slopes. So the red unit on Phobos could be ancient, or it could be recently exposed primitive material, or even a different composition entirely.

Jocelyn: Wait, that's backwards from what we usually hear about space weathering. So how do they decide between those possibilities?

Subrahmanyan: That's where they bring in the organic absorption feature from the companion paper. The Blue Unit shows a statistically significant absorption at three point three seven microns, which they say is consistent with aliphatic organic material and insoluble organic matter found in CI/CM carbonaceous chondrites, while the Red Unit lacks it. But laboratory irradiation experiments show that energetic processing can restructure and dehydrogenate carbonaceous materials and substantially weaken that absorption, so the contrast between units could mean different starting compositions or just different degrees of surface processing.

Vera: So the picture is more complicated than a simple weathered-versus-fresh story. They end the section by saying that high infrared-to-visible ratios do not uniquely indicate a specific composition, and that these ratios need to be interpreted together with detailed spectroscopy. That's a nice honest summary of where they stand after all that analysis.

Jocelyn: And I suppose that's exactly what the MIRS instrument on the MMX mission is designed to do, right?

Subrahmanyan: Yes, they close by saying that measurements in the zero point nine to three point six micron range by MIRS will be essential for distinguishing between these possibilities. So page seven is essentially the part where they admit how far the data can take us and where it stops, and they point to the next spacecraft to carry the investigation forward.

Conclusion: Vera: So, that brings us to the end of our discussion on "Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids."

Jocelyn: It really does. The key takeaway for me is that Phobos doesn't fit neatly into any single asteroid category — each of its spectral units overlaps with the primitive asteroid populations in a different way, but the D- and T-type asteroids are the closest overall match.

Vera: And that's exactly why the result matters. The infrared-to-visible albedo ratio gives us a new, population-level way to test the captured asteroid hypothesis, and while it doesn't settle the origin debate, it does show that Phobos' surface is comfortably within the range of primitive outer solar system material.

Subrahmanyan: I think the most exciting part is how the Blue and Red units bracket such different regions of that parameter space — it really emphasises that Phobos is not one simple object, but a patchwork of terrains with different histories.

Jocelyn: Absolutely, and that's why the MMX mission and its MIRS instrument are going to be so valuable — the broad-band ratios can point us in the right direction, but we need the detailed spectroscopy to actually tease apart composition from space weathering.

Vera: Well said. So, with that, we thank our guest Subrahmanyan for joining us, and we thank our listeners for staying with us through this one.

Jocelyn: We'll be back shortly with our next paper, so stay tuned — we've got more of the universe to get through.

Vera: Until then, keep looking up.

T. J. Dyer, J. Beccarelli, M. Delbo, C. Avdellidou, M. Pajola, A. Milton, A. Lucchetti, G. Munaretto

Université Côte d’Azur · Centre national d’études spatiales · Istituto Nazionale di Astrofisica · University of Leicester

astro-ph.EP

Submitted: 2026-08-12

Updated: 2026-08-13

Comments: 9 pages, 3 figures, 1 table, accepted for A&A

License: http://creativecommons.org/licenses/by-nc-nd/4.0/

Importance score: 75/100

The gist: The paper "Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids" by T.

Key concepts

Infrared-to-visible albedo ratio (pIR/pV): A simple broadband measure of how much brighter a surface appears in infrared (e.g., 3.4 microns) compared to visible light. It serves as a proxy for spectral slope, helping classify surfaces when detailed spectral features are absent, as with Phobos.
Spectral units (Blue, Transitional, Red): Distinct regions on Phobos defined by how steeply their reflectance rises toward longer wavelengths. Red units are steepest, Blue are flattest, and Transitional are intermediate. These units show different infrared-to-visible ratios, indicating varied surface properties.
Phase reddening: The effect where a surface appears redder when observed at high phase angles (Sun-object-observer angle). The authors corrected Phobos spectra for this using coefficients from the CaSSIS camera, ensuring fair comparison with asteroid observations made at lower phase angles.
Synthetic photometry: A method of computing what a telescope would measure by folding a spectrum through a filter's passband, weighted by solar spectrum and detector response. Here, it converts OMEGA spectra into V-band and W1-band reflectances, enabling direct comparison with asteroid catalog data.

Terminology

Core — 3× in this paper · explained in 2 episodes
Calibration — 1× in this paper · explained in 7 episodes
Clustering — 1× in this paper · explained in 2 episodes
Consistency — 1× in this paper · explained in 3 episodes
European Space Agency — 1× in this paper
Trace — 1× in this paper · explained in 2 episodes
Wide-field Infrared Survey Explorer — 1× in this paper

Summary

The paper Infrared-to-visible albedo ratio of Phobos: Comparison with primitive asteroids by T. J. Dyer et al. aims to assess whether the different spectral units of Phobos occupy the same spectral parameter space as primitive asteroid populations, in order to confirm or not its possible asteroidal origin. The ratio between infrared and visible albedo, pIR /pV, provides a simple proxy for the spectral slope of asteroid surfaces between the visible and near-infrared wavelength ranges.

For each of the three spectral units of Phobos (Red, Transitional and Blue), the authors extracted averaged, normalised reflectance spectra by comparing normalised reflectance in the visible (0.50–0.60 µm) and near-infrared (3.1-3.8 µm) wavelength ranges from the Mars Express OMEGA instrument, to compute a proxy for the infrared-visible albedo ratio (RP). Then, using the database of visible and infrared albedos compiled in the Minor Planet Physical Properties Catalogue (MP3C), they computed asteroid pIR /pV values (RA), to compare to those for Phobos.

The RP for the Red, Transitional and Blue regions vary between 2.90 and 3.12, 2.32 and 2.35, 1.64 and 1.85 respectively. These values span a range occupied by primitive asteroid populations, with the Transitional and Red regions coinciding with the high ratio regime within the pIR /pV domain, which is commonly associated with red-sloped primitive asteroids, including D- and T-type objects as well as Jupiter Trojans.

The asteroid sample was taken from MP3C, using infrared geometric albedo (solely from NEOWISE) and visible geometric albedo from extensive literature sources. Only low albedo asteroids (pV < 0.12) were retained. The taxonomic classes considered were C-complex (780 objects), dark X-complex/P-types (794 objects), D/T-type (464 objects), and Z-types per the Mahlke taxonomy (32 objects with finite RA values). The mean infrared-to-visible albedo ratios are RA = 1.16 ± 0.02 for the C-complex, 1.47 ± 0.02 for dark X/P-types, 2.12 ± 0.15 for Z-types, and 2.29 ± 0.03 for D/T-types.

The OMEGA data used were acquired on the 22nd August 2004 at approximately 151.9 km, resulting in a spatial resolution of 171.1 m/px. The thermal excess was estimated and removed using the procedure described in Beccarelli et al. 2026b, relying on the empirical correction developed by Clark et al. 2011. Regions of Interest (ROIs) were independently selected for each spectral unit: three for the Red unit, two for the Transitional unit, and two for the Blue unit. A phase-reddening correction was applied using CaSSIS-derived phase-reddening coefficients (Munaretto et al. 2025), since the OMEGA data were acquired at a high phase angle of 64◦ while WISE/NEOWISE observations were at 15◦ − 35◦.

Synthetic V- and W1-band reflectances were computed using filter-weighted mean reflectances, with the Bessell V and WISE W1 relative system response curves. The response-weighted effective wavelengths were 0.5526 µm for V and 3.3643 µm for W1. The infrared-to-visible reflectance ratio was defined as RP = ⟨R⟩W1 / ⟨R⟩V. The uncertainties on RP were propagated from the wavelength-dependent uncertainties of the OMEGA spectra.

The results show a clear progression in infrared-to-visible albedo behaviour, from blue material to redder material (in the Transitional Units), to even redder material (in the Red Units). When compared to asteroid populations, the C-complex population is concentrated predominantly at RA values below the Phobos measurements, with its high-RA tail overlapping the two Blue Unit ROIs. The dark X/P-type population extends to higher ratios and shows overlap with both Blue Unit ROIs, with the Transitional Unit ROIs intersecting its sparse high-RA tail, and the Red Unit ROIs lying beyond its upper end. All the Blue, Transitional, and Red Unit ROIs fall within the main D/T-type distribution, although the Blue Unit measurements occur towards its lower-ratio side. The Z-type sample spans the RA values measured for all three Phobos spectral units, but the Transitional and Red Unit reference values fall within the comparatively sparse, nearly flat high-RA tail of the distribution.

The comparison of visible geometric albedo as a function of RA (using pV = 8.37±0.05% for the Stickney rim corresponding to the Blue Unit and 6.57±0.05% for the Stickney floor corresponding to the Red Unit, from Fornasier et al. 2024) shows that BU1 and BU2 fall within a region occupied by several primitive asteroid populations, predominantly D/T-types, the sparsely populated edge of the dark X/P-type distribution, and more marginally the C-complex. The three Red Unit ROIs form a compact group at RP ≃ 2.9–3.1 and pV ≃ 0.066, a region populated predominantly by D/T-type asteroids, but outside their core distribution.

The authors conclude that all three Phobos units overlap the parameter space occupied by primitive asteroids, with the strongest correspondence to D/T-type asteroids. Z-types also offer a potential match, but the limited number of objects in this taxonomic class does not currently allow a robust comparison. However, none of the Phobos spectral units exhibits a unique association with a single taxonomic class, as partial overlap is also found with some dark X/P-types and less so with C-complex asteroids.

The results are broadly consistent with the findings of Takir et al. (2021), who found that the 0.7–2.52 µm portions of Phobos' disk-integrated spectra were consistent with D-type asteroids. The authors note that the spectral differences between the Phobos units may reflect variations in surface maturity, compositional heterogeneity, or a combination of both. The companion OMEGA analysis by Beccarelli et al. (2026b) shows that the Blue Unit exhibits a statistically significant absorption feature at 3.37 µm, interpreted as being consistent with aliphatic organic material found in CI/CM carbonaceous chondrites, whereas the Red Unit lacks this feature.

The authors emphasize that high infrared-to-visible ratios do not uniquely indicate a specific composition, and the ratio provides a population-level measure of integrated spectral behaviour that must be interpreted together with detailed spectroscopy. Measurements in the 0.9–3.6 µm range by the MIRS instrument onboard MMX will be essential for distinguishing between the possibilities of space weathering, surface maturity, or mixtures of endogenous and exogenous material.

Improvements for AI systems

Improvements to AI Systems:

Taxonomic Classification with Uncertainty-Aware Overlap Detection

Enhance asteroid taxonomy classifiers to output not just a single class label, but a probability distribution across multiple classes (e.g., C-complex, X/P, D/T, Z) with explicit overlap boundaries, using the measured pIR/pV ratio ranges and their uncertainties.
The improved system can flag when a new object's spectral parameters fall in overlapping regions (e.g., between D/T and Z-types) and recommend additional observations (e.g., 0.9–3.6 µm spectroscopy) to disambiguate.

Phase-Reddening Correction Module for Remote Sensing Data

Integrate a learned phase-reddening correction function (trained on CaSSIS-derived coefficients) that automatically adjusts visible-to-infrared reflectance ratios for high-phase-angle observations (e.g., >60°) before comparing to low-phase-angle catalogs.
The improved system can process raw spacecraft spectra (e.g., from OMEGA, MIRS) and output phase-corrected albedo ratios, reducing systematic biases in cross-mission comparisons.

Albedo-Ratio-Based Compositional Inference Engine

Build a probabilistic inference engine that maps a measured pIR/pV ratio (with uncertainty) to a likelihood of belonging to each primitive asteroid class, incorporating the full MP3C distribution shapes (not just means).
The improved system can, for a given surface unit (e.g., Phobos Blue vs. Red), output a ranked list of plausible taxonomic affinities with confidence intervals, and highlight when no unique match exists (as found for Phobos units).

Spectral Unit Segmentation with Thermal-Excess Removal

Improve unsupervised clustering algorithms for planetary surface spectra by embedding the empirical thermal correction (Clark et al. 2011) as a differentiable preprocessing layer, allowing joint optimization of unit boundaries and thermal parameters.
The improved system can automatically delineate spectral units (Red, Transitional, Blue) from raw infrared data, with per-pixel uncertainty propagation, and generate synthetic band ratios (e.g., V/W1) directly.

Cross-Dataset Albedo Harmonization

Develop a calibration module that reconciles albedo measurements from different instruments (e.g., OMEGA vs. NEOWISE) by learning instrument-specific bias functions using overlapping targets (e.g., Phobos units observed by both).
The improved system can merge multi-mission photometry into a unified albedo catalog, reducing systematic offsets and enabling more robust population-level comparisons.

Organic-Material Detection via Spectral Feature Correlation

Train a feature-detection network that correlates the presence of a 3.37 µm absorption (aliphatic organics) with the pIR/pV ratio and taxonomic class, using the Beccarelli et al. finding as a labeled example.
The improved system can predict the likelihood of organic content on unresolved bodies from broadband albedo ratios alone, flagging targets for follow-up spectroscopy (e.g., with MIRS on MMX).

Simulation-Based Prior for Space Weathering vs. Compositional Heterogeneity

Implement a generative model that simulates surface evolution (space weathering, mixing of exogenous material) and predicts how pIR/pV ratios shift across a body, trained on Phobos unit measurements.
The improved system can, given a set of spectral units on an asteroid, infer the relative contributions of maturity vs. intrinsic composition, and predict whether the body likely originated as a captured asteroid or is a reaccumulated fragment.

What the Improved AI System Can Do:

Given a new spectral measurement of a small body (from any telescope or spacecraft), it can automatically output:
Phase-corrected, thermal-corrected pIR/pV ratio with full uncertainty.
A probabilistic taxonomic classification (e.g., 70% D/T, 20% Z, 10% X/P) with explicit overlap warnings.
A recommendation for which additional wavelength ranges (e.g., 0.9–3.6 µm) are most informative to reduce classification ambiguity.
A prediction of organic-matter likelihood based on the ratio’s position relative to the Phobos Blue/Red dichotomy.
It can also process full hyperspectral cubes (e.g., from MMX/MIRS) to map spatial variations in taxonomic affinity and organic content across an entire moon or asteroid surface, enabling tests of origin hypotheses (captured vs. co-formed) with quantified confidence.

Abstract

Aims. The ratio between infrared and visible albedo, pIR/pV, provides a simple proxy for the spectral slope of asteroid surfaces between the visible and near-infrared wavelength ranges. The aim of this work is to assess whether the different spectral units of Phobos occupy the same spectral parameter space as the primitive asteroid populations in order to confirm or not its possible asteroidal origin. Methods. For each of the three spectral units of Phobos (Red, Transitional and Blue), we extracted averaged, normalised reflectance spectra by comparing normalised reflectance in the visible (0.50-0.60 mu m) and near-infrared (3.1-3.8 mu m) wavelength ranges from the Mars Express OMEGA instrument, to compute a proxy for the infrared-visible albedo ratio (R P). Then, using the database of visible and infrared albedos compiled in the Minor Planet Physical Properties Catalogue (MP3C), we computed asteroid pIR/pV values (R A), to compare to those for Phobos. Results. The R P for the Red, Transitional and Blue regions vary between 2.90 and 3.12, 2.32 and 2.35, 1.64 and 1.85 respectively. These values span a range occupied by primitive asteroid populations, with the Transitional and Red regions coinciding with the high ratio regime within the pIR/pV domain, which is commonly associated with red-sloped primitive asteroids, including D- and T-type objects as well as Jupiter Trojans.