Photometric analysis

Introduction

The retrieval of photometric behavior of parameters characterizing small bodies spectra gives us information about optical and physical properties of their surface.

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Study on NIRS3 data:

• I/F at 1.9 μm
• 2.7 and 2.8 μm band depths
• Spectral slope between 1.9 and 2.5 μm

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Study on ONC data:

• v-band I/F (Ryugu average)
• v-band I/F (SCI crater – before and after impact)

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Dataset

• NIRS3

Entire dataset acquired between TD1 (February 2019) and TD2 (July 2019), including 10⁵ spectra and phase angle range between 15° and 40°

• ONC (Ryugu average)

CRA-1, SCI and CRA-2 mission stages (March/April 2019), including 40 million pixels and phase angle range between 15° and 40°

• ONC (SCI crater)

16 selected images (8 before and 8 after the impact), including 70 thousand pixel (35 before and 35 after the impact) and phase angles up to 40°

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Method

The empirical method has been successfully applied to several bodies: Vesta, Lutetia, Churyumov-Gerasimenko, Ceres (Longobardo et al., 2014; 2016, 2017; 2019). It consists of the following steps:

1. Application of the Akimov disk function to remove topography influence (i.e. incidence and emission effects)
2. Retrieval of the median spectral parameter behavior as a function of the phase angle
3. Retrieval of phase function

4. Comparison with other bodies

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Photometric parameters

We compared disk-resolved asteroid’s phase functions by defining the two following photometric parameters on I/F phase functions:

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• R20/R30 is the visible I/F observed at 20° or 30° phase angle
• PCS₁₅₄₀ is the phase function steepness (in percent) between 15° and 40° phase angles

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NIRS3 and ONC phase function

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NIRS3 and ONC phase function

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The two phase functions are consistent within two times the error, despite the different spectral range and the different spatial resolution.

This is expected from the similar albedo in the two ranges.

Comparison in the NIR

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Ryugu is photometrically similar to Ceres, as expected

Comparison in the VIS

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Ryugu PCS values

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• 45% (this work), found on I/F values corrected for shape model
• 45% (Tatsumi et al., 2020), found on I/F values corrected for shape model
• 60% (Tatsumi et al., 2020; Ishiguro et al., 2014), found on I/F values NOT corrected for shape model

The role of spatial resolution

• Bright asteroids: increasing spatial resolution steepens phase function (i.e., PCS increases) because it highlights albedo heterogeneities. This is observed on Vesta and Ida (Longobardo et al., 2016).
• Dark asteroids: increasing spatial resolution results in a phase function flattening (i.e., PCS decreases) because it reduces the role of shadowing. This is observed not only on Ryugu (this work), but also on Ceres (Longobardo et al., 2019) and Bennu (Goulish et al., 2020).

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PCS variations are observed between spatial resolution larger and smaller than 100m, respectively

Photometry of SCI crater

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Before impact

After impact

Within errors, phase curves are similar.

However, after the impact phase curve is steeper, according to previous results (Honda et al., 2021)

Interpretation and implications

• Interpretation.
• Larger roughness (up to pixel scale, i.e., 20m), according to Yokota et al., (2022).
• Filling factor decrease (i.e., larger porosity), according to the opposition width decrease.

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• Implications
• Exposed subsurface is rougher than surface, according to Ryugu formation model and observations of other impact craters (Sakatani et al., 2021)
• This roughness change is less important than other impact craters, also according to small variation of thermal inertia (Sakatani et al., 2021) and boulders size frequency distribution (Grott et al., 2020) observed between the crater interior and exterior.

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2.7 and 2.8 μm band depths

The two band depths have a similar behavior, i.e., decrease with increasing phase angle.

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This behavior has been never observed in the asteroids visited so far.

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Comparison with other small bodies

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Bands and their phase variations are suppressed with albedo decreasing.

When albedo further decreases, the bands at higher phase angles are more suppressed, generating a decreasing trend (e.g., Murchison, Cloutis et al., 2018)

Infrared slope

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Phase reddening

Phase reddening value is very similar to that found by Tatsumi et al. (2020) on ONC data (visible range). This is a different behavior wrt other dark bodies (Ceres, 67P), where the visible phase reddening is about 3 times the infrared phase reddening.

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Phase reddening: possible explanations

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• No multiple scattering -> quite constant phase function from VIS to IR -> constant phase reddening

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• Visible and infrared radiation “see” the same grain roughness:
• Smooth particles
• Sub-micron roughness masked by the low albedo

This would result in similar response to visible and infrared radiation -> constant phase reddening

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Conclusions

• The visible and NIR phase functions of Ryugu are similar
• Ryugu is photometrically similar to other dark asteroids
• Improving spatial resolution flattens phase curve of dark small bodies
• SCI crater’s phase curve suggests a rougher exposed subsurface
• The band depths photometric behavior is driven by low albedo
• Phase reddening is consistent with constant particle phase function and/or particles microscopically smooth

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