Mercury phase functions from MESSENGER/MDIS spectrophotometry: First results

Abstract. We analyze spectrophotometric observations of Mercury acquired from the Mercury Dual Imaging System (MDIS) on board NASA’s MErcury Surface, Space Environment, Geochemistry and Ranging mission’s spacecraft (MESSENGER). The spectrophotometric properties provide insight into regolith composition and structure, which in turn provide insight into surface processing. Using the Lommel-Seeliger reflection coefficient, we derive first estimates for the single-scattering phase function and the disk-integrated phase curve in eight colors. The results help us prepare for the interpretation of the Mercury observations by the ESA/JAXA BepiColombo mission.

1. Introduction

Planet Mercury continues to challenge scientific interpretation. Mercury’s principal rotational characteristics were unveiled only in 1960’s and its surface composition remains largely open. The elemental chemistry instruments on board the MESSENGER spacecraft provided constraints on many elements [1-6], however the surface mineral composition remains a mystery [7]. Mercury will soon be scrutinized by the BepiColombo mission that will examine the surface over a broader range of wavelengths, promising to identify the mineral composition.

Spectrophotometry provides constraints on the physical and chemical properties of the regolith (e.g., [8]). We analyze MDIS eight-color disk-resolved photometry with the Lommel-Seeliger reflection coefficient (LS), the simplest model valid for regoliths with dark constituent materials. The LS coefficient has been recently utilized in, e.g., asteroid lightcurve inversion using disk-integrated photometry from the ESA Gaia mission [9]. The Gaia photometry unveils asteroid phase curve slopes in the phase-angle range of 15-35^o. Examination of the Mercury photometry using the LS coefficient allows for the determination of the single-scattering phase function over MDIS’ wavelength range, paving the way for the assessment of BepiColombo Mercury data [10,11].

2. Modeling, results, and discussion

We make use of the LS reflection coefficient with the single-scattering phase function as in [9]. The disk-integrated phase function is introduced as part of the single-scattering phase function using a divisor that corresponds to the disk-integrated phase function for isotropic scattering. It follows that the data points themselves suggest the shape of the phase functions.

Figure 1 shows the single-scattering phase functions as point clouds when reduced from the full MDIS data set (e.g., [8]). The filter wavelengths are as follows (in nm): λ(F)=433.2, λ(C)=479.9, λ(D)=558.9, λ(E)=628.8, λ(G)=748.7, λ(L)=828.4, λ(J)=898.8, λ(I)=996.2. In order to minimize surface roughness effects, we have required that the incidence angle is less than 40^o.

Figure 1: Single-scattering phase functions as indicated by the processed data points.

In Figure 2, we show the disk-integrated phase curves. The phase curves look realistic for phase angles <60^o, whereas, for phase angles >60^o, there is a  bump that can be due to the simplicity of the LS coefficient.

Figure 2: Disk-integrated phase curve as indicated by the processed data points.

 

Figure 3 shows linear magnitude-scale fits to the data, with normalization to zero magnitude at 30^o phase angle, when both incidence and emergence angles are required to be less than 40^o. Color dependence is evident and there are indications of reddening as a function of phase angle. Although exceptions are present, the slopes tend to become shallower with increasing wavelength, in agreement with the increasing reflectance of Mercury with wavelength [12].

Figure 3: Linear fits to the single-scattering phase function data for incidence and emergence angles <40^o.

3. Conclusion

We have carried out a preliminary analysis of the MESSENGER observations. By concentrating on data at small incidence and emergence angles, we obtain realistic first estimates for the single-scattering phase function. This, in turn, allows us to estimate the disk-integrated phase curve of Mercury. We detect reddening with the phase angle and flattening with increasing spectral reflectance. The features relate to the properties of regolith particles. We will continue the analysis by accounting for surface roughness and porosity (e.g., [13]) and recent advances in space weathering modeling [14,15].

Acknowledgments. Research by KM, VB, and AP supported, in part, by the Academy of Finland (grant 325805). DD is supported by NASA's Solar System Working program (grant 80NSSC18K0521).

References

 [1] Nittler, L. R., et al.: The major-element composition of Mercury’s surface from MESSENGER X-ray spectrometry, Science, Vol. 333, pp. 1847–1850, 2011.

[2] Peplowski, P. N., et al.: Enhanced sodium abundance in Mercury's north polar region revealed by the MESSENGER Gamma-Ray Spectrometer, Icarus, Vol. 228, pp. 86-95, 2014.

[3] Peplowski, P. N., et al.: Radioactive elements on Mercury’s surface from MESSENGER: Implications for the planet’s formation and evolution, Science, Vol. 333, pp. 1850–1852, 2011.

[4] Peplowski, P. N., et al.: Variations in the abundances of potassium and thorium on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer, J. Geophys. Res., Vol. 117, E00L04, 2012.

[5] Weider, S. Z., et al.: Chemical heterogeneity on Mercury’s surface revealed by the MESSENGER X-Ray Spectrometer, J. Geophys. Res., Vol. 117, E00L05, 2012.

[6] Evans, L. G., et al.: Major-element abundances on the surface of Mercury: Results from the MESSENGER Gamma-Ray Spectrometer, J. Geophys. Res., Vol. 117, E00L07, 2012.

[7] Izenberg, N. R., et al.: The low-iron, reduced surface of Mercury as seen in spectral reflectance by MESSENGER, Icarus, Vol 228, pp. 364–374, 2014.

[8] Domingue, D. L., et al.: Application of multiple photometric models to disk-resolved measurements of Mercury’s surface: Insights into Mercury’s regolith characteristics, Icarus, Vol. 268, pp. 172-203, 2016.

[9] Muinonen, K., et al.: Asteroid lightcurve inversion with Bayesian inference, Astronomy & Astrophysics, in revision, 18 p., 2020.

[10] Cremonese, G., et al.: SIMBIO-SYS: Scientific cameas and spectrometer for the BepiColombo mission, Vol. 216, 75, pp. 1-78, 2020.

[11] Rothery, D., et al.: Rationale for BepiColombo studies of Mercury’s surface and composition, Space Science Reviews, Vol. 216, 66, pp. 1-46, 2020.

[12] McClintock, W. E., et al.: Spectroscopic observations of Mercury’s surface reflectance during MESSENGER’s first Mercury flyby, Science, Vol. 321, pp. 62-65, 2008.

[13] Wilkman, O., et al.: Photometry of dark atmosphereless planetary bodies: an efficient numerical model, Planetary and Space Science, Vol. 118, pp. 250-255, 2015.

[14] Penttilä, A., et al.: Rigorous light-scattering simulations of space-weathering effects on reflectance spectra. Icarus, Vol. 345, 113727, pp. 1-8, 2019.

[15] Muinonen, K., et al.: Scattering and absorption of light in planetary regoliths. J. Visual. Exper., Vol 149, e59607, doi:10.3791/59607, 2019.