Dramatic Changes Observed in Pluto’s Haze Opacity from 2002–2022

The New Horizons flyby (14-JUL-2015) imaged Pluto’s haze layers in detail [1,2]. Analysis of the haze’s scattering properties suggests a bimodal population of haze particles [3]. The constituents in Pluto’s atmosphere are roughly similar to Titan’s: a predominantly nitrogen atmosphere with trace abundances of methane and other hydrocarbons. This context leads to ongoing work on haze formation on Pluto [e.g., 4,5,6,7]. It is striking, then, that stellar occultation lightcurves over the past twenty years show evidence of large variations in Pluto’s haze optical depths on timescales of a few years.

We report on six occultations by Pluto observed from 2002 to 2022; noteworthy events because they were either observed in two wavelengths or contained central flash features. Multiple wavelengths let us separate opacity due to haze particles from light attenuation due to differential refraction: haze scattering efficiencies increase at shorter wavelengths, while differential refraction (due to inflections in the vertical T(z) profile) is barely affected by wavelength. Similarly, the amplitudes of central flashes are very sensitive to haze opacities.

The 2002 occultation was observed from five telescopes on Mauna Kea in wavelengths ranging from 0.6 to 2.3 μm [8,9]. The mid-event baseline flux increased with wavelength, consistent with a one-way zenith optical depth due to haze of = 0.2+0.011/-0.007 at at λ = 0.6 μm. In contrast, a two-color lightcurve obtained in 2007 from the Mt John observatory in New Zealand detected no haze, only an upper limit of < 0.0065. A 2011 lightcurve also detected no haze (an upper limit of < 0.027), while the New Horizons flyby detected a haze opacity of = 0.018 in Pluto’s northern hemisphere. Occultations in 2018 and 2022 provided central flash lightcurves that let us retrieve haze optical depths. We will discuss and rule out some possible explanations for the large variations in haze opacity over timescales of a few years.

1. Cheng, A. F. et al. Icarus 290, 112–133 (2017).

2. Gladstone, G. R. et al. Science 351, aad8866 (2016).

3. Fan, S., Gao, P., Zhang, X. et al. Nat Commun 13, 240 (2022). https://doi.org/10.1038/s41467-021-27811-6.

4. Gao, P. et al. Icarus 287, 116–123 (2017).

5. Young, L. A. et al. Icarus 300, 174–199 (2018).

6. Lavvas, P. et al. Nat. Astron. 5, 289–297 (2021).

7. Wong, M. L. et al. Icarus 287, 110–115 (2017).

8. Elliot, J. L. et al. Nature, Volume 424, Issue 6945, pp. 165-168 (2003).

9. Sicardy, B. et al., Nature, Volume 424, Issue 6945, pp. 168-170 (2003).