Showing papers by "Carly Howett published in 2022"

A Near-surface Temperature Model of Arrokoth

Abstract: A near-surface thermal model for Arrokoth is developed based on the recently released 105 facet model of the body. This thermal solution takes into account Arrokoth’s surface reradiation back onto itself. The solution method exploits Arrokoth’s periodic orbital character to develop a thermal response using a time-asymptotic solution method, which involves a Fourier transform solution of the heat equation, an approach recently used by others. We display detailed thermal solutions assuming that Arrokoth’s near-surface material’s thermal inertia = 2.5 W/m−2 K−1 s1/2. We predict that at New Horizons’ encounter with Arrokoth, its encounter hemisphere surface temperatures were ∼57–59 K in its polar regions, 30–40 K in its equatorial zones, and 11–13 K for its winter hemisphere. Arrokoth’s orbitally averaged temperatures are around 30–35 K in its polar regions and closer to 40 K near its equatorial zones. Thermal reradiation from the surrounding surface amounts to less than 5% of the total energy budget, while the total energy ensconced into and exhumed out of Arrokoth’s interior via thermal conduction over one orbit is about 0.5% of the total energy budget. As a generalized application of this thermal modeling together with other Kuiper Belt object origins considerations, we favor the interpretation that New Horizons’ REX instrument’s 29 ± 5 K brightness temperature measurement is consistent with Arrokoth’s near-surface material being made of sub-to-few-millimeter-size tholin-coated amorphous H2O ice grains with 1 W/m−2 K−1 s1/2 << 10–20 W/m−2 K−1 s1/2 and which are characterized by an X-band emissivity in the range 0.9 and 1.

Extreme Exospheric Dynamics at Charon: Implications for the Red Spot

Abstract: Charon's exosphere may exhibit extreme seasonal dynamics, with centuries of quiescence punctuated by short lived (∼4 earth years) exospheric surges near the equinoxes, as spring sunrise bi‐annually drives frozen methane off the polar night zones. Charon's pole‐centric red spot has been proposed to be the product of Ly‐α photolysis of frozen methane into refractory hydrocarbon “tholins”, but the role of exospheric dynamics in the red material's formation has not been investigated. We show with exospheric modeling that methane “polar‐swap”, in which exospheric CH4 sublimated from the spring polar zone is rapidly re‐frozen onto the autumn hemisphere, deposits ∼30 μm polar frosts too thick for Ly‐α light to penetrate. Ethane, the primary methane photoproduct under these conditions, may unlike methane remain frozen decades after polar sunrise under solar wind exposure. Solar wind radiolysis of polar ethane frost synthesizes higher‐order refractories that may contribute to the coloration of Charon's polar zones.

Dione’s Thermal Inertia and Bolometric Bond Albedo Derived from Cassini/CIRS Observations of Solar Eclipse Ingress

Abstract: On 2010 May 18 Cassini’s Composite Infrared Spectrometer (CIRS) observed Dione’s leading hemisphere as its surface went into solar eclipse. Surface temperatures derived from each of CIRS’ focal plane 3 (FP3, 600−1100 cm−1) show a rapid decrease in Dione’s surface temperature upon eclipse ingress. This change was compared to the model surface emission to constrain bolometric Bond albedo and thermal inertia. Seven FP3 detectors were able to constrain the observed surface’s thermophysical properties. The bolometric Bond albedo derived from these detectors are consistent with one another (0.54 ± 0.05 to 0.62 ± 0.03) and that of diurnal studies (e.g., 0.49 ± 0.11, Howett et al. 2014). This indicates that Dione’s albedo is uniform to within the uncertainties across the observed region of its leading hemisphere. The derived thermal inertias are consistent across detectors, 9 ± 4 J m−2 K−1 s−1/2 (MKS) to 16 ± 8 MKS, and with previous diurnal studies (e.g., 8 to 12 MKS, Howett et al. 2014). The skin depth probed by the eclipse thermal wave is ∼0.6–1 mm, which is much shallower than that probed by diurnal cycles (∼50 mm). Thus, the agreement in thermal inertia between the eclipse and diurnal studies indicates that Dione’s subsurface structure is uniform from submillimeter to subcentimeter depths. This is different from the Jovian system, where eclipse-derived thermal inertias are much lower than those derived from diurnal studies. The cause of this difference is not known, but one possibility is that the E-ring grains that bombard Dione’s leading hemisphere overturn it, causing uniformity to centimeter depths.

Charon’s refractory factory

Abstract: We combine novel laboratory experiments and exospheric modeling to reveal that “dynamic” Ly-α photolysis of Plutonian methane generates a photolytic refractory distribution on Charon that increases with latitude, consistent with poleward darkening observed in the New Horizons images. The flux ratio of the condensing methane to the interplanetary medium Ly-α photons, φ, controls the distribution and composition of Charon’s photoproducts. Mid-latitude regions are likely to host complex refractories emerging from low-φ photolysis, while high-φ photolysis at the polar zones primarily generate ethane. However, ethane being colorless does not contribute to the reddish polar hue. Solar wind radiolysis of Ly-α–cooked polar frost past spring sunrise may synthesize increasingly complex, redder refractories responsible for the unique albedo on this enigmatic moon.