ROCKS AND ROUGHNESS: TIR MEASUREMENTS OF RYUGU FROM HAYABUSA-2
J. L. Bandfield, Space Science Institute, Boulder, CO, 80301, USA.

Brightness Temperature (K)Brightness Temperature (K)The Hayabusa-2 mission will return the most detailed thermal infrared measurements ever acquired of anasteroid; both from the thermal infrared imager (TIR) on the spacecraft itself and the Mobile Asteroid Surface
Scout (MASCOT) lander [1]. These measurements will enable a better understanding of the composition andthermophysical properties of asteroid 162173 Ryugu and their underlying processes. The combination of in-situ
and remotely sensed observations will provide an unprecedented set of observations that link small scale surface
properties to large scale remotely sensed measurements. These measurements can be also used as a framework
for understanding telescopic and spacecraft datasets of other airless bodies throughout the solar system.
In many cases, TIR observations of airless bodies have displayed confusing or surprising characteristics. In
particular, anisothermality can have large and often unanticipated effects on thermal infrared measurements ofplanetary surfaces. The effects of surface roughness, layering, rocks, and directional emissivity in conjunction
with viewing geometry must be considered for the proper interpretation of the measurements.
A combination surface roughness and thermal model has been developed for the analysis of lunar andmartian thermal infrared datasets [e.g., 2–3] and has been adapted for application to Hayabusa-2 TIR Ryugu
observations using orbital, shape, and thermophysical properties as they are currently known or estimated [4–8].
The roughness model assumes a simple Gaussian distribution of slopes. For each individual surface location, thetemperatures of 738 slope and azimuth combinations are modeled using a one-dimensional heat diffusion model[9]. These temperatures are converted to radiance and added in proportion to their statistical occurrence andweighted by their projection to the field of view of the observer (i.e., slopes facing away from the observer
contribute less to the measurement than slopes facing the observer).
The spacecraft will typically have a synoptic view from the day side of the asteroid, providing excellent
diurnal and seasonal thermal imaging coverage. However, the typically fixed vantage of Ryugu results in mostsurfaces being imaged from a variable off-nadir orientation. In addition, periodic descent observations will allow
for portions of the asteroid to be measured down to cm scales. With the presence of a potentially rough,heterogeneous surface, the variable emission and azimuth viewing angle needs to be taken into account and themeasurements cannot be treated in the same manner as nadir observations that are common with other spacecraft
investigations. In a sense, the Hayabusa-2 TIR images will have a vantage similar to telescopic observations
because of its non-orbiting, Earth-pointed orientation.
A series of modeled observations have been produced to better understand how emissivity, surface
roughness, thermal inertia, vertical layering ofthermophysically distinct units, and rock abundance
300
Nadir LT:will influence TIR measurements. For example,
1200 Hsurface roughness and viewing orientation can
280
influence measured brightness temperatures by asmuch as 40K (Fig. 1). Viewing geometry alone can260
cause brightness temperature differences that are
RMS Slopes = 0°240
comparable to variations in thermophysical properties.
10°
Roughness, rock abundance, thermal inertia,
20°
220
layering, and directional emissivity effects all30°
Nadir Viewing - RMS Slopes = 20°
influence brightness temperatures and do so in a200
unique manner when taking into account the range of7
9111315
local times, seasons, and viewing angles that will be
320
acquired by Hayabusa-2. Although the relationships
300
Nadir LT:between these factors can be complicated, the0920 H
280modeling shows that there is a wealth of useful
information to be gleaned from the measurements.
260
References: [1] Tsuda Y. (2013) Acta. Astron.,240
10.1016/j.actaastro.2013.06.028. [2] Bandfield J. L. andRMS Slopes = 0°
10°
Edwards C. S. (2008) Icarus, 10.1016/j.icarus.2007.08.
22020°
028. [3] Bandfield J. L. et al. (2015), Icarus 10.1016/30°
200j.icarus.2014.11.009. [4] Müller T. G. et al. (2011) A&A,
Nadir Viewing - RMS Slopes = 20°
A14510.1051/0004-6361/201015599. [5] Kawakami K.
1805791113et al. (2010) Japan. Soc. Planet. Sci., 19, 4–11. [6]Local Time (H)Moskovitz N. A. et al. (2013) Icarus, 10. 1016/j.icarus. Fig 1. The effects of surface roughness on modeled
2013.02.009. [7] Kim M. -J. et al. (2013) A&A, brightness temperatures of Ryugu at 0°N and northern
L1110.1051/0004-6361/201220673. [8] Hasegawa S. T. fall equinox. Both plots show the same surface (TI ofG. et al. (2008) Ast. Soc. Japan, 60, 399-405. [9] 200 J m-2 K-1 s-½) at the same season, but the top plot isKieffer H. H. (2013) JGR, 10.1029/ 2012JE004164.
at 0° and the bottom plot is at 40° phase angle.