At 525 km in mean diameter, Vesta is the second-most massive object in the main asteroid belt of our Solar System. At all scales, pyroxene absorptions are the most prominent spectral features on Vesta and overall, Vesta mineralogy indicates a complex magmatic evolution that led to a differentiated crust and mantle [1]. The thermal behavior of areas of unusual albedo seen on the surface at the local scale can be related to physical properties that can provide information about the origin of those materials. Dawn's Visible and Infrared Mapping Spectrometer (VIR) [2] hyperspectral images are routinely used, by means of temperature-retrieval algorithms, to compute surface temperatures along with spectral emissivities. Here we present temperature maps of several local-scale features of Vesta that were observed by Dawn under different illumination conditions and different local solar times

Ammannito, E.

Blewett, D. T.

Capaccioni, F.

Capria, M. T.

Combe, J.-Ph.

Denevi, B. W.

DeSanctis, M. C.

Li, J.-Y.

Mittlefehldt, D. W.

Palmer, E.

Palomba, E.

Raymond, C. A.

Russell, C. T.

Sunshine, J. M.

Titus, T. N.

Tosi, F.

Zambon, F.

NASA Technical Reports Server

THERMAL ANALYSIS OF UNUSUAL LOCAL-SCALE FEATURES 
ON THE SURFACE OF VESTA 
 
F. Tosi1, M.T. Capria1, M.C. De Sanctis1, F. Capaccioni1, E. Palomba1, F. Zambon1, E. Ammannito1, 
D.T. Blewett2, J.-Ph. Combe3, B.W. Denevi2, J.-Y. Li4, D.W. Mittlefehldt5, E. Palmer4, J.M. Sunshine6, 
T.N. Titus7, C.A. Raymond8, C.T. Russell9 
 
1INAF-IAPS, Via del Fosso del Cavaliere 100, I-00133 Rome, Italy, federico.tosi@iaps.inaf.it. 2The Johns 
Hopkins University Applied Physics Laboratory, Laurel, MD, USA. 3Bear Fight Institute, Winthrop, WA, USA. 
4Planetary Science Institute, Tucson, AZ, USA. 5NASA Johnson Space Center,  Houston, TX, USA. 6University 
of Maryland at College Park, MD, USA. 7USGS Astrogeology Science Center, Flagstaff, AZ, USA. 8NASA/Jet 
Propulsion Laboratory and California Institute of Technology, Pasadena, CA, USA. 9University of California at 
Los Angeles, Los Angeles, CA, USA. 
 
ABSTRACT 
 
At 525 km in mean diameter, Vesta is the second-most 
massive object in the main asteroid belt of our Solar System. 
At all scales, pyroxene absorptions are the most prominent 
spectral features on Vesta and overall, Vesta mineralogy 
indicates a complex magmatic evolution that led to a 
differentiated crust and mantle [1]. 
The thermal behavior of areas of unusual albedo seen on 
the surface at the local scale can be related to physical 
properties that can provide information about the origin of 
those materials. Dawn’s Visible and Infrared Mapping 
Spectrometer (VIR) [2] hyperspectral images are routinely 
used, by means of temperature-retrieval algorithms, to 
compute surface temperatures along with spectral 
emissivities. 
Here we present temperature maps of several local-scale 
features of Vesta that were observed by Dawn under 
different illumination conditions and different local solar 
times. 
 
Index Terms— Asteroids, Remote sensing, Infrared 
spectroscopy, Surface temperature, Thermal properties. 
 
1. INTRODUCTION 
 
Localized dark and bright materials, often with extremely 
different albedos, were found on Vesta’s surface. The range 
of albedos is among the largest observed on Solar System 
rocky bodies [3]. Evidence for mineralogical stratigraphic 
layering was observed on crater walls and in ejecta [1]. Dark 
materials, often associated with craters, appear in ejecta and 
crater walls, and their pyroxene absorption strengths are 
correlated with material brightness. It is plausible that 
enough dark material to match what we observe could be 
delivered on Vesta by low albedo (probably carbonaceous) 
asteroids over time [3]. The identity of the bright 
endmember is probably the intrinsic Vesta basaltic soil, rich 
in unaltered, crystalline pyroxenes [3]. It has been argued 
that, for the most part, Vesta’s surface material can actually 
be thought of as having two spectral components—the dark 
and bright endmember spectra—in different proportions [3]. 
Pitted terrain is characterized by irregular rimless 
depressions found in and around some impact craters on 
Vesta, with a distinct morphology not observed on other 
airless bodies [4]. Similar terrains are associated with 
numerous Martian craters, where pits are thought to form 
through degassing of volatile-bearing material heated by the 
impact [4]. The thermal behavior of pitted terrains found on 
Vesta can be related to physical properties that may provide 
some information about the origin of those materials. VIR 
hyperspectral images can be used to retrieve surface 
temperatures, with high accuracy (<1 K) as long as 
temperatures are greater than ∼180 K, which constrains this 
analysis to observations of the illuminated surface. 
Bright and dark surface materials on Vesta, as well as 
pitted materials, are currently being investigated by the 
Dawn team [3,4]. We present temperature maps and spectral 
profiles of some of these features as derived from VIR data 
in different phases of the mission. Such information may 
constrain their thermophysical properties and provide 
insight on their composition and physical structure. 
 
2. DATA SET AND ANALYSIS 
 
After the initial Survey Orbit (11 through 31 August 2011) 
and High Altitude Mapping Orbit (HAMO-1, 29 September 
through 1 November 2011), Dawn spiraled down to its 210-
km above mean surface Low Altitude Mapping Orbit 
(LAMO), which was by far the longest phase of the Vesta 
mission (12 December 2011 through 30 April 2012). The 
Dawn spacecraft eventually raised its altitude back to 685 
km above mean surface to perform a second High Altitude 
Mapping Orbit (HAMO-2, 15 June to 25 July 2012), prior to 
https://ntrs.nasa.gov/search.jsp?R=20140001397 2019-08-29T14:59:43+00:00Z
its departure from Vesta. During Dawn’s one-year mission 
at Vesta, the heliocentric distance of the asteroid increased 
from 2.23 to 2.55 Astronomical Units (AU) – the latter 
value being close to aphelion. 
Data acquired by the Framing Camera (FC) in the 
Survey phase first revealed the existence of several unusual 
surface features: 1) bright and dark material deposits, 2) 
spectrally distinct ejecta, 3) regions suggesting finer-grained 
materials. Terrains with a distinct pitted morphology were 
identified later, on the basis of FC clear-filter images 
obtained during LAMO [4]. 
Here we calculate surface temperatures of several local-
scale features, particularly dark and bright material deposits 
and pitted terrains, using VIR data acquired especially in the 
Survey and High Altitude mapping orbits. In the Survey 
phase, VIR obtained resolved hyperspectral images of more 
than 65% of the surface of Vesta with average spatial 
resolution of ∼0.68 km. In the HAMO-1 and -2 phases, VIR 
acquired hyperspectral images at a roughly constant pixel 
resolution of 0.17 km. Even though footprints were mostly 
non-continuous due to the higher instantaneous speed of the 
ground tracks, some of the dark/bright material deposits and 
the four known pitted-terrain sites (Marcia, Cornelia, 
Licinia, Numisia) were re-observed in greater detail. VIR 
data acquired in LAMO have much reduced spatial coverage 
and larger phase angles (resulting in long shadows) with 
respect to Survey and HAMO data, and as such LAMO data 
will not be discussed here. 
On the illuminated side of Vesta, the region of the 
infrared spectrum beyond ∼3.5 µm is dominated by thermal 
emission from the asteroid’s surface, which can be used to 
determine surface temperature by means of temperature-
retrieval algorithms. To calculate surface temperatures, we 
applied a Bayesian approach to nonlinear inversion [5] 
based on the Kirchhoff law rλ=1-ελ and the Planck function. 
Results were compared with those provided by the 
application of alternative methods (e.g, [6]). In all cases, the 
minimum retrievable temperature (∼180 K) is set by the 
Noise Equivalent Spectral Radiance (NESR), i.e. the RMS 
noise of the in-flight measurements expressed in units of 
spectral radiance. On the other hand, for a given local solar 
time (LST), the maximum temperature depends on 
incidence angle and surface properties such as thermal 
inertia and albedo. 
 
3. RESULTS 
 
Some bright features of Vesta have an overall albedo in the 
visible as much as 40% brighter than surrounding areas. 
Data from the IR channel of VIR show that bright regions 
generally correspond to regions with lower thermal 
emission, i.e. lower temperature, while dark regions 
correspond to areas with higher thermal emission, i.e. higher 
temperature (Fig. 1). This behavior confirms that many of 
the dark appearances in the VIS mainly reflect albedo 
variations, and not, for example, shadowing. 
 
Fig. 1. Spectral comparison between very dark material (cube: 373312662) 
and very bright material (cube: 394683501) observed respectively in 
HAMO-1 and HAMO-2 by VIR in its infrared domain. Very dark material 
(red dots) shows a thermal emission significantly greater than very bright 
material (blue dots) observed at the same spatial scale and under similar 
solar illumination and local solar time. 
 
In particular, during maximum daily insolation, dark 
features in the equatorial region may show temperatures 
greater than 270 K during the observed range of heliocentric 
distances, while bright features seen at comparable spatial 
resolution and local solar time are at least 10 K cooler. 
However, individual features may show different thermal 
behaviors due to differences in composition (revealed also 
by the emissivity) and/or structure (e.g, average grain size of 
the surface regolith, porosity, etc.). 
 
Fig. 2. Spectral comparison based on VIR infrared data between pitted 
terrain in crater Marcia (blue curve) and very bright material in nearby 
crater Calpurnia (red curve). Pitted terrain, despite its lower albedo, 
exhibits a significantly reduced thermal emission compared to very bright 
material, even under comparable solar illumination and local solar time. 
Pitted terrains display distinct thermal properties 
compared to other terrains on Vesta. They show a 
significatly reduced thermal emission at infrared 
wavelengths longward of 4 µm (Fig. 2), have distinct 
margins in the temperature images, and are cooler than 
nearby terrains even though they show similar albedos and 
are observed at similar solar illumination conditions. 
 
 
Fig. 3. Simple cylindrical projection of a portion of Marcia crater, observed 
by VIR (cube: 366641356) during the Survey phase (680 m/pixel). The 
upper panel shows the region as seen at the near-infrared wavelength of 1.4 
µm. The middle panel shows a temperature map of the same area, as 
derived from VIR infrared spectra using the method described in [5] in the 
range 4.5-5.1 µm, where thermal emission is important. The bottom panel 
shows the local solar incidence angle measured from the surface normal to 
a detailed shape model. Materials illuminated at high incidence angles are 
generally cooler than materials at low incidence angles. The spectrally 
distinct material associated with Marcia’s floor (lower left side of the 
projected image), observed at 10.4 h LST, is ~10 K colder than the rest of 
the crater where illumination conditions are similar, and has similiar to 
slightly lower temperatures than Marcia’s dark ejecta blanket. The average 
surface temperature on the crater’s floor where the pitted material is located 
is 234 K, lower than the average temperature of 241 K found on the floor of 
the nearby crater Calpurnia (center of the image) observed at 11.0 LST and 
the temperature of ~260 K found in the northern wall of Calpurnia 
observed at 11.1 h LST. 
 
 
Fig. 4. Simple cylindrical projection of a portion of Cornelia crater, 
observed by VIR (cube: 371813008) during the HAMO-1 phase (171  
m/pixel). The upper panel shows the region as seen at the near-infrared 
wavelength of 1.4 µm. The lower panel shows a temperature map of the 
same area, as derived from VIR infrared data. Pitted material associated 
with Cornelia’s floor, observed at 9.5 h LST, has an average temperature of 
229 K and shows distinct margins in the thermal image, clearly standing 
out compared to the lit portion of the crater and particularly to the dark 
material exposed in its northern inner wall, which has an average 
temperature of 256 K. 
 
Pitted terrains can be >10 K cooler than the 
surroundings, and even cooler than bright material deposits 
found on Vesta, despite their lower albedo (Figs. 3, 4). 
Spectral emissivity profiles retrieved for pitted terrains 
found in the floors of Marcia and Cornelia reveal prominent 
differences with respect to other distinct features discovered 
on Vesta, most notably dark and bright material deposits 
(Fig. 5). These are likely related to differences in both 
composition and physical structure of the surface material. 
The lower temperatures suggest a higher thermal inertia, i.e. 
a slower response to insolation, which may be due to 
reduced local porosity (i.e., increased local density) and/or 
an increase in the local thermal conductivity. 
These might be caused by rapid degassing of volatile-
bearing materials: local cavities at micrometer to centimeter 
scale gradually disappear, leaving a compact layer. An 
increase of both density and thermal conductivity may 
actually be responsible for the observed behavior. This is 
consistent with the lower albedo of compact material 
compared to the unconsolidated material widespread on the 
surface of Vesta. 
 
 
Fig. 5. Comparison of spectral emissivities, retrieved from VIR infrared 
data, between pitted terrain in the floor of Cornelia (blue dots) and dark 
material found in the wall of the same crater (red dots). The interval 4.9-5.1 
µm is unsuitable for pitted material due to the superposition of solar 
reflectance and thermal emission, which yields no emissivity information. 
These two profiles reveal an anti-correlation at wavelengths shorter than 
4.9 µm, which may be related to a difference in composition. 
 
4. ACKNOWLEDGMENTS 
 
This work was supported by the Italian Space Agency 
(ASI), ASI-INAF Contract I/004/12/0. Support of the Dawn 
Science, Instrument, Operations Teams, as well as of the 
Dawn at Vesta Participating Scientist program, is gratefully 
acknowledged. We warmly thank Robert Gaskell (PSI) for 
providing a detailed shape model of Vesta that was used in 
this work. 
 
5. REFERENCES 
 
[1] De Sanctis, M.C., et al. (2012), “Spectroscopic characterization 
of mineralogy and its diversity across Vesta”. Science 336, pp. 
697-700. 
 
[2] De Sanctis, M.C., et al. (2011), “The VIR Spectrometer”. Space 
Science Reviews 163 (1-4), pp. 329-369. 
 
[3] McCord, T.B., et al. (2012), “Dark material on Vesta from the 
infall of carbonaceous volatile-rich material”. Nature 491 (7422), 
pp. 83-86. 
 
[4] Denevi, B.W., et al. (2012), “Pitted terrain on Vesta and 
implications for the presence of volatiles”. Science 338, pp. 246-
249. 
 
[5] Keihm, S., et al. (2012), “Interpretation of combined infrared, 
submillimeter, and millimeter thermal flux data obtained during the 
Rosetta fly-by of Asteroid (21) Lutetia”. Icarus 221, pp. 395-404. 
 
[6] Clark, R.N., et al. (2011), “Thermal removal from near-infrared 
imaging spectroscopy data of the Moon”. Journal of Geophysical 
Research 116, CiteID E00G16. 
 


Thermal Analysis of Unusual Local-scale Features on the Surface of Vesta

Thermal Analysis of Unusual Local-scale Features on the Surface of Vesta

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