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| Titel |
Insolation and Resulting Surface Temperatures of Study Regions on Mercury. |
| VerfasserIn |
Karin Bauch, Harald Hiesinger, Jörn Helbert |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054514
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| Zusammenfassung |
| The imaging spectrometer MERTIS (Mercury Radiometer and Thermal Infrared
Spectrometer) is part of the payload of ESA’s BepiColombo mission, which is scheduled for
launch in 2014 [1]. The instrument consists of an IR-spectrometer and radiometer, which
observe the surface in the wavelength range of 7-14 and 7-40μm, respectively. The four
scientific objectives are to a) study Mercury’s surface composition, b) identify rock-forming
minerals, c) globally map the surface mineralogy and d) study surface temperature and
thermal inertia [1, 2].
In preparation of the MERTIS experiment, we performed detailed thermal models of the
lunar surface, which we extrapolated to Mercury. When calculated with lunar parameters, this
allows us to compare the results to lunar temperature measurements of the Apollo,
Clementine, Chandrayaan, and Lunar Reconnaissance Orbiter missions [e.g., 3-6]. For our
simulation, we use topography data from the Moon and idealized crater geometries and
transfer them as model regions to the surface of Mercury. It also allows a direct comparison
of the insolation and thermal variation between craters on the lunar and Mercurian
surface.
Previous studies of the lunar surface have shown that thermal emission contributes to the
observed signal from the surface and can influence the spectral characteristics,
e.g., the depth of absorption bands [e.g., 5, 7, 8]. Therefore accurate knowledge of
the solar insolation and resulting thermal variations is necessary for the correct
interpretation of long-wavelength spectral data. In order to calculate insolation and surface
temperatures, we use a numerical model which has been described by [9]. Surface
temperatures are dependent on the surface and subsurface bulk thermophysical
properties, such as bulk density, heat capacity, thermal conductivity, emissivity,
and albedo. Topography also influences surface temperatures, as it changes the
angle of solar incidence, but also leads to shadowed areas, e.g., the floors of polar
craters.
Lunar and Mercurian surface temperatures show the same general characteristics. Both
have very steep temperature gradients at sunrise and sunset, due to the lack of an atmosphere.
However, there are major differences due to the orbital characteristics. At local noon, the
near- and farside of the Moon receive sunlight under similar solar elevation angles. However,
at this time of the lunar day, the surface on the farside is slightly warmer than the nearside,
because of the shorter distance to the Sun. During the orbit around the Sun, the distance
varies due to the
eccentricity of the Earth-Moon-System, which results in different temperatures during a
year.
On Mercury the 3:2 resonant rotation rate and the eccentric orbit cause distinct
characteristics. At longitudes 0Ë and 180Ë local noon coincides with perihelion, which leads
to a “warm pole”. At longitudes 90Ë and 270Ë local noon coincides with aphelion,
which results in a “cold pole”. At these longitudes secondary sunrises and sunsets
are visible, when Mercury’s orbital angular velocity exceeds the spin rate during
perihelion.
The slow rotation and close distance of Mercury to the Sun require a detailed
analysis of shadowing effects at low elevation angles. During these times of the
day, a fraction of the solar disk is below the horizon and the solar constant must
be modified. The Sun can not be treated as a point source, as it would indicate
darkness for areas where the sun is partially eclipsed. On the Moon this effect is less
pronounced. Due to the larger distance the angular radius of the Sun appears much smaller
and the faster rotation period leads to relatively quick sunrises. However, when
investigating polar areas, where the Sun is only partially visible over long times, or areas at
local sunrise or sunset this effect needs to be included in the computation of solar
insolation.
References: [1]Â Hiesinger, H. et al. (2010), PSS 58, 144-165. [2] Helbert, J. et al. (2005),
LPSC XXXVI, Abstract #1753. [3] Keihm, S.J. and Langseth, M.G. (1973), Proc. Lunar Sci.
Conf. 4th, 2503-2513. [4] Lawson, S.L. et al. (2000), JGR 105, E5, 4273-4290. [5] Pieters,
C.M. et al. (2009), Science 326, 568-572. [6] Paige, D.A. et al. (2010), Space Sci. Rev 150,
125-160. [7] Clark, R.N. (2009), Science 326, 562-564. [8] Sunshine, J.M. et al. (2009),
Science 326, 565-568. [9] Bauch, K.E. et al. (2009), LPSC XL, Abstract #1789. |
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