Comparative planetology aims to unveil the common origin processes and divergent evolutionary paths of different bodies in the solar system. The surface of a planetary body reveals important information about its state and evolution. For example, it yields useful data for studying the object's interior, atmosphere, and interaction with the space environment, which in turn provides insight on how the object formed. Often, the external view of a planet is the only available data source for these analyses. Spacecraft can gather the most detailed facts about celestial objects from their immediate vicinity in terms of spatial, temporal, and chemophysical properties.
Among various advanced remote sensing methods, visible/IR (VIS/IR) spectroscopy is a key technology for exploring planetary surfaces and atmospheres. This approach requires close, concerted efforts in planetary science and engineering aimed at generating a maximum of scientific requests and space-flight-qualified instrumental solutions. In the framework of these requirements, different European planetary spectrometers1–7 have been successfully developed, applied, and operated in planetary exploration missions over the last two decades. They can be divided into three types: imaging spectrometers for the 0.25–5μm range,1, 3–5 imaging spectroscopy in the 7–14μm region,6, 7 and interferometers from 1.25 up to 45μm2.
False color image (left) and surface temperature map (right, red 170K≤T≤243Kwhite) of the asteroid 21 Lutetia taken by the Visible and Infrared Imaging System (VIRTIS) sensor on the Rosetta spacecraft.8
The overlay represents a typical VIRTIS spectrum from 21 Lutetia.
As part of an international team led by Angioletta Coradini, we developed the pushbroom operating sensor VIRTIS (Visible and Infrared Thermal Imaging Spectrometer, 0.25–5μm)3 for the ongoing European Space Agency (ESA) cornerstone mission Rosetta to comet 67P/Churyumov-Gerasimenko. During its long journey, the spacecraft observed the asteroids 2867 Šteins in 2008 and 21 Lutetia (see Figure 1) in 2010, studying their surface composition, temperature, and texture. Reaching the target in about 4 astronomical units (1 astronomical unit =149:6 million kilometers), VIRTIS will start observations in March 2014 while orbiting the comet. Meanwhile, Rosetta will follow the comet through its path in the inner solar system. Being the oldest objects in the solar system, comets allow the study of the source material of planetary formation and differentiation.
The VIRTIS technology has been adapted for the VIS/IR mapping spectrometer5 on NASA's Dawn spacecraft, which will analyze the asteroids 4 Vesta in 2011–2012 and 1 Ceres in 2015. Through an international cooperation led by Giuseppe Piccioni and Pierre Drossart, we have adapted the technology for ESA's Venus Express (VEX) mission4 to observe Venus's atmosphere, as well as its selected surface properties. VIRTIS/VEX is the first mission to systematically investigate the deep atmosphere and surface features of Venus within its narrow night side atmospheric windows from an orbit around the planet. Figure 2 shows the night side surface window radiance of Venus measured by VIRTIS that allows us to extract surface temperature and emissivity data.9–11
Figure 2. Night side radiance within the near-IR atmospheric windows of Venus obtained by VIRTIS/VEX.
Features such as a very hot climate driven by a runaway greenhouse effect, dense CO2 atmosphere, and atmospheric superrotation distinguish Venus from the other terrestrial planets. Although its mass and density differ only slightly from that of Earth, both took completely different evolutionary paths. Understanding these divergences requires more knowledge of Venusian surface material, which has been poorly studied until now. Consequently, the VIRTIX/VEX surface data is crucial for discussing the processes and models of the planet's evolution.
In contrast to Venus, Mercury, which was formed close to the Sun, has a mass only 0.055 times that of Earth and has not developed an atmosphere comparable to other terrestrial planets. However, it shows an unusually high mean density, which indicates a massive iron-rich core consisting of about two-thirds of the planet's mass. NASA's MESSENGER mission started spectral observations of Mercury's surface in the VIR/near-IR wavelength range in March of this year.
Figure 3. Mercury Radiometer and Thermal-infrared Imaging Spectrometer (MERTIS) for the BepiColombo mission. (Background photo copyright NASA.)
We plan to complete these observations with mid-IR investigations of Mercury's surface with the Mercury Radiometer and Thermal-infrared Imaging Spectrometer (MERTIS)6, 7 (see Figure 3). This instrument is under development for ESA's BepiColombo mission, which is scheduled to launch in 2014. MERTIS combines a pushbroom IR grating spectrometer (TIS) with a radiometer (TIR) sharing the same optics, instrument electronics, and in-flight calibration components for the whole wavelength range of 7–14μm (TIS) and 7–40μm (TIR).6, 7 Within the mid-IR range, the instrument will record the temperature, texture, and mineralogical characteristics of Mercury's surface, contributing to a better understanding of the planet's genesis.
Gabriele E. Arnold
German Aerospace Center e.V. Institute of Planetary Research
Gabriele Arnold is a staff member at the German Aerospace Center. She has published widely in the fields of planetary remote sensing and spectral studies of planetary surfaces, and is involved in numerous deep space missions such as Mars/Venus Express, Rosetta, and BepiColombo.
1. J.-P. Bibring, Y. Langevin, A. Gendrin, B. Condet, F. Poulet, M. Berthé, A. Soufflot, R. Arvidson, N. Mangold, P. Drossart, Mars surface diversity as revealed by the OMEGA/Mars Express observations, Science 11, pp. 1576-1581, 2005.
2. V. Formisano, F. Angrilli, G. Arnold, S. Atreya, G. Bianchini, D. Biondi, A. Blanco, The Planetary Fourier Spectrometer (PFS) onboard the European Mars Express mission, Planet. Space Sci. 53, pp. 963-974, 2005.
3. A. Coradini, F. Capaccioni, P. Drossart, S. Erard, G. Filacchione, M. C. De Sanctis, M. T. Capria, VIRTIS: an imaging spectrometer for the ROSETTA mission, Rosetta: ESA's Mission to the Origin of the Solar System, pp. 565-587, Springer, 2009.
4. G. Piccioni, P. Drossart, A. Sanchez-Lavega, R. Hueso, F. W. Taylor, C. F. Wilson, D. Grassi, South-polar features on Venus similar to those near the north pole, Nature 450, pp. 637-640, 2007.
5. P. Drossart, G. Piccioni, J. C. Gerard, M. A. Lopez-Valverde, A. Sanchez-Lavega, L. Zazova, R. Hueso, A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express, Nature 450, pp. 641-645, 2007.
6. G. Arnold, J. Helbert, H. Hiesinger, H. Hirsch, E. Jessberger, G. Peter, I. Walter, Mercury Radiometer and Thermal Infrared Spectrometer: a novel thermal imaging spectrometer for the exploration of Mercury, J. Appl. Rem. Sens.
2, pp. 023528, 2008. doi:10.1117/1.2961041
7. H. Hiesinger, J. Helbert, The Mercury Radiometer and Thermal Infrared Spectrometer (MERTIS) for the Bepi Colombo mission, Planet. Space Sci. 58, pp. 144-165, 2010.
8. A. Coradini, F. Capaccioni, S. Erard, G. Arnold, M. C. De Sanctis, The surface composition and temperature of asteroid 21 Lutetia as observed by ROSETTA/VIRTIS, Science, submitted.
9. G. Arnold, R. Haus, D. Kappel, P. Drossart, G. Piccioni, Venus surface data extraction from VIRTIS/VEX measurements: estimation of a quantitative approach, J. Geophys. Res.
13, pp. E00B10, 2008. doi:10.1029/2008JE003087
10. R. Haus, G. Arnold, Radiative transfer in the atmosphere of Venus and first surface emissivity retrievals from VIRTIS/VEX measurements, Planet. Space Sci
. 58, pp. 1578-1598, 2010. doi:10.1016/j.pss.2010.08.001
11. R. W. Carlson, F. W. Taylor, The Galileo encounter with Venus: results from the Near-infrared Mapping Spectrometer, Planet. Space Sci. 41, pp. 475-476, 1993.