Science requirements and description of the 1 μm camera onboard the Akatsuki Venus Orbiter
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The 1 εm camera onboard the Japanese Venus Mission/Akatsuki scheduled to arrive at Venus in 2015 or 2016 is described. In addition to its scientific goals, the optical and mechanical designs, the calibration procedures, and some results of feasibility studies are presented. Those studies are about the source of the contrast in the dayside image, the resolution of the H2O detection on the nightside surface, and a possibility of finding an active volcano.
Key wordsVenus infrared imaging
1. Scientific Goals
Venus is a next-door planet of the Earth in the solar system with a similar mass and a little shorter distance from the sun. However, its surface environments are astonishingly different from those of the Earth. The atmosphere is as thick as 90 atm and as hot as 740 K on the surface (see, for example, “Venus” University of Arizona Press, 1983). From the atmospheric dynamics’ point of view, there is an interesting phenomenon called the ‘super-rotation’. This is a longstanding problem for meteorologists for more than 40 years (e.g. Gierasch, 1975). The atmosphere at the cloud top (at around 70 km above the surface) is rotating 60 times faster than the solid surface. There must be an effective mechanism to pump up the angular momentum of the surface to the atmosphere in the cloud top region. Various generation mechanisms have been proposed so far. The most common among them is so-called the “Gierasch Mechanism” where the angular momentum is pumped up from the surface by the meridional circulation (Gierasch, 1975). In other mechanisms the angular momentum is transported by equatorial atmospheric waves (Fels and Lindzen, 1973), by thermal tides (Baker and Leovy, 1987), and so on. However, its actual process has not been understood yet mostly because the thick clouds prevent mankind from looking deep into the atmosphere. However, in the middle of 1980s a break through was found by Allen and Crawford (1984) that those regions below the clouds can be seen through several spectral windows in the 1–2 εm region.
The main target of the present Venus mission/Akatsuki is to understand the generation mechanism of the superrotation of the Venus atmosphere by using those windows. For this purpose, five cameras utilizing different wavelengths and a radio beacon for occultation measurement are on board. Those different wavelengths provide information of different heights. Although the height resolution of about 10 km by those nadir-looking cameras are rather poor, it may be compensated by a better height resolution of 1 km by the radio occultation. The dynamical information obtained at the acceleration region (between the ground and the cloud-top region) will give us insights into the generation mechanism of this interesting phenomenon. Although an overview of the mission and the scientific instrumentation has already been described by Nakamura et al. (2007), the principal parts are noted below briefly.
The 1 εm camera named IR1 works both on the dayside and the nightside. On the dayside, it measures the 0.90 εm solar radiation scattered by the clouds, and quantifies the horizontal wind vectors by using the cloud-tracking technique. By combining the information obtained at various heights by the other cameras and by the radio occultation, meteorological information such as the wind field and the distribution of eddy diffusion may be deduced. This will make it possible to investigate the generation mechanism of the super-rotation.
On the nightside, it has three channels of 0.90, 0.97 and 1.01 εm to detect thermal emission mostly from the surface and a little from the lowest atmosphere. The latter two channels are a differential absorption pair for measuring the surface H2O abundance with the 1.01 εm channel as a reference. Although the center of the H2O band is located at 0.94 εm, the 0.97 εm channel is suitable for measuring the H2O abundance because of moderate absorption (it is too strong at the band center). H2O is one of the most important minor constituents in the lower atmosphere because of various reactions with surface minerals and a role for the greenhouse effects. Also it is related to the chemistry of the clouds, which are mostly made of H2SO4 and H2O.
These nightside channels are also used to investigate the distribution of surface emissivity. Surface emissivities may be deduced from the measured radiances with a known surface temperature and a correction of the influence of overlying clouds (Hashimoto and Sugita, 2003). Spatial variations of surface emissivities at near-infrared wavelengths were studied (e.g., Hashimoto et al., 2008); however, little has been discussed about its wavelength dependence so far. Since each material shows each wavelength dependence of emissivity, observation of wavelength dependence will help us to constrain the surface material more confidently. Especially, the spectra of ferrous mineral such as olivine = (Mg, Fe)2SiO4 and ferric mineral such as hematite = Fe2O3 show different wavelength dependence each other in the 0.9 to 1.0 εm region (see Fig. 8); it may be possible to discuss the redox state of Venus surface. Since redox state of Venus’ surface is likely related to the escape of water, the evolution of Venus would be inferred from IR1 observation.
Also IR1 will search for an active volcanism (Hashimoto and Imamura, 2001). On Venus, there are lots of landforms, which are related to volcanic activity; however, no active one has been found so far. Although Venus Express has not found any sign of active volcanism (Mueller et al., 2008), IR1 still has a chance to find it because of its much wider coverage of the surface owing to the equatorial orbit. Detection of active volcanism on Venus has a huge impact on the theory of evolution of planetary interior.
IR1 camera specifications.
center 0.90 µm, width 0.01 µm for dayside
center 0.90 µm, width 0.03 µm for nightside
center 0.97 µm, width 0.04 µm for nightside
center 1.01 µm, width 0.04 µm for nightside
Field of view: 12° × 12°
Pixel resolution: 0.012° × 0.012°
Noise level: 0.77 mW cm−2 str−1εm−1 at 260 K (dayside)
1.3 εWcm−2 str−1εm−1 at 260 K (nightside)
Detector size: 18 mm × 18 mm
Pixel format: 1024 × 1024
Pixel size: 17 εm × 17 εm
Full well: 700000 e−
Dynamic range: 14 bits
Focal length: 84 mm
F number: 8
Exposure time: 3–30 sec
Weight: sensor 2.3 kg
electronics (common for two cameras) 3.7 kg
Size: sensor 51 cm × 28 cm × 21 cm
electronics (common for two cameras) 30 cm × 22 cm × 12 cm
Power: sensor 2.6 W (operation)
electronics (common for two cameras) 40.4 W (operation)
The flat-field measurement and determination of the absolute sensitivity were carried out at Tsukuba Space Center by using a one meter integration sphere with a known radiance. IR1 has a diffuser to obtain a flat-field with a spatial scale of one degree or so during the flight with the Venus’ dayside as a light source. However, a flat-field with a larger spatial scale of several degrees is difficult to be obtained in flight, and is measured with the integration sphere. The sensitivity fluctuations measured are about 3% peak-to-peak in the main part of the field of view.
For the 0.90 εm dayside channel, input radiance of 37 mW cm−2εm−1 str−1 (77% of the nominal input) resulted an output of 633 ADU/s (analogue to digital conversion unit) = 44300 e−/s (65% of the nominal output); that is, the measured sensitivity was proved to be 84% of the designed. In case of IR1 camera 1 ADU = 70 e−.For the 0.90 εm nightside channel, an input radiance of 55 εW cm−2εm−1 str−1 (37 times as large as the nominal input) resulted an output of 565 ADU/s = 39600 e−/s (27 times as large as the nominal output); that is, the measured sensitivity was proved to be 73% of the designed. The nominal exposure durations are 3–6 s and 10–30 s, respectively, for the dayside and nightside measurements.
The thermal noise measured is 130 ADU/s at 290 K, which will be reduced to 1/10 when the detector is cooled down to 260 K. The depth of the well is measured to be around 10000 ADU = 700000 e−.
The S/N ratio of the actual measurements is mostly determined by the statistical noise. The thermal noise of 15 ADU/s at 260 K is usually unimportant. The expected S/N ratio for the measurements may be estimated as follows: Since the expected dayside output is 3000 ADU/3 s = 210000 e− /3 s, the expected S/N ratio due to statistical noise is 0.2% (=(210000)−1/2); this satisfies the scientific requirement of S/N = 300 for the 0.90 εm dayside measurements. The value 3000 ADU/3 s is based on the expected dayside radiance and the measured sensitivity noted above. Since the expected nightside output is 210 ADU/10 s = 15000 e−/10 s, the expected S/N ratio due to statistical noise is 0.8% (=(15000)−1/2); this satisfies the scientific requirement of S/N = 100 for the 0.90 εm nightside measurements. The value 210 ADU/10 s is also based on the expected nightside radiance and the measured sensitivity.
4. Feasibility Studies
4.1 Source of contrast for the 0.90 εm dayside image and the representative height
On the dayside, IR1 camera images the clouds illuminated by the sunlight, and various meteorological parameters such as the horizontal wind velocity are derived by tracking features on the clouds. Therefore, it is important to identify the source of such features and their representative height seen in the 0.90 εm image. As shown by the Venus’ global image taken during the Galileo’s flyby, the contrast found in the 986 nm dayside images was as small as 3% (Belton et al., 1991). They estimated the representative height for such 986 nm contrast as 50–55 km. This is based on the consideration about the wind speed difference between those seen in the UV and IR images, and the wind speed gradient with height found by the entry probes of Venera and Pioneer Venus (Schubert, 1983). Since detailed description has been published elsewhere (Takagi and Iwagami, 2011), some elementary results are noted below.
The dayside spectrum in the 0.9–1.0 εm region calculated is shown in Fig. 3. This is based upon a line-byline calculation using a molecular database HITRAN 2004 (Rothman et al., 2005), the VIRA1985 model atmosphere (Keating et al., 1985), a baseline cloud model (table 2a of Pollack et al., 1993), and a radiative transfer code RSTAR (Nakajima and Tanaka, 1986) to calculate the scattering process by the cloud particles. The molecular absorptions are so weak in this wavelength region that the solar spectrum appears almost as it is.
The representative height is investigated by using a mean cloud model composed of the lower (48–50 km), middle (50–57 km) and upper (57–70 km) clouds and the upper haze (70–84 km). Such mean cloud model and the expected changes are taken from the entry probe data summarized in table 4a of James et al. (1997). According to the present preliminary radiative transfer calculation, it is found that any layer (upper, middle and lower) except for the upper haze has a possibility to cause such a change of 3% in the measured radiance. The variability of the lower cloud is certainly larger than those of the others as seen in the entry probe data noted above; however, the optical thickness above the lower cloud is also larger than those of the others, and hides the variability most seriously. That is, it failed to identify the representative height based on the mean cloud model with its expected change and a radiative transfer calculation. Another source of information is needed for farther discussion.
The horizontal scale of the feature may provide a key. The scale of the 986 nm features tracked by Galileo was in the order of 300 km (this is read on figure 1 of Belton et al., 1991). By comparing the clouds data measured by a pair of entry probes worked nearly the same time (such as listed in table 4a of James et al., 1997), it is found that the lower clouds appear to be most variable as far as the distance between the entry probes is in the order of several hundred km (such as the case of Venera 11 and 12), and that whole the layers are variable if the distance exceeds 1000 km (such as the case of Venera 9 and 10). This fact is suggestive that the representative height of the 0.90 εm dayside images is in the lower cloud region 48–50 km. However, the number of examples is so limited that a definitive conclusion cannot be drawn.
4.2 H2O detection near the surface
The H2O abundance near the nightside surface has been measured in the 1.18 εm region based on the Galileo data (Drossart et al., 1993), and mixing ratios of 30±15 ppm with a latitudinal variation of 20% have been reported. Based on the Venus Express data, mixing ratios of 44±9 ppm with a latitudinal variation of ±1.5% (this corresponds to a variation of 44 × 0.03 = 1.3 ppm) over 60°S–25°N latitudes have been reported (Bezard et al., 2009). The resolution of 5 ppm by IR1 may not enough to find inhomogeneity in the H2O abundance if the latter estimation is true. However, the representative height for the IR1 measurement at 0.97/1.01 εm regions is lower than that of the 1.18 εm region by a few km, and may succeed to detect inhomogeneity in the H2O abundance in the lowest atmosphere. Longer exposure duration such as 100 s instead of 10 s may be effective to improve the resolution.
4.3 Surface characterization and volcano quest
The 1 εm camera onboard the Venus Mission/Akatsuki is ready to start observation. On the dayside, it images features due to inhomogeneity in the clouds’ thickness to derive the horizontal wind velocity. It will provide useful information to investigate the acceleration mechanism for the super-rotation of the atmosphere. On the nightside, it also measures the thermal emission from the surface and the lower atmosphere to investigate the H2O abundance near the surface with a resolution of 5 ppm, and the distribution of the surface emissivity, which will be an important key to investigate the internal activity and the thermal history of Venus.
The authors thank to the people working for manufacturing the camera. They are of Nikon Co. for the optics, Fujitoku Co. (Barr Associates Inc.) for the filters, Magoshi Co. for the baffle, Mitsubishi Electric Co. for the sensor, Sumitomo Heavy Industries Ltd. for the whole IR1system and NEC-Toshiba Space Systems Co. for the whole mission. They also thank to all the people of ISAS (Institute of Space Aeronautical Science) working for the mission.
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