Earth Planets Space, 50, 183–188, 1998 Observations of Mars and its satellites by the Mars Imaging Camera (MIC) on Planet-B Tadashi Mukai1, Tokuhide Akabane2, Tatsuaki Hashimoto3, Hiroshi Ishimoto1, Sho Sasaki4, Ai Inada1, Anthony Toigo5, Masato Nakamura6, Yutaka Abe6, Kei Kurita6, and Takeshi Imamura6 1The Graduate School of Science and Technology, Kobe University, Kobe 657-8501, Japan 2Hida Observatory, Kyoto University, Kamitakara, Gifu 506-13, Japan 3Institute of Space and Astronautical Science, Kanagawa 229-8510, Japan 4Geological Institute, University of Tokyo, Tokyo 113, Japan 5Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A. 6Space and Planetary Science, University of Tokyo, Tokyo 113, Japan (Received August 7, 1997; Revised December 16, 1997; Accepted January 30, 1998) We present the specifications of the Mars Imaging Camera (MIC) on the Planet-B spin-stabilized spacecraft, and key scientific objectives of MIC observations. A non-sun-synchronous orbit of Planet-B with a large eccentricity of about 0.87 around Mars provides the opportunities (1) to observe the same region of Mars at various times of day and various solar phase angles with spatial resolution of about 60 m from a distance of 150 km altitude (at periapsis), and (2) to monitor changes of global atmospheric conditions on Mars near an apoapsis of 15 Mars radii. In addition, (3) several encounters of Planet-B with each of the two Martian satellites are scheduled during the mission lifetime of two years from October 1999 to observe their shapes and surface structures with three color filters, centered on 450, 550, and 650 nm. (4) A search for hypothetical dust rings along the orbits of two satellites will be tried from the forward-scattering region of sunlight. 1. Introduction 630∼680 nm. MIC uses tri-color linear CCDs, consisting A visual wavelength camera to be launched with the of each 1 × 2560 pixel arrays. It scans the azimuth direc- Planet-B spacecraft is called the Mars Imaging Camera tion (orthogonal to the CCD pixels) with spacecraft spin at (MIC). The capability of the MIC to wholy map the sur- a rotation speed of 7.5 rpm. Its total field of view along the face of Mars is rather poor, compared with other planned array is 54.2◦ (2560 pixels in elevation direction)×360◦ (az- spacecraft(s) in orbit around Mars during the period of about imuth direction). However, actual image size is limited by two years from October 1999, e.g. Mars Surveyor ’98 orbiter the memory of the camera system, e.g. 1 Mbyte on-board in an orbit being near sun-synchronous, polar, near circular at memory which represents about 11◦ (512 pixels in elevation about 405 km altitude. However, the uniqueness of the MIC direction) × 11◦ (512 lines in azimuth direction at a rota- observations results from large elongated orbits scheduled in tion speed of 7.5 rpm and 0.49 ms integration time). Res- the mission of Planet-B, i.e. its expected orbital parameters olution of the image corresponds to about 80 arc seconds are; periapsis of 150 km, apoapsis of 15 RM (RM denotes a in both elevation and azimuth direction at 7.5 rpm. Pixel radius of Mars), period of about 34 hours. Planet-B’s non- data is quantized into 8 bits via an A/D converter. MIC has sun-synchronous orbit allows it to observe the same region lossy image compression capability. An image Compression at various times of day and various solar phase angles. In ad- Module provided by CNES under international collaboration dition, the large distance of its apoapsis provides occasions performs JPEG image compression from 3 to 100 times at for encounters with the two Martian satellites, Phobos and 4 Mpixels s−1 at 14 MHz. The compression ratio can be cho- Deimos. MIC specifications are noted in Section 2, and key sen by command from the ground. In standard operation, the scientific objectives of MIC are summarized in Section 3. In imaging area is designated by command as the azimuth start Section 4, a brief summary of MIC is shown. angle, the azimuth image size, the elevation start angle, and the elevation image size. Azimuth angle is defined as the 2. Instruments phase angle of the spacecraft rotation whose original point The size of MIC is approximately 90 mm × 150 mm × is Sun direction. Elevation angle corresponds to pixel num- 253 mm, and its weight is about 2.7 kg (see Fig. 1). Its power ber of the CCD. Image size of each direction can be chosen consumption is about 14 W while imaging. MIC will do opti- from 1024, 512 and 256 lines (azimuth) or pixels (elevation). cal imaging of Mars itself and the two Martian satellites with Gain level and image compression parameters are also set by three visible color bands; 440∼480 nm, 520∼580 nm, and command from the ground. MIC also has other special onboard functions: “Automatic Copy rightc The Society of Geomagnetism and Earth, Planetary and Space Sciences bright-object imaging” and “Autonomous tracking”. The (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; former is realized by the hardware circuits, which search for a The Geodetic Society of Japan; The Japanese Society for Planetary Sciences. pixel brighter than threshold level and then obtains the image 183 184 T. MUKAI ET AL.: OBSERVATIONS OF MARS AND ITS SATELLITES BY THE MARS IMAGING CAMERA ON PLANET-B MIC, as noted above about 80 arc seconds, yields the spatial resolution of about 60 m/pixel from a distance of 150 km al- titude (at periapsis of Planet-B). But Mars Global Surveyor will perform detailed morphology mapping prior to Planet- B. However, MIC has many opportunities to obtain images at low solar angle illumination where small undulations are emphasized by longer shadow zones. Moreover, using space- craft spin, MIC can obtain images in oblique view although it is not equipped with a gimbal mechanism. This may allow MIC to take oblique and sometimes stereographic surface images around periapsis. The following are examples of the proposed observation plan: characterization of rampart craters and terrain soft- ening as a key for underground water distribution, volcanic and hydrothermal study on Arsia Mons (121◦W, 9 ◦S), stere- ographic pictures of canyon systems, and relation between aeolean morphology and present wind system. Figure 2 shows MIC surface resolution, plotted from a compilation of many expected orbits of Planet-B. Planet-B’s orbital period is about 34 hours, and subsequently the lon- gitude on Mars during periapsis shifts about 150◦ with each successive orbit. Since uncertainties that affect the orbital elements of Planet-B exist around Mars, we have to note that this map is a very preliminary one. As shown in Fig. 2, Arsia Mons is the only polygenetic volcano in middle-low latitude of southern hemisphere which will be available in the high resolution images of MIC (≤100 Fig. 1. The Mars Imaging Camera (MIC). Its size is approximately m/pixel). Mapping of magnetic fields, by using MGF on 90 mm × 150 mm × 253 mm. Planet-B, associated with the imaging of lava flows around Arsia Mons by MIC will show tectonic features originated by volcanic activity, and clarify their evolution. of neighboring area. Its purpose is for easy operation of Mars 3.2 Meteorologic objectives global imaging from apoapsis. The latter is realized by the In this section we express unsolved problems which are onboard software, which finds the direction of the center of important to investigate Martian atmospheric phenomena. the object, calculates the orbital motion of it, and estimates In mid-spring to mid-summer in the northern hemisphere of the direction of it at the next exposure timing. This function Mars, the equatorial region is surrounded by a cloud belt ex- ◦ ◦ is prepared for fly-by imaging of Phobos and Deimos. tending from 10 Sto30 N latitudes (Clancy et al., 1996). In the Tharsis area (125◦–101◦W, 12 ◦S–16◦N), being a part of 3. Science Objectives the equatorial cloud belt, many bright clouds appear. They Since Planet-B takes a very elliptic retrograde orbit, MIC are classified into morning, afternoon, and evening clouds. can take global images of Mars. It can monitor changes Since high volcano tops are not covered with those clouds, of global atmospheric conditions on Mars: e.g., dust storm it is estimated that the cloud height is lower than 20 km. growth, change of atmospheric opacity due to dust and thin The morning clouds disappear near noon, and simultane- hazy clouds, change of cloud features, growth and retreat ously the afternoon clouds appear. They have a tendency to of polar caps, development and thickness of polar hood. In grow on the huge volcanoes. The afternoon clouds stand out addition, the planned orbits will make close encounters of until about Martian local time of 15:00, and get mixed with Planet-B with Phobos and Deimos several times during the evening clouds. However, detailed characteristics of these mission lifetime, and consequently, MIC can observe the clouds are not yet known. There is a possibility that morning shapes and the surface structures of two satellites in detail. clouds could be derived from moisture trapped in the surface When Planet-B is close to Mars near its periapsis, MIC regolith. Monitoring those and other regions at twilight, MIC can observe the planetary limb in spatial resolution as high may clarify the occurrence and nature of the phenomena. as 2 km, enough for investigating the vertical atmospheric Martian clouds are bright and conspicuous in blue. How- structure.