Cassini Huygens EECS 823 DIVYA CHALLA

OUTLINE Mission and its Objectives Saturn Rings study Cassini Radar – Titan Coverage Radiometer Design System Parameters Cassini Radiometer and calibration

MISSION AND ITS OBJECTIVES To study the Saturn system – atmosphere, its rings and moons (Titan). Launched on 15th October, 1997. Orbital insertion: July 1, 2004 (Cassini orbiter) and January 14, 2005 (Huygens – Titan lander). Total of 3 missions – prime (2004-2008), the Equinox (2008-2010), and the Solstice (2010-2017) missions. System designers: Giovanni Domenico Cassini and Christiaan Huygens. 12 instruments for the Cassini orbiter and 6 for Huygens.

SATURN RINGS Comprises majorly water ice and small fraction of non-icy material (single scattering albedos are low for these particles). Largest particle size ˂ 10m. High Brightness at Azimuth suggesting high porosity of particles for C ring (might be silicate core). Cassini radiometer operates at λ where absorptivity of water ice negligible. Amount of non-icy material: 6-11 % by volume. Determines ice-to-dust ratio of particles as a function of distance from Saturn.

A: Calibrated Antenna temperature map B: Brightness temperature map obtained through calibration [2] Brightness temperature profile for low resolution scans [2]

Cassini RADAR – Titan coverage: Objective: determine physical state, topography, & composition and global temperatures. Idea: Microwave radiometry to map the surface and combine with Cassini optical remote sensing instrument’s data. Radiometry Mode: Produce brightness temperature maps of surface of Titan and its dependence on polarization and angle of incidence is used to discriminate between smooth and broken ice fields. Emissivity (a function of density) is used to interpret the surface density map. By physical temperature contrasts between different regions and between day and night, global circulation and thermal transport are estimated. Cycle of seasons is studied and each season lasts about 7.5 earth years. Thermal properties like ability to retain heat: temperature measurements at various local times. Atmospheric gases like propylene, Hydrogen cyanide, methane, nitrogen are detected. Cassini’s gravity measurements of Titan revealed that it is hiding water and ammonia ocean beneath surface.

Titan’s surface as seen by the Huygens probe Surface of Titan before and after Cassini arrival at Saturn NASA images

Radiometer Design System Parameters F =13.8 GHz (Ku-band) Wavelength = 2.2cm Bandwidth = 135 MHz Along-track look angle: ±60 to ±120 Cross-track look angle: ±60 to ±120 Number of beams: All 5 Full Beam width at half-power : 0.360 (beam 3) Brightness temperature error (∆T) = ˂ 3K Azimuth Resolution = 6-600 km Range Resolution = 6-600 km All parameters taken from [2]

Cassini Radiometer and Calibration Operates in burst mode Antenna integration time varies based on active mode data collection (35ms to 1s) Cassini radar can be separated into active and passive radiometry mode. Calibration: Determine a calibration factor that converts output voltage to radiance. Cold reference: sky (2.7 K) and Warm reference: known source For absolute calibration, data of Titan surface from Huygens is considered as reference. The calibration scale is considered linear. Challenges like aging of components, changes in reference temperatures are also included.

a: Initial scale calibration a: Initial scale calibration b: model convolutions to estimate side lobe contributions c: derived zero offset correction d:final calibrated antenna temperature [3]

TO DO How the parameters of interest are extracted exactly. Examples of measured data. Assessment of capabilities and limitations.

REFERENCES https://saturn.jpl.nasa.gov/science/rings/ Z. Zhang, A.G. hayes, M.A. Janssen, et al. “Cassini microwave observations provide clues to the origin of Saturn's C ring”, Vol 281, January 2017 L. Borgarelli, E.Z. faustini, Alenia Spazio S.p.A, “Casssini Radar Instrument: Radiometer Mode”, Geoscience and Remote Sensing Symposium, 1995. IGARSS ‘95. http://www.space.com/34825-cassini-saturn-ring-dives-begin.html http://www.thetimenow.com/astronomy/cassini-huygens.php