Strategy of meteorological study in Venus Climate Orbiter mission T. Imamura, M. Nakamura Institute of Space and Astronautical Sciences

Venus Climate Orbiter (Planet-C) project: Status and schedule The VCO mission was approved by the Space Development Committee of the government in Budget request for the prototype model study in 2003 is being made. The spacecraft will be launched in 2008 and arrive at Venus in The mission life will be more than than 2 earth years.

SCIENCE BACKGROUND

Earth and Venus They have almost the same size and mass. Surface environments are completely different. How does the climate system depend on planetary parameters?

Temperature (K) Altitude (km) Earth Venus Pressure (atm) H 2 SO 4 Cloud Haze Thermal structures of Earth and Venus

General circulation of terrestrial planetary atmospheres: how they work? EarthVenus

Super-rotation of Venus’ atmosphere Angular momentum flux Viscosity ? Although the period of planetary rotation is 243 days, the atmosphere near the cloud top circles around the planet once every 4 days.

Cyclostrophic balance of Venus’ atmosphere Pole EQ Cool Hot Pole Strong zonal wind Large contrifugal force Weak zonal wind Small contrifugal force These two torques are balanced each other.

Similar wind system in Titan’s stratosphere? S.Pole EQ N.Pole Brightness temperature (K) Rotation period= 16 days Assuming cyclostrophic balance, the rotation period of the upper atmosphere is 4 days.

Net transport of angular momentum : UPWARD A hypothesis for super-rotation: Gierasch’s mechanism Horizontal viscosity transports angular momentum equatorward Hadley cell transports angular momentum upward at low latitudes and downward at high latitudes Direct or indirect cells? Momentum carrier?

Meridional circulation Winter Pole EQ Summer Pole Earth: 3-cells exist in each hemisphere Shaded: Clockwise White: Anti- clockwise Venus.. ?

Cloud layer Tidal wave Excitation of eastward-propagating tidal wave accelerates the cloud layer westward. Acceleration Acceleration by thermal tide Heating region Motion of the sun relative to cloud layer

Model prediction for thermal tide Zonal wind Meridional wind Vertical wind Temperature T×√p Phase Vertical structure of semi-diurnal tide (Takagi, 2001)

Goals of the mission Mechanism of super-rotation Structure of meridional circulation Hierarchy of atmospheric motion Lightning Cloud physics Plasma environment Detection of active volcanism Meteorology Others

STRATEGY

Requirements for meteorological study Determination of wind field below cloud top Covering both dayside and nightside  Zonally-averaged circulation and momentum flux Multiple altitude levels including sub-cloud region  Vertical structure Covering from meso-scale to planetary-scale  Cross-scale coupling SOLUTION: Continuous high-resolution global imaging from a meteorological satellite (like METEOSAT!)

Near-IR windows 2.3  m (Galileo flyby) Leakage of thermal emission from the hot lower atmosphere Visible-UV

Wind speed (m s -1 ) (km) Zonal wind Cloud layer Angular momentum transport Viscosity ? Altitude regions to be covered Sounding region Radio occultation CO (Near-IR) Lower cloud (Near-IR) Airglow (Visible) 2 2 Lightning

Platform for imaging observation cameras Solar cell HGA North South 360 deg ± 10 deg MGA500N thruster 12 deg FOV, 1000x1000 pixels

Synchronization with the super-rotation Example: Earth cloud movie Time (hours) Angle from apoapsis (deg) Spacecraft motion Air motion at 50 km altitude Orbital period = 30 h Orbit: 300 km x 13 Venus radii Inclination 172°  detect small deviations of atmospheric motion from the background zonal flow

km Movement with time Continuous global viewing  Cloud motion vectors Cloud tracked winds on the Earth Derivation of wind field

What can be seen in high-resolution lower-cloud movie? - Synoptic/planetary-scale waves - Cloud organization - Gravity waves - Other meso-scale phenomena 2.3  m Images by Ground-based observation (Crisp et al. 1991) Morphology of lower clouds

INSTRUMENTS

Cameras (1) Near IR camera 1 (IR1) 1.0  m (near-IR window) 1024 x 1024 pixels, FOV 12deg, SiCCD  Cloud distribution, fine structure of lower cloud (dayside)  Surface emission including active volcanism (nightside) Near IR camera 2 (IR2) 1.7, 2.3, 2.4  m (near-IR window), 2.0  m (CO 2 absorption) 1040 x 1040 pixels, FOV 12deg, PtSi   Cloud distribution and particle size (nightside)   Cloud top height (dayside, 2.0  m)   Carbon monooxide (nightside) Galileo (2.3  m)

IR2 thermal test model Detector housingFilter wheel Optics Aperture Venus image taken with IR2 test filter (Okayama Astronomical Observatory) Stirling cooler Dayside Nightside

Cameras (2) UV camera (UVI) 280, 320 nm  1024 x 1024 pixels, FOV 12deg, SiCCD   SO 2 and unknown UV absorber near the cloud top (dayside) Longwave IR camera (LIR)  9-11  m  240 x 240 pixels, FOV 12deg, Uncooled bolometer   Cloud top temperature (day/night) Lightning and Airglow camera (LAC)  777, 551, 558 nm  8 x 8 pixels, FOV 12deg, Photo diode   High-speed sampling of lightning flashes (nightside)   O 2 / O airglows (nightside) PVO (North pole) Mariner 10

Operation of cameras Whole disk in the field of view over 70% of the orbital period  Development/decay of planetary- scale features in both hemispheres  Precise mapping of each pixel onto planetary surface Acquisition every few minutes- few hours (nominal: 2 hours) Spatial resolution is <16 km Near-IR (nightside) Lightning/Airglow Near-IR (dayside) Ultraviolet Long-IR 12 deg FOV

Radio occultation (USO) Spacecraft motion To the earth Atmosphere Temperature profiles at two opposite longitudes in the low latitude  Zonal propagation of planetary-scale waves H 2 SO 4 vapor profile Ionosphere Pole

0 km 50 km km 90 km 70 km NightsideDayside SO 2, Unknown absorber (UV ） Cloud top temperature （ Mid-IR ） Lower clouds （ Near-IR ） CO （ Near-IR ） Temperature, H 2 SO 4 vapor （ Radio occultation ） Cloud motion vectors Airglow （ Visible ） Lightning （ Visible ） Surface （ Near-IR ） 3-D viewing Cloud top height （ Near-IR ）

Optical sounding of ground surface Search for hot lava erupted from active volcano by taking global pictures at 1.0  m every half a day Emissivity distribution of the ground surface

Summary The spacecraft will be launched in 2008, arrive at Venus in 2009, and observe meteorological processes more than 2 years. The mission is optimized for observing atmospheric dynamics in the low/mid-latitudes. Science payloads will be multi-wavelength cameras covering wavelengths from UV to IR, USO, plasma detectors, and magnetometer. Collaboration with complementary VEX measurements is strongly needed.

VEX and VCO Optimization:Spectroscopy  Imaging Orbit:Polar  Equatorial Global images:High latitudes  Low latitudes

Possible collaboration Complementary information on the general circulation and cloud chemistry

Origin of ultraviolet contrast Cloud height or UV absorber Mechanism of producing inhomogeneity Chemical species related with cloud formation (VEX) Spatial correlation between cloud top height and UV contrast (VCO)

Possible collaboration Cloud morphology in both low and high latitudes To constrain the VCO sounding region using the VEX spectroscopic data Collaboration in receiving downlink (Radio science) Mutual comparison of the tools for data analysis –Radiative transfer code –Cloud tracking algorithm –General circulation model European instruments onboard VCO Complementary information on the general circulation and cloud chemistry

Model predictions for “horizontal viscosity” Two-dimensional turbulence in Venus-like mechanical model (Iga, 2001) Phase velocity-latitude cross section of meridional momentum flux u’v’ in Venus-like GCM (Yamamoto and Takahashi, 2003)

Energy cycle of Earth climate system Axi-symmetric potential energy 33.5x10 5 J/m 2 Axi-symmetric kinetic energy 3.6x10 5 J/m 2 Disturbance potential energy 15.6x10 5 J/m 2 Disturbance kinetic energy 8.8x10 5 J/m 2 Solar energy 1.5 W/m 2 Solar energy 0.7 W/m W/m W/m W/m W/m 2 Dissipation 0.1 W/m 2 Dissipation 1.9 W/m 2 Venus?

VEX VCO Forbes (2002) Gravity waves at low latitude (radio occult.) Meridional drift velocity at low latitude H 2 SO 4 vapor at low latitude by radio occult. Polar collar Polar dipole Meridional transport of trace gases Meridional drift velocity at high latitude H 2 SO 4 vapor at high latitude by radio occult. Gravity waves at high latitude (radio occult.) Equatorial waves Planetary waves driving the circulation