Strategy of meteorological study in Venus Climate Orbiter mission
T. Imamura, M. NakamuraInstitute 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 2001.
• Budget request for the prototype model study in 2003 is being made.
• The spacecraft will be launched in 2008 and arrive at Venus in 2009.
• 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)
A
ltit
ude
(km
)
Earth
Venus
P
ress
ure
(atm
)H2SO4 Cloud
Haze
Thermal structures of Earth and Venus
General circulation of terrestrial planetary atmospheres: how they work?
Earth Venus
Super-rotation of Venus’ atmosphere
Angular momentum fluxViscosity
?
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 EQ
Cool
Hot
PoleStrong zonal wind
Large contrifugal forceWeak
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 tem
perature (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
Tidal wave
Excitation of eastward-propagating tidal wave accelerates the cloud layer westward.
Acceleration
Acceleration
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
Venus wind system
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
2.3m (Galileo flyby)
Leakage of thermal emission from the hot lower atmosphere
Visible-UV
0 50 100 Wind speed (m s-1)
(km)100
80
60
40
20
0
Zonal wind
Cloud layer
Angular momentum transport
Viscosity ?
Altitude regions to be covered
Sounding regionR
adio occultation
CO
(Near-IR
)
Low
er cloud (Near-IR
)
Airglow
(Visible)
SO
/Unknow
n absorber
(UV
)
Cloud top tem
perature
(Long-
IR) 2
CO
absorption
(Near-IR
) 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 movieTime (hours)
Ang
le f
rom
apo
apsi
s (d
eg)
Spacecraft motion
Air motion at 50 km altitude
Orbital period = 30 h
Orbit: 300 km x 13 Venus radiiInclination 172°
detect small deviations of atmospheric motion from the background zonal flow
100-300 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
2.3m 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 (CO2 absorption)
1040 x 1040 pixels, FOV 12deg, PtSi Cloud distribution and particle size (nightside) Cloud top height (dayside, 2.0m) Carbon monooxide (nightside)
Galileo (2.3m)
IR2 thermal test model
Detector housingFilter wheel Optics
Aperture
Venus image taken with IR2 test filter (Okayama Astronomical Observatory)
Stirling coolerDayside Nightside
Cameras (2)UV camera (UVI) 280, 320 nm 1024 x 1024 pixels, FOV 12deg, SiCCD SO2 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) O2 / 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
• H2SO4 vapor profile
• Ionosphere
Pole
0 km
50 km
35-50 km
90 km
70 km
NightsideDayside
SO2, Unknown absorber (UV )
Cloud top temperature( Mid-IR )
Lower clouds ( Near-IR ) CO ( Near-IR )
Temperature, H2SO4 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 1.0m 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
33.5x105 J/m2
Axi-symmetric kinetic energy
3.6x105 J/m2
Disturbance potential energy
15.6x105 J/m2
Disturbance kinetic energy
8.8x105 J/m2
Solar energy1.5 W/m2
Solar energy 0.7 W/m2
1.5 W/m2
0.3 W/m2
2.2 W/m20.2 W/m2
Dissipation0.1 W/m2
Dissipation1.9 W/m2
Venus?
VEX VCO
Forbes (2002)Gravity waves at low latitude (radio occult.)
Meridional drift velocity at low latitude
H2SO4 vapor at low latitude by radio occult.
Polar collar Polar dipole
Meridional transport of trace gases
Meridional transport of trace gases
Meridional drift velocity at high latitude
H2SO4 vapor at high latitude by radio occult.
Gravity waves at high latitude (radio occult.)
Equatorial waves
Planetary waves driving the circulation