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