Analysis of Rosetta/VIRTIS spectra of earth using observations from ENVISAT/AATSR,TERRA/MODIS and ENVISAT/SCIAMACHY,and radiative-transfer simulations

Rosetta,theSolarSystemcornerstonemissionofESA'sHorizon2000programme,consistsofanorbiter
and alander,andisduetoarriveatthecomet67P/Churyumov-Gerasimenko inMay2014.Followingits
2004launch,Rosettacarriedoutaseriesofplanetary fly-bysandgravitationalassists.Ontheseclose
fly-bysoftheEarth,measurementsweretakenbytheVisibleInfraredThermalImagingSpectrometer
(VIRTIS). AnalysisofthesespectraandcomparisonwithspectraacquiredbyEarth-observingsatellites
can supporttheverification oftheinflight calibrationofRosetta/VIRTIS.
In thispaper,measurementstakenbyVIRTISinNovember2009arecomparedwithsuitable
coincident datafromEarth-observinginstruments(ESA-ENVISAT/AATSRandSCIAMACHY,andEOS-
TERRA/MODIS). RadiativetransfersimulationsusingNEMESIS(Irwin etal.,2008) are fit tothe fly-bydata
taken byVIRTIS,usingrepresentativeatmosphericandsurfaceparameters.VIRTISmeasurements
correlate90%withAATSR's,85-94% withMODIS,and82-88% withSCIAMACHYs.
The VIRTISspectraarereproducibleinthe1-5 ?m region,exceptinthe1.4 ?m deepwatervapour
spectral absorptionbandinthenear-infraredincasesinwhichtheradianceisverylow(cloud-free
topographies),whereVIRTISconsistentlyregistersmoreradiancethandoMODISandSCIAMACHY.Over
these cloud-freeregions,VIRTISregistersradiancesafactorof3-10largerthanSCIAMACHYandof3-8
greaterthanMODIS.Itisspeculatedthatthisdiscrepancycouldbeduetoaspectrallightleakoriginating
from reflections fromtheorder-sorting filters abovethedetectoraround1.4 ?m.