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.