Converting THEMIS VIS to Albedo
Sean MarshallAdvised byPhil Christensen
Albedo M-W: “fraction of
incident radiation… reflected by a surface”– Values from 0 to
1– Depends on wavelength, and on direction– Helps determine composition
Albedo data from Thermal Emission Spectrometer (TES) on Mars Global Surveyor
Methods Visible Imaging
Subsystem (VIS) of the Thermal Emission Imaging System (THEMIS) on 2001 Mars Odyssey
Visible camera – detects reflected sunlight
Five different bands Very well calibrated (see
McConnochie et al., 2006) Calculate albedo from VIS
Calculating albedo
Radiance of the Sun (W/m2/sr):Irradiance/flux from the Sun at 1 AU
(W/m2):
Irradiance from the Sun at Mars(W/m2):
= Sun’s luminosity (3.846e26 W)= Sun’s radius (6.96e8 m) = solid angle subtended by the Sun at
1 AU = Sun-Mars distance (in AU)
Calculating albedo
Radiance of Mars if it were a perfect reflector (W/m2/sr):
= Sun’s solid angle at 1 AU
= Sun-Mars distance
Wavelength dependence – spectral radiance in W/m2/sr/μm
Calibrated spectral radiance (L) detected by THEMIS VIS: from RDR (simple conversion)
Lambert albedo:= Sun’s radiance
= Sun’s flux at Mars
θ = solar incidence angle
Getting data Calculate spectral radiance of Sun Load THEMIS VIS image in Davinci Calculate average radiance from all or
part of the image (in band 3) Find region in JMARS and get TES
albedo data Copy relevant parameters to
spreadsheet
Getting data
Analysis Albedo values are
consistently high– Typically by
>50% Using band 3
(red)– But Mars is mostly
red… Using other
bands– Points color coded
Colors matter!
AcknowledgmentsPhil Christensen,
Greg Mehall– Advising
Saadat Anwar, Chris Edwards, Kim Murray, Dale Noss– Consulting
McConnochie et al. JMARS & Davinci
teamsEveryone at Space
Grant!
TES albedo consistency
Using individual TES stamps