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