What we did

• Analyzed MGS thermal inertia, albedo, and elevation data*
• Cross-correlated datasets to define thermophysical units
• Examined units on global, regional, and local scales

   * TES- and MOLA-derived maps from Mellon et al (2002)

What we found

• An unusual low thermal inertia, low albedo region poleward of 65°S
• Refined TES-based thermal inertia and albedo for past lander sites

• Surface temperatures are driven by albedo, dust opacity, atmospheric pressure, and thermal inertia
• Thermal inertia is the key control on diurnal temperature oscillations:

  I ≡ ( k ρ C )^1/2    [ J m^-2 K^-1 s^-1/2 ]

  where:
  

  k = bulk thermal conductivity ≈ k_g + k_s + k_r
  ρ = bulk density
  C = heat capacity
  

  I depends primarily on physical properties:

  increases with particle size and degree of induration

Global Thermal Inertia from  Mellon et al (2002)

Global TES Albedo

Thermophysical Unit Definition

Global Thermophysical Unit Map

Southern Hemisphere Thermal Inertia

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90270
180

MGS vs. Viking Thermal Inertia: 60°S-80°S

Southern Hemisphere Albedo

0
90270
180

Southern Hemisphere Thermophysical Unit Map

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90270
180

Thermal inertia can be lowered by:

• Fining particles (lowers k_g)
• Reducing bulk density (lowers ρ, k_eff)
• Changing composition (can lower {ρ C} by ×2 max )
• Removing cementation (lowers ρ, k_s)

Albedo can be lowered by:

• Coarsening particles (reduces backscattering)
• Darkening material:
• Different bulk composition
• Coating particles
• Thermally thin (~ few μm) layer
• Cementation

"Fluffed" soil theory

Thermal inertia reduction by ρ reduction in material ~ Unit B

• Presley & Christensen (1997) show for Mars conditions:
     k × 1.25 ~ ρ × 2
  for 23-30 μm particles.  Thus:
     I ÷ 2 requires ~ ρ ÷ 2.5



• "Soil" is fluffed by ground ice which subsequently sublimes



• Neutron data show ice up to 75% by volume in polar region (Boynton et al, 2002)

Dark dust theories

Albedo reduction in material with I ~ Unit A

• Darker dust (new bulk composition)
• Need source distinct from bright dust


• Darker layer over bright dust ~ Unit A
• Thermally thin (~few μm) - I controlled by bright dust
• Areal extent large - unstable for long times?

Global storms expected to homogenize dust over years to decades

• Darker coating on bright dust ~ Unit A
• Darkening process must be reversible

Inertia-Albedo Histogram for 30°S to 80°S

Inertia-Albedo Histogram for 60°S to 80°S

Inertia-Albedo Histogram for 70°S to 80°S

Low thermal inertia, low albedo region poleward of 65°S

• Consistent with Viking thermal inertias (Paige & Keegan, 1994)
• Differs from classic low thermal inertia regions (low I, high A)
• Several hypotheses examined:
• Soil "fluffing":  would require ~ ρ ÷ 2.5
• Grain surface darkening:  must be reversible
• Darker dust:  unlikely due to global circulation
• Continuing analysis will incorporate color data and MOC images

MGS-TES updated lander site data

Site	Inertia*	Albedo	Unit
VL-1	283	0.22	C
VL-2	234	0.24	C
MPF	387	0.19	F

* Units: J m^-2 K^-1 s^-1/2