Size Distribution of Swiss Cheese Features (SCFs) on Mars

Melissa J. StrausbergDivision of Geological and Planetary Sciences

California Institute of Technology

Pasadena, CA

Outline

• What are SCFs? Where, when and how do we find them?

• How do we think that they formed?

• Size measurement and distribution

• Age distribution

• Implications for initiating phenomenon

What are SCFs?

• Small circular or quasi-circular pits in the permanent CO2 layer on the Martian south polar cap

• First discussed in 2000 (Thomas et al., Nature 2000) with return of high-resolution MOC images

• Cause of formation still debated– postulated to be due to spatially varying dust cover, which gives rise to spatially varying albedo (Byrne & Ingersoll)

Characteristics of SCFs

Different shapes

Different sizes

Different separation distances

Area of Study

Latitude: 87º S

Longitude: 3.3º to 8.3º W

Methods

• Measured SCF diameters using Matlab (1089 features); analyzed data using CurveFit

• Assumed error of 2 pixels on each measurement (regardless of pixel width)

• Reduced 2 between 0.5 and 5 for all measurements, indicating appropriate error estimation

Size Distribution

• Distribution ~same for all images in region, so reasonable to combine results

• Could be normal, linear with extra “tail”, quadratic with extra “tail”– all of these fits have reduced 2 < 5

Age Distribution

• Most studies assume linear growth of 0.5 – 2.5 m/Martian year (Malin et al., Byrne & Ingersoll)

• Applying those hypotheses to this size data:

Growth rate 0.5 m/year 2.5 m/year

Mean age 177.3 Mars years

35.5 Mars years

Standard Deviation

42.7 Mars years

8.5 Mars years

Comparison to Other Studies

• Byrne & Ingersoll (2003) found average size of 217 m, standard deviation of 35 m (tight normal distribution) in same area of residual cap

• Their study encompassed a contiguous region centered near 87º S, 5º W. This study included geographically dispersed images at 87º S between 3.3º W and 8.3º W

Conclusions

• Must consider how to reconcile discrepancy between this study and Byrne & Ingersoll (2003):– Process that induced formation of SCFs was spatially

variable on short length scales; or– Albedo of substrate below CO2 layer is highly variable,

giving rise to different growth rates even in proximate regions

• The wider distribution of sizes found in this study may indicate longer duration of “initiating event” and/or temporally variable strength

• Open question: is growth rate size-dependent?