size distribution of swiss cheese features (scfs) on mars

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?