Learning Objectives
- Calculate crater density for equal-area samples of planetary surfaces.
- Use Poisson counting uncertainty to compare relative ages.
- Explain why crater counts require provenance, diameter threshold, and geologic context.
Preparation
Review Lectures 5 and 7 on terrestrial surfaces, airless worlds, crater saturation, resurfacing, and relative chronology.
Materials and Data
../data/lab-03-crater-counts.csv, an instructor-count table from named real mosaics: LROC WAC global lunar mosaic and Mars Odyssey THEMIS daytime IR mosaic.- Calculator or spreadsheet.
- Optional instructor-provided image windows for annotation; the numerical task can be completed from the CSV.
Apparatus and Setup
- Open the crater-count CSV and confirm that all four samples use 10,000 km2 counted areas.
- Create columns for crater density above 1 km, crater density above 5 km, Poisson uncertainty, and relative-age rank.
- Record the processing state as instructor-count table from real NASA mosaics, not a published crater chronology catalog.
Procedure
- For each region, calculate crater density \(N/A\) for D >= 1 km and D >= 5 km craters.
- Calculate 1-sigma Poisson uncertainty as \(\sqrt{N}/A\) for both diameter thresholds.
- Rank the four surfaces from youngest to oldest using D >= 1 km density. Repeat using D >= 5 km density.
- Identify whether either ranking is ambiguous once uncertainty bars are considered.
- Write a process interpretation for each region: volcanic resurfacing, ancient highlands, young plains, or heavily cratered terrain.
Measurement Record
| Region | N(D >= 1 km)/A | 1-sigma uncertainty | N(D >= 5 km)/A | Relative age rank |
|---|---|---|---|---|
| Mare Imbrium sample | ||||
| Lunar southern highlands sample | ||||
| Elysium Planitia sample | ||||
| Noachis Terra sample |
Analysis and Uncertainty
Report crater density in craters per 10,000 km2 and craters per km2. Use \(\sqrt{N}\) as the counting uncertainty, then divide by area. Discuss at least one non-statistical uncertainty: secondary craters, eroded crater rims, illumination angle, image resolution, or geologic-unit boundary placement.
Deliverables
- Completed crater-density table with uncertainty for both crater thresholds.
- Bar chart or dot plot with uncertainty bars for D >= 1 km crater density.
- Annotated ranking statement that identifies any overlap in uncertainty.
- Provenance note naming LROC WAC, Mars Odyssey THEMIS, and
lab-03-crater-counts.csv.
Assessment Criteria
- Correct crater-density and Poisson calculations: 30%.
- Graph includes uncertainty bars and units: 20%.
- Relative-age interpretation follows the evidence and notes ambiguity: 25%.
- Non-statistical uncertainty is specific to crater counting: 15%.
- Complete data provenance: 10%.
References, Software, and Data Provenance
Data file: materials/ASTR120/data/lab-03-crater-counts.csv. Named image products: NASA Lunar Reconnaissance Orbiter Camera Wide Angle Camera global mosaic and NASA Mars Odyssey THEMIS daytime infrared mosaic. Processing state: instructor-created crater-count teaching table from real mosaics, with counts simplified for introductory comparative analysis.