Dust and Dirt

Dust, dirt, and soil can be important evidence in an investigation. Try the following activity to see how dirt can be used to solve a crime.

Materials

- dirt from three location
- plastic bags
- marking pen
- 3 sheet of white paper
- magnifying lens

Procedure

NOTE: This is an outdoor activity.

1. Collect dirt from three locations. Place it in the plastic bags and mark the location on each bag.
2. At home, place a dirt sample from each bag on separate sheets of white paper. Examine the samples with the magnifying lens.
3. How are the dirt samples different? Is the soil light or dark? What does the soil look like? What size are the gains in the soil? Do you see any stones? Is there any decaying plant material in the soil? What characteristics could you use identify the dirt from a particular place?

Explanation

Soil is one-half air and water, both of which are necessary for plants and animals to live. The rest of soil consists of recycling organisms-such as fungi, molds, bacteria, and earthworms- pieces of rock, and humus (decaying plant and animal matter). The size of the soil particles determines whether soil is sand, silt, or clay. Sands has the largest soil particles and clay has the smallest.

All this information can be used to study soil samples found at a crime scene. Dust and dirt are common as trace evidence because they are easily carried from the scene of a crime in the soles of shoes, on clothing, or on the tires of a vehicle. Forensic scientists compare the dust and dirt found at a crime scene with selected samples to determine where they came from.

Detective Science in Action

In 1977, Janie Sheperd had been missing for four days. Her car, discovered on a London street, had been driven roughly 75 miles (120 km) from her home. By analyzing thick deposits of mud found undermeath the vehicle, investigators determined that the car had been driven in one of four surrounding counties, which greatly narrowed the search area.