**2. Observed properties of Martian hematite spherules**

Within a few days of operations, the Opportunity rover surprised the science community by sending pictures of a large number of spherules on Mars, now commonly known as blueberries [18]. The follow-up investigation [11, 13] by the NASA science team found several interesting observations of blueberries. Some of these observations are as follows: (i) the primary carriers of hematite at the landing site are blueberries and their fragments; (ii) the hematite was located mostly on the surface of the landscape; (iii) the deeper soil is mostly basaltic sand and is free of hematite; (iv) the blueberries are mostly perfect spheres; (v) the typical diameters of the blueberries are 4–6 mm; (vi) the blueberries are hard; (vii) the blueberries are made of very fine grain material; and (vii) the blueberries have no internal structure. In addition, all the hematite spherules appear to be located within the upper 10 mm thickness of the surface soil.

**Figure 4** shows the result of the RAT used on the Martian surface to cut open some of the soil-embedded spherules. The spherules were found to be hard and difficult to cut. The spherules showed no internal structure with grain size below the detection limit (31 μm) of the Microscopic Imager (MI). Further investigation of the spherules found that there are two types of blueberries: larger blueberries with average size of 4 mm in diameter and microberries with average size of 0.79 mm in diameter. All blueberries were smaller than 6.2 mm in diameter with a median size of 4.2 mm.

#### **Figure 4.**

*Rock abrasion tool (RAT) was used to cut some of the spherules embedded in the soil. The hematite spherules show no internal structure. (Image courtesy of NASA/JPL).*

#### **Figure 5.**

*Jammerbugt (sol 842) showing the trenches dug by the Opportunity rover. All hematite is located on the Martian surface and no blueberry was found in the deeper soil. (Image courtesy of NASA/JPL).*

On sol 833, Opportunity got stuck in a fine-grained soil (named Jammerbugt) and it took 8 days for the operation team to free the rover. During this process, the rover's wheel dug deep trenches on Martian soil. **Figure 5** shows images of Jammerbugt taken by the rover on sol 842 (June 7, 2006) using the panoramic camera. **Figure 5** shows that all the hematite (blueberries, microberries, and fine dust) is limited to the top surface and trenches dug by the rover showed no sign of blueberries in deeper soil.

In summary, a very large amount of Martian hematite spherules was found to be mostly perfect hard spheres less than 6 mm in diameter with fine grain, no internal structure, and located within 10 mm of the Martian surface.
