Explanation: How were these unusual Martian spherules created? Thousands of unusual gray spherules made of iron and rock and dubbed blueberries were found embedded in and surrounding rocks near the landing site of the robot Opportunity rover on Mars in 2004. To help investigate their origin, Opportunity found a surface dubbed the Berry Bowl with an indentation that was rich in the Martian orbs . The Berry Bowl is pictured here , imaged during rover's 48th Martian day . The average size of a Martian blueberry rock is only about 4 millimeters across. By analyzing a circular patch in the rock surface to the left of the densest patch of spherules , Opportunity obtained data showing that the underlying rock has a much different composition than the hematite rich blueberries . This information contributes to the growing consensus that these small, strange, gray orbs were slowly deposited from a bath of dirty water .

APOD Turns 30!: Free Public Lecture in Cork, Ireland on Tuesday, June 24 at 7 pm