are between 2–3 m in diameter and are bounded by troughs that are 20 to 50 cm deep. These shapes are caused by ice in the soil reacting to major temperature changes. The top of the soil has a crust. The microscope showed that the soil is composed of flat particles (probably a type of clay) and rounded particles. When the soil is scooped up, it clumps together. Although other landers in other places on Mars have seen many ripples and dunes, no ripples or dunes are visible in the area of Phoenix. Ice is present a few inches below the surface in the middle of the polygons. Along the edge of the polygons the ice is at least 8 inches deep. When the ice is exposed to the Martian atmosphere it slowly disappears. In the winter there would be accumulations of snow on the surface. Surface chemistry Results published in the journal Science after the Phoenix mission ended reported that chloride, bicarbonate, magnesium, sodium, potassium, calcium, and possibly sulfate were detected in the samples. The pH was narrowed down to 7.7 + or - 0.5. Perchlorate (ClO4), a strong oxidizer, was detected. This was a significant discovery. The chemical has the potential of being used for rocket fuel and as a source of oxygen for future colonists. Under certain conditions perchlorate can inhibit life; however some microorganisms obtain energy from the substance (by anaerobic reduction). The chemical when mixed with water can greatly lower freezing points, in a manner similar to how salt is applied to roads to melt ice. Perchlorate strongly attracts water; consequently it could pull humidity from the air and produce a small amount of liquid water on Mars today. Gullies, which are common in certain areas of Mars, may have formed from perchlorate melting ice and causing water to erode soil on steep slopes. Two sets of experiments demonstrated that the soil contains 3-5% calcium carbonate. When a sample was slowly heated in the Thermal and Evolved-Gas Analyzer (TEGA), a peak occurred at 725 °C, which is what would happen if calcium carbonate were present. In a second experiment acid was added to a soil sample in the Wet Chemistry Laboratory (WCL) while a pH electrode measured the pH. Since the pH rose from 3.3 to 7.7, it was concluded that calcium carbonate was present. Calcium carbonate changes the texture of soil by cementing particles.