Pavonis Mons

Four exceptionally large volcanoes in a region called Tharsis are unique to the western hemisphere of Mars. Three of the Tharsis volcanoes, Ascraeus Mons, Pavonis Mons, and Arsia Mons, are aligned along a NE - SW trend, with Pavonis in the middle, straddling the equator. Olympus Mons, the fourth Tharsis volcano and the largest in the solar system, is located NW of Pavonis Mons. At the top right of the image, the rim of the caldera of Pavonis Mons is just barely visible, with steep NE-facing cliffs formed by the collapse of a portion of the volcano's summit. At the southwest edge of the caldera, additional fractures are apparent and may someday collapse, making the summit caldera even larger. This image of Pavonis Mons also demonstrates some of the distinctive characteristics of the martian surface in the Tharsis region. Tharsis is very dusty; the dust covers everything like fresh snow, which is the reason why there is very little contrast in the surface materials as compared to other THEMIS images that show apparently bright and dark surfaces in the same picture. This dust cover makes it difficult to distinguish different geologic or geomorphic units in the area, and even the piles of lava flows that constructed this volcano are difficult to make out. Most of the craters on the volcano are small, a few tens of meters to kilometers in diameter, suggesting that this surface is a relatively young one on Mars (the older a surface is, the more and larger craters it has). In the lower third of the image, linear arrangements of small, round pits can be seen. These features are commonly called 'pit chains' and most likely represent the collapse of lava tubes. Lava tubes are like a subway, allowing molten rock to move from place to place underground. A particularly large pit near the bottom center of the image looks a lot like a crater. However, the lack of degradation of the rim of this feature suggests that if it were an impact crater, it would be relatively young, and an ejecta blanket of debris should be visible. Because there is no apparent sign of an ejecta blanket, it is more likely that this and nearby similar features are simply the result of larger collapses.