NASA/JPL/University of Arizona
Martian Volcanic Vent with Elevated Rim
The two aligned pits are clearly volcanic vents, as they sit on top of small (for Mars) shield volcanoes and are the sources for many lava flows. The vent at bottom also has an elevated rim, as do fresh impact craters. The presence or absence of raised rims is often used as a criteria to help distinguish between volcanic and impact craters, but isn't always reliable. The rims of volcanic craters may become elevated by repeated overflowing of a lava lake or from the ballistic deposition of molten lava or "spatter". The rims of impact craters are raised by rebound from the impact and from overturned flaps of ejecta.
Could these vents be the source of atmospheric methane that has been recently detected on Mars? No, they are old and dusty, like every volcanic vent imaged so far on Mars.
Written by: Alfred McEwen