NASA has lost contact with one of its three spacecraft orbiting Mars, the agency announced Tuesday. Meanwhile, a second Mars orbiter is perilously close to running out of fuel, and the third mission is running well past its warranty.

Ground teams last heard from the Mars Atmosphere and Volatile Evolution, or MAVEN, spacecraft on Saturday, December 6. “Telemetry from MAVEN had showed all subsystems working normally before it orbited behind the red planet,” NASA said in a short statement. “After the spacecraft emerged from behind Mars, NASA’s Deep Space Network did not observe a signal.”

NASA said mission controllers are “investigating the anomaly to address the situation. More information will be shared once it becomes available.”

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Our exploration of the outer Solar System has revealed a host of icy moons, many with surface features that suggest a complex geology. In some cases, these features—most notably the geysers of Enceladus—hint at the presence of oceans beneath the icy surfaces. These oceans have been ascribed to gravitational interactions that cause flexing and friction within the moon, creating enough heat to melt the body’s interior.

Something that has received a bit less attention is that some of these orbital interactions are temporary or cyclical. The orbits of any body are not always regular and often have long-term cycles. That’s also true for the other moons that provide the gravitational stress. As a result, the internal oceans may actually come and go, as the interiors of the moons melt and refreeze.

A new study, released today by Nature Astronomy, looks at one of the consequences of the difference in density between liquid water and ice (about 10 percent): the potential for the moon’s interior to shrink as it melts, leaving an area of low pressure immediately below its icy shell. If the moon is small enough, this study suggests, that could cause the surface of the ocean to boil.

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