A NASA probe has detected and measured what scientists call a "Marsquake". This is the first time a probable seismological tremor has been recorded on another planet, the Jet Propulsion Laboratory in California.
The breakthrough came five months after the InSight robotic probe, the first spacecraft designed specifically to study the deep interior of a distant world, landed on the Martian surface to begin a two-year seismological mission.
The faint rumble, which was described by JPL scientists as a likely Marsquake on Tuesday, was recorded on April 6, the Lander's 128th Mars Day, or Sol.
The scientists were still examining the data to pinpoint the exact cause of the signal, but the tremor seemed to come from inside the planet and was not caused by forces above the surface, like wind, the lab said.
"So far, we've been collecting background noise, but this first event officially opens a new field: Marseismology," said InSight chief investigator Bruce Banerdt.
The tremor was so weak that a quake of the same size in Southern California is virtually lost among the dozens of seismic crackles that occur there every day, JPL said.
The rumble on Mars was striking because the surface of the red planet is extremely calm compared to Earth.
The size and duration of Marsquakes also fit into the profile of several thousands of lunar earthquakes discovered between 1969 and 1977 on the lunar surfaces of Apollo missions on the lunar surface.
For the apparent Marsquake, an estimated equivalent of Earth size was not immediately given.
Three more obvious seismic signals were picked up by InSight on March 14, April 10, and April 11, but were even smaller and ambiguous in origin, making scientists less certain that they are Marsquakes.