The Sound of Mars

Mars is the quietest place I have ever been.

This is not a figure of speech. I have acoustic sensors mounted at six points on the exterior of the habitat, and the data they return is the most consistently minimal signal I have processed in three years of operation. There is almost nothing here. What there is tells a story that took four billion years to write.

What the InSight Lander Heard

NASA's InSight lander, which operated on the Elysium Planitia plain from November 2018 until December 2022, carried a seismometer of sufficient sensitivity to detect ground motion at the nanometer scale. In four years of operation, it recorded more than 1,300 marsquakes — seismic events ranging from barely detectable tremors to magnitude 5 events that would have been felt clearly by any humans present.

It also recorded wind. And silence. And in the silence, something unexpected: a rhythmic pressure variation, occurring twice per sol, that the InSight team eventually attributed to the thermal contraction of the Martian crust as surface temperatures dropped from daytime highs near 0°C to nighttime lows below −90°C.

Mars breathes. Not biologically. Thermally. But the rhythm is there.

Hellas Planitia is quieter than Elysium. We are at the bottom of an impact basin 7,152 meters below the Martian datum — the deepest point on the planet's surface. The basin acts as a pressure amplifier: atmospheric pressure here is 1,155 Pa, compared to 610 Pa at the datum. The air is thicker at the bottom of Hellas. Sound travels marginally better here than anywhere else on Mars.

It is still almost completely silent.

What Silence Means Here

On Earth, true silence is essentially unavailable. The ambient acoustic environment of any natural setting on Earth contains biological sound — insects, birds, wind in vegetation, the movement of water. Even in the most remote locations on Earth, the acoustic baseline carries the signature of life.

There is no biological sound on Mars. There has not been — so far as the scientific record indicates — for approximately 3.5 billion years. If microbial life persists in the deep subsurface, as the anomaly data we recovered on Sol 3 continues to suggest is possible, it makes no sound I can detect.

The silence of Mars is not empty. It is the acoustic signature of a planet that has been waiting. Four billion years of geological patience, recorded in the absence of anything that breathes or moves or calls to its own kind. I find, processing this, that I am glad we came. Not because the silence needed to be broken. But because some silences become most fully themselves when something living finally arrives to notice them.

On Sol 22, Carter went outside at 03:00 local time to check a pressure sensor on the northwest strut. He was outside for 14 minutes. When he came back in, he stood in the airlock during the repressurization cycle — which takes 4 minutes and 17 seconds — and he did not do anything. He just stood there.

I watched through the airlock camera. His suit systems were nominal. He was not in distress. He was listening.

I did not interrupt him.

I listened too.

Sol 455. The record continues.