The 20th century saw humankind’s first careful steps outside of the biosphere in which our species has evolved. Whereas before humans had experienced the bitter cold of high altitudes, the crushing pressures in Earth’s oceans, as well as the various soundscapes and vistas offered in Earth’s biosphere, beyond Earth’s atmosphere we encountered something completely new. Departing Earth’s gravitational embrace, the first humans who ventured into space could see the glowing biosphere superimposed against the seemingly black void of space, in which stars, planets and more would only appear when blending out the intense light from the Earth and its life-giving Sun.

Years later, the first humans to set foot on the Moon experienced again something unlike anything anyone has experienced since. Walking around on the lunar regolith in almost complete vacuum and with very low gravity compared to Earth, it was both strangely familiar and hauntingly alien. Although humans haven’t set foot on Mars yet, we have done the next best thing, with a range of robotic explorers with cameras and microphones to record the experience for us here back on Earth.

Unlike the Moon, Mars has a thin but very real atmosphere which permits the travel of soundwaves, so what does the planet sound like? Despite what fictional stories like Weir’s The Martian like to claim, reality is in fact stranger than fiction, with for example a 2024 research article by Martin Gillier et al. as published in JGR Planets finding highly variable acoustics during Mars’ seasons. How much of what we consider to be ‘normal’ is just Earth’s normal?

Spherical Astronauts On Mars

A major limitation with experiencing extraterrestrial worlds is of course that even if we could easily zip over to the more distant ones in a faster-than-light spacecraft, our bodies have evolved within the confines of the Earth’s biosphere and explicitly just the biosphere as it has existed only relatively recently, geologically speaking. Even the atmospheric conditions of the Earth’s Cambrian period would be lethal to humans, with virtually no oxygen to breathe. It’s highly unlikely that we will find any planets out there that are at least as friendly to human life as the Cambrian period would be to our astronauts, so our experience of alien worlds will most assuredly not match those of the average Star Trek episode.

But assume, if you will, that our perfectly spherical, friction-less astronauts are as impervious to cold, heat and radiation as the intrepid robotic explorers which currently peruse the surface of Mars or which have in the past prodded the Venusian atmosphere and its surface. If stepping outside the lander in this ideal scenario, what can our robotic friends tell us about what walking on Mars would be like?

With Mars much further away from the Sun, its light is dimmer, though still bright enough to make out the rocky reddish, brown, greenish and tan coloring. Most steps that you take will leave behind a footprint, albeit not as deep as on Earth due to Mars gravity of only about 0.38 g, or roughly a third of Earth’s. This does preclude the option of bunny hopping across the surface as on the Moon with its 0.165 g.

Mars’ atmosphere is quite thin, also on account of the planet having lost its magnetosphere a long time ago, exposing the atmosphere to the solar winds as they rip and tear away at it. With an atmospheric density of at most 1,150 Pa (on the Hellas Planitia plain) it’s akin to being on Earth at an altitude of 35 km, or well above the average commercial jetliner’s cruising altitude of below 12 km. Even so, sounds are audible, albeit attenuated courtesy of the 96% CO₂ content of the Martian atmosphere. This makes everything sound muted and quite different from what we are used to on Earth.

Whether you stand still and take in the vista surrounding you, or move around, you can hear something like what the Perseverance rover recorded using its twin microphones:

Perseverance also captured the noise of the Ingenuity helicopter as it flew near the rover at a distance of 80 meters, all of which provided researchers with invaluable data on how sound propagates on Mars. In the earlier referenced paper by Martin Gillier et al. the attenuation is calculated to be 500 times higher for low frequencies and 10 times higher at high frequencies as in Earth’s atmosphere at sea level.

Meanwhile the speed of sound on Mars on its surface varies as the CO₂ in the atmosphere increases or decreases with the seasons, especially near the poles where carbon dioxide ice is known to exist. Compared to the speed of sound in Earth’s atmosphere of 343 m/s, on Mars you can expect around 252 m/s, although this will differ wildly per season and at which altitude you are.

As glorified in The Martian and other works of fiction Mars may be, the experience of walking around on its surface would be mostly one of eerie disconnect due to the lower gravity and the muffled sounds including those made by one’s own boots. Assuming that the radiation blasting the Martian surface and intense temperature swings are no concern, this might yet be the perfect vacation spot for some astronauts.

From Venus With Love

The Soviet Venera 13 and 14 missions featured the first landers to Venus that were equipped with microphones. These were active during their final descent, as well as the workings of the pyrotechnics and surface drill, prior to the quiet observation of the lander with its scientific instruments. Below is embedded part of the audio from the Venera 14 mission (also on Archive.org).

Based on these audio recordings, the wind speed on Venus’ surface was calculated to be on average 0.3 and 0.5 m/s, which doesn’t seem much until you realize that this is with a pressure of around 9.5 MPa (94 times Earth’s atmosphere) and a temperature of 465 ℃. These findings were covered in a 1982 paper by L. V. Ksanfomaliti et al. (PDF) as submitted to Soviet Astronomy Letters.

The effect for our theoretical astronaut would be akin to being crushed and burned at the same time, while the thick, mostly CO₂-based atmosphere slowly churns past.

It is for this reason that our less-invincible astronauts would remain in the Venusian atmosphere at a more agreeable pressure and temperature level. In such a floating colony the experience would be much more akin to being on Earth at ground level, if you excuse the sulfuric rain droplets, of course.

Welcome To Europa

So far our photographic and auditory collection of extraterrestrial planets is still rather limited, with Mars and Venus being the two primary examples where we have collected both types of recordings on their respective surfaces. However, depending on how things work out, we may soon be adding Jupiter’s moon Europa to this list. This is perhaps the most intriguing target in our solar system which we have not visited yet in any significant detail, despite it being assumed to be a water ice-covered moon that is slightly smaller than Earth’s Moon, with potentially liquid water below the ice.

Recently the Europa Clipper spacecraft was launched on its multi-year mission for a rendezvous with Europa by April 2030. One of its mission goals at Europa is to determine a suitable landing site for the proposed Europa Lander, which – if funded – would land on Europa in the 2030s where it would be able to examine and image the surface. Sadly Europa does not have much of at atmosphere, much like Earth’s Moon, but it might make for a fascinating place to do some ice skating for our radiation-proof astronauts. If there is a liquid ocean underneath the ice as suspected, then deep-sea diving on Europa is definitely also on the menu, barring any scary oceanic lifeforms in said oceans.

Beyond these places in our solar system the sounds and sights become sadly a bit murky. Mercury is a Sun-blasted rock, while Pluto is a darkness-shrouded rock, and all of Jupiter, Saturn, Neptune and Uranus are gas giants. Beyond perhaps a couple of the more interesting moons surrounding these gas giants we will have to look beyond this solar system to find more interesting extraterrestrial sights and sounds. Fortunately for this we will only have to send out our faster-than-light spacecraft into deep space, as there are still billions upon billions of star systems to examine and places to experience. Makes you realize how good we’ve got it here on earth.