What is it like to be on Mars or Venus’s surface? Or even further out, like on Pluto or Saturn’s moon Titan? Since the launch of Sputnik 1 65 years ago, this curiosity has driven advances in space exploration.
However, we are only scratching the surface of what is known about other planetary bodies in the Solar System.
Our new study, published today in Nature Astronomy, demonstrates how some unlikely candidates, namely sand dunes, can provide insight into what weather and conditions you might encounter if you stood on a distant planetary body.
What exactly is a grain of sand? William Blake, an English poet, famously wondered what it meant to “see a world in a grain of sand.”
We took this quite literally in our research. The idea was to use the presence of sand dunes to learn about the conditions on the planet’s surface.
There are two “Goldilocks” criteria that must be met in order for dunes to exist. The first requirement is a supply of erodible but durable grains.
Winds must also be strong enough to cause the grains to hop across the ground, but not strong enough to carry them high into the atmosphere.
Direct measurements of winds and sediment have previously only been possible on Earth and Mars.
Wind-blown sediment features on other bodies (including comets) have been observed by satellite.
The presence of such dunes on these bodies indicates that the Goldilocks conditions have been met.
We concentrated our efforts on Venus, Earth, Mars, Titan, Triton (Neptune’s largest moon), and Pluto. For decades, these bodies have been the subject of unresolved debates.
How do we reconcile the apparent wind-blown features on the surfaces of Triton and Pluto with their thin, tenuous atmospheres? Why is there so much sand and dust activity on Mars, despite measuring winds that appear to be too weak to sustain it? And does Venus’s thick and oppressively hot atmosphere move sand in the same way that air or water does on Earth? Adding to the discussion Our research predicts the winds needed to move sediment on these bodies, as well as how easily that sediment would break apart in those winds.
We built these predictions by combining findings from a variety of other research papers and testing them against all available experimental data.
The theories were then applied to each of the six bodies, using telescope and satellite measurements of variables such as gravity, atmospheric composition, surface temperature, and sediment strength.
Previous research has focused on either the wind speed threshold required to move sand or the strength of different sediment particles.
Our research looked at how easily particles could break apart in sand-transporting weather on these bodies.
We know Titan’s equator has sand dunes, but we don’t know what sediment surrounds the equator.
Is this pure organic haze falling from the sky, or is it mixed with denser ice? We discovered that loose aggregates of organic haze would disintegrate upon collision if blown by the winds at Titan’s equator.
This implies that Titan’s dunes aren’t made entirely of organic haze. To form a dune, sediment must be blown around in the wind for an extended period of time (some dune sands on Earth are over a million years old).
We also discovered that wind speeds on Pluto would have to be extremely high in order to transport either methane or nitrogen ice (which is what Pluto’s dune sediments were thought to be).
This calls into question the existence of “dunes” on Pluto’s plain, Sputnik Planitia.
They could be sublimation waves instead. These are dune-like landforms formed by material sublimation rather than sediment erosion (as seen on Mars’ north polar cap).
Our results for Mars indicate that wind-blown sand transport generates more dust on Mars than on Earth.
This suggests that our models of Mars’ atmosphere may be failing to capture the planet’s strong “katabatic” winds, which are cold gusts that blow downhill at night.
This research comes at an exciting time in the history of space exploration.
We have a relative abundance of observations for Mars; five space agencies are conducting active missions in orbit or on the ground. Studies like ours help shape the goals of these missions and the paths taken by rovers like Perseverance and Zhurong.
Triton has not been observed in detail in the Solar System’s outer reaches since the NASA Voyager 2 flyby in 1989.
A mission proposal is currently being considered, which would see a probe launched in 2031 to study Triton before annihilating itself by flying into Neptune’s atmosphere.
Missions to Venus and Titan planned for the next decade will transform our understanding of these two planets.
The Dragonfly mission, which is scheduled to launch in 2027 and arrive on Titan in 2034, will land an unmanned helicopter on the moon’s dunes.
Pluto was observed by NASA’s ongoing New Horizons mission during a flyby in 2015, but there are no plans to return.
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