How does it feel to be on the surface of Mars or Venus? Or even further, like on Pluto or Titan, the moon of Saturn?
This curiosity has advanced space exploration since the launch of Sputnik 1 65 years from. But we are only beginning to scratch the surface of what is knowable about the other planetary bodies in the solar system.
Our new studypublished today in Nature Astronomy, shows how some unlikely candidates — namely sand dunes — can provide insight into the weather and conditions you might encounter if you were standing on a distant planetary body.
What’s in a grain of sand?
English poet William Blake wondered what it means to “see a world in a grain of sand”.
In our research, we took this at face value. The idea was to use the mere presence of sand dunes to understand what conditions exist on the surface of a world.
For the dunes to even exist, there is a pair of “Goldilocks” criteria that must be met. The first is a supply of erodible but durable grains. There must also be winds fast enough to blow those grains to the ground, but not fast enough to carry them high into the atmosphere.
Until now, direct measurement of winds and sediments was only possible on Earth and Mars. However, we observed windblown sediment features on several other bodies (and even comets) by satellite. The very presence of such dunes on these bodies implies that the Goldilocks conditions are met.
Our work has focused on Venus, Earth, Mars, Titan, Triton (Neptune’s largest moon) and Pluto. Unresolved debates over these bodies have been going on for decades.
How do you reconcile the seemingly windblown features on the surfaces of Triton and Pluto with their thin, tenuous atmospheres? Why do we see such prolific sand and dust activity on Mars, despite measured winds that seem too weak to sustain it?
And does Venus’ thick, sweltering atmosphere move sand the way air or water moves on Earth?
Advancing the debate
Our study offers predictions of the winds needed to move sediment over these bodies and how easily these sediments would break up in these winds.
We built these predictions by pulling together results from a host of other research papers and testing them against all the experimental data we could get our hands on.
We then applied the theories to each of the six bodies, relying on telescope and satellite measurements of variables such as gravity, atmospheric composition, surface temperature and sediment resistance.
Studies prior to ours have focused on either the threshold wind speed required to move sand or the strength of various sediment particles. Our work combined them – looking at how easily particles could shatter in sand transport weather conditions on these bodies.
For example, we know that Titan’s equator has sand dunes, but we don’t know what sediments encircle the equator. Is it pure organic mist is it raining from the atmosphere, or is it mixed with denser ice?
It turns out that we discovered that loose aggregates of organic mist would disintegrate in a collision if blown by winds at Titan’s equator.
This implies that the dunes of Titan are probably not made up of purely organic mist. To build a dune, sediments must be blown away for a long time (some of the sands in Earth’s dunes are a million years old).
We also discovered that the wind speed must be excessively fast on Pluto to transport methane or nitrogen ice (which was assumed to be the sediments of Pluto’s dunes). This calls into question whether the “dunes” of the plain of Pluto, Sputnik Planitiaare dunes at all.
They can rather be sublimation waves. These are dune-like landforms resulting from the sublimation of materials, rather than the erosion of sediments (like those seen on the northern polar cap of Mars).
Our results for Mars suggest that more dust is generated by windblown sand transport on Mars than on Earth. This suggests that our models of the Martian atmosphere may not be effectively capturing the strong “katabatic“winds, which are cold gusts that blow downhill at night.
Space exploration potential
This study comes at an interesting stage in space exploration.
For Mars, we have a relative abundance of observations; five space agencies conduct active missions in orbit, or in situ. Studies like ours help illuminate the goals of these missions and the paths taken by rovers such as Perseverance and Zhurong.
At the edge of the solar system, Triton has not been observed in detail since NASA’s Voyager 2 flyby in 1989. There is currently a mission proposal which, if selected, would have a probe launched in 2031 to study Triton, before annihilating itself while flying through Neptune’s atmosphere.
Planned missions to Venus and Titan over the next decade will revolutionize our understanding of both. from NASA Dragonfly The mission, which is expected to leave Earth in 2027 and arrive on Titan in 2034, will land an unmanned helicopter on the moon’s dunes.
Pluto was observed during a 2015 overview by NASA’s ongoing New Horizons mission, but there are no plans to return.
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