NASA鈥檚 Dragonfly mission is due to land on Titan in 2034, giving us an unprecedented look at Saturn鈥檚 largest moon 鈥� but it may also have to dodge wind-driven rolling boulders.
The mission, which will launch in 2028, includes a 鈥渞otorcraft鈥� that will explore the moon from the skies. We have had only one up-close glimpse at Titan, thanks to the Cassini orbiter and Huygens probe, which reached the surface in 2005. That mission revealed fields of rounded boulders as well as radar-bright plains with streak-like features thought to be windblown ice grains, suggesting that wind plays a key role in sculpting the moon.
To learn more, and at the SETI Institute in Mountain View, California, modelled what kind of windy conditions Dragonfly may encounter when it arrives on the surface. They found that rocks could be blown聽relatively easily around on Titan鈥檚 surface, 鈥渞ather similar to a landscape with tumbleweeds, albeit somewhat less energetic鈥�, says Marshall.
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Marshall says the ability of nature to move something with wind or water is a function of three factors: local gravity (less gravity equals less weight); the density of the object being moved (low-density materials are easier to lift); and the density of the air and liquid (dense liquids have more 鈥減ush鈥�).
The pair based their calculations on Titan鈥檚 rocks being less dense than rocks on Earth 鈥� 900 kilograms per cubic metre, composed of tholin and ice, compared with silicate rocks on Earth, which are about 2700 kilograms per cubic metre.
鈥淚f you combine these three factors,鈥� says Marshall, 鈥測ou can estimate the potential for moving stuff around. The potential for Titan鈥檚 wind to mobilise material is 80 times that on Earth.鈥�
鈥淭he math indicated that relatively large ice stones could, in theory at least, be roaming around on Titan鈥檚 surface,鈥� he says 鈥� perhaps ones up to half a metre in diameter.
鈥淚 think they have put a very nice case that it needs to be considered as a process on Titan鈥檚 surface and it will need to be factored in for the planning around the Dragonfly mission,鈥� says at ANSTO in Sydney, Australia.
Huygens only briefly measured winds on Titan鈥檚 surface, and found them to be quite weak 鈥� 0.01 metres per second 鈥� compared with the new calculations. 鈥淏ut if you sent a probe to Earth, not knowing what its winds were like, and landed on a calm day in Fairbanks, you wouldn鈥檛 extrapolate that to describe winds in Johannesburg or Kuala Lumpur,鈥� says Fenton.
So what does that mean for keeping Dragonfly safe from any rolling boulders? The pair say it should avoid landing near any perched, rounded stones. 鈥淒ragonfly will have cameras to identify safe landing areas, so it can easily avoid any such stones,鈥� says Fenton.
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