Why Titan is awesome #8

Editions onetwothreefourfivesix and seven.

A new study has used radar data obtained by the Cassini probe to determine that the winds blowing on Saturn’s moon Titan aren’t that strong. This is because waves on the surface of the three largest lakes near the moon’s north pole probably aren’t that high, at least in the “early summer”. The lakes are Kraken Mare, Ligeia Mare and Punga Mare. A University of Texas press release accompanying the paper has extrapolated its findings to suggest that future probes to the moon’s surface will be “in for a smooth landing”. (This is probably why Gizmodo has a weird headline for its piece discussing this work: Saturn’s Moon Titan May Have the Perfect Landing Spot For Spacecraft. I’m not sure how a study about the entire surface area of three of the largest Titanic lakes leads to talk about a tiny landing spot for a single mare explorer, but sure. PTI also, btw.)

These findings aren’t exactly pioneering. In 2008, scientists used Cassini radar data to measure the height of waves on Ontario Lacus, a lake near Titan’s south pole. In 2011, the probe was used to analyse the surfaces of Kraken Mare, one of the three lakes considered in the new study, and Jingpo Lacus (both near the north pole). In 2013, a fresh set of Cassini readings were used to study the surface of Ligeia Mare – yet another of the three lakes featured in the new study. And the new study confirms what we’ve known for the last ~decade or so: the waves on the moon’s supercold liquid-methane bodies don’t rise up by more than a few centimetres. About 1 cm, according to one reading, with the wave itself being about 20 cm long.

The real bit of novelty was two things. First, the use of a technique developed in a 2012 to determine surface roughness, called radar statistical reconnaissance (RSR), incidentally by the same person listed as the first author: a UTexas research associate named Cyril Grima (no offence to him but his name keeps making me imagine he looks like Brad Dourif). RSR uses a set of mathematical techniques to analyse the coherence, and incoherence, of radar waves reflected by rough surfaces. It was among the suite of techniques used decide a landing spot for the NASA InSight lander to be launched in May 2018. The second bit of novelty in the new study is the conclusion that the early summer period in Titan’s northern hemisphere cannot be as windy as it has been believed or the waves would be much bigger.

However, there are some caveats, as usual ignored by most media reports. The first giveaway is that RSR is a statistical technique and, sure enough, towards the end of the paper, its authors write (with ref to the image immediately below):

Credit: Earth and Planetary Science Letters
Credit: Earth and Planetary Science Letters

If smaller patches of higher roughness should occur within those tracks [shown in colour], they might not produce enough surface echoes to significantly change the trend of the amplitude distributions, making them invisible through the RSR process. In the best case, two regions with different but equally-dominant scattering regimes might produce a bi-modal distribution that is not obvious on our measurements. Hence, quiet sea surfaces might be a dominant trend during the northern early summer, but fields of waves activity might occur over local patches and/or might not be sustained over significant periods of time.

(Emphasis added.) This I think could turn the Gizmodo-type (and similar) headlines on their heads, to: “The surfaces of Titan’s lakes are usually calm – but a freak wave could sink NASA’s fancy probe.” But to be realistic, and taking a cue from the InSight paper, figuring out the perfect landing spot for a mare explorer will take a lot more analysis, especially since Cassini is going to be out of commission pretty soon. The Grima et al study is set to be published in the September 15 edition of the journal Earth and Planetary Science Letters.