Lava channel, Sinuous rille

Classic lava channels with steep sides and flat floors. They do not wind very far, and they usually start in a round depression.

Lava channels are much alike on Earth, Venus, the Moon and Mars (Kargel et al. 1993). On the Moon they are called (sinuous) rilles.

On Venus they are 10–300 km long, up to 5 km wide and up to 500 m deep. This is much deeper than rivers on Earth or the canali.

Unlike the canali, they may also have been shaped by thermal erosion, that is, by melting of the ground. More than half of them are linked to coronae and arachnoids. So their formation may be tied to mantle diapirs, which produce long-lasting lava flows with high eruption rates.

One of the best-known lunar lava channels is Hadley Rille. It was probably a lava tube below the surface at first. The tube formed inside a thick, inflated pāhoehoe lava flow, one that swelled as lava thickened beneath its crust. Later its roof collapsed (Keszthelyi 2008). A similar origin can fairly be assumed for lava flows on Venus.

The rounded depressions at the heads of lava channels are where lava reached the surface (lava fountains). Here the lava deepened its bed by thermal erosion and pooled into a lava lake. In the end the lake drained down the lava channel (Head and Wilson 1980).

Some lava channels have been studied in detail. In general, however, several questions are open. Are they made by lava flowing on the surface or below it? Is the channel carved by mechanical or by thermal erosion? And are the channels formed by erosion at all, or by construction – a lava flow inside which the channel forms between lava levees?

An interesting historical note: sinuous lunar lava channels were first recognised by Johann Hieronymus Schröter in 1787. He called them Rille, German for “furrow”. Julius Schmidt published their first catalogue in 1866, when several hundred were already known. A fine lunar example is Schröter’s Valley.

Because they meander, they were long thought to be river channels – by some, even in the 1970s. Yet Beer and Mädler, the most precise mappers of the Moon and Mars, had already ruled this out in 1838, because the rilles do not run down from mountains. The idea of meandering valleys fitted well with an old explanation of the lunar maria. In that view the basins once held water, and we now see the sediments of the sea floor. But the Moon never had liquid water: it has no atmosphere.