Canali

Canali are perhaps the most remarkable features on Venus. They are lava channels without branches and with a constant width. The longest one winds for 6000 km, across wrinkle ridges and often across lava plains whose surface has since sunk or risen.

Venus is close to 500°C, so it is (almost) impossible that water carved these channels. Yet no lava flow on any other planet is known to have run this far. Venus also has “ordinary” lava channels, which start in a round depression (see there). But these meander in a different way. Planetary geologists mapping Venus have found almost 50 canali so far.

The English term is the plural of the Italian word for channel, canale (Baker et al. 1992). It links Venus with the history of Mars exploration. In the 19th century Schiaparelli used the same word for the Martian canals he mapped – most of which do not exist.

Canali have no tributaries. Their sides usually lack the raised levees that build up from lava along the channels of lava flows. They are simply cut into the material of the plain. In places, abandoned channels, branches, point bars, deltas and lava ridges have formed along them.

Their sources and ends are usually invisible. They appear at full width in the lava plains without any warning, and they vanish the same way. Often they reappear a little farther on, again “out of nowhere”. In a few places, however, a canale can be seen to start in a fracture and end in a delta (Baker et al. 1992, Komatsu et al. 1993).

In places a canale runs uphill. This means that the surface has risen since it formed (and has sunk elsewhere). It is also evidence of vertical crustal movements (Jindal 2019). All this suggests that canali are fairly old structures.

Unlike river channels cut by water, canali keep almost the same width (1–4 km) for as much as 1000 km. Their depth is tiny compared with their width: barely 50 m. Deposits on the inner bends (point bars) suggest that erosion and deposition were in balance for a long time in places. Nothing like this is known from lava channels on Earth. Estimates of how long the longest canale, Baltis Vallis (see the plate), was active range from 1 to 800 thousand years.

What flowed in the canali? If they are lava channels, the lava must have been very fluid: ultramafic komatiite, tholeiitic basalt, sulphur lava or carbonatite lava. Some researchers think they are valleys cut by water before the runaway greenhouse era (Jones and Pickering 2003). The most exotic theory says that a flow of dry sand could have made them in the hot, dense atmosphere of Venus. Like muddy turbidity currents on the sea floor, the sand would have kept swirling downslope in the dense medium (Waltham et al. 2008).