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Tides probably once allowed life to conquer the land via inter-tidal zones, and today they increase the richness of marine life by nutrient mixing. Drastic variations in their size by location has led oceanographers to wonder how the tides we know compare with those of the past, or even on other worlds.
Tides in the Mediterranean are so puny Julius Caesar failed to take them into account when invading Britain, undermining his first invasion. By contrast in Canada's Bay of Fundy, differences between high and low tides can reach an unmissable 16 meters (53 feet). Large tides are partially caused by local features that funnel water in and out of one location, but they are also shaped by the size of the ocean basin. Professor Mattias Green of Bangor University, Wales, modeled 120 different continental configurations to explore the potential range.
"Earth's current tides are the biggest we've found in 750 million years. I certainly think the tides now may be among the biggest in Earth's history," Green said in a statement. Consequently, the first fishes to colonize the inter-tidal zone, before eventually moving onto land entirely, almost certainly had to make do with a smaller amount of territory in which to play.
This is one case where bigger is not necessarily better – most of the world's really big tides are around the Atlantic, rather than the much larger Pacific Ocean. "The Atlantic is an almost perfectly tuned organ pipe for the tide. It resonates," Green added. “The Pacific is poorly tuned.” Indeed, Green thinks the Earth's current tides are close to the maximum possible without a larger or closer Moon.
On geologic scales, the Atlantic has only recently obtained its present size, and the oceans of earlier eras were generally either smaller or much, much larger, like Panthalassa, when a single mighty ocean covered most of the Earth.
Green and colleagues simulated not only the placement of the continents over the period for which we can map them accurately, but also worlds where only a few islands poke above the waterline, and others where a single ocean covers just 10 percent of the planet.
The results have been published in Geophysical Research Letters and Green said the team was surprised by just how big a difference continental placement can make. An Earth-sized water world with a moon the same size and distance as our own would have tides that carry 1,000 times less energy – calling into question whether volcanic islands would ever play host to life.
"If you're just one big ocean it's difficult to have a big tide,” Green noted. “Adding one New Zealand-sized continent doesn't make much difference, but add a couple New Zealands and you get tides 100 times more energetic."
The findings help explain one long-standing mystery. The energy dissipated in the tides pushes the Moon further away from Earth. However, the Moon is currently moving away from Earth at a puzzlingly fast rate. Given its age, it must have moved far more slowly over most of its history, or it would now be much more distant. If previous continental basins produced smaller tides, the recession would indeed have been slower, and the current location makes sense.
Without a large satellite, an alien world would have little in the way of tides, even if continents were well placed. Objects as large as the Moon are thought to be rare circling rocky planets, so we have long known tides elsewhere were probably smaller. Green's work reveals we are doubly lucky to experience those we do.
