Our experts examine the hottest new research
CUTTING EDGE
Tide lines show Moon’s move
Marks left by tides 3.2 billion years ago suggest the Moon was once much closer
The gravitational pull of the Moon hauls up Earth’s oceans into two bulges on opposite sides of the planet. As Earth rotates beneath these twin bulges, sea levels along the coastlines rise and fall, creating the tides. Much of the world’s shores, including around the UK, experience two cycles of high and low tides roughly equal in magnitude every day.
The Sun’s gravity also has an effect on the ocean’s tides, and roughly twice a month (one lunar orbit), when both the Moon and Sun are in line with Earth, their gravitational effects combine to create a much larger range between high and low water, known as the spring tide. Conversely, when the Moon and Sun are at 90° to each other the tides are weaker, what’s called a neap tide.
One factor that has affected the tides over longer timescales is that the rotation of Earth has decelerated over the planet’s history, and the Moon has slowly spiralled ever further out in its orbit. Today we know, thanks to measurements taken by a device placed on the Moon by Apollo astronauts, called the Lunar Laser Ranging Experiment, that the Moon is drifting away at a rate of 3.8cm per year.
However, just after the Moon’s formation, our satellite circled much, much closer, and each day on Earth was only around four hours long. But what’s not clear is exactly how the Earth–Moon system has evolved over the 4.5 billion years since: how have the time Earth takes to rotate and the Moon takes to orbit changed over time? Computer modelling studies widely disagree. What’s needed are some actual data points from Earth’s deep history.
Sandstone tells a story
And this is where geology can provide crucial insights. Certain kinds of rock were formed from submerged dunes in shallow coastal waters, and show alternating layers of deposited sand and mud, created by strong and weak currents respectively, at different times of the tidal cycle. Tom Eulenfeld and Christoph Heubeck, both at the Institute for Geosciences, Friedrich Schiller University Jena, Germany, have reexamined the oldest example of this in the geological record. It’s known as the Moodies Group sandstone in South Africa, and dates back a staggering 3.22 billion years. The thickness of these alternating layers cycles every 15 layers, believed to be due to the varying current strengths over the cycle between spring and neap tides over a month.
“What’s not clear is exactly how the Earth–Moon system has evolved since the Moon’s formation. Geology can provide crucial insights”
These geological measurements, combined with the application of Kepler’s third law of planetary motion, enabled Eulenfeld and Heubeck to reconstruct the rate of Earth’s spin and Moon’s orbital period at the time these ancient rocks were deposited. They calculate that 3.2 billion years ago the Earth–Moon distance was around 70 per cent of the current value, and that Earth’s rotation rate then resulted in a year of about 700 days, with each day lasting around 13 hours. Previous measurements of 650-million-year-old rocks from South Australia place the Earth–Moon distance at 97 per cent of today’s separation at that time. With these points to fill in the gaps, computer models can begin to build a much better picture of how the dance of the Moon around Earth has changed over time.
Prof Lewis Dartnell is an astrobiologist at the University of Westminster.
Lewis Dartnell was reading… Constraints on Moon’s Orbit 3.2 Billion Years Ago from Tidal Bundle Data by Tom Eulenfeld and Christoph Heubeck. Read it online at: arxiv.org/abs/2207.05464