Mar 10 2020

Day Was Shorter 70 Million Years Ago

What does an extinct mollusk have to do with the Moon? This is one of those amazing science stories that ties together multiple disciplines and lines of evidence into one elegant narrative. In this case a detailed analysis of a 70 million year old mollusk shell has given scientists a critical piece of information that will help them model the Earth-Moon system.

Let’s start with the Moon – astronomers know that the Moon is moving farther away from the Earth at a constant rate, 3.82 centimeters per year. We can precisely measure this because the Apollo missions left corner reflectors on the surface of the Moon, and we can shoot lasers off those reflectors and measure the round-trip travel time. Because scientists have also precisely measured the speed of light, we can use this round-trip time to calculate the exact distance between the laser on Earth and the reflector on the Moon.

Why is the Moon moving away from the Earth? In a word – tides. Tidal forces from nearby large objects causes a bulge to form. We are most familiar with this phenomenon because of the bulge in the ocean caused mostly by the Moon (and to a lesser degree the Sun) which we experience locally as a rising and falling of the sea. The tidal bulge on the Earth is slightly ahead of the Moon in its orbit, because the Earth is spinning faster than the Moon. This leading bulge tugs slightly on the Moon, accelerating it into a higher orbit farther from the Earth. This represents a transfer of momentum from the Earth to the Moon via gravity, which not only moves the Moon farther away, but slows down the rotation of the Earth (and the conservation of angular momentum is obeyed).

The same thing happened the other way around – the tidal bulge on the Moon also pushed the Earth farther away, slowing down the rotation of the Moon until it was “tidally locked” with the Earth. That is why the same side of the Moon always faces the Earth. This also applies to planets close to their parent stars. We are finding many exoplanets close to their stars, and it is likely that they are all tidally locked, with the same side facing their sun at all times.

So far, so good, but there is a math problem with the Earth-Moon model. If we extrapolate back in time, assuming a constant rate for the movement of the Moon, then the Moon would have been inside the Earth 1.4 billion years ago. We know from other evidence, however (such as dating the Moon rocks), that the Moon is probably 4.5 billion years old. So clearly the Moon’s movement away from the Earth has been accelerating and is not constant. If only we could precisely measure the length of a day in the distant past we could plug that into our models of the Earth-Moon system and straighten everything out. This is where mollusks enter the story.

Actually, before we get to the mollusks, scientists have already been able to estimate the number of days in a year by counting coral rings. Corals lay down a layer of calcium carbonate every day, and fossil corals preserve those daily growth lines. There are also seasonal variations that allow for the determination of a year – so you can count the number of days in an ancient year. The length of a year is constant over time, so this also allows scientists to calculate the length of a day.

So the new study is not totally new, but it is a new and more precise technique, and is focusing on a different animal – Torreites sanchezi, a rudist bivalve (see the picture above). They report:

A combination of layer counting, spectral analysis of chemical cyclicity and chemical layer counting shows that the rudist precipitated 372 daily laminae per year, demonstrating that length of day has increased since the Late Cretaceous, as predicted by astronomical models.

So they used a combination of techniques, which showed several things. First, there were 372 days in a year 70 million years ago, which translates to a day 23.5 hours long. But further, the daily variation in growth rate was more extreme than expected. The authors hypothesize that this is evidence for photosynthesis, and that we are seeing the light-dark patterns in the growth rings. Mollusks are animals and can’t photosynthesize, so this in turn means they had photosynthetic symbionts living in their shells. This is circumstantial evidence, but is the strongest evidence we have so far for this.

These bivalves have something else in common with corals – they were the reef-builders of their day. They basically filled the niche that corals serve elsewhere. They died out in the Cretaceous–Paleogene (K–Pg) extinction event that wiped out the non-avian dinosaurs 66 million years ago.

The researchers hope to use their techniques on other similar fossils from different time periods, to get more data points on the length of the day over history. This will really help revise the physical models of the Earth-Moon system. But also, this data gives us information about not only the climate but the weather in the past. The researchers state that their techniques give them about 5 data points per day. Since the growth rates appear to be dependent primarily on light levels, we can essentially tell that it was cloudy on a particular day 70 million years in the past. We can also infer the water temperatures from the shell data.

Shells in general are great sources of fossil evidence from the past. They are produced by a living process but are hard and survive for millions of years. They record precise data, giving us a powerful window into the ancient past. This is an amazing story unto itself, but I love how this story weaves together so many aspects of science. It is a great example of the power of consilience in science.


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