One of the puzzles that geologists occasionally ponder is the nature of eskers and drumlins.
Eskers are winding ridges a few tens of metres high that look remarkably like railway embankments. Indeed they are often used as readymade roads and run up and down hills over distances that sometimes stretch to hundreds kilometres.
Drumlins, on the other hand, are tear drop-shaped hills a few tens of metres high and a hundreds of metres long. They often appear in large numbers with the same orientation in drumlin fields .
Geologists have long assumed that eskers and drumlins are formed by glaciers and left behind after these ice giants retreated.
There are essentially two problems. The first is the internal structure of these formations. Eskers and drumlins have have an outer layer of water-borne clay and silt with attendant fossil debris. This covers an inner core made of unsorted boulders and rocks which are entirely free of fossils. These inner cores do not appear to have been affected by the action of water. How does this structure arise?
The second is that if glaciers are responsible for eskers and drumlins, they ought to be forming now. And yet nobody can find anywhere on Earth where these structures are currently forming.
Today, Milton Zysman and Frank Wallace publish on the arXiv their explanation for the formation of these objects and it makes for fascinating, if not entirely convincing, reading.
Zysman and Wallace say that eskers and drumlins are the debris left on Earth after our planet repeatedly passed through the tail of a giant comet. They say this explains the distribution of eskers and drumlins, which often form in roughly parallel lines.
It also explains their internal structure. The rocky core of these objects is pure cometry debris which explains the absence of fossils. The outer layer built up later through the action of water and ice.
The cometary origin, they say, also explains the rather unique shape of the individual rocks in the cores and the striations that mark them predominantly in line with their longest axis. (Apparently, these markings are consistent with the process of erosion that must occur in comet tails.)
Zysman and Wallace also point out that the ice age that is associated with esker and drumlin formation must have been caused by the comet tail, which would have enveloped Earth in a layer of dust that rapidly cooled the planet.
This is not an entirely new idea. Various commentators have suggested that many of Earth’s rocks and much of its water and atmosphere may have come from comets. And indeed this paper is an edited version of one the authors originally gave in 1997.
However, Zysman and Wallace’s idea as it stands is little more than an interesting guess. What of isotopic evidence? Presumably the isotopic content of the rocky cores should differ in a measurable way from material on Earth that has other origins. If this work has been done, they make no mention of it.
And the fact that we have not seen eskers and drumlins forming in the two hundred years that we’ve been looking does not mean they did not form in the past, during the many millennia that glaciers were ravaging the Earth. (In fact, there are recent reports that scientists have seen a drumlin forming for the first time in Antarctica.)
And finally, it’s hard to imagine that the debris from a comet tail hitting the atmosphere at several tens of kilometres per second would then land in a tear drop shape just a few tens of metres in size or form a line a few tens of metres wide but hundreds of kilometres long.
It should be straightforward to refute or dismiss this idea by simulating of the kind of debris patterns that this kind of event would produce. And in any case, the heat generated when rocks enter the Earth’s atmosphere melts their outer surface, giving them a “fusion crust” that is easy to identify. Why don’t the rocks in esker and drumlin cores have fusion crusts?
Putting Zysman and Wallace aside, however, it is still possible that the Earth has been shaped by extraterrestrial forces in ways that we are only beginning to grasp. For example, there is growing evidence that the Solar System has been regularly disturbed by passing stars and their accompanying discs of ice and dust. These events must have had a dramatic impact on our world.
It is becoming increasingly clear that conditions on Earth are a product of the interplanetary and interstellar environment in ways we are only beginning to understand. And of course we need new hypotheses to explore this idea to its fullest extent.
Ref: arxiv.org/abs/1004.0416: Tails of a Recent Comet: The Role Cometary Jets Play in Crustal Formation Esker/ Drumlin Swarms