Biological transfer on a non-contact Rocheworld

Question

I've been pondering for some time an alternate-history story where the Earth as we know it is replaced by a Rocheworld, of the canonical variety introduced in the eponymous Robert Forward novel where the two lobes share a common atmosphere but the solid and liquid surfaces do not touch, and in which the "New World" (i.e., the Americas) is literally a different world--the other lobe of the Rocheworld, inaccessible by land or sea from Eurasia, Africa, and Australia.

The two lobes are separated by approximately 150 miles. While that's well above the conventional edge of space, the lower gravity and correspondingly larger scale height of the atmosphere at the interior poles should result in an atmosphere that remains dense enough to breathe all the way through the gap.

So, I figure the occasional bird every few hundred, or maybe thousand, years will manage to fly or get swept from one lobe to the other, along with plenty of airborne microbes, flying insects, and whatever seeds or other small animals the birds might carry with them... but just how much viable biological material, of what maximum complexity, should I expect to be regularly exchanged? And as a result, just how weird should the flora and fauna of the New World be expected to be?

Answer 1

In terms of biological transfer there are a number of possibilities

Smooth still world with high pressure bridge
The remoteness would be similar to that between America and Europe on our Earth. Airborne seeds, spores, birds and anything that was small enough to hitch a ride on a bird such as insect eggs and bacteria etc would be able to make the journey.

Smooth still world with low pressure bridge
The remoteness would be greater than that between America and Europe. Birds would not be able to survive the low pressure environment, but airborne seeds, spores and bacteria should be able to make the crossing depending on how much weather we allow on “smooth still” world.

Real world
Real worlds don’t tend to be smooth or still. They tend to have mountains and weather and both of these would cause problems. The presence of mountains would create also sorts of tiny gravitational anomalies which would help to destabilize the Roche world. But the weather would be the real killer. The presence of Earth sized storms migrating across the planet into an area of low gravity will cause chaos.

In an area with little net gravity, gravitation would not play the leading role. Instead other forces such as storm fronts would. Even at 1g on the Earth the sea can be whipped up to 10’s of meters high and rocks and boulders from the sea floor can be thrown into the air. Such a storm moving into the air bridge area under perhaps just 0.05g would carve up the sea into enormous mountains and the higher the seas got the lower the gravitational attraction would be. It is highly likely that sea water at some level would be transferred. So some lower forms of sea life might also make the journey.

Best guess world
Given the propensity for things that can move freely to end up spinning when a force is applied (like stars, planets and storm systems). I suspect the air bridge area itself would suffer the most horrific perpetual circulating storm which would suck up water and seafloor materials and fling it out into the bridge. Some of the smaller solid and liquid particles could easily be transferred between worlds.

So in summary in best guess world seeds, spores and bacteria for sure. Other microscopic life forms such plankton very probably. Larger life forms such as insects, possibly especially if carried aloft inside a small branch or similar in egg form. Macro life forms such as fish and birds: I doubt it due to the violent turbulence and the unpleasant mixture of mud, water and air that would span large parts of the bridge.

Of course it is entirely likely that something would evolve on one of the worlds to live in such an environment. But that’s another story.

Answer 2

Good grief, um okay part of this question can be, somewhat, answered, the rest of it is on you, it's too big to answer outside of a complex history which you are going to have to pen. The answer that I can give you is about the amount and complexity of transfer which sod-all and most of it very simple, fruit eating birds don't tend to migrate and so don't find themselves in a position to be "blown off course" the main migratory species are seabirds and grazing species neither of which would carry a major payload of viable material. So the transfer is limited to aeolian transfer of dust, pollen and microbes, there could be a lot or very very little of this activity depending on the atmospheric interface at the air bridge, I'd guess the latter but have no way of knowing or modelling a definite answer.

Even if there was regular transfer of airborne material like pollen there's no guarantee that any crossover could result, if the two worlds have a history of life that precedes their current shared situation then they could in fact share no structural biology in common at all. Planet A may in fact be composed of left-handed proteins while Planet B uses a right-hand twist instead and life could never bridge the gap. If only one planet had life while the other was lifeless then the lifeless world's new ecosystem is going to be built on material blown in from it's lush partner but will be badly behind due to the minimal transfer of material.