William B Stoecker
November 27, 2019 | 7 comments
Image Credit: NASA/JPL-Caltech
Are we all "aliens"? Did all life on Earth originate elsewhere, with one-celled extremophile bacteria (and perhaps more complex, multi-celled organisms) spreading through space? Or is the situation even stranger and more complex than that, with organisms from other worlds arriving here, and bacteria and other life forms from Earth spreading out to "seed" other planets, moons, and comets? Meteorites that originated on our Moon and on Mars have been found on Earth, usually in Antarctica where they are easily spotted on parts of the ice cap that have not been "contaminated" with rocks formed here. Large asteroid impacts, it seems, can blast surface material from a planet in an upward direction exceeding the planet's escape velocity, and some of that material drifts through space and may fall on another world. Despite the pressures and high temperatures of the impacts (both when the rocks are blasted off their home worlds and when they impact with their new home), researchers believe a few organisms might survive. European researchers used a two stage light gas gun to fire frozen algae pellets into water at almost seven kilometers per second... and, incredibly, some were still alive.
In fact, the idea of life spreading through space, called "panspermia," dates back to ancient times, but the first fully developed theory to explain it was the work of Swedish chemist Svante Arrhenius in 1903. He theorized that microorganisms might be carried to extreme altitudes by violent storms and then be propelled by the pressure of sunlight beyond their home world's escape velocity and eventually drift down through the atmosphere of another world. In modern times the most famous proponents of this theory have been the late British astronomer Fred Hoyle, and his colleague Chandra Wickramasinghe. Bacteria and phytoplankton have been found on the outside of orbiting spacecraft, still viable and able to be revived, whether they originated elsewhere or came from Earth. NASA has done tests as well, proving that many one celled organisms, and not just extremophile bacteria, can survive in the vacuum and radiation in space.
Planets capable of supporting the kind of ecosystem we have on Earth, with photosynthesizing plants producing large amounts of oxygen, and the oxygen being breathed in by animals that feed on the plants (and on one another) may be rare, but there is reason to suspect that bacteria and perhaps more complex organisms may exist in worlds long believed to be lifeless, like the moons of the outer planets in our Solar System, and perhaps in Mars and even in our own Moon. Astronomers speak of the "Goldilocks zone," a region neither too close nor too far from a star, where the stellar light can produce temperatures suitable for our kind of life. But there are also internal Goldilocks zones within planets and moons and dwarf planets. Like our own Earth, the cores of many worlds are very hot, often so hot no life can survive. But at more moderate depths, even on worlds far from the Sun, where surface temperatures are close to absolute zero, moderate temperatures exist. Extremophile bacteria have even been found alive in rocks so deep in Earth's crust that the temperature there is above the sea level boiling point of water. Only the immense pressure at depth keeps water in a liquid state. And of course, bacteria thrive in hot springs here on Earth, and around "black smokers," deep sea vents that emit superheated water.
The exact reason for the internal heat of planets and stars is not fully understood, but hydrogen fusion is believed to power stars, and worlds not massive enough to sustain fusion reactions have heat left over from their initial formation, and from the decay of radioisotopes. In addition, moons orbiting close to planets, especially if their orbits are eccentric, are stressed by tidal forces, generating a great deal of heat. In general, large, massive bodies are hotter than smaller ones; stars are hotter and more massive than planets; massive Jupiter's core is hotter than Earth's; and the Moon's core is cooler than ours. In addition to the known sources of heat, there may be some unknown energy permeating all of space that is absorbed by planets and stars. Jupiter and the other giant planets may have no internal habitable zone, for they have no solid surface, and their turbulent atmospheres would tend to carry any organisms down to regions where even extremophiles could not survive the heat.
But there is strong evidence that smaller objects in our Solar System have internal Goldilocks zones. Jupiter's moon Europa, with a diameter over 3,000 kilometers, is warm enough inside to have cryovolcanos. On cold outer worlds, water ice is a "rock" or mineral, and the "lava" is liquid water. Jupiter's moon Ganymede is the largest in our Solar System, with a 5,268 kilometer diameter, and NASA scientists believe that it has an internal ocean of liquid water. Titan, a moon of Saturn, is almost as large as Ganymede, and is so cold on the surface that there are oceans of liquid methane and ethane, but there is evidence of warmer temperatures and liquid water underground. Saturn's moon Enceladus has cryovolcanos that eject water and water vapor far into space; if there are living organisms in it they could drift to Earth and other worlds or land on comets that might carry them Earthward. Pluto, a dwarf planet far out on the edge of the Kuiper Belt, has a surface temperature only a few degrees above absolute zero, and its crust is composed of ice. But the smooth surface and other features have convinced astronomers that Pluto probably has an ocean of liquid water beneath its frozen surface, which is partly covered with organic compounds. Ceres, a dwarf planet in the main asteroid belt between Mars and Jupiter, has a diameter of only 965 kilometers, but its mostly smooth surface and possible cryovolcanos indicate warmer temperatures and liquid water beneath the crust.
All of space contains organic molecules, as do comets, and we cannot rule out the possibility that organisms live within the comets, thawing and springing to life whenever the comet comes close enough to the Sun to melt the water ice which is known to be present in virtually all of them. If one celled organisms are present, many of them would be ejected into the comet's tail... and the Earth has passed through comet tails many times.
It is even possible that multi-celled plants and animals might originate on other worlds and survive the journey through space to arrive on Earth. Consider the tardigrades, tiny (barely visible to an unaided human eye) eight legged animals with primitive, lens-less eyes that live on damp moss and similar habitats, eating one celled plants and animals and other tardigrades. In tests, they have survived the vacuum, extreme temperatures, and radiation in space, entering a state of suspended animation and reviving if placed in liquid water. Biologists have tentatively classed the tardigrades by placing them in their own phylum, Tardigradia, and assuming that it belongs in the super-phylum Ecdysozoa, which includes Arthropods, and nematode worms among other phyla. But this is pretty much a wild guess. Tardigrades might very well have originated, for all we know, inside Enceladus, been ejected into space, and carried by comets to our vicinity.
Understand that conditions suitable for life do not prove that life exists in these worlds. But our universe, often very hostile to life, with black holes and gamma ray bursts and supernovae, naturally produces organic compounds, composed of some of the most common elements, principally hydrogen, oxygen, carbon, and nitrogen. And beyond Pluto in our Solar System, scientists have already discovered other dwarf planets. Since comets are more common than planets and planets are more common than stars (just as grains of dust outnumber boulders), can we rule out the possibility that interstellar space is filled with all manner of objects, including dwarf planets, worlds the size of Earth, and even Jupiter-sized planets? And if many of these have internal Goldilocks zones, there may be more life in interstellar space than within planetary systems orbiting stars. And that life may spread in all directions, with all the creatures in the universe (including us) being distantly related.
William B Stoecker