Why Comet Oxygen is Bad for the Search for Aliens – Europeโs Rosetta mission has made the surprise discovery that Comet 67P/Churyumov-Gerasimenko contains significant quantities of molecular oxygen.
The discovery rules out O2-forming mechanisms through some chemical interactions at the cometโs surface; this oxygen comes from inside the cometary material before it had a chance to combine with hydrogen to form water molecules, originating from when the comet was first formed billions of years ago inside the gas cloud that was left over after the formation of our sun.
This discovery is perplexing in many ways, primarily because astronomical studies of star-forming clouds have turned up empty-handed in the search for molecular oxygen. If thereโs little sign of molecular oxygen in stellar nurseries, where did the oxygen in 67P/C-G come from? It is a highly reactive molecule, meaning it quickly breaks down, combining with other chemicals. Obviously something is amiss, throwing star system evolution theories into a spin โ the environment surrounding our primordial sun must have been somehow different than classical theories predict.
โThis is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of solar system evolution,โ said Matt Taylor, Rosettaโs project scientist, in an ESA news release.
Even though we often look toward comets as being the possible seed for life (after all, they are known to also harbor water ice and chemicals that form the building blocks for life on Earth), the implication that comets (not just 67P/C-G) could be reservoirs of primordial molecular oxygen might actually be a downer for astronomical searches for extraterrestrial biosignatures.
The next generation of space telescopes, such as NASAโs James Webb Space Telescope (JWST) that is planned for launch in 2018, will begin a new era of seeking out extraterrestrial life, or, more specifically, biosignatures โ gases that are linked with biology as we know it. Imagine if we found an Earth-sized planet orbiting a sun-like star within that starโs habitable zone, with an atmosphere composed of nitrogen, oxygen, methane, carbon dioxide and other trace gases associated with life?
This would be a new era of discovery and may throw up a new class of exoplanet โ exoplanets that not only orbit within their starsโ habitable zones, exoplanets that not only are small and (probably) rocky, but exoplanets that possess water and exoplanets that possess biosignatures. Just because a candidate exoplanet possesses all these Earth-like qualities, however, we have to be cautious and 67P/C-G is another lesson why jumping to the โaliensโ conclusion would be a really bad idea.
โIf we look at exoplanets, our goal of course will be to detect biosignatures, to see if the planet contains life,โ said Kathrin Altwegg, Rosetta scientist with the Physics Institute and Center for Space and Habitability at the University of Bern in Germany. โAnd as far as I know, so far the combination of methane and O2 was a hint that you have life underneath it. On the comet, we have both methane and O2, but we donโt have life. So itโs probably not a very good biosignature.โ
Exocomets Gone Wild
In recent years, astronomers have gotten better at probing the inter(exo)planetary space surrounding stars. Our solar system isnโt unique; other stars that are similar to ours have systems of planets and thereโs strong evidence for asteroids and comets, particularly around tumultuous young stars that are undergoing some gravitational jiggles.
A few hundred million years after the formation of Earth, the planets werenโt as well behaved as they are now โ planetary migrations, particularly by massive planet Jupiter, stirred up the orbits of minor bodies (such as asteroids and protoplanets), causing collisions and likely hurling a few into interstellar space, ejecting them completely from the sunโs gravitational pull.
These gravitational misadventures can be witnessed around other star systems too โ dusty clouds reveal continuous asteroid collisions and infrared telescopes have highlighted recent (in cosmic timescales) planetary collisions. The extremely eccentric orbits of some exoplanets are a testament to some gravitational hardship.
And comets, or more aptly โexocometsโ, have been detected around other stars. But these systems are often the ones that have the most extreme cometary activity, likely younger stars, going through gravitational growing pains, or perhaps starsโ exo-Oort clouds getting perturbed by another passing star. (Interestingly, the recent interest in the star KIC 8462852 that was discovered to have a peculiar transit signal โ causing fascinating speculation about alien megastructures โ may have its roots in a cloud of exocomets getting booted from the outermost regions of the star system by another passing star, creating a powerful transit event that we were lucky enough to witness.)
So we already know that our solar system isnโt so unique, and other star systems posses similar celestial objects, only in varying quantities and ages (and therefore activities), but going back to the question of seeking out extraterrestrial biosignatures, how might these familiar objects interfere with our search?
Whether we can see them from afar or not, we can be certain that comets are a common element of most star systems and their signature may obscure the question of life, or at least the detection of biosignatures as we think we know them.
A Recent Encounter
When Comet Siding Spring zipped past Mars in October 2014, we were fortunate enough to have an armada of robotic eyes in that location to observe the spectacle. This unprecedented event was met with unprescedented observations of cometary interactions with a planetary atmosphere. NASAโs MAVEN spacecraft detected sodium, magnesium, aluminum, chromium, nickel, copper, zinc, iron and other metals from the cometโs dust sprinkle through the planetโs upper atmosphere โ these elements were deposited there by what would have been a โmind-blowing meteor shower,โ remarked study scientists.
So as we look toward other stars and develop the ability to look, with a higher precision, at the spectroscopic signature of the light reflected and absorbed by distant planetary atmospheres, how do we know that atmosphere isnโt being polluted with the debris ejected by passing comets? Could this signal be strong enough to dupe us into thinking weโre seeing alien biospheres, particularly if these comets are of a similar composition to Comet 67P/Churyumov-Gerasimenko?
Needless to say, more work needs to be done, and we need to understand whether Rosettaโs data is unique to 67P/C-G or whether more comets contain similar quantities of elements and molecules that are often associated with life.
So for now, this has been a steep learning curve for astronomers and yet another lesson not to take astronomical phenomena at face value; just because we may find a potentially habitable world that appears to have an atmosphere containing elements that could pertain to a thriving biosphere, we first have to rule out the cometary interactions, a facet to the life-hunting game that has, until now, not been fully realized.
