On Aug. 13, Comet 67P/Churyumov-Gerasimenko will reach perihelion (the closest point the comet will comet to the sun during its orbit), and with it the orbiting European Rosetta mission. Here's a brief rundown of Rosetta's observations of the icy body in the run-up to its day in the sun.
After a 10-year circuitous journey covering more than 4 billion miles, on Aug. 6, 2014, the European Space Agency's Rosetta spacecraft arrived at the oddly duck-shaped Comet 67P/Churyumov-Gerasimenko. This image was taken on Aug. 22 when Rosetta was 39 miles from the center of the comet's nucleus. A region of cliffs, named Hathor, is visible in the comet’s smaller lobe on the left.MORE: Rosetta's Comet Fires its Most Spectacular Jet Yet
The surface of Comet 67P proved to be much more varied than scientists expected. This image, taken on Oct 7, shows the smooth region, called Imhotep, on the underside of the comet’s larger lobe. Rosetta snapped the picture from just 11 miles away.MORE: Philae Comet Lander Back in Touch With Mothership
Scientists decided the Philae lander, which piggybacked a ride to the comet aboard Rosetta, should touch down on a relatively smooth region on the comet's head named Agilkia, but 67P proved a tough nut to crack. Unable to fire its ice harpoons to anchor into the comet’s surface, Philae bounced several times before coming to a stop at another, still unknown location, more than a half-mile away. Rosetta took this picture about 30 minutes before Philae’s landing. Agilkia is pictured at the top right.MORE: Rosetta's Comet Does Battle With the Solar Wind
While engineers tried to locate Philae, scientists turned their attention back to the primary mission of Rosetta, which is to characterize and map the comet and study how increased heating from the sun changes the comet over time. A closeup of the neck region connecting the two parts of the duck-shaped comet revealed a region flecked with boulders. The image was taken in January when Rosetta was circling about 19 miles away from the comet.MORE: Rosetta Watches Comet Erupt With a Dusty Surprise
By March, regions of the comet that were previously shadowed began to emerge as 67P drew closer to the sun. This image, taken on March 20, shows a smooth material nestled between more rugged features on the comet's large lobe. By the end of the month, the comet was kicking up so much dust that flight controllers decided to move Rosetta to a safer orbit up to 125 miles away.MORE: Rosetta Captures Comet Lander Philae's Big Adventure
With just three months remaining before 67P reaches perihelion, or its closest point to the sun, Rosetta snaps a picture showing dust blasting off all the sunlit portions of the comet. The image was taken on May 4 when Rosetta was about 148 kilometers away. The comet’s smaller lobe is to the right and the large lobe is left.MORE: Rosetta's Comet May be Made of Pebbles
ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
Rosetta spied a burst of gas and dust from a rugged region, known as Anuket, in the comet’s neck. The outburst, imaged on July 29, is the result of the comet heating up as it reaches perihelion, the closest point to the sun in its 6.5-year orbit. Scientists were surprised to learn that the jet temporarily overpowered the solar wind magnetic field which is wrapped around the comet’s nucleus. 67P’s perihelion is on Aug. 13.MORE: Meet Rosetta's Beautiful Lumpy Comet
Europe’s 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 comet’s 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 there’s 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 — the environment surrounding our primordial sun must have been somehow different than classical theories predict.
“This is an intriguing result for studies both within and beyond the comet community, with possible implications for our models of solar system evolution,” said Matt Taylor, Rosetta’s 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 NASA’s 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 — 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 star’s 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 — exoplanets that not only orbit within their stars’ 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 “aliens” conclusion would be a really bad idea.
“If we look at exoplanets, our goal of course will be to detect biosignatures, to see if the planet contains life,” said Kathrin Altwegg, Rosetta scientist with the Physics Institute and Center for Space and Habitability at the University of Bern in Germany. “And 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 don’t have life. So it’s probably not a very good biosignature.”
Exocomets Gone Wild
In recent years, astronomers have gotten better at probing the inter(exo)planetary space surrounding stars. Our solar system isn’t unique; other stars that are similar to ours have systems of planets and there’s 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 weren’t as well behaved as they are now — 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 sun’s gravitational pull.
These gravitational misadventures can be witnessed around other star systems too — 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 “exocomets”, 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 stars’ 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 — causing fascinating speculation about alien megastructures — 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 isn’t 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. NASA’s MAVEN spacecraft detected sodium, magnesium, aluminum, chromium, nickel, copper, zinc, iron and other metals from the comet’s dust sprinkle through the planet’s upper atmosphere — these elements were deposited there by what would have been a “mind-blowing meteor shower,” 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 isn’t being polluted with the debris ejected by passing comets? Could this signal be strong enough to dupe us into thinking we’re 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 Rosetta’s 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.