MESSENGER and Cassini’s landmark discoveries in 2014
For NASA, 2014 marked the 10th anniversary of MESSENGER in space and the tenth anniversary of Cassini at Saturn. Throughout the year, the two probes returned valuable scientific data to Earth while scientists revealed the fruits of the probes’ labors through several announcements of groundbreaking discoveries at the two planets, including new information about solar neutrons (Mercury) and the many moons (and one new one) of Saturn.
Orbiting the closest planet to the Sun, the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft, currently in its second and final mission extension, spent 2014 in its third Earth year of orbital operations at Mercury.
Since arriving at Mercury on 18 March 2011, MESSENGER has completed both its primary mission (which ended on 16 March 2012) and its first mission extension (which ended on 17 March 2013).
Now in its second mission extension, MESSENGER’s scientists have followed-up on and made new discoveries about Mercury and our solar system’s inner-most area.
In January, scientists writing the Journal of Geophysical Research: Space Physics revealed information gathered from MESSENGER regarding solar wind flows and swirls.
According the journal article, the solar wind of particles streaming off the sun helps drive flows and swirls in space that are actually just as complicated as terrestrial weather patterns observed on Earth, Venus, Mars, Jupiter, Saturn, Uranus, and Neptune.
Using data collected from MESSENGER, scientists were able to identified for the first time at Mercury a classic space weather event called a Hot Flow Anomaly (HFA), which had previously been spotted at Earth, Venus, Saturn, and Mars.
As stated by Vadim Uritsky at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, “Planets have a bow shock the same way a supersonic jet does. These hot flow anomalies are made of very hot solar wind deflected off the bow shock.”
In order to find these HFAs at Mercury, the analysis team used MESSENGER to detect the presence of two HFA signatures.
The first signature led to the detection of magnetic fields that can be used to detect giant electric current sheets that lead to HFAs.
The second observation looked for the heating signature of the charged particles.
Scientists were then able analyze this information to quantify the kind of turbulence that existed in the region. This led to the discovery of HFAs at Mercury.
These observations not only helped prove the existence of HFAs at Mercury, but also helped create a more complete understanding of this type of space weather in general
In total, HFAs have been observed in a variety of scale sizes – from around 600 miles across at Venus to closer to 60,000 miles across at Saturn.
The new study of information gathered by MESSENGER suggests that the most important factor for determining HFA size is the geometry and size of the planet’s bow shock.
Following from this announcement, scientists reported in July in the Journal of Geophysical Research: Space Physics, that MESSENGER had directly observed solar neutrons.
Solar neutrons are created during solar flare events and carry vital information regarding the power of solar flares – the intense bursts of radiation from the Sun during times of magnetic energy release – and the processes of solar flares that accelerate solar particles.
The issue regarding solar neutrons, up until now, is that they have a very short lifespan of approximately 15 minutes and are thus very difficult to detect or observe directly because how far they travel out into space depends on the velocity at which they are ejected from the Sun.
Based on their lifespan, solar neutrons almost universally die out before reaching near-Earth space (if they’re even ejected in Earth’s direction to begin with).
To this end, no neutron detectors on Earth or in near-Earth space have ever observed a solar neutron.
However, MESSENGER orbits Mercury at a distance from the Sun of approximately 28 million miles, much closer than the approximate 93 million mile distance between the Earth and the Sun.
As stated by project scientist David Lawrence at The Johns Hopkins Applied Physics Lab, “To understand all the processes on the sun, we look at as many different particles coming from the sun as we can – photons, electrons, protons, neutrons, gamma rays – to gather different kinds of information.
“Closer to Earth, we can observe charged particles from the sun, but analyzing them can be a challenge as their journey is affected by magnetic fields.”
Charged particles tend to twirl and gyrate around the magnetic field lines created by the vast magnetic systems that surround the Sun and the Earth.
However, neutrons are not electrically charged and travel in straight lines from the flaring region. Therefore, they can carry information about flare processes unperturbed by the environments through which they move.
This is important because solar neutrons can then be used by scientists to decipher one aspect of the complicated acceleration processes that are responsible for the creation of highly energetic and fast solar particles.
To this end, scientists used data collected by MESSENGER between 4-5 June 2011 of a solar flare accompanied by fast-moving, energetic charged particles to search for any elusive solar neutrons that might have encountered MESSENGER as they passed by.
This data was then compared to information regarding the solar flare captured by NASA’s solar telescope STEREO (Solar Terrestrial Relations Observatory).
By combining data from the two spacecraft, scientists were able to determine that MESSENGER detected an increase in the number of solar neutrons at Mercury’s orbit hours before the large number of charged particles (that had been slowed down by their interaction with the Sun’s magnetosphere) associated with the flare reached the spacecraft.
This indicated that the neutrons were most likely produced by accelerated flare particles striking the lower solar atmosphere, releasing neutrons as a result of high-energy collisions.
Thus, the MESSENGER and STEREO data together provided new information about how particles are accelerated in solar flares.
As scientists continued to sift through the data being collected by MESSENGER, its operations team turned their attention toward the probe’s 10-year anniversary on 3 August.
Over the course of the probe’s 10 years in space, MESSENGER travelled 8 billion millions in 29 trips around the sun, returned 255,858 images to Earth, conducted 6 flybys of three of the four inner planets (not Mars), and performed 3,308 orbits of Mercury spanning 7 Mercury solar days and 1,232 Earth days.
During those orbits of Mercury, MESSENGER conducted 35 million shots of the Mercury Laser Altimeter, returned 10 terabytes of scientific information to Earth that has so far been publically released, and has averaged a speed of 91,730 mph (relative to the Sun).
Marking the occasion, MESSENGER Mission Operations Manager Andy Calloway, of the Johns Hopkins University Applied Physics Laboratory stated, “We have operated successfully in orbit for more than three Earth years and more than 14 Mercury years as we celebrate this amazing 10th anniversary milestone.
“The MESSENGER spacecraft operates in one of the most challenging and demanding space environments in our Solar System, and we have met that challenge directly through innovation and hard work, as exemplified by the stunning discoveries and data return achievements.
“Our only regret is that we have insufficient propellant to operate another 10 years, but we look forward to the incredible science returns planned for the final eight months of the mission.”
And as those final eight months of operations began, MESSENGER’s team continued to advance the mission beyond its original goals.
On 8 October 2014, MESSENGER’s team turned the spacecraft’s camera away from Mercury and toward Earth to observe the 8 October 2014 lunar eclipse.
At the time of the eclipse, MESSENGER was 107 million km (66 million miles) from Earth, and its viewing angle was nearly perfect as MESSENGER was almost but not quite directly between the Earth and the Sun.
Thus, from MESSENGER’s viewpoint, the Earth and moon were nearly full.
The observation was in part a test to see what could be observed as well as part of a growing effort by the scientific community to use still operational but past their original mission timeframes spacecraft to perform non-traditional observations of solar system bodies.
These observations are designed to help planetary scientists understand what various stellar events (that we concretely know about) look like from long distances and how our current technology observes them to help in the detection of such phenomenon in extra-solar planetary systems.
For example, NASA has used Cassini at Saturn to observe the Transit of Venus (as seen from Saturn) to learn more about and validate extra-solar planetary atmospheric analyses obtained through extra-solar planetary transits.
In MESSENGER’s case, the observation of the lunar eclipse on 8 October was, in a way, part of a growing desire to detect extra-solar moons either by transit of the moons just before or after the transit of their host planet in front of the host planet’s star or by eclipses of those moons into their host planet’s umbral shadow.
MESSENGER’s observation of the eclipse was a success, with MESSENGER’s cameras capturing both the Earth and the disappearing moon during the eclipse.
But for MESSENGER, the end if nearing.
At this time, the probe has enough onboard fuel to perform just one more orbit-raising maneuver – currently scheduled for 21 January 2015.
Once this burn is complete, MESSENGER will ostensibly be out of fuel.
Because the probe is in an orbit with a low periapsis (point in an orbit of closest approach to the center of mass of a stellar body other than the Earth or Sun), these regular orbit-raising maneuvers are needed to keep the spacecraft in a stable orbit.
Once MESSENGER runs out of fuel, its orbit will slowly decay, and the spacecraft will end its tenure at the innermost planet of our solar system with a crash landing onto the surface of Mercury.
That crash landing is expected in March 2015.
Cassini: 10 years exploring the Saturnian system:
Like MESSENGER, Cassini’s operational team was also looking at 2014 for the commemoration of their craft’s 10 year anniversary at its scientific target: Saturn.
As Cassini entered 2014 having spent 9.5 years in Saturn orbit, scientists on Earth were busy analyzing data collected by the probe in previous years.
The first 2014 scientific announcement from Cassini data related to a solar event (which created distinct auroras at Saturn’s poles) in April and May 2013 witnessed by both Cassini at Saturn and Hubble from Earth orbit.
Hubble witnessed the event in ultraviolet wavelength while Cassini garnered close-up views in infrared, visible light, and ultraviolet wavelengths.
As a result of this detailed study, scientists were able to obtain a step-by-step choreography of the auroras’ movements throughout Saturn’s complex magnetic environment.
Discussing the results of the observation, Jonathan Nichols of the University of Leicester in England, stated that “Saturn’s auroras can be fickle – you may see fireworks, you may see nothing. In 2013, we were treated to a veritable smorgasbord of dancing auroras, from steadily shining rings to super-fast bursts of light shooting across the pole.”
Images from Cassini’s Ultraviolet Imaging Spectrometer (UVIS) provided scientists with a look at the changing patterns of faint emissions on scales of a few hundred miles and specifically tied the changes in the auroras to the fluctuating wind of charged particles blowing off the sun and flowing past Saturn.
The data captured by Cassini suggest that one way the bright auroral storms are produced is by the formation of new connections between magnetic field lines in Saturn’s magnetosphere – the same process that causes auroral storms in the magnetic field around Earth.
More interestingly for the auroras at Saturn was the discovery that one persistent bright patch of aurora rotated around the planet’s pole in lockstep with the orbital position of Saturn’s moon Mimas.
Previous observations of auroras at Saturn had shown an intermittently bright spot magnetically linked with the moon Enceladus, but this was the first time that an auroral bright spot was linked with Mimas as well as linked in lockstep rotation for a prolonged amount of time with any one of Saturn’s moons.
Moreover, these new observations gave scientists unexpected clues to a long-standing mystery about the atmospheres of the giant outer planets.
“Scientists have wondered why the high atmospheres of Saturn and other gas giants are heated far beyond what might normally be expected by their distance from the sun,” said Sarah Badman, a Cassini visual and infrared mapping spectrometer team associate at Lancaster University, England.
“By looking at these long sequences of images taken by different instruments, we can discover where the aurora heats the atmosphere as the particles dive into it and how long the cooking occurs.”
With these observations and discoveries in hand, scientists stated in February that they were continuing to evaluate the Hubble and Cassini data of the overall event with the hope of discovering how clouds of charged particles move around Saturn as it spins and receives blasts of solar material from the sun.
“As we move into the part of the 11-year solar cycle where the sun is sending out more blobs of plasma, we hope to sort out the differences between the effects of solar activity and the internal dynamics of the Saturn system”, said Marcia Burton, a Cassini fields and particles scientist at NASA’s Jet Propulsion Laboratory.
As these results from 2013 data were released, Cassini’s operational team continued to gather scientific data that scientists would use to unlock the secrets of the second largest planet in our solar system and its complex system of moons.
Specifically, of the two moons of primary scientific interest for Cassini and its team, the 100th flyby of Titan was quickly approaching.
On 6 March 2014, the Cassini spacecraft dived to within 933 miles (15,000 km) of Titan to perform the landmark 100th flyby of the Saturn moon.
In the previous 99 flybys of Titan, Cassini had helped unlock many of the mysteries surrounding this moon, which a decade earlier was just a fuzzy orange ball about the size of Mercury with a nitrogen atmosphere.
With these first 99 flybys, knowledge of Titan increased dramatically with the categorization of most of the moon’s surface features, including its liquid methane and ethane lakes, and the categorization of the seasonal terrain changes on the moon as seasonal floods of liquid methane and ethane carve new patterns into Titan’s surface.
Following this 100th flyby of Titan, Cassini and its teams re-oriented the spacecraft for continual observation of Saturn while setting up for another flyby of Titan in June.
As Cassini continued observations, scientists announced in April that years of collective data from the Cassini orbiter had concretely found evidence for a large underground ocean of liquid water on Saturn’s moon Enceladus – the second of the two moons of primary scientific interest for Cassini.
A large subterranean saltwater ocean on Enceladus had long been suspected from data returned by Cassini through close flybys of the moon in previous years.
Previous flybys of Enceladus between 2009 and 2012 provided stronger evidence that the moon harbors a large, salt-water ocean beneath its surface.
This strong evidence was finally confirmed by NASA in April 2014.
This significant discovery, along with the confirmation (through analysis of the ejecta material from the cryo-geysers) that the ocean harbors organic molecules and nutrients and has a permanent heat source (caused by gravitational heating by Saturn as Enceladus orbits the massive planet and is therefore squeezed and elongated) threw Enceladus to the near top of the list of “best places in the solar system to host alien microbial life.”
Following very close to this confirmation, NASA scientists also revealed that Cassini had captured the birth of a new moon in one of Saturn’s rings.
Based on images captured on 15 April 2013, NASA scientists revealed on 14 April 2014 that disturbances seen by Cassini at the very edge of Saturn’s A-ring – the outermost of the planet’s large rings – was in fact the formation of a small icy object that may be a new moon.
The object in question is 20 percent brighter than the surrounding material, is 750 miles (1,200 km) long and 6 miles (10 km) wide, and was found by the detection of unusual protuberances in the usually smooth profile of the A-ring’s edge.
“We have not seen anything like this before,” said Carl Murray of Queen Mary University of London, lead author of the report that identified the possible moon. “We may be looking at the act of birth, where this object is just leaving the rings and heading off to be a moon in its own right.”
Continuing the series of revelations about Saturn’s moon came the announcement that scientists, working with data from Cassini, had developed a new way to understand the atmospheres of exoplanets by using Titan as a stand-in.
This new understanding demonstrates the significant impact of hazy skies on our ability to learn about the worlds orbiting distant stars.
Currently, scientists are able to discern characteristics of an exoplanet by collecting the light that passes through that exoplanet’s atmosphere as it transits in front of its host star.
As that transit occurs, some of the star’s light travels through the exoplanet’s atmosphere where it is changed in subtle, but measurable, ways. As this happens, the exoplanet’s atmospheric and terrestrial (if it is a terrestrial planet) characteristics are imprinted onto that light.
The light is then captured by telescopes and broken down into its spectra – where the recorded imprint is then studied.
One particular problem with this method of teasing out an exoplanet’s characteristics is that overly hazy atmospheres can distort some of the data that get imprinted onto the light spectra.
But the significance of the data distortion was not well known.
To accurately determine how overly hazy atmospheres distort that data, scientists turned to Cassini’s observations of Titan.
Specifically, the scientific team exploited a similarity between exoplanet transits and sunsets witnessed by the Cassini spacecraft at Titan. These observations by Cassini, called solar occultations (in this case, when Titan passed directly between Cassini and its line-of-sight to the Sun), effectively allowed scientists to observe Titan as if it was a transiting exoplanet.
As the alignment between Cassini, Titan, and the Sun took Titan across the disc of the Sun, Cassini collected the light data that streamed through Titan’s atmosphere. Scientists then took that data (which, at the time of observation between 2006 and 2011 was not actually being collected for this particular work) and broke the light down into its spectra.
Scientists then examined the characteristics of the light spectra and ran an analysis of Titan’s characteristics as if it was a completely unknown exoplanet. They then compared that data to the definitive characteristics of Titan that have been directly observed.
In the process, Titan’s sunsets revealed just how dramatic the effects of hazes can be.
In all, it was found that hazes high above some transiting exoplanets might strictly limit what their spectra can reveal to planet transit observers. Basically, the observations might only be able to gather information from a planet’s upper atmosphere.
On Titan, that corresponds to about 90 to 190 miles (150 to 300 kilometers) above the moon’s surface, high above the bulk of its dense and complex atmosphere.
Moreover, hazes more strongly affect shorter wavelengths, or bluer, colors of light. Studies of exoplanet spectra have commonly assumed that hazes would affect all colors of light in similar ways.
Studying sunsets through Titan’s hazes has revealed that this is not the case.
“People had dreamed up rules for how planets would behave when seen in transit, but Titan didn’t get the memo,” said Mark Marley at NASA Ames. “It looks nothing like some of the previous suggestions, and it’s because of the haze.”
The team’s technique applies equally well to similar observations taken from orbit around any world, not just Titan. This means that researchers could study the atmospheres of planets like Mars and Saturn in the context of exoplanet atmospheres as well to learn more about how different atmospheric compositions effect light spectra data on exoplanetary characteristics.
Meanwhile, as this information about Titan was released, Cassini’s operations team was busy preparing the spacecraft for its 18 June flyby of Titan, the 102nd flyby of Titan for the spacecraft.
This flyby, coupled with the 101st flyby on 17 May, continued a radio science experiment to bounce signals off the surface and through the atmosphere of the moon.
This new method of scientific observation at Titan helped reveal fluctuations in temperature at various levels of Titan’s atmosphere and to record changes in the northern hemisphere features of the moon.
Immediately following this flyby came the announcement from the Southwest Research Institute in San Antonio, Texas, and the National Center for Scientific Research and Observatoire de Paris that Cassini had found firm evidence that nitrogen in Titan’s atmosphere originated in conditions similar to the cold birthplace of the most ancient comets in the Oort cloud.
Previously, scientists had believed that Titan’s building blocks had formed within the warm disc of material thought to have surrounded the infant planet Saturn during its formation.
However, this new discovery disproved that theory and solidified the implication that Titan’s building blocks formed early in the solar system’s history in the cold disc of gas and dust that formed the sun.
This was realized by looking at the isotopic ratio of nitrogen-14 to nitrogen-15 in Titan’s atmosphere.
When investigating the mystery of how the solar system formed, isotope ratios are one of the most valuable clues.
In planetary atmospheres and surface materials, the specific amount of one form of an element relative to another form of that same element can be a powerful diagnostic tool because it is closely tied to the conditions under which materials form.
Moreover, the results of this study have reaching implications for Earth as well as Titan as the results support an emerging view that ammonia ice from comets is not likely to be the primary source of Earth’s nitrogen.
Previously, researchers had assumed that comets delivered the bulk of Titan and Earth’s nitrogen. If this were the case, then the nitrogen isotope ratio in Titan’s original atmosphere would be the same as that ratio is on Earth today.
However, measurements of the nitrogen isotope ratio at Titan by Cassini show that this is not the case. The ratio is different on Titan and Earth. However, measurements of the nitrogen ratio in comets have borne out their connection to Titan but not Earth.
This means the sources of Earth’s and Titan’s nitrogen must be different.
As June drew to a close, NASA and the Cassini team turned their attention toward the celebration of the 10th anniversary of Cassini’s arrival at Saturn.
On 30 June 2014, that anniversary arrived for a probe that’s primary mission was only supposed to last until 2008.
In its 10 years of exploration at Saturn, Cassini executed close to 2 million commands, gathering 514 GB of scientific data while traveling 2 billion miles since its arrival in Saturn orbit.
In those 10 years, Cassini performed 132 close flybys of Saturn’s moons, discovered seven previously unknown moons of the second largest planet in the solar system, contributed to the publication of 3,039 scientific papers, took 332,000 photographs of Saturn and its moon over 206 orbits of the planet, and performed 291 engine burns to facilitate scientific research from 26 participating nations on Earth.
Speaking for the Cassini team, Linda Spilker, Cassini project scientist, said that “It’s incredibly difficult to sum up 10 extraordinary years of discovery in a short list, but it’s an interesting exercise to think about what the mission will be best remembered for many years in the future.
“Having a healthy, long-lived spacecraft at Saturn has afforded us a precious opportunity.
“By having a decade there with Cassini, we have been privileged to witness never-before-seen events that are changing our understanding of how planetary systems form and what conditions might lead to habitats for life.”
As part of the 10 year festivities, NASA took the opportunity to announce the name of Cassini’s final mission extension that will see the spacecraft through operations to its destructive entry into Saturn’s atmosphere in 2017.
The mission, as some had suspected, will be called the Grand Finale. To set up for this mission, on 9 August 2014, Cassini conducted the largest burn of its remaining mission.
The burn saw Cassini fire its main engine for just about one minute to provide a change in velocity of 41 ft/s.
It was the largest maneuver by Cassini in five years, and none of the remaining maneuvers will come close to the duration and velocity change experienced by the spacecraft during this burn.
According to NASA, the burn was conducted not only to set up Cassini’s need for the 103rd flyby of Titan on 21 August, but also to “begin the process of ‘cranking down’ Cassini’s orbit so that the spacecraft circles nearer to the plane of the rings and moons.”
By the end of August, Cassini’s team revealed that the spacecraft’s late-July flyby of Titan showed development of cloud structures and storms over the northern hydrocarbon seas, indicating the onset of summer storms that researchers had considered long overdue.
Following this announcement came the end of September statement that Cassini had been monitoring the evolution of a mysterious feature in the liquid hydrocarbon sea on Titan called Ligeia Mare.
In total, the mysterious feature covers an area of approximately 100 square miles (260 square km).
Observations of the feature began on 10 July 2013 when it was first detected through photo comparisons of the same region obtained on 26 April 2007. The subsequent flyby of Titan by Cassini on 21 August revealed that this new feature had changed in appearance from July 2013.
The feature was discovered using Cassini’s Synthetic Aperture Radar (SAR), and the research team has concluded that it is not a flaw in the radar data. Furthermore, there is no evidence that its appearance is the result of an evaporation event of part of the hydrocarbon sea as the overall shoreline of Ligeia Mare has not noticeably changed.
Currently, it is believed that the feature is in some way related to the changing seasons on Titan as summer draws near in the moon’s northern hemisphere.
At this time, scientists believe that the feature could be surface waves, rising bubbles, floating solids, solids suspended just below the surface, or perhaps something more exotic.
“Science loves a mystery, and with this enigmatic feature, we have a thrilling example of ongoing change on Titan,” said Stephen Wall, deputy team lead of Cassini’s radar team.
In mid-October, attention shifted to an announcement of a scientific discovery related to another one of Saturn’s moons, Hyperion.
On 26 September 2005 as Cassini flew by the chaotically tumbling moon Hyperion, it detected a strong electrostatic charge on the moon’s surface.
This electrostatic charge on Hyperion marks the first time that this type of charge has been seen on any other body in the solar system aside from Earth’s moon and indeed also marks the first confirmed detection of a charged surface on any other moon in the solar system.
Measurements made by several of Cassini’s instruments during a close encounter with Hyperion in September 2005 indicate that something unexpected took place in the charged particle environment around the spacecraft – that the spacecraft was magnetically connected to the surface of Hyperion for a brief period, allowing electrons to escape from the moon toward Cassini.
Hyperion in fact is a porous and icy world, with a sponge-like appearance. Its surface is continuously bombarded by ultraviolet light from the sun and exposed to a rain of charged particles – electrons and ions – within the invisible bubble generated by Saturn’s magnetosphere.
Scientists now believe that Hyperion’s exposure to this hostile space environment is the source of the particle beam that struck Cassini back in 2005.
In all, this discovery surprised the scientific community as many had thought Hyperion to be a simple inert object, which would not undergo any strong interactions with the Saturnian magnetosphere.
Moreover, on the same day as the announcement about the discovery at Hyperion, NASA also announced a new study regarding another one of Saturn’s moons, Mimas.
Based on information garnered from a flyby of the moon by Cassini on 13 February 2010, scientists now believe that Mimas’ external surface is covering up one of two things: either the moon’s frozen core is shaped like an American football, or the moon contains a liquid water ocean.
This prediction stemmed from an analysis of Mimas’ wobble as it orbits Saturn.
“The data suggest that something is not right, so to speak, inside Mimas,” said Radwan Tajeddine, a Cassini research associate at Cornell University.
“The amount of wobble we measured is double what was predicted.”
This directly suggests that either Mimas’ frozen core is not spherical in nature and is actually shaped more like an American football, or that Mimas contains a liquid water ocean under its surface.
Either possibility for the interior of Mimas would be intriguing as the moon’s heavily cratered outward appearance does not suggest that anything unusual lies beneath its surface.
Because Mimas formed more than four billion years ago, scientists would expect its core to have relaxed into a more or less spherical shape by now. So if Mimas’ core is oblong in shape, it likely represents a record of the moon’s formation, frozen in time.
Conversely, if Mimas hides a subterranean ocean under its surface, it would join an exclusive club of “ocean worlds” that includes several moons of Jupiter and two other Saturn moons, Enceladus and Titan.
Meanwhile, as October continued, NASA scientists announced the identification of a high-altitude methane ice cloud on Titan.
Methane clouds had previously been thought unlikely in Titan’s stratosphere because the moon’s troposphere traps most of the moisture and because stratospheric clouds require extremely cold temperatures (below minus 333 degrees Fahrenheit – minus 203 degrees Celsius) to form.
However, a continued examination of images obtained by Cassini of Titan’s northern polar region from eight years ago led to the identification of the high-altitude clouds as being composed primarily of methane.
Subsequent data from Cassini’s Composite Infrared Spectrometer and the spacecraft’s radio science instrument revealed that the high-altitude temperature near the north pole is actually 11 degrees Fahrenheit colder than at the same level in the atmosphere just south of Titan’s equator.
This 11 degree Fahrenheit temperature difference is enough to allow methane ice to form in Titan’s stratosphere.
Moreover, the mechanism for forming these high-altitude clouds appears to be different from what happens during cloud formation in the troposphere.
According to the data returned by Cassini, Titan has a global circulation pattern in which warm air in the summer hemisphere wells up from the surface and enters the stratosphere, slowly making its way to the winter pole.
Once that warm air reaches the winter pole, the air mass sinks back down, cooling as it descends, which allows the stratospheric methane clouds to form.
Continuously, as November began, scientists announced the initial results of the 21 August flyby of Titan by Cassini.
Piquing the curiosity of scientists in the radar and image data returned by Cassini during that flyby was the appearance of two new bright futures in Titan’s largest sea, Kraken Mare.
Unlike the mysterious object that was first seen in Ligeia Mare in 2013, the new features in Kraken Mare were observed in both radar data and images from Cassini’s Visible and Infrared Mapping Spectrometer (VIMS).
The observation of these features at two different wavelengths provided researchers with important clues as to the nature of the objects’ composition.
Specifically, the VIMS data showed the features’ similarities to waves or wet ground that Cassini researchers had already identified near the shoreline of Titan’s seas.
Analysis of the two different wavelength observations of the Kraken Mare objects revealed that they are likely waves or floating debris.
While researchers will continue to painstakingly assess the data returned by Cassini of the Kraken Mare objects from the 21 August flyby, that information will be the only information researchers have about these new objects as all subsequent flybys of Titan by Cassini will not place Kraken Mare in the observational corridor that Cassini will have of Titan’s surface.
And there is no chance of altering Cassini’s flight path to place these objects back into the observational corridor for subsequent flybys because Cassini’s orbital characteristics and flybys of Titan now have to be carefully designed to keep the spacecraft on course for Saturn’s atmosphere for a destructive entry into 2017.
However, an upcoming flyby of Titan in January 2015 will allow Cassini and its researchers to observe for one final time the original magic island feature in the Ligeia Mare to hopefully provide more information on this mysterious object’s identity.
This inability to view these sea regions in greater detail is perhaps one of the highlighting aspects of scientific objectives vs. end of mission needs for Cassini.
Above all other scientific needs is the necessity to preserve Saturn’s potentially life harboring moons from any contamination that Cassini may have brought with it from Earth.
Thus, mission controllers must set Cassini up for a destructive entry into Saturn’s atmosphere at the end is its mission at the expense of some wanted new observations of Titan and the other moons of Saturn.
(Images via NASA).