The Cassini-Huygens Mission to Saturn and Titan

Following the painstaking experience watching Titan on Netflix last night (I highly recommend you don’t watch it) which was about the possibility of life on Saturn’ moon following an existential crises plaguing earth, out of sheer agitation I decided to look into this theme and find out more. The Cassini-Huygens mission was a seven year journey of the NASA Cassini Orbiter along with the European Space Agency’ Huygens Probe intended to travel to the Saturn system. Titan IVB-Centaur rocket was launched from Cape Canaveral carrying the orbiter and probe in 1997 and passing Venus, Earth and our moon, an Asteroid Belt and Jupiter, the Huygens Probe successfully landed on Saturn’s moon Titan in 2005. The Cassini spacecraft also managed to capture new and detailed information and images including a number of new moons such as Methone and Pallene, as well as the first spacecraft to orbit Saturn that showcased surprising activity on Enceladus, new rings around Saturn, and a plethora of additional information not previously known by scientists. The 2 hour 27 minute descent to the surface was matched with 72 minutes on the frozen ground until contact with Cassini was lost, and the probe managed to obtain images of Titan’s geology and meteorology to reveal astonishing similarities to Earth. But is Titan really similar to Earth?

The primary scientific goal of the mission was to explore Saturn’s interior and atmosphere, the ring system and the magnetosphere and plasma environment, as well as the magnetic field and origins of Saturn. In addition, a strong focus Saturn’ second largest moon, Titan when the Huygens probe was launched into the moon through the thick atmosphere during its descent to the surface. The probe was intended to study both the atmosphere and the surface composition and contain six instruments that enabled this, such as the Huygens Atmospheric Structure Instrument (HASI) designed to measure the electrical and physical properties of Titan.[1] It was additionally equipped with the following experiments:

Surface Science Package (SSP) aimed at determining the properties of the surface where the probe landed in order to ascertain further details of its composition and measure other aspects of the landing site.

Descent Imager Spectral Radiometer (DISR) that used sensors to uncover spectral measurements of the surface and as it descended toward the surface took images of the spectra.

Gas Chromatograph and Mass Spectrometer (GCMS) that attempted to identify gaseous atmospheric properties and captured material that it analysed as it descended to the surface.

Aerosol Collector and Pyrolyser (ACP) is a device that collected chemical and aerosol samples of the atmosphere and analysed the material taken from the atmosphere.

Doppler Wind Experiment (DWE) detected Doppler shifts that the probe experienced and caused by the atmosphere and winds was measured along with other properties through radio signals.[2]

The atmosphere of Titan contains the chemical composition mostly of nitrogen, but also a mixture of methane, ethane and other hydrocarbons, which makes the composition itself similar to that of Earth that contains 80% nitrogen,[3] particularly as it is the only other planetary body in our solar system that has evidence of liquid on the surface. Images from Cassini show how the surface of the moon contains rivers of ethane and methane and explains why it is the only moon that has clouds and a thick atmosphere. The orbital period of Titan around Saturn nears 16 days and is tidally-locked to Saturn, while the distance to the sun is almost ten times further than earth at 9.54 AU making the solar energy captured by the moon at a much lower rate, however the atmosphere enables Titan to capture solar energy. “It’s partial transparency to significant amounts of sunlight, and its high opacity to longer wavelengths… [s]ome of the energy from the Sun reaches the surface of Titan because solar radiation consists mostly of photons at near-visible wavelengths and the atmosphere is partially transparent at those wavelengths,”[4] that allows only about 10% of the energy to reach the surface. This energy is then absorbed by the surface that releases infrared heat out and back into the atmosphere that re-radiates by out and back down and the phenomenon of this greenhouse effect contributes to the temperature, which is at 92k (-180 degrees Celsius). The atmospheric pressure of Titan is 60% greater than Earth and the radius is almost half, but as the mass of Titan is 1.3452 x 10^23 kg compared to this size, the gravity cannot hold the gas and thus the atmosphere is much greater than that of Earth.

Seasonal variations to do exist on Titan and the Cassini mission aided researchers to ameliorate their understanding of the patterns of atmospheric changes, arriving on the northern hemisphere of Titan during winter.[5] Seasonal changes, however, are incredibly slow as it is driven by the eccentricity of Saturn’ 29.5 yearlong orbit around the sun, and the atmosphere responds to the effects of these changes in rising and falling temperatures as well as day to night variations. The orbital configuration has resulted in an imbalance of methane lakes and rivers in the northern and southern hemispheres of Titan that transports chemicals through evaporation and precipitation differently. The atmospheric pressure on the surface is at 1500mbar and researchers have confirmed the moon experiences cryovolcanism captured by observations from Cassini where the eruption style and composition is similar to volcanoes erupting liquid instead of lava.[6] Among the other important findings include the origins of Titan’ methane and the chemistry of the atmosphere as the gas increased during the descent of the probe, until it reached the surface when a spike was detected at almost 40% increase in methane suggest the possibility during the formation and accretion period, methane became trapped in the ice and reached the surface through cryovolcanism. In addition, radiogenic argon-40 (40Ar) was also detected that further confirms details of Titan’ interior since 40Ar can only form through the decay of potassium-40 (40K) that is found in rocks.[7]

HASI findings also enabled scientists to measure the density of the upper atmosphere and showed the thermosphere to be warmer and with a greater density. Atmospheric circulation and the transportation of heat was captured by the DISR by the “imbalance seen in the radiative flux measurements”[8] and it indicates winds verified by the probe as it encountered the flow from west to east and therefore the same direction as the rotation of Titan. Models have since showed that the captured data confirmed a reversal of direction and different points during the descent of the probe caused by temperature variations between the northern and southern hemispheres and within the Hadley cell that circulates from the north and south poles.[9] The super rotating winds measured the Doppler shift during the descent of the probe and measured large variations in wind speeds as it decreased the closer it reached the surface and the findings also suggest that Titan does not have a mesosphere as was predicted.

The atmosphere of Titan was found to be stratified comparatively to Earth’ troposphere where variations in temperature produce layers and while the amount of sunlight is minimal, the icy moon still contains wind and clouds, where clear by images by Cassini indicate massive dunes on the surface.[10] This provides greater details of the structure of the atmosphere including its temperature, as the DISR captured images of the surface and terrain and showed a plateau of dried river beds and lakes with narrow channels and dendritic networks that parallel the fluvial configuration you’ll find on Earth particularly erosion which DISR detected and indicative of such activity. Buried below the icy surface together with the discovery of what is known as Schumann resonance, very low radio signals in the atmosphere (around 36 Hertz) and for such signals to be reflected, “an ocean of water and ammonia which is buried at a depth of 55-80km below a non-conducting, icy crust”[11] could explain this.

There were a number of reasons for destroying the Cassini orbiter by incinerating it through the powerful atmosphere of Saturn. Not only will it provide additional information to scientists both gaining closer access to the rings of Saturn and the upper atmosphere that provide a glimpse of the interior structure and the opportunity to accurately ascertain the age and composition through closer inspection of the mass of the rings surrounding the planet, but also because of regulations vis-à-vis interplanetary contamination. The preservation of the integrity of the solar system in order to prevent and potentially damage both other planets and our own has led to all spacecraft to undergo sterilisation processes to avoid microbe contamination, particularly of planets or moons that may be capable of organic habitat. The Cassini spacecraft detected a number of new finds on Enceladus that verifies the possible conditions for contamination of microbes from Earth. Cassini itself captured vapour being released from Enceladus that confirmed an oceanic subsurface Knowing that the craft itself has a maximum output of fuel, to prevent any risk of it contaminating the integrity of the moon, scientists decided to plan the death of the device.[12]

While Titan does have similarities to Earth, particularly for having a thick atmosphere and a high percentage of nitrogen, but a number of other missing or differing constituents make these similarities marginal at best. The benefits of the Cassini-Huygens mission to Saturn and Titan enabled scientists to gain further insight to clarify these terrestrial and atmospheric differences.

[1]Fulchignoni, M., Ferri, F., Angrilli, F. et al. Space Science Reviews (2002) 104: 395. https://doi.org/10.1023/A:1023688607077:
[2] Patrick Irwin, Giant Planets of Our Solar System: Atmospheres, Composition, and Structure, Springer Science & Business Media (2003) 330
[3] http://www.lmd.jussieu.fr/~sllmd/pub/REF/2012Icar..218..707L.pdf
[4] Athena Coustenis, Fred Taylor, F. W. Taylor, Titan: The Earth-like Moon World Scientific (1999) 62
[5] Ingo Müller-Wodarg, Caitlin A. Griffith, Emmanuel Lellouch, Thomas E. Cravens, Titan: Interior, Surface, Atmosphere, and Space Environment, Cambridge University Press (2014) 215
[6] R. M. C. Lopes, Cryovolcanism on Titan: New results from Cassini RADAR and VIMS: https://doi.org/10.1002/jgre.20062
[7] Robert Brown, Jean Pierre Lebreton, Hunter Waite, Titan from Cassini-Huygens, Springer Science & Business Media (2009) 182
[8]Ibid., Titan from Cassini-Huygens, 345
[9] https://www.nasa.gov/mission_pages/cassini/whycassini/cassinif-20070601-05.html
[10] http://pirlwww.lpl.arizona.edu/~jani/radebaugh-titandunes-aeolian13.pdf
[11] http://sci.esa.int/cassini-huygens/55230-science-highlights-from-huygens-9-schumann-like-resonances-hints-of-a-subsurface-ocean/
[12] http://www.spacesafetymagazine.com/space-exploration/extraterrestrial-life/cassini-huygens-preventing-biological-contamination/

The ‘Seeds’ of Super Massive Black Holes

Stellar black holes are scattered throughout the universe, formed under the right conditions when stars reach the end of their life cycle as it collapses into itself when it no longer contains the fuel to counteract the pressure of gravity and resist compression. In order to produce a black hole, the exploding star would need to have a mass greater than our sun (at least twenty times greater) so that there is enough material – which is dispersed following the supernova – to form a black hole and will continue to grow as it consumes material such as gas and stars that draws them into the dense space. Super Massive Black Holes (SMBH) are comparatively galactic monsters that live in the centre of most galaxies and while their origins are indeed more difficult to ascertain, a number of theoretical possibilities continue to elude astronomers. Do they having the humble beginning by starting as stellar black holes that grow over billions of years through accretion and even mergers with other black holes? The confirmation that Sagittarius A* located 26,000 light years away is a SMBH located in our very own galaxy and a number of techniques have been used to ascertain its position, particularly through comparative observations found in the properties of other host galaxies and quasars. The recent quasar, J1342+0928, has given astronomers a glimpse into the possible causes of such huge SMBH.

Albert Einstein predicted the existence of black holes through his general theory of relativity, however it was not until Sir Martin Rees proposed the idea that extremely massive black holes could exist within the Active Galactic Nuclei (AGN)[1] of our own and most galaxies. His theory pieced together a detection of an unknown radio source by Bruce Balick and Robert Brown at the centre of the Milky Way and a possible demonstration that the object with such powerful gravitational force could be caused by an SMBH. AGNs are the centre of an active galaxy and depending on the properties and activity that occur within name, AGNs can be called Quasars, Seyfert Nuclei, Blazers, Liners, Radio Galaxies and BL-LAC objects and a number of other names that verifies the diversity of activity the centre of galaxies can have.[2] Host galaxies themselves also present unique characteristics and types including the elliptical, spiral and irregular galaxies and each have different components. Classification of galaxies was initially developed by Edwin Hubble that divided and then subdivided commonly found features that exhibit unique properties the he coded into a general system; a spiral galaxy, for instance, contains a bulge at the centre, surrounded by a halo and a disk structured as arms like spirals around the galaxy. Further classifications developed as the study of galaxies improved including de Vaucouleurs’s classification of galaxies that helped classify unusual properties or peculiarities, such as Quasars.[3]

R0CjB

Following Balick and Brown’ discovery, there were clear limitations to radio observations to verify whether the source was specifically a SMBH and continued efforts led to NASA’s Chandra X-Ray Observatory to spot never before seen x-ray emissions by penetrating the galactic dust and clouds that blurred the possibility of a closer investigation to this blackness at the heart of our galaxy using radio sources, giving us more insight about the activity and behaviour of SMBH.  The Very Large Array as part of the National Radio Astronomy Observatory (NRAO) includes 27 radio antennas configured to provide images that would give the resolution of one dish at almost 130 metres in diameter.[4] The image below is one such image taken from the observatory in New Mexio and shows a central source – now known to be Sag A* – has forced ionized gas into a mini-spiral rotating around centre and revealing the possible features of a concentration of dark matter, but whether this concentration was a SMBH was not confirmed as well as raising the question and nature of SMBH as apparently motionless. “The implied minimum dark matter density of ~3×10.9 M  pc^-3, however, still allowed a cluster of dark objects, such as neutron stars or stellar mass black holes, as one of the alternatives to a single supermassive black hole because the measurements did not force the cluster’s lifetime to be shorter than the age of the Galaxy.”[5]

Picture 1

As observations have also included evidence of the emission of radiation including Infrared, X-ray and Gamma-Ray sources, other telescopes including the Very Large Telescope (VLA) managed by the European Southern Observatory in the Atacama Desert of northern Chile and the 10m W. M. Keck I telescope on the summit of Mauna Kea in Hawaii has mapped the orbits of stars and objects that are within one parsec (about 3.26 light-years) of the central dark object, enabling astronomers to measure the mass of Sgr A* utilising Kepler’s Laws that calculate the length of time it takes for the orbiting object to encircle the galactic centre together with the semi-major axis, leading astronomers to believe the centre of our galaxy is contained by a SMBH equivalent to 4 million solar masses.[6] “Orbits are derived simultaneously so that they jointly constrain the central dark objects properties: its mass, its position, and, for the first time using orbits, its motion on the plane of the sky.”[7] This is because the dark central object has a powerful mass within such a small radius nearing 100 AU (1 AU is the distance between Earth and the Sun or about 150 million kilometres) that suggested dark matter to be confined in a space of 0.015pc ruling out the possibility of a cluster of stellar black holes as the source, leading to the conclusion that it is a SMBH. The Fermion Ball Hypothesis offers an alternative possibility for the blackness problem as it attempts to explain the supermassive dark object at the centre to be a ball of self-gravitating, non-interacting, degenerate fermions.[8] Fermion balls may have been formed in the early universe and studies show the analysis of the orbits of stars S0-1 and S0-2 around Sgr A* was initially consistent to the FB scenario as the pressure from the degeneracy maintains a balance with the gravitational attraction to the fermions.[9] Continued observations of the S0-2 orbit revealed that a mass of 3.6‡ 0.6 x 10^6 MΘ  is located within a sphere radius of 0.6mps Sgr A*[10] that excludes the possibility of the FB Hypothesis as such a mass density would impact on the fermion ball particles.

These advances in telescopic technology including the use of Very-Long Baseline Interferometry (VLBI) techniques that captures high resolution radio sources from different locations before being combined into following the meticulous measurement of time differences all fed into one central location on a supercomputer with enormous data capacity to enable this to occur efficiently have recently made it possible to capture the shadow of the event horizon, the boundary surrounding the SMBH. As the diameter of the telescope increases the resolution, sensitivity and baseline, and the Event Horizon Telescope (EHT) and the Global mm-VLBI Array (GMVA) combines multiple telescopes around earth to simulate the power that an earth-sized telescope would capture. The boundary surrounding a black hole is an event horizon because while gravity is very strong, objects can still escape from the gravitational pull and thus energy can be detected near the horizon while anything that travels beyond that an observer cannot see.[11]

The Event Horizon Telescope and Global mm-VLBI Array on the Eart

Quasars are the most luminous AGN and they have a very strong correlation to SMBH as the latter is required to give Quasars the immense power that they project. “Quasars rank amongst the most luminous sources of radiation in the universe and are believed to be powered by SMBH.”[12] M–σ or the M-Sigma relation indicates a correlation between the mass of a SMBH and the stellar velocity dispersion in their host galaxies[13] and confirm SMBH to be a fundamental element of host galaxies with Quasars. Quasars are said to exist in predominantly larger galaxies during the active phase of gas accretion and therefore the early life of a galaxy, where particles or matter are heated and accelerated away like a jet or stream of light from the ionization around the boundary of an SMBH at velocities almost at the speed of light and emitting powerful energy and therefore luminosity that exceeds all the sources of light within the entire galaxy.[14] In December 2017, the most distant quasar J1342+0928 at a redshift of z=7.54 was found and provided a glimpse into the cosmological timeline by using Planck parameters that confirmed the age of galaxy to be barely 690 Myr after the big bang[15] with an SMBH that contains a mass of 8e8 Msun (800 million solar masses!).[16] The existence of a SMBH during such an early epoch of the universe’ existence confirms a number of models about how black holes can grow to such a supermassive size as it captures nearby material and ultimately engines the power of a quasar. The standard view is that following supernova explosions, the gravitational interactions form stellar black holes and continue to grow through mergers and accretion, however the accretion theory does not explain the SMBH in quasars so old and powerful as J1342+0928 known as the quasar seed problem, indicating that the conditions to have formed them must have been achieved by direct collapse.[17] A large ‘seed’ to form a black hole as colossal as 100,000+ solar masses and therefore thousands of times bigger than black holes shaped by a supernova is possible in the right conditions during the early universe as it required an equally colossal star collapse that would supress star formation because the temperature of the ultraviolet photos ensure that the surrounding gas remains hot enough. Stars usually form when the cloud cools as the gas is dispersed, but the conditions in the early universe confirm that such a supermassive ‘seed’ could be possible before it begins to attract matter and gas to grow over billions of years into its colossal size.[18]

The catalyst that enables supermassive black holes to form remains unconfirmed, but as technology enables astronomers to locate and verify new hints about the galaxy, we are inching closer and closer to verification of theories

 

[1] Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research, Ferrarese & Ford 2005, Space Science Reviews, Volume 116, Issue 3-4, pp. 523-624
[2] Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research, Ferrarese & Ford 2005, Space Science Reviews, Volume 116, Issue 3-4, pp. 523-624
[3] Sidney Van den Bergh, Galaxy Morphology and Classification, Cambridge University Press (1998) 13
[4] http://www.vla.nrao.edu/
[5] Stellar Orbits around the Galactic Center Black Hole, Ghez et al. 2005, The Astrophysical Journal, v.620, p.74
[6] Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research, Ferrarese & Ford 2005, Space Science Reviews, Volume 116, Issue 3-4, pp. 523-624
[7] Stellar Orbits around the Galactic Center Black Hole, Ghez et al. 2005, The Astrophysical Journal, v.620, p.74
[8] Neven Bilic, Supermassive Fermion Balls and Constrains From Stellar Dynamics Near Sgr A∗: https://arxiv.org/pdf/astro-ph/0310172
[9] http://iopscience.iop.org/article/10.1086/323949/fulltext/
[10] Siegfried Röser, From Cosmological Structures to the Milky Way, John Wiley & Sons (2006) 196
[11] Jim Al-Khalili, Black Holes, Wormholes and Time Machines, Taylor & Francis (2016) 64
[12] Formation of z~6 Quasars from Hierarchical Galaxy Mergers, Li et al. 2007, The Astrophysical Journal, v.665, p.187
[13] Raphael Sadoun, M- sigma relation between SMBHs and the velocity dispersion of globular cluster systems: arXiv:1204.0144 [astro-ph.CO]
[14] Andrew C. Fabian, Active Galactic Nuclei: Proc Natl Acad Sci U S A. 1999 Apr 27; 96(9): 4749–4751.
[15] Fulvio Melia, J1342+0928 Confirms the Cosmological Timeline in R_h=ct: arXiv:1712.03306 [astro-ph.CO]
[16] E. Bañados,, An 800 million solar mass black hole in a significantly neutral universe at redshift 7.5: arXiv:1712.01860 [astro-ph.GA]
[17] Bhaskar Agarwal, Formation of massive seed black holes by direct collapse in the early Universe, Dissertation (2013) pp 24
[18] Silvia Bonoli, Massive black hole seeds born via direct gas collapse in galaxy mergers: their properties, statistics and environment, https://arxiv.org/pdf/1211.3752.pdf