by Paul Gilster | Nov 8, 2017 | Outer Solar System |
How to explain the water vapor and ice blown off in the form of geyser-like jets from Enceladus? It’s a question we need to answer, because we’re learning just how interesting the icy moons of gas giants can be, with the potential for biological activity far from the Sun.
In the case of Enceladus, though, the average global thickness of the ice is thought to be 20 to 25 kilometers. What has thinned the ice at the south pole, where warm fractures expel mineral-rich water into space? An unusual amount of heat is demanded to sustain this ongoing activity, along with a mechanism to explain what is happening within the moon.
The heat in question is likely the result of friction, according to a new study published in Nature Astronomy. The work of Gaël Choblet (University of Nantes, France) and colleagues, the investigation involved modeling by Cassini researchers in Europe and the U.S., tapping the abundant data that the Saturn orbiter returned to Earth in its 13 years at the planet. It was Cassini that first showed us the jets of water vapor and simple organics.
Image: Dramatic plumes, both large and small, spray water ice out from many locations along the famed “tiger stripes” near the south pole of Saturn’s moon Enceladus. The tiger stripes are fissures that spray icy particles, water vapor and organic compounds. More than 30 individual jets of different sizes can be seen in this image and more than 20 of them had not been identified before. At least one jet spouting prominently in previous images now appears less powerful. This mosaic was created from two high-resolution images that were captured by the narrow-angle camera when NASA’s Cassini spacecraft flew past Enceladus and through the jets on Nov. 21, 2009. Credit: NASA/JPL/Space Science Institute.
The authors find evidence in the Cassini data that hydrothermal activity takes place on the seafloor under Enceladus’ ice. Rock grains thought to be the result of this activity show chemistry occurring at temperatures of at least 90 degrees Celsius. What’s intriguing here is that Choblet and team believe the energy required to produce these temperatures exceeds what could be expected from the decay of radioactive elements within Enceladus. Says Choblet:
“Where Enceladus gets the sustained power to remain active has always been a bit of a mystery, but we’ve now considered in greater detail how the structure and composition of the moon’s rocky core could play a key role in generating the necessary energy.”
The result: An Enceladus whose core is relatively loose and rocky, with 20 to 30 percent empty space. The orbiting moon would experience tidal forces that cause rock in the relatively porous core to flex and abrade against adjacent rock, generating the required heat. This internal scenario also accounts for the rise of heated water from the ocean, moving upward and interacting chemically with the rocks. From the paper’s abstract:
Water transport in the tidally heated permeable core results in hot narrow upwellings with temperatures exceeding 363?K, characterized by powerful (1–5?GW) hotspots at the seafloor, particularly at the south pole. The release of heat in narrow regions favours intense interaction between water and rock, and the transport of hydrothermal products from the core to the plume sources. We are thus able to explain the main global characteristics of Enceladus: global ocean, strong dissipation, reduced ice-shell thickness at the south pole and seafloor activity.
If Choblet’s models are correct, we should expect maximum activity from this process at the moon’s poles, with mineral-rich water thinning the ice shell to as little as 1 kilometer at the south pole, and being expelled into space through fractures in the ice. This analysis would explain why Enceladus has thinner ice at the south pole, but it does not explain why north and south poles are so utterly different. Unlike the south pole, Enceladus’ north pole is cratered and ancient.
The circulation of the global ocean over millions of years would mean that the entire volume of the ocean passes through the moon’s core, an amount estimated to equal 2 percent of the volume of Earth’s oceans. The new study goes past earlier work in modeling tidal friction at a higher level of complexity — earlier models found that tidal heating would be insufficient to keep the ocean liquid. The porous core model thus accounts for the plumes we see today, which may result from thinner ice in the south to begin with, causing runaway heating over time.
The paper is Choblet et al., “Powering prolonged hydrothermal activity inside Enceladus,” Nature Astronomy 6 November 2017 (abstract).
by Paul Gilster | Nov 7, 2017 | Advanced Rocketry |
When we kick around ideas for deep space propulsion, we have to keep in mind that the best solutions may involve hybrid technologies, leveraging the best of several methods. JAXA (Japan Aerospace Exploration Agency) demonstrates this with its ambitious plan to take a sail like IKAROS to Jupiter, for a study of the Jupiter trojan asteroids. Like the original, the upgraded IKAROS will use liquid crystal reflectivity control devices as a means of attitude control.
But operating at the limits of solar sail functionality, the new JAXA sail will also carry a high specific impulse ion engine to facilitate its maneuvers among the trojans. Here we have a mission that couldn’t be flown with just a sail, because the numerous trajectory changes required at destination demand a reliable thruster, one that in this case will be fed by some 30,000 solar panels in the form of thin film solar cells attached to the sail membrane.
What of missions into still deeper space? We’d like to return with robust technologies to the outer planets, not to mention the need to follow up the Voyagers and New Horizons with craft specifically designed to study the interstellar medium beyond the heliosphere.
For that matter, we’re seeing increased interest in exploring the Sun’s gravity lens, whose effects can be examined beginning at 550 AU. That last is a hot and controversial topic, as the spirited debate between Slava Turyshev (JPL) and Geoff Landis (NASA GRC) showed at the recent TVIW conference in Huntsville (keep an eye on the TVIW 2017 video page, where these discussions will soon be available). To resolve the matter, we need to actually go there.
JPL’s John Brophy has been exploring another form of hybrid technology to make such missions possible — I talked about this one when it surfaced last April (see NIAC 2017: Interstellar Implications). Now working under a Phase 1 grant from the NASA Innovative Advanced Concepts (NIAC) program, Brophy proposes using lasers to provide the power source for a spacecraft’s ion engines, beaming to solar panels on the craft. The idea here is to take advantage of a power source separate from the spacecraft in return for major gains in weight and efficiency.
Brophy’s ion engine infrastructure depends upon a 10-kilometer 100 MW laser array capable of beaming power across the Solar System. The beam would be captured by a 70% efficient photovoltaic array tuned to the laser frequency and producing power at 12 kV, according to this precis prepared for the NIAC program. As Brophy has noted, the array output here far surpasses our best solar arrays today, found on the International Space Station, which produce 160 volts. With an areal density of 200 grams per square meter, the 10-kilometer array would be heavier than today’s solar sails but substantially lighter than existing solar arrays.
The laser array in question has its roots in high-power laser concepts of the kind Philip Lubin has advocated for Breakthrough Starshot, only Brophy’s array is in space, avoiding the numerous issues raised by Starshot’s ground-based installation (and creating construction issues of its own). What you achieve with this kind of configuration is the ability to deploy powerful ion engines in the outer Solar System, as Brophy is quick to note in his NIAC precis:
Our innovation is the recognition that such an array increases the power density of photons available to a spacecraft illuminated by the laser beam by two orders of magnitude relative to solar insolation at all the solar system distances beyond 5 AU, and that this enormous power can then be used to great effect by driving a highly-advanced ion propulsion system.
Image: The laser-powered ion thruster concept as developed by John Brophy and colleagues.
The thruster being powered by the laser beam is a lithium-fueled gridded ion propulsion system that does away with what would otherwise be heavy power processing hardware and associated thermal radiators. The 58,000 second specific impulse — compare this to Dawn’s 3,000 seconds — goes well beyond current state of the art in spacecraft systems — about 20 times — and takes advantage of the fact that lithium is both easily ionized and easily stored. The result:
This allows the thruster to be operated with nearly 100% ionization of the propellant which effectively eliminates neutral gas leakage from the thruster and the production of charge-exchange ions that are responsible for thruster erosion and current collection on the photovoltaic arrays. This key benefit enables very long thruster life and facilitates the development of the 12-kV photovoltaic array.
Brophy believes such a system could achieve velocities of 260 kilometers per second, which would make missions to Jupiter feasible within one year of flight time, while reaching the gravity focus would be a matter of 10 to 12 years. If that isn’t tantalizing enough, he also talks about a Pluto orbiter mission with a travel time in the area of 4 years. Thus the hybrid concept ramps up the performance of ion engines in places far enough from the Sun to pose serious power issues, and also taps into a laser infrastructure that could one day drive missions system-wide.
by Paul Gilster | Nov 6, 2017 | Advanced Rocketry |
Although I want to start the week by looking at hybrid propulsion technologies, let’s start by considering developments in ion propulsion before going on to see how they can be adapted for future deep space missions. Hall thrusters are a type of ion engine that uses electric and magnetic fields to manipulate inert gas propellants like xenon. The electric field turns the propellant into a charged plasma which is then accelerated by means of a magnetic field [see reader ‘Supernaut’s correction to this in the comments below].
We’ve seen the benefits of ion engines in missions like Dawn, which has recently received a second mission extension to continue its work around the asteroid Ceres. Now we learn that a Hall thruster called X3, developed at the University of Michigan by Alec Gallimore, has received continuing design modification by researchers at NASA Glenn and the US Air Force, scaling up the low thrust levels produced by conventional ion engines. A series of tests have demonstrated results that have implications for future manned space missions.
In fact, the X3 breaks all previous Hall thruster records, producing 5.4 newtons of force compared with the earlier 3.3 newtons. As noted in this University of Michigan news release, the X3 design also doubles the operating current record, reaching 250 amperes vs. 112 amperes, while running at slightly higher power levels than previous designs. A Hall thruster with higher power and improved thrust levels could shorten travel times, a significant factor as we work to mitigate radiation problems for human crews on long interplanetary missions.
At Glenn Research Center, doctoral student Scott Hall (University of Michigan) worked with NASA’s Hani Kamhawi on experiments to test the improved thruster, using the only vacuum chamber in the United States large enough to cope with the X3’s exhaust, even though the sheer amount of xenon can still cause some of it to drift back into the plasma plume, affecting the results. An upgraded vacuum chamber at the University of Michigan should be ready in early 2018.
Image: A side shot of the X3 firing at 50 kilowatts. Credit: NASA.
The work at Glenn involved four weeks to set up the thrust stand, mount and connect the thruster with propellant and power supplies, deploying a custom thrust stand to bear the X3’s weight. 25 days of testing then produced the results above in a project funded by NASA’s Next Space Technologies for Exploration Partnership. The next step here will be to integrate the X3 with power supplies now being developed by Aerojet Rocketdyne. By the spring of next year, new tests at NASA Glenn using the Aerojet Rocketdyne power processing system are expected.
Image: The X3 nested-channel Hall thruster with all three channels firing at 30 kW total discharge power. Credit: University of Michigan.
These developments in current Hall thruster technology are exciting in themselves and have implications for the near-term in missions to destinations like Mars. But I’m also interested in pursuing how we might move ion technologies in new directions by creating hybrid designs, with Kuiper Belt objects and the gravitational focus at 550 AU as potential destinations. With laser methods now in the spotlight as Breakthrough Starshot continues its analysis of a mission to Proxima Centauri, hybrid ion engine designs boosted by laser power are coming into consideration. I’ll take a look at the possibilities in tomorrow’s post.
by Paul Gilster | Nov 3, 2017 | Exoplanetary Science |
Just how elaborate is the planetary system around the nearest star? It’s a question rendered more interesting this morning by the news that the ALMA Observatory in Chile has now detected dust in the system in an area one to four times as far from Proxima Centauri as the Earth is from the Sun. Moreover, there are signs of what may be an outer dust belt, an indication that while we have already discovered Proxima Centauri b, we are looking at a system in which cold particles and debris that could have formed other planets continue to accompany the star.
Image: This artist’s impression shows how the newly discovered belts of dust around the closest star to the Solar System, Proxima Centauri, may look. ALMA observations revealed the glow coming from cold dust in a region between one to four times as far from Proxima Centauri as the Earth is from the Sun. The data also hint at the presence of an even cooler outer dust belt and indicate the presence of an elaborate planetary system. These structures are similar to the much larger belts in the Solar System and are also expected to be made from particles of rock and ice that failed to form planets. Note that this sketch is not to scale — to make Proxima b clearly visible it has been shown further from the star and larger than it is in reality. Credit: ESO/M. Kornmesser.
It would surprise no one if Guillem Anglada-Escudé were the lead author of this work, as Anglada-Escudé led the effort that discovered Proxima b. But in this case, the Guillem Anglada who led the dust work, based at the Instituto de Astrofísica de Andalucía, Granada, Spain, simply shares Anglada-Escudé’s name — the two are not related. Further complicating matters is the fact that Proxima b discoverer Anglada-Escudé is a co-author of the paper on dust.
Image: This picture combines a view of the southern skies over the ESO 3.6-metre telescope at the La Silla Observatory in Chile with images of the stars Proxima Centauri (lower-right) and the double star Alpha Centauri AB (lower-left) from the NASA/ESA Hubble Space Telescope. Proxima Centauri is the closest star to the Solar System and is orbited by the planet Proxima b, which was discovered using the HARPS instrument on the ESO 3.6-metre telescope. Credit: Y. Beletsky (LCO)/ESO/ESA/NASA/M. Zamani.
Says lead author Anglada:
“The dust around Proxima is important because, following the discovery of the terrestrial planet Proxima b, it’s the first indication of the presence of an elaborate planetary system, and not just a single planet, around the star closest to our Sun.”
Given Proxima Centauri’s age, at least as old as the Solar System and perhaps older, we’re probably detecting residual dust from regions something like the Kuiper Belt and main asteroid belts in our own system. Such dust can also produce zodiacal light, which can be seen in our own system as sunlight is scattered by interplanetary dust, a faint but observable phenomenon.
Image: This image of the sky around the bright star Alpha Centauri AB also shows the much fainter red dwarf star, Proxima Centauri, the closest star to the Solar System. The picture was created from pictures forming part of the Digitized Sky Survey 2. The blue halo around Alpha Centauri AB is an artifact of the photographic process, the star is really pale yellow in colour like the Sun. Credit: Digitized Sky Survey 2. Acknowledgement: Davide De Martin/Mahdi Zamani.
The total mass of the inner belt appears to be about 1/100th of Earth’s mass, at a temperature of about -230 degrees Celsius, roughly the temperature in the Solar System’s Kuiper Belt. The inner belt appears to extend a few hundred million kilometres from Proxima Centauri, and could be quite a useful find if it helps us estimate the inclination of the Proxima Centauri system. The dust is assumed to be elliptical given the tilt of what is probably a circular ring around the star. Such a determination would offer us a way to tighten mass estimates of Proxima b.
And are there other planets waiting to be discovered here? Anglada again:
“This result suggests that Proxima Centauri may have a multiple planet system with a rich history of interactions that resulted in the formation of a dust belt. Further study may also provide information that might point to the locations of as yet unidentified additional planets.”
The paper speculates interestingly about a “compact source of 1.3 mm emission at a projected distance of ? 1.2” SE from the star…” which may or may not be an artifact in the equipment, but if real could be anything from a background galaxy (the authors consider this remote) to a Saturn-mass planet with a temperature around 1000 K. The problem: No radial velocity signature of such a planet has yet been found, although if it were there, it should be detectable.
Image: Figure 4 from the paper. Sketch (not to scale) of the proposed components in the Proxima Centauri planetary system. Question marks indicate marginally detected features. Credit: Anglada et al.
The paper is Anglada et al., “ALMA Discovery of Dust Belts Around Proxima Centauri,” accepted at Astrophysical Journal Letters (preprint).
by Paul Gilster | Nov 2, 2017 | Exoplanetary Science |
The other day I looked at how we can use transit spectroscopy to study the atmospheres of exoplanets. Consider this a matter of eclipses, the first occurring when the planet moves in front of its star as seen from Earth. We can measure the size of the planet and also see light from the star as it moves through the planetary atmosphere, giving us information about its composition. The secondary eclipse, when the planet disappears behind the star, is also quite useful. Here, we can study the atmosphere in terms of its thermal variations.
In my recent post, I used a diagram from Sara Seager to show primary and secondary eclipse in relation to a host star. The image below, by Josh Winn, is useful because it drills down into the specifics.
Image: A comparison between transits and secondary eclipses (also sometimes called occultations). In a planetary transit, the planet crosses in front of the star (see lower dip) blocking a fraction of the star’s brightness. In a secondary eclipse, the planet crosses behind the star, blocking the planet’s brightness (see dip in the middle). The latter dip in brightness is fainter due to the faintness of the planet. Credit: Josh Winn. See A New Discovery of a Secondary Eclipse for more background as it applies to the HAT-P-11 system.
Secondary eclipses have been significant in the study of Kepler-13Ab, a world where conditions could not be more different on the planet’s nightside vs. its dayside. A ‘hot Jupiter’ some 1730 light years from Earth, this is a world close enough to its parent star that it is tidally locked. Researchers led by Thomas Beatty (Pennsylvania State) have used the Hubble Space Telescope to determine that that the dayside here can surpass a blistering 3000 Kelvin.
By contrast, the nightside of Kepler-13Ab, turned forever away from the star, is a place where titanium oxide snow can fall. The process is intriguing: Any titanium oxide gas on the star-facing side is carried by strong winds around to the nightside, where the gas condenses into clouds and eventually falls as snow. A gravitational tug six times greater than Jupiter’s pulls the titanium oxide into the lower atmosphere, forming a ‘cold trap’ — an atmospheric layer that is colder than the layers both below and above it. Ascending gases drop back into the trap.
Image: This illustration shows the seething hot planet Kepler-13Ab that circles very close to its host star, Kepler-13A. Seen in the background is the star’s binary companion, Kepler-13B, and the third member of the multiple-star system is the orange dwarf star Kepler-13C. Credit: NASA, ESA, and G. Bacon (STScI).
These Hubble observations are the first time a cold trap has been detected on an exoplanet. The secondary eclipse data reveal that the atmospheric temperature on the dayside actually grows colder with increasing altitude, lacking titanium oxide to absorb light and re-radiate it as heat. Normally, titanium oxide in a hot Jupiter makes the atmosphere warmer at higher altitudes. But larger hot Jupiters possess the characteristics needed for cold traps to form.
From the paper:
We contend that the dual facts that Kepler-13Ab possesses a decreasing temperature-pressure profile and a relatively high surface gravity support the hypothesis of Beatty et al. (2016) that both surface gravity and temperature play a role in determining the presence of a stratospheric temperature inversion in hot Jupiters. Specifically, in high-surface-gravity planets such as Kepler-13Ab, the characteristic freefall time within the atmosphere is substantially shorter (Equation (11)). This should, in turn, substantially increase the efficiency of a day-night (Parmentier et al. 2013) cold-trap process, thereby sequestering the TiO/VO molecules available to cause an inversion in the interior of the planet.
In other words, many of the hot Jupiters we’ve observed likely have precipitation like this, but in those with lower surface gravity than Kepler-13Ab, the titanium oxide snow doesn’t fall far enough to form a cold trap before being drawn back to the dayside of the planet. Note how much information we’re teasing out of secondary transits of a planet we cannot actually see and it should be clear that Beatty is right in referring to the current studies of hot Jupiters as testbeds for how we are going to analyze the atmospheres of terrestrial planets in the future.
The paper is Beatty et al., “Evidence for Atmospheric Cold-trap Processes in the Noninverted Emission Spectrum of Kepler-13Ab Using HST/WFC3,” Astronomical Journal Vol. 154, No. 4 (22 September 2017). Abstract / preprint.
