Mountains on Titan

Titan is by far the largest moon in orbit around Saturn and the 2nd largest moon in the Solar System. It has a diameter of 5,152 km, making it nearly 1.5 times the size of Earth's Moon. Titan has a thick atmosphere and opaque haze layers obscure its entire surface. From inside out, the bulk of Titan is believed to be comprised of a partially differentiated interior of rock and ice, a high pressure ice layer (consisting of ice III, V, and VI), a subsurface ocean of liquid water and an outer ice I shell. Ice III, V, and VI are high pressure phases of ice which do not occur naturally on Earth. Ice I is basically normal ice and all naturally occurring ice on Earth is ice I.

Figure 1: Saturn’s fourth-largest moon, Dione, can be seen through the haze of the planet’s largest moon, Titan, in this view of the two posing before the planet and its rings from an image taken by the Cassini spacecraft. Credit: NASA/JPL-Caltech/Space Science Institute.

Figure 2: Artist’s concept showing a possible scenario for the internal structure of Titan, as suggested by data from the Cassini spacecraft.

A thermal model of the interior of Titan developed by Mitri et al. (2010) show that the long term cooling of Titan can cause a global volume contraction of ~0.01. As Titan cools, the base of its subsurface ocean would freeze onto the top of its high pressure ice layer while the top of its subsurface ocean would freeze onto the underside of its outer ice I shell. Because high pressure ice is a lot denser than liquid water (~10 to 30 percent denser) and ice I is only marginally less dense than liquid water (<10 percent less dense), the gradual freezing of Titan's subsurface ocean into high pressure ice and ice I would cause an overall reduction in the volume of Titan.

It seems that the presence or absence of a high pressure ice layer in the interior of an object can determine whether or not it will undergo global contraction or expansion during cooling. For example, the interior of Jupiter's moon Europa is comprised of a rocky interior, an overlying subsurface ocean and an outer ice I shell. Unlike Titan, Europa does not have a high pressure ice layer. Since its outer ice I shell has a lower density than the underlying subsurface liquid water ocean (i.e. water is less dense than ice), the long term cooling of Europa will cause the outer ice I shell to thicken and result in overall global volume expansion.

Figure 3: A model of the topography produced by the contractional deformation of Titan's icy lithosphere. (Mitri et al., 2010)

Figure 4: Cassini radar imagery showing three elongated radar bright features that may be fold ridges formed from the contractional deformation of Titan's icy lithosphere. A topographic profile across one of the ridges (black rectangle) show that it has a height of 1,930 m. (Mitri et al., 2010)

The global volume contraction of Titan leads to contractional deformation of Titan's icy lithosphere, producing fold features (i.e. mountains). These fold features can reach topographic heights of up to several kilometres, especially so if Titan underwent more rapid cooling in the early Solar System and thereby experienced more contraction. The radar instrument on the Cassini spacecraft has imaged mountainous topography on Titan that is consistent with fold features produced by the contractional deformation of Titan's icy lithosphere. Perhaps, such a contractional deformation process may have formed most of Titan's mountains.

Reference:

Mitri et al., “Mountains on Titan: Modelling and Observations”, Journal of Geophysical Research: Planets, Volume 115, Issue E10, October 2010

Shockwaves from the Rheasilvia Impact

Vesta is one of the largest asteroids in the Solar System, measuring 573 km by 557 km by 446 km in size. It is a member of the main asteroid belt and it circles the Sun between the orbits of Mars and Jupiter. From July 2011 to September 2012, NASA’s Dawn spacecraft was in orbit around Vesta and the spacecraft conducted numerous observations of this large asteroid. Centred on the south pole of Vesta is a large impact feature known as the Rheasilvia basin. The basin has a depth of ~20 km and a diameter of ~500 km, nearly as large as Vesta itself.

 Figure 1: This topographic map from NASA’s Dawn spacecraft shows the two large impact basins in the southern hemisphere of Vesta. The map is colour-coded by elevation, with red showing the higher areas and blue showing the lower areas. Rheasilvia, the largest impact basin on Vesta, is ~500 km in diameter. It is estimated to have formed no more than ~1 billion years ago by counting the number of smaller craters that have formed on top of it. The other basin, Veneneia, is ~400 km in diameter and it lies partially beneath Rheasilvia. Veneneia is estimated to be at least ~2 billion years old. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.

Figure 2: This image obtained by the framing camera on NASA’s Dawn spacecraft shows the south pole of the giant asteroid Vesta. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Observations by NASA’s Dawn spacecraft suggests that the impact which excavated the Rheasilvia basin may have been sufficiently large to create disrupted terrains at the impact antipode, which is the area on Vesta opposite to the point of impact. Compared to the age of the Solar System, the Rheasilvia basin is relatively young, estimated to be no more than ~1 billion years old. Modelling work performed by Bowling et al. (2013) show that the degree of antipodal deformation is very sensitive to the mantle porosity and core strength of Vesta.

In the “control” model with zero mantle porosity and a strong rock-like core, the shockwaves from the Rheasilvia impact passes through the mantle and core of Vesta with little attenuation. The shockwaves eventually converge around the antipode with sufficient energy to uplift enough material to create a ~6 km tall antipodal peak. More realistically, the presence of mantle porosity and/or a weaker core would result in a smaller degree of antipodal deformation. In fact, the models show that unless the mantle porosity is relatively low and the core is relatively strong, no antipodal deformation would occur.

Figure 3: Modelled antipodal topography 1500 seconds after the Rheasilvia impact. All simulations in this series were run with a strong, rock-like core. (T. J. Bowling et al., 2013)

Topographic maps of Vesta’s north pole, acquired by NASA’s Dawn spacecraft, show an area near the impact antipode that is ~5 to 10 km higher than the surrounding plains. However, the antipodal point itself lies within a ~63 km diameter crater named Pomponia. Pomponia is believed to have formed more recently than the Rheasilvia basin and its formation would have obliterated much of the predicted antipodal topographic uplift from the Rheasilvia impact. Additionally, a ~90 km diameter crater named Albana lies right next to Pomponia.

Figure 4: Topography at the north pole of Vesta. The white dot represents the approximate location of the impact antipode corresponding to the Rheasilvia basin. The region marked ‘2’ indicates the area in which a crater size frequency distribution was produced. (T. J. Bowling et al., 2013)

If the ~5 to 10 km elevated area near the impact antipode is a product of the Rheasilvia impact, it would suggest that Vesta has a low mantle porosity and a core of considerable strength. Unfortunately, the presence of the craters Pomponia and Albana make it difficult to determine what portion of the elevated area is a product of topographic uplift from the Rheasilvia impact and what portion is due to more recent impacts.

Nevertheless, a study of the crater size frequency distribution in an area near the impact antipode shows a deficiency of smaller craters with diameters between 3 km to 9 km. In comparison, craters with diameters larger than ~10 km are as common around the impact antipode as elsewhere on Vesta. The deficiency of smaller craters is evidence that some degree of antipodal deformation from the Rheasilvia impact did occur. This is because the powerful converging shockwaves from the Rheasilvia impact around the antipodal point would have erased small craters more effectively than larger ones.

The very presence of antipodal deformations from the Rheasilvia impact indicates that Vesta must have relatively low mantle porosity and a relatively strong core. This study by Bowling et al. (2013) show that the features observed at the antipode of the Rheasilvia impact can serve as a crude method of constraining the internal structural properties of Vesta. Finally, this method may also be used to constrain the internal properties of some other objects in the Solar System that have craters large enough to have perhaps produced deformation features at their antipodes. An example of one such object is Saturn’s icy moon Mimas with its relatively large crater named Herschel.

Figure 5: An image of Saturn’s moon Mimas taken by the Cassini spacecraft on 13 February 2010. The large crater on the left is Herschel. In the background is the enormous bulk of the planet Saturn. With a diameter of 396 km, Mimas is thought to be about the smallest an object can be and still crunch itself into a near-spherical shape. Credit: NASA/JPL/Space Science Institute.

Reference:

T. J. Bowling et al., “Antipodal terrains created by the Rheasilvia basin forming impact on asteroid 4 Vesta”, Journal of Geophysical Research: Planets, Volume 118, Issue 9, pages 1821-1834, September 2013

Heat Redistribution on WASP-18b

WASP-18b is a massive extrasolar planet with a mass equal to 10 Jupiter masses and it is in a close-in 0.94-day orbit around an F6V parent star - a star that is somewhat hotter and larger than the Sun. The planet is believed to be tidally-locked with the same side permanently facing its parent star. As a result, temperatures on the day-side of WASP-18b can get as high as ~3000 K. Like Jupiter, WASP-18b is a gas giant planet comprised primarily of hydrogen and helium.

Figure 1: Artist’s impression of HAT-P-7b. Like WASP-18b, HAT-P-7b is gas giant planet in a close-in orbit around its parent star. Credit: NASA, ESA, and G. Bacon (STScI).

Observations of WASP-18b together with atmospheric models show nearly no day-side to night-side redistribution of heat. Any winds transporting heat away from the planet’s day-side is expected to be very weak. In an atmospheric model of WASP-18b that is consistent with almost no heat redistribution, the planet’s day-to-night temperature difference is around 2000 K to 3000 K (Figure 2). In another atmospheric model of WASP-18b, this time with 6.5 km/s superrotating winds racing around the planet from day-side to night-side, the day-to-night temperature difference drops to ~800 K (Figure 3). The effect of the superrotating winds is more efficient heat redistribution, resulting in a lower day-to-night temperature contrast.

Figure 2: Modelled atmospheric thermal profiles for selected longitudes for the case with almost no redistribution of heat. (N. Iro, P.F.L. Maxted, 2013)

Figure 3: Modelled atmospheric thermal profiles for selected longitudes for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. (N. Iro, P.F.L. Maxted, 2013)

Using the Spitzer space telescope, observations of the secondary eclipses of WASP-18b (i.e. when the planet passes behind its parent star and is blocked) show a relatively high planet-to-star flux ratio that is consistent with a very hot day-side, indicating nearly no redistribution of heat (Figure 4). This is because, with efficient heat redistribution, like in the atmospheric model with superrotating winds, the planet’s day-side would not be as hot, resulting in a lower planet-to-star flux ratio (Figure 5).

Figure 4: Modelled planet-to-star flux ratio as a function of wavelength for the case with almost no redistribution of heat. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

Figure 5: Modelled planet-to-star flux ratio as a function of wavelength for the case with 6.5 km/s superrotating winds redistributing heat from day-side to night-side. The red circles with error bars represent actual data points from observations by the Spitzer space telescope. (N. Iro, P.F.L. Maxted, 2013)

References:

- N. Iro, P.F.L. Maxted, “On the heat redistribution of the hot transiting exoplanet WASP-18b”, Icarus 226 (2013) 1719-1723

- Nymeyer et al. (2011), “Spitzer Secondary Eclipses of WASP-18b”, arXiv:1005.1017 [astro-ph.EP]

Giant Planet in the Habitable Zone

Gravitational microlensing has led to the detection of planets with masses ranging from more than Jupiter to a few times the mass of Earth. It involves measuring the magnification of light from a distant background star due to the lensing effect by the gravitational field of a foreground star. During a microlensing event, a lightcurve of the background star is produced as the foreground star crosses in front of it. The presence of planets around the foreground star can produce sharp deviations in the otherwise smooth and symmetric lightcurve of the background star.

Figure 1: Artist’s impression of a habitable Earth-like moon around a gas giant planet.

Figure 2: An illustration of a microlensing event of a distant background star by a foreground star with and without a planet.

Using the Keck telescope near the summit of Mauna Kea in Hawaii, a team of astronomers observed a microlensing event and reported the discovery of a gas giant planet that is probably within the habitable zone of its parent star. This planet is identified as MOA-2011-BLG-293Lb. It has a mass of 4.8 ± 0.3 Jupiter mass and orbits its parent star at a distance of 1.1 ± 0.1 AU, where one AU is the average Earth-Sun separation distance. The planet’s parent star is a Sun-like star with a mass of 0.86 ± 0.06 solar mass. MOA-2011-BLG-293Lb and its parent star are located at an estimated distance of 25,000 light years away, near the centre of the Milky Way galaxy.

MOA-2011-BLG-293Lb is interesting because a hypothetical terrestrial-size moon in orbit around it can support Earth-like conditions and may be potentially habitable. Since MOA-2011-BLG-293Lb circles its parent star near the outer edge of the habitable zone, a terrestrial-size moon around this giant planet would require some sort of greenhouse warming effect to keep its surface warm enough to sustain liquid water. MOA-2011-BLG-293Lb is also one of the furthest planets discovered to date.

Reference:

V. Batista et al. (14 October 2013), “MOA-2011-BLG-293Lb: First Microlensing Planet possibly in the Habitable Zone”, arXiv:1310.3706 [astro-ph.EP]

Io’s Global Magma Ocean

Figure 1: Io and Jupiter. Io has a diameter of 3,642 km, making it slightly larger than Earth’s Moon.

Io, a moon of Jupiter, is the most volcanically active object in the Solar System. This is due to the large amount of tidal heating being generated within the moon’s interior as it is pulled between Jupiter and the other Galilean satellites - Europa, Ganymede and Callisto. The extensive volcanism and high temperature lavas on Io suggest the presence of a global layer of magma beneath its crust.

The existence of such a subsurface global ocean of magma was determined by using Jupiter’s powerful magnetic field as a probe. At Io, induction caused by Jupiter’s magnetic field can be used to infer the conductivities and hence the properties of its subsurface layers. This is because the conductivity of rock material depends on its temperature and melt state. For instance, in comparison to solid rocks, fully or partially molten rocks have dramatically higher conductivities.

Figure 2: This cross-sectional visualization shows the internal structure of Jupiter’s moon Io as revealed by data from NASA’s Galileo spacecraft. A global magma ocean that is believed to be more than 50 km thick (shown in red-brown) underlies a low-density crust about 30 to 50 km thick (shown in gray). Io’s core, measuring about 1200 to 1800 km in diameter, is composed of iron and iron sulphide (shown in a metallic silver hue). Credit: NASA/JPL/University of Michigan/UCLA.

Data from magnetic observations carried out by the Galileo spacecraft during its flybys near Io in October 1999 and February 2000 show the presence of electromagnetic induction from a highly conductive global layer beneath the surface of Io. This global conducting layer is consistent with a subsurface magma ocean with a thickness of over 50 km and a rock melt fraction of 20 percent or more. The global magma ocean has an estimated temperature exceeding 1200°C and it exists beneath a low density crust 30 to 50 km thick.

“The hot magma in Io’s ocean is millions of times better at conducting electricity than rocks typically found on the Earth’s surface” said the study’s lead author, Krishan Khurana, a former co-investigator on the Galileo magnetometer team and a research geophysicist with UCLA’s Institute of Geophysics and Planetary Physics. “Just like the waves beamed from an airport metal detector bounce off metallic coins in your pocket, betraying their presence to the detector, Jupiter’s rotating magnetic field continually bounces off the molten rocks in Io’s interior. The bounced signal can be detected by a magnetometer on a passing spacecraft.”

The high conductivity of Io’s global magma ocean shields the interior of the moon from Jupiter’s powerfully magnetic field. As a result, the magnetic field inside Io maintains a vertical orientation despite the varying orientations of the external magnetic field (i.e. Jupiter’s magnetic field). This study was published in 2011 in the journal Science and it explains why Io is the most volcanic object known in the solar system.

Reference:

KK Khurana et al., “Evidence of a Global Magma Ocean in Io’s Interior”, Science 3 June 1011: Vol. 332 pp. 1186-1189

Smashing onto a Magnetar

Neutron stars are very compact objects that form from the gravitational collapse of massive stars. A typical neutron star packs as much mass as half-million Earths within a diameter of only ~20 km. Magnetars are part of a very rare group of neutron stars that have extremely powerful magnetic fields. Occasionally, magnetars exhibit ‘glitches’ that are observed as sudden spin-ups of these compact objects. Glitches are believed to be caused by the sudden transfer of angular momentum from the faster rotating superfluid interior to the slower rotating solid outer crust of a magnetar.

Figure 1: Artist’s impression of a neutron star shown to scale with Manhattan Island. Credit: NASA.

In the May 30 issue of the journal Nature, R. F. Archibald et al. (2013) report the discovery of an ‘anti-glitch’ (i.e. a sudden spin-down) of the magnetar 1E 2259+586. This unexpected sudden spin-down is contrary to the spin-ups caused by glitches. Ordinarily, the magnetar has a spin period of 7 seconds, but the anti-glitch slowed its spin by 2 millionths of a second. In another paper by Huang and Geng (2013), the authors suggest that the sudden spin-down of 1E 2259+586 is caused by the collision of a solid object with the magnetar. The solid object came in from a direction that is opposite to the spin of the magnetar, collided with the magnetar and led to the sudden spin-up.

Observations of 1E 2259+586 reveal a decaying X-ray afterglow that is associated with the anti-glitch. Based on the energy released, the mass of the colliding solid object is estimated to be ~1/5,000,000th the mass of Earth. If the solid object is a dense iron-nickel body, it would have a diameter of 64 km. The impact of the solid object onto the surface of the magnetar is an incredibly violent process. As the solid object approaches, the immense gravity of the magnetar would stretch the object into an elongated shape before breaking it up entirely. Material from the destructed object is then compressed to ultra-high densities before slamming onto the surface of the magnetar. This process releases a huge amount of energy in a very short period of time. In fact, an intense burst of hard X-rays lasting 36 milliseconds was detected by the Fermi Gamma-ray Burst Monitor on 21 Aril 2012, consistant with the timing of the anti-glitch.

Figure 2: Artist’s impression of a magnetar. Credit: ESA.

Observations have already confirmed the existence of planetary systems around neutron stars. As a result, there are a number of ways in which a solid object, like an asteroid, can be placed on a collision trajectory with a neutron star. Firstly, the presence of planets can gravitationally perturb and scatter asteroids towards the central neutron star. Secondly, like for the Solar System, an extended cloud of small objects might also exist around the neutron star and some of them could fall towards the neutron star due to disturbances from nearby stars.

Thirdly, in a system with multiple planets, planets may collide, throwing off chunks of material where some would eventually impact the central neutron star. Lastly, the neutron star could be speeding on an escape trajectory out from its own planetary system due to the large velocity ‘kick’ it received at birth from asymmetric gravitational collapse. As the neutron star speeds through its planetary system, it can capture and ram into small objects that happen to lie in its path, resulting in the anti-glitch and X-ray burst observed for the magnetar 1E 2259+586.

References:

- R. F. Archibald et al., “An anti-glitch in a magnetar”, Nature 497, 591-593 (30 May 2013)

- Y. F. Huang and J. J. Geng (12 October 2013), “Anti-glitch induced by collision of a solid body with the magnetar 1E 2259+586”, arXiv:1310.3324 [astro-ph.HE]

Habitability vs. Colonizability

“Do there exist many worlds, or is there but a single world? This is one of the most noble and exalted questions in the study of Nature.”

- St. Albertus Magnus (13th century)

In the article “A Tale of Two Worlds” by novelist Karl Schroeder, the author states that in the detection and characterization of planets around other stars, habitability and colonizability are not the same thing. NASA’s Kepler space telescope has shown that Earth-size planets that are neither too hot nor too cold to support life are surprisingly common. These potentially habitable planets may at first seem to be where humans and their machines could one day settle. However, Schroeder mentions that the current definition of whether a planet is habitable has nearly nothing to do with its colonizability.

Take the exoplanets Kepler-62e and Kepler-78b as examples. Kepler-62e is a super-Earth in orbit around a star that is somewhat cooler than the Sun. It has 1.61 times the Earth’s diameter and is located at a comfortable distance from its parent star such that temperatures are just right to support life. Kepler-62e possesses the right properties for it to be a potentially Earth-like habitable planet. In contrast, Kepler-78b, formerly known as KIC 8435766 b, is an Earth-size planet in an extremely close-in 8.5-hour orbit around a Sun-like star. This planet is expected to be tidally-locked with one side permanently facing it parent star and experiencing hellish temperatures of 2300 K to 3100 K. Being so close to its parent star, any breathable atmosphere or liquid water is unlikely to be present on Kepler-78b. Nevertheless, the permanent night side of Kepler-78b is believed to be much cooler and temperatures there may even dip below freezing in the absence of any appreciable atmosphere to transport heat over from the scorching dayside.

Figure 1: Four potentially habitable exoplanets shown to scale alongside the Earth. Left to right: Kepler-22b, Kepler-69c, Kepler-62e, Kepler-62f, and Earth (except for Earth, these are all artists’ renditions). Credit: NASA Ames/JPL-Caltech.

 Figure 2: Artist’s depiction of Kepler-62e, a super-Earth in the habitable zone of a star that is smaller and cooler than the Sun. Credit: NASA Ames/JPL-Caltech.

Figure 3: Artist’s depiction of Kepler-20e - a planet with a smaller radius than Earth in a close-in orbit around a Sun-like star. Kepler-20e is believed to be tidally-locked with the same hemisphere always facing its parent star. The planet’s dayside temperature is estimated to be over 1000 K while its night side is much cooler. Kepler-78b is quite similar to Kepler-20e, just that it has a much hotter dayside. Credit: NASA/Ames/JPL-Caltech.

Kepler-62e is a potentially habitable planet while Kepler-78b is most certainly not. However, this may not imply that Kepler-62e is more colonisable than Kepler-78b. In fact, Kepler-78b may be more promising when it comes to colonizability. Assuming that Kepler-62e has the same density as Earth, its surface gravity will be 1.6 times of Earth’s. The stronger gravity will place an unavoidable permanent strain on humans and their machines. Even if the stronger gravity may be bearable, getting stuff off the surface of Kepler-62e into space will require exponentially more energy compared to Earth.

A study by L. Kaltenegger et al. (2013) suggests that planets in the size range of Kepler-62e are likely to be completely covered by ocean with no land in sight. The absence of land may yet again lower its potential for colonization even though the planet’s ocean may be a perfect environment for its local life. Actually, if life exists on a planet, it may immediately deem the planet unsuitable for colonization, regardless of the planet’s physical properties. This is because life on another world is likely to operate on a different biochemistry that is incompatible and possibly hostile to Earthly life. Furthermore, colonization also raises the problem of contaminating a pristine alien biosphere. Based on these considerations, an Earth-like habitable planet that is teeming with life (i.e. an Earth analogue) is almost certainly unsuitable for colonization.

Figure 4: Artist’s impression of an Earth-like planet.

Figure 5: Artist’s impression of a potentially habitable planet.

Compared to Kepler-62e, the planet Kepler-78b may appear inhospitable due to its superheated dayside. However, Kepler-78b is tidally-locked and the other half of the planet never faces its parent star. One can imagine conditions there being somewhat like within the cold permanently shaded craters at Mercury’s poles, but encompassing half a planet. An airtight habitat containing a breathable atmosphere could easily find its place on the cool night side of Kepler-78b. On a side note, the sight of its parent star from the stupendously hot dayside of Kepler-78b would certainly be terrifyingly spectacular. The huge temperature difference between the dayside and night side of Kepler-78b provides an enormous potential to move heat around, thereby generating power. Additionally, Kepler-78b is approximately the same size as Earth and this makes getting stuff off the planet’s surface into space no more difficult than it is for Earth, unless Kepler-78b is unusually dense.

Although Kepler-62e is undoubtedly well suited to support life as a habitable planet, the seemingly inhospitable Kepler-78b looks more promising with regard to its colonizability. In short, besides habitability, colonizability should also be used to judge the value of planets around other stars. Nevertheless, Kepler-62e and Kepler-78b are mere examples to distinguish between habitability and colonizability. Both planets are in no way prime interstellar destinations since they are located several hundred light years away. From here to there, there are innumerable stars with planets just like Kepler-62e and Kepler-78b.

With regard to habitability, the ‘habitable zone’ is generally defined as a region around a star where temperatures are neither too hot nor to cold for a planet to have liquid water on its surface and thus capable of supporting life. On the contrary, a ‘colonizable zone’ does not have the same limitations as a ‘habitable zone’ since it depends on a planet by planet basis and may not be required to be around a star at all. A study by Strigari et al. (2012) show that for ever star in the galaxy, there may be as many as ~10,000 unbound objects with masses ranging from Pluto to somewhat larger than Jupiter. These objects are sometimes termed free-floating planets or rogue planets. Such worlds may serve as colonizable “pit stops” in the vast distances between stars.

Figure 6: Artist’s impression of a Pluto-like object and its large moon, orbiting far from its parent star.

In the solar system, objects including Mercury, Earth’s Moon and Pluto may turn out to be excellent places for colonization in a novel method proposed by K. L. Roy et al. (2009). The authors propose creating habitable environments for humans by enclosing airless and otherwise sterile planets, moons, large asteroids, and even free-floating planets within engineered shells. Within such a shell, an environment could be created that is similar in almost all respects to that of Earth except for gravity. These “shell worlds” could be constructed just about anywhere with a suitable planet, moon or large asteroid. It allows humans and their machines to colonize any star system without interfering with or contaminating a planet that has already developed life (i.e. a habitable planet).

References:

-  W. J. Borucki et al. (2013), “Kepler-62: A Five-Planet System with Planets of 1.4 and 1.6 Earth Radii in the Habitable Zone”, arXiv:1304.7387 [astro-ph.EP]

- Sanchis-Ojeda et al. (2013), “Transits and Occultations of an Earth-Sized Planet in an 8.5-Hour Orbit”, arXiv:1305.4180 [astro-ph.EP]

- L. Kaltenegger et al. (2013), “Water-Planets in the Habitable Zone: Atmospheric Chemistry, Observable Features, and the case of Kepler-62e and -62f”, arXiv:1304.5058 [astro-ph.EP]

- Strigari et al. (2012), “Nomads of the Galaxy”, arXiv:1201.2687 [astro-ph.GA]

- K. L. Roy et al. (2009), “Shell Worlds: An Approach to Terraforming Moons, Small Planets, and Plutoids”, JBIS Vol. 62, pp. 32-38

Nucleosynthesis of Gold in Neutron Star Collisions

Gold is rare on Earth and it is also rare in the Universe. Unlike elements like carbon, silicon or iron, gold cannot be created within the core of a star. Instead, the creation of gold requires a more energetically cataclysmic event. Short-duration gamma-ray bursts (SGRBs) are intense flashes of gamma-rays lasting less than ~2 seconds. They are believed to be produced following the merger of compact object binaries involving two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH).

A compact object binary forms when both massive stars in a binary system separately explode as supernovae and leave behind their collapsed cores as a tightly bound NS-NS, NS-BH or BH-BH pair. As both compact objects circle each other, they radiate away gravitational waves and draw closer to each other until they eventually collide. Nevertheless, only collisions involving NS-NS and NS-BH pairs can produce SGRBs. “It’s a very fast, catastrophic, extremely energetic type of explosion,” says Edo Berger, an astronomer at the Harvard-Smithsonian Center for Astrophysics (CfA).

NS-NS and NS-BH mergers are expected to create significant quantities of neutron-rich radioactive nuclei via the r-process, also known as the rapid neutron capture process, from the ejection of neutron-rich material. These radioactive nuclei will decay and produce a faint transient, known as a “kilonova”, in the days following the SGRB. It is believed that NS-NS and NS-BH mergers may be the dominant source for stable r-process elements in the Universe. All r-process elements are heavier than iron, a list that includes gold, mercury, platinum, uranium, thorium and more.

Berger et al. (2013) present the first detection of a kilonova following a SGRB. The SGRB is identified as GRB 130603B and it was initially detected by NASA’s Swift satellite on 3 June 2013 at 15:49:14 UTC. Although the burst event itself lasted for less than two-tenths of a second, GRB 130603B displayed a slowly fading afterglow dominated by infrared light. Over the next few days, telescopes in Chile and the Hubble Space Telescope (HST) performed optical and near-infrared observations of the afterglow.

A kilonova model with estimated ejecta mass ~10,000 to 30,000 Earth masses travelling at ~10 to 30 percent the speed of light is consistant with the observed properties of the afterglow from GRB 130603B. Assuming 10 parts per million of the ejecta mass is in the form of gold, that works out to ~10 times the mass of the Moon in gold alone. In comparison, the total amount of gold that have been mined in human history is roughly equivalent in terms of volume to a cube 21 metres on a side.

GRB 130603B is the first SGRB with evidence for r-process rich ejecta and it provides a clear signature for a compact object merger event involving either a NS-NS collision or a NS-BH collision. Based on the ejecta mass estimated for GRB 130603B and on the known frequency of SGRBs, compact object mergers are likely to be the dominant site for the nucleosynthesis of stable r-process elements in the Universe. “It’s possible that supernovae still produce a small contribution, but they do not appear to be the dominant process,” says Berger. After being created and ejected outward, these heavy elements eventually become incorporated into the formation of subsequent generations of stars and planets elsewhere in the galaxy. “To paraphrase Carl Sagan, we are all star stuff, and our jewellery is colliding-star stuff,” says Berger.

Reference:

Berger et al. (2013), “An r-Process Kilonova Associated with the Short-Hard GRB 130603B”, arXiv:1306.3960 [astro-ph.HE]

A System of Low Density Worlds

Using data collected by NASA’s Kepler space telescope, four transiting planetary candidates were found around the star KOI-152. The four planet candidates, identified as KOI-152 b, c, d and e, range in size from 3.5 to 7 times the size of Earth. They circle KOI-152 with orbital periods of 13.5, 27.4, 52.1 and 81.1 days - near a 1:2:4:6 chain of commensurability. All four planet candidates orbit their parent star in a region that is tighter than Mercury’s orbit around the Sun. The planet candidates gravitationally perturb one another and cause the transit timing of each planet candidate to exhibit variations. By analysing the transit timing variations, the masses of all four planet candidates were found to be rather small for their sizes. This means that all four planet candidates around KOI-152 have low densities (Figure 3).

Figure 1: Artist’s impression of a low density planet with a substantial gaseous envelope.

Figure 2: Artist’s impression of a low density planet with a substantial gaseous envelope.

Figure 3: The planet candidates of KOI-152, with masses and radii in Earth units. The final column shows the amount of stellar flux each planet candidate receives, where a value of 1 represents the amount of flux Earth receives from the Sun.

The planet candidates around KOI-152 have masses ranging from 4.1 to 10.9 times the mass of Earth. Planets with such masses have no known analogues in the Solar System because the Solar System has no object intermediate in mass between Earth and Uranus/Neptune, both of which are more than 14 times the mass of Earth. Of the four planet candidates, KOI-152 b has the highest bulk density even it is similar in size to KOI-152 c and e. This suggests that KOI-152 b is comprised of either a substantial mass fraction of water and/or a relatively thin hydrogen-helium envelope, with a denser rocky interior. The less dense KOI-152 c and e are likely to have more voluminous envelopes of water and/or hydrogen-helium.

Finally, the largest of the four planet candidates, KOI-152 d, has a remarkably low density of just 9 percent the density of liquid water. KOI-152 d is expected to have a very voluminous hydrogen-helium envelope comprising more than 10 percent, but less than 50 percent of its mass. Compared to dense rocky planets such as Earth with 5.5 times the density of liquid water and Kepler-10b with 8.8 times the density of liquid water, the low density planet candidates around KOI-152, especially the extremely low density KOI-152 d, show that there is a tremendous compositional variation amongst planets with ~1 to 10 times Earth’s mass.

Reference:

Daniel Jontof-Hutter, Jack J. Lissauer, Jason F. Rowe1 and Daniel C. Fabrycky (2013), “KOI-152’s Low Density Planets”, arXiv:1310.2642 [astro-ph.EP]

A Trio of Inflated Hot Jupiters

Figure 1: Artist’s impression of a hot Jupiter.

As part of the SuperWASP (Wide Angle Search for Planets) program, an international team of researches have reported the discovery of three highly irradiated and bloated hot Jupiters. Like Jupiter, these planets are gas giants and are comprised almost entirely of hydrogen and helium. However, unlike Jupiter, all three planets circle in very tight orbits around stars that are hotter and larger than the Sun - F5 to F7-type stars with effective surface temperatures of 6,250 K to 6,500 K. In comparison, the Sun is a cooler G2-type star with an effective surface temperature of 5,780 K. The three planets, identified as WASP-76 b, WASP-82 b and WASP-90 b, take just 1.81, 2.71 and 3.92 days respectively to circle their parent stars.

All three planets are bloated, with diameters of 1.6 to 1.8 times the diameter of Jupiter, and their intensely irradiated day-sides are scorched to temperatures of ~2,000 K. WASP-76 b, WASP-82 b and WASP-90 b belong to a class of inflated hot Jupiters. Planets of this nature tend to orbit in very close proximity to stars that are somewhat hotter than the Sun. In fact, planets with as much as ~2 times the diameter of Jupiter have been found. An example is the hyper-inflated WASP-17 b. It appears that stellar irradiation plays a key role in determining where a hot Jupiter is inflated, because all known inflated hot Jupiters receive more than 150 times the amount of stellar irradiation Earth gets from the Sun (or 4,000 times the stellar irradiation Jupiter gets from the Sun). There is an extensive literature regarding the mechanisms for inflating hot Jupiters. Such mechanisms include tidal heating and Ohmic dissipation.

Figure 2: Parameters for WASP-76 b, WASP-82 b and WASP-90 b.

 Figure 3: A number of planets with measured mass, radius and stellar incident flux. The black points are planets with less than 150 times the mass of Earth and the red points are planets with more than 150 times the mass of Earth (i.e. giant planets). Giant planets with inflated radii are shown at the top-right corner of the plot. (Weiss et al., 2013)

References:

- West et al. (2013), “Three irradiated and bloated hot Jupiters: WASP-76b, WASP-82b & WASP-90b”, arXiv:1310.5607 [astro-ph.EP]

- Weiss et al., “The Mass of KOI-94d and a Relation for Planet Radius, Mass, and Incident Flux”, 2013 ApJ 768: 14

Pan and Daphnis

Figure 1: An image of Saturn and its rings taken by the Cassini spacecraft on 15 April 2008. Credit: NASA/JPL/Space Science Institute.

 Figure 2: A mosaic of a portion of Saturn’s rings, showing the A Ring, and the positions of the Encke Gap and Keeler Gap. Credit: NASA/JPL/Space Science Institute.

Pan and Daphnis are two inner moons of Saturn that orbit within gaps in Saturn’s A Ring. Pan orbits within the 325 km wide Encke Gap and Daphnis orbits within the 42 km wide Keeler Gap. Pan is a small walnut-shaped moon measuring 34.4 by 31.4 by 20.8 km in size. Daphnis is smaller than Pan and it measures 8.6 by 8.2 by 6.4 km in size. Pan was discovered in 1990 from images taken by the Voyager 2 spacecraft and Daphnis was discovered in 2005 from images taken by the Cassini spacecraft.

Pan’s walnut-shape is due to the presence of an equatorial ridge that was formed when the moon swept up ring material from the Encke Gap. Daphnis probably has an equatorial ridge as well. Pan takes 13.8 hours to circle Saturn while Daphnis takes 14.3 hours. The orbits of both moons have slight inclinations which cause then to move above and below Saturn’s ring plane. Pan speeds around Saturn at 19.9 km/s and from its surface, Saturn would span a whopping 53.6°.

Figure 3: Saturn’s moon Pan casting a slender shadow onto the A Ring. This image was taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

 Figure 4: The gravitational pull from Saturn’s moon Daphnis perturbs the orbits of the ring particles along the Keeler Gap and sculpts them into vertical structures. These structures cast long shadows across the A Ring in this image taken by the Cassini spacecraft as Saturn was approaching its August 2009 equinox. Credit: NASA/JPL/Space Science Institute.

References:

- Thomas, P. C. (July 2010), “Sizes, shapes, and derived properties of the saturnian satellites after the Cassini nominal mission”, Icarus 208 (1): 395-401

- Porco, C.C.; Thomas, P.C.; Weiss, J.W.; Richardson, D.C. (2007), “Saturn’s Small Inner Satellites: Clues to Their Origins”, Science 318: 1602-1607

A Very Rare Type of Wolf-Rayet Star

Massive stars with more than 20 times the Sun’s mass are exceedingly rare. These stars are estimated to be as rare as ~1 in 1,000,000, possibly rarer. Nature does not like to make massive stars. Those that are formed, burn fast, shine bright and have brief lives of just a few million years. Despite their rarity, massive stars come in a very diverse range of stellar types. Some of these stellar types are so uncommon that they are represented by only a few known individual stars in a galaxy containing many billions of stars.

The Large Magellanic Cloud (LMC) is an irregular galaxy and also a satellite galaxy of the Milky Way. It is estimated to contain about 10 billion times the mass of the Sun. A region of the LMC known as Lucke-Hodge 41 (LH41, also known as NGC 1910) has a rich population of massive stars. This region is home to a high concentration of very rare stars including a few luminous blue variables (LBV), a yellow supergiant, a few Wolf-Rayet (WR) stars, as well as a number of other stellar oddities.  These stars are all massive stars at various stages of evolution.

WR stars represent advanced evolutionary stages in the evolution of massive stars. These stars shed mass rapidly by means of a very powerful stellar wind. WR stars are extremely hot and can have surface temperatures as high as ~200,000 K. They are also extremely luminous, ranging from tens of thousands to a few million times the luminosity of the Sun. Due to the extreme mass loss, WR stars are basically stripped-down versions of highly evolved massive stars. These stars have lost the bulk of their outer envelopes to reveal the products of nucleosynthesis in their interiors.

All WR stars show helium emission lines in their spectra. Depending on their evolutionary stage, the spectra of WR stars are dominated by emission lines of nitrogen, carbon or oxygen. As such, WR stars are classified accordingly as WN, WC or WO types. WO stars are believed to be WR stars that have evolved past the WC stage. They represent the most advanced and short-lived evolutionary stage in the life of a massive star before it explodes as a supernova.

While surveying the population of massive stars in LH41, a team of astronomers discovered a new type of WR star with strong enough emission lines from high ionization stages of oxygen and carbon to be classified as a WO star. This newly discovered star is identified as LH41-1042 and further classified as a WO4 star. LH41-1042 is the second known WO star in the LMC and its first known WO4 star, the other being a WO3 star.

Although most of the WR stars in the LMC have already been discovered, a number of WR stars might still await discovery in the most crowded regions of the LMC. Since so few WO stars are currently known, the discovery of another would provide a good opportunity to better understand these short-lived and thus exceedingly rare stars.

Reference:

Kathryn F. Neugent et al. (2012), “The Discovery of a Rare WO-type Wolf-Rayet Star in the Large Magellanic Cloud”, The Astronomical Journal 144: 162 (4pp)

Patchy Clouds on an Exotic World

Figure 1: Artist’s impression of Kepler-7b (left), a gas giant planet 1.6 times the radius of Jupiter (right). Kepler-7b is the first exoplanet to have its clouds mapped. The cloud map was produced using data from NASA’s Kepler and Spitzer space telescopes. Credit: NASA/JPL-Caltech/MIT.

Using data from NASA’s Kepler and Spitzer space telescopes, a team of astronomers have created a cloud map of a scorchingly hot gas giant planet known as Kepler-7b. This planet has a mass of 0.44 ± 0.04 Jupiter mass and a size of 1.61 ± 0.02 Jupiter radii. The low mass and large size gives Kepler-7b an exceptionally low density of just 14 percent the density of liquid water. Kepler-7b circles its host star in a tight 4.89-day orbit. By detecting infrared light from Kepler-7b, Spitzer was able to measure the planet’s temperature, estimating it to be between 1,100 K and 1,300 K. This is somewhat too cool for a planet that orbits so close to its host star. However, this can be explained by the high reflectivity observed for Kepler-7b, where the planet reflects a larger faction of light coming from its host star and so does not heat up as much. Such a high reflectivity is believed to be due to the presence of reflective high altitude clouds in the planet’s atmosphere.

In fact, Kepler-7b’s measure temperature places it within an exceptionally rich region of condensation phase space where silicate clouds can potentially form in the upper, observable portion of the planet’s atmosphere. The same would not be true for a warmer planet (temperatures on the dayside would be too hot for silicate clouds to condense) or a cooler planet (silicate clouds would only be present in the deep unobservable layers of the atmosphere). Hence, Kepler-7b is neither too hot nor too cold for silicate clouds to form in its observable atmosphere.

Observations of Kepler-7b’s dayside by Kepler and Spitzer reveal the presence of reflective high altitude clouds located west of the planet’s substellar point. “By observing this planet with Spitzer and Kepler [telescopes] for more than three years, we were able to produce a very low-resolution ‘map’ of this giant, gaseous planet,” study co-author Brice-Olivier Demory of the Massachusetts Institute of Technology in Cambridge said in a statement. “We wouldn't expect to see oceans or continents on this type of world, but we detected a clear, reflective signature that we interpreted as clouds,” he said.

Figure 2: Models of the dayside temperature structure of Kepler-7b. Both a cloud-free model (orange) and cloudy model (blue) are shown. (Demory et al., 2013)

Figure 3: Models of the dayside planet/star flux ratio for Kepler-7b. Compared to the cloudy model (blue), the cloud-free model (orange) is fainter in the optical but brighter in the mid-infrared. The cloudy model is brighter in the optical due to the scattering of light by clouds. Dashed curves represent the thermal emission component (i.e. heat from the planet) and solid curves represent the total flux. The optical detection in the Kepler band (red) is shown, along with the Spitzer 1-σ (cyan) and 3-σ (red) upper limits. (Demory et al., 2013)

The tell-tale sign for clouds first came from a westward shift seen in the Kepler visible light curve of Kepler-7b. This corresponds to a bright region on Kepler-7b that is centred 41 ± 12° west of the substellar point. Previously, this bright region was thought to be a more intensely heated part on the planet. However, observations by Spitzer show a lack of thermal emission from Kepler-7b and this suggests that the bright region is largely caused by reflected light rather than heat. The reflected light is believed to be light from the planet’s host star being scattered back into space by reflective high altitude clouds, with the most likely candidate being silicate clouds. East of the substellar point, the skies appear to be relatively cloud-free. Being so close to its host star, Kepler-7b is tidally-locked where the same side of the planet always faces its host star, resulting in a permanent dayside and a permanent night side. As a result, the cloud patterns on Kepler-7b are not expected to change much over time, unlike those on Earth.

Reference:

Demory et al. (2013), “Inference of Inhomogeneous Clouds in an Exoplanet Atmosphere”, arXiv:1309.7894 [astro-ph.EP]

An Unequal Pair of ‘Identical Twin’ Stars

Figure 1: Artist’s impression of an eclipsing binary system.

In 2008, a team of astronomers announced the discovery of a pair of newborn stellar twins residing in the Orion Nebula. The stellar twins exist as a binary star system, identified as Par 1802. Both stars formed at around the same time from the same natal material, and each star in the binary has a mass of 0.41 ± 0.01 solar masses, identical to within 2 percent. As such, they are expected to possess identical physical attributes and are virtually ‘identical twins’. However, in the study, the team reported that these twin stars have surface temperatures differing b ~300 K and luminosities differing by ~50 percent. Furthermore, the sizes of both stars differ by 5 to 10 percent.

Physical parameters of Par 1802:
	Primary Component			Secondary Component
Mass (Sun = 1)	0.414 ± 0.015			0.406 ± 0.014
Surface Temperature (K)	3,945 ± 15			3,655 ± 15
Luminosity (Sun = 1)	0.72 ± 0.11			0.46 ± 0.12
Radius (Sun = 1)	1.82 ± 0.05			1.69 ± 0.05

Figure 2: Light curve of Par 1802 - an eclipsing binary system with a period of 4.67 days. The ratio of eclipse depths provides a direct measure of the ratio of surface temperatures, with the deeper eclipse corresponding to the eclipse of the hotter component (primary) by the cooler component (secondary). (K.G. Stassun et al., 2008)

Par 1802 is a very young equal-mass eclipsing binary system and its estimated age is ~1 million years. In an eclipsing binary system, both stars periodically eclipse each other as they circle around their common centre of mass. The variation in combined brightness when one component eclipses the other can reveal a lot about Par 1802. Here, the equal-mass stars of Par 1802 clearly show unequal surface temperatures, luminosities and sizes. This can be explained by stellar evolution models for young stars with ~0.4 times the Sun’s mass. The models predict that such stars undergo a brief period of rapid evolution at an age of ~1 million years. As a result, the warmer, more luminous and larger star (primary component) can be interpreted as being slightly younger than its companion. An age gap of only a few hundred thousand years is sufficient.

Figure 3: Comparison of the observed physical properties of Par 1802 with theoretical predictions. The measured properties of the primary and secondary components of Par 1802 are shown as green and red symbols, respectively. (K.G. Stassun et al., 2008)

Considering that stars with ~0.4 times the Sun’s mass evolve most rapidly during the first few million years after formation and that such stars can live for many billions of years, an age difference of only a few hundred thousand years in an equal-mass binary system is observationally detectable only during the first few million years of its evolution. At later times, the physical signs of unequal ages become less observable. Par 1802 is an example of how birth order in ‘identical twin’ stars, with a lag of only a few hundred thousand years, can manifest itself as observable physical differences between the two stars - at least when they are very young.

Reference:

K.G. Stassun et al., “Surprising dissimilarities in a newly formed pair of ‘identical twin’ stars”, Nature, 450, 1979-1082 (19 June 2008)