Low Density Planets of Kepler-51

Kepler-51 is a fairly young star with an estimated age of ~300 million years and it is also slightly more luminous than the Sun. Observations of Kepler-51 by NASA’s Kepler space telescope found that it hosts three transiting planet candidates - Kepler-51 b, Kepler-51 c and KOI-620.02. The three planets have orbital periods of 45.2 days (Kepler-51 b), 85.3 days (Kepler-51 c) and 130.2 days (KOI-620.02), placing them close to a 1:2:3 resonance. By measuring the amount of light each planet blocks as it transits its host star, the size of each planet is found to be 7.1 (Kepler-51 b), 9.0 (Kepler-51 c) and 9.7 (KOI-620.02) times the Earth’s diameter.

Figure 1: Artist’s impression of a gaseous planet.

Figure 2: Phase-folded transit light curves of Kepler-51 b (top), Kepler-51 c (middle) and KOI-620.02 (bottom). Black dots are the observed fluxes and coloured solid lines show the best-fit models.

As the three planets circle their host star, they gravitationally perturb one another. This leads to transit timing variations (TTVs) where each planet transits the host star at slightly earlier or later timings, deviating somewhat from strictly periodic transit intervals. By studying the TTVs, Masuda (2014) derived the mass for each of the three planets to be 2.1 (Kepler-51 b), 4.0 (Kepler-51 c) and 7.6 (KOI-620.02) times the Earth’s mass. With the size and mass of each planet known, all three planets were found to have remarkably low densities of less than 5 percent the density of water, possibly the lowest densities yet determined for exoplanets. In comparison, the Earth has a mean bulk density of 5.52 times the density of water. With this finding, the Kepler-51 system serves as yet another example of a very low-density compact multi-transiting planetary system.

The planets around Kepler-51 have mean densities that are much lower than any of the planets in the solar system. To explain their “puffiness”, each planet probably possesses an extended outer hydrogen-helium envelop surrounding a denser core. Assuming the planetary system has an age of ~300 million years; calculations show that the observed radii of the Kepler-51 planets can be explained if they have about 10 percent (Kepler-51b), 30 percent (Kepler-51c) and 40 percent (KOI-620.02) of their masses in their hydrogen-helium envelopes. All three planets are unlikely to be habitable, at least for the type of life found on Earth, given that the planets have thick gaseous envelopes and equilibrium temperatures that exceed 100°C.

Reference:

Masuda (2014), “Very Low-Density Planets around Kepler-51 Revealed with Transit Timing Variations and an Anomaly Similar to a Planet-Planet Eclipse Event”, arXiv:1401.2885 [astro-ph.EP]

At the Edge of Destruction

M. Gillon et al. (2014) report the discovery of WASP-103 b, an ultra-short-period planet at the edge of tidal disruption. WASP-103 b orbits an F-type star at a distance of just ~2 stellar radii from the star's surface, taking a mere 22.2 hours to complete an orbit. The WASP transit survey is sensitive to detecting ultra-short-period giant planets when these planets happen to cross in front of their host stars. WASP-103 b has 1.49 times the mass and 1.53 times the diameter of Jupiter. This newfound planet joins a small group of gas giants that are known to be at the verge of being tidally disrupted by their host stars. The group include planets such as WASP-12 b and WASP-19 b.

Artist’s impression of a gas giant. Credit: Daniel Mallia.

WASP-103 b is significantly inflated and has a bulk density that is only 55 percent the density of water. The low density of WASP-103 b is not just because of the intense irradiation it receives due to its extreme closeness to its host star. Tidal heating is also expected to contribute significantly to the planet's "bloatedness" since the planet's orbit is only 15 to 20 percent away the Roche Limit. Any closer, the planet is expected to be tidally destructed by the gravity of its host star.

Ultra-short-period gas giants that are right at the edge of being tidally disrupted might experience mass loss and significant tidal distortion. One such planet, WASP-12 b, is known to be surrounded by planetary material that has escaped it. In the case of WASP-103 b, the extreme irradiation it receives, the planet's inflated size and the brightness of its host star makes it favourable for atmospheric characterisation with existing ground-based and space-based telescopes. Observing signs of mass loss and tidal distortion for such extreme planets can shed light on the final stages in the lives of hot-Jupiters.

Reference:

M. Gillon et al. (2014), "WASP-103b: a new planet at the edge of tidal disruption", arXiv:1401.2784 [astro-ph.EP]

Birth of a Brown Dwarf

Brown dwarfs are sub-stellar objects that are not massive enough to fuse hydrogen in their interiors and shine as full-fledged stars. Nevertheless, brown dwarfs are thought to form in the same way as stars do - from collapsing clouds of gas and dust. A study by Lee et al. (2013) of an isolated dense molecular cloud core, L328, shows that it contains three sub-cores. One of which, identified as L328-IRS, is a Very Low Luminosity Object (VeLLO) that is believed to be in the process of collapsing to form a brown dwarf.

Artist’s impression of a young brown dwarf that is in the process of accreting matter. A pair of bipolar jets can be seen stemming from it. Credit: ESO.

Observations of carbon monoxide as a tracer for the motion of matter reveal a bipolar outflow stemming from L328-IRS. By analysing the outflow, the accretion rate of the proto-brown dwarf is found to be an order of magnitude less than the accretion rate for standard star formation, consistant with the formation of a brown dwarf. Based on the accretion rate, L328-IRS is expected to grow to no more than ~0.05 solar mass. However, the accretion rate may be uncertain due to several unknown factors of the outflow itself.

Nonetheless, L328-IRS has a small total envelop mass of ~0.09 solar mass and ~100 percent star formation efficiency is also unlikely. As a result, L328-IRS is expected to be a proto-brown dwarf since it is unlikely to accrete more than ~0.08 solar mass, which is the minimum mass necessary to become a full-fledged star. The three sub-cores in L328 are though to have formed concurrently in a gravitational fragmentation process. In one of the sub-cores, global contraction of the gaseous envelop is underway to form the proto-brown dwarf L328-IRS. All these indicate that the formation of L328-IRS is consistant with the idea that brown dwarfs form like normal stars.

Reference:

Chang Won Lee et al., “Early Star-forming Processes in Dense Molecular Cloud L328; Identification of L328-IRS as a Proto-brown Dwarf”, ApJ, 777:50 (15pp), 2013 November 1

CoRoT-27b: A Massive and Dense Planet

Parviainen H. et al. (2014) report the discovery of a massive high-density planet on a close-in 3.58 day orbit around a 4.2 billion year old Sun-like star. The planet is identified as CoRoT-27b. Like Jupiter, CoRoT-27b is a gas-giant planet. Its presence was detected by the CoRoT space telescope as the planet periodically transits its parent star and blocks a small fraction of the star’s light. CoRoT-27b weighs in at 10.39 ± 0.55 Jupiter-masses and has 1.01 ± 0.04 times the radius of Jupiter. This gives CoRoT-27b a mean density of 12.6 times the density of water, which is more than twice the mean density of Earth and almost 10 times the mean density of Jupiter.

Figure 1: Artist’s impression of a gas-giant planet.

Like Jupiter, CoRoT-27b is a gaseous planet comprised primarily of hydrogen and helium. The structure and composition of CoRoT-27b can be inferred from two models. For the first model, the planet is assumed to be made of a central rocky core surrounded by an extensive hydrogen-helium envelop. The 1st model is consistant with a heavy element mass fraction of 0.11, representing a core mass of 366 Earth-masses. For the second model, a central rocky core is absent and the heavy elements are present throughout the hydrogen-helium envelop. The 2nd model is consistant with a heavy element mass fraction of 0.07, representing a heavy element mass of 219 Earth-masses.

CoRoT-27b falls within a sparsely populated overlapping mass regime between the most massive planets and brown dwarfs. Given its high mass, gravity on the “surface” of CoRoT-27b is 27 times the surface gravity on Earth. Technically, CoRoT-27b does not have a surface since it is gaseous through, right down to a central rocky core, if one is present. Being so near to its parent star, the equilibrium temperature on CoRoT-27b is estimated to be 1500 ± 130 K. The discovery of CoRoT-27b is an important addition to a scarcely populated class of massive close-in planets.

Figure 2: Radial velocity curve showing how much CoRoT-27b gravitationally tugs at its parent star. This information allows the planet’s mass to be estimated. Parviainen H. et al. (2014).

Figure 3: Transit light curve showing the amount of dimming of the parent star when CoRoT-27b passes in front of it. This information allows the size of the planet to be measured. Parviainen H. et al. (2014).

Figure 4: CoRoT-27b mass, period and density compared with the population of confirmed transiting exoplanets. Parviainen H. et al. (2014).

Reference:

Parviainen H. et al. (2014), “Transiting exoplanets from the CoRoT space mission XXVII. CoRoT-27b: a massive and dense planet on a short-period orbit”, arXiv:1401.1122 [astro-ph.EP]

A Planet on the Verge of Engulfment

Figure 1: Artist’s impression of a hot-Jupiter transiting its host star. Credit: Mark A. Garlick.

The exoplanet Kepler-91 b orbits around an evolved K3 host star that is in the process of transforming into a red giant. Observations show that Kepler-91 b is a gas-giant planet measuring 0.88 times the mass and 1.38 times the radius of Jupiter. Its host star has 1.3 times the mass and 6.3 times the radius of the Sun. Kepler-91 b circles around its host star in a slightly eccentric, close-in orbit with a period of 6.25 days. Given the planetary mass and radius, the mean density of Kepler-91 b works out to be 0.33 times the density of Jupiter. This low density suggests that Kepler-91 b is somewhat inflated due to the strong stellar irradiation from its host star.

Although the orbit of Kepler-91 b is nowhere near the shortest for exoplanets, the sheer size of its host star means that Kepler-91 b is a mere 1.32 stellar radii from the surface of its host star at closest approach. As the host star continues to expand into a red giant, estimates show that Kepler-91 b is expected to be swallowed in less than 55 million years - a mere blink of the eye on astronomical scales. Even that is considered as an upper limit to the planet’s life. The equilibrium temperature of Kepler-91 b is estimated to be over 2000 K.

Figure 2: Best-fit solutions for the transit of Kepler-91 b in front of its host star. Source: Lillo-Box et al. (2013).

 Figure 3: Diagram illustrating the irradiation of Kepler-91 b by its host star. The red lines represent the boundaries of the stellar irradiation that hits the planet’s surface. The yellow part represents the dayside of the planet. The black part represents the night side and the red part is the extra region illuminated due to the close-in orbit and the large stellar radii of the host star. Source: Lillo-Box et al. (2013).

The close-in orbit of Kepler-91 b and the sheer size of its host star result in more than half of the planet being illuminated by the host star (Figure 3). In fact, around 70 percent of the planet is illuminated by the host star. When Kepler-91 b is at closest approach, its host star would appear to subtend a remarkable 48 degrees, covering around 10 percent of the sky as seen from the planet. In comparison, the Sun covers only 0.0005 percent of the sky as seen from Earth. Kepler-91 b is indeed on the verge of being swallowed by its host star.

Reference:

Lillo-Box et al. (2013), “Kepler-91b: a giant planet at the end of its life”, arXiv:1312.3943 [astro-ph.EP]

Deep Alien Biospheres

Life on Earth not only exists on the surface, but it also includes a subsurface biosphere extending several kilometres in depth. At such depths, the only reasonable source of energy to sustain life comes from the planet's own internal heat. Indeed, a planet that is located far from its host star, resulting in surface temperatures too cold to support life, can potentially harbour a thriving subsurface biosphere that is sustained solely by the planet's own internal heat.

Figure 1: Artist’s impression of a terrestrial planet.

 Figure 2: Artist’s impression of a terrestrial planet. Credit: Kevin Sherman.

A study by S. McMahon et al (2013) show that subsurface liquid water maintained by the internal heat of a planet can support an underground biosphere even if the planet is too far from its host star to support life on the planet's surface. The authors introduce a term known as the “subsurface-habitability zone” (SSHZ) to denote the range of distances from a star where a terrestrial planet (i.e. a rocky planet like the Earth) can sustain a subsurface biosphere at any depth below the surface down to a certain maximum habitable depth. This maximum depth depends on numerous factors, but in general, it is the depth where the enormous pressure starts to make the material too compact for life to infiltrate.

Based on the premises that the global average temperature of a terrestrial planet (1) decreases with increasing distance from its host star and (2) increases with depth beneath the planet's surface, the inner (i.e. closer to the host star) and outer (i.e. further from the host star) boundaries of the SSHZ can be determined. The outer edge of the SSHZ is where temperatures are below the freezing point of water at all depths down to the maximum habitable depth. The inner edge of the SSHZ is where the average surface temperature reaches the boiling point of water.

Figure 3: If the maximum habitable depth for an Earth-analogue planet is 5 km, the outer edge of the SSHZ would be at 3.2 AU. For a maximum habitable depth of 10 km, the outer edge of the SSHZ would be at 12.6 AU. At a maximum habitable depth of 15.4 km, the outer edge of the SSHZ tends towards infinity. Credit: S. McMahon et al (2013).

 Figure 4: The relationship between subsurface habitability and surface albedo (i.e. surface reflectivity of the planet). Two extremes of planetary albedo are shown: a = 0.9 (high reflectivity) and a = 0 (zero reflectivity). Other than surface albedo, the calculations assume a planet with the Earth’s current size, bulk density, heat production per unit mass and emissivity. Credit: S. McMahon et al (2013).

Figure 5: Subsurface habitability for three planetary masses of 0.1, 1.0 and 10 Earth-masses. Other than planet mass, the calculations assume a planet with the Earth’s current bulk density, heat production per unit mass, albedo and emissivity. Credit: S. McMahon et al (2013).

Results from the study show that for a planet with high albedo (high reflectivity), the SSHZ is narrower and closer to the star than for a planet with low albedo (low reflectivity) (Figure 4). Furthermore, planets with larger mass have subsurface biospheres that are thinner, shallower and less sensitive to the heat flux from the host star (Figure 5). This is because a more massive planet is expected to have a steeper geothermal gradient whereby the temperature rises more rapidly with increasing depth as compared to a less massive planet. In fact, a 10 Earth-mass planet can support a ~1.5 km thick subsurface biosphere less than ~6 km below its surface even if the planet is at an arbitrarily large distance from its host star.

The possibilities for subsurface biospheres mean that a planet whose surface is too cold for life can still support a deep biosphere that derives its energy and warmth from the planet's own internal heat. An advantage that life in a subsurface biosphere has is that it is well protected from ionizing stellar and cosmic radiation by the overlying rock layers. Since the SSHZ is vastly greater in extent than the traditional habitable zone, cold planets with subsurface biospheres may turn out to be much more common than planets with surface biospheres. Nevertheless, detecting the biosignature of a subsurface biosphere from remote sensing will be more challenging than for a surface biosphere.

Reference:

McMahon et al., “Circumstellar habitable zones for deep terrestrial biospheres”, Planetary and Space Science 85 (2013) 312-318

Habitability of Large Exomoons

Large exomoons around giant planets in the habitable zone of their host stars could serve as habitats for extraterrestrial life. Such an exomoon would need to have at least twice the mass of Mars or so (i.e. ~0.2 Earth masses) for it to be habitable. For comparison, Ganymede, the largest moon in the Solar System, is roughly 1/40 the mass of Earth. In addition, habitability requires a surface temperature that cannot be too high or too low. This is governed not just by stellar radiation from the host star, but also by stellar light reflected from the giant planet, thermal radiation from the giant planet itself and tidal heating.

Figure 1: Artist’s impression of a giant planet hosting a system of moons. Credit: Kevin Sherman.

Over time, a gaseous giant planet contracts and releases thermal energy as it converts gravitational potential energy into heat. In a paper by Heller & Barnes (2013), the authors investigate how thermal radiation from a shrinking gaseous giant planet could drive a runaway greenhouse effect for an Earth-like exomoon if it is in a close enough orbit around the giant planet. This effect is particularly significant for a young giant planet during the first few hundred million years or so. During this period, the young and hot giant planet is cooling at a more rapid rate, and consequently releases a greater deal of thermal radiation.

To illustrate the combined effects of stellar radiation, thermal radiation from the giant planet and tidal heating, Heller & Barnes (2013) introduced five possible states for an exomoon: (1) Tidal Venus, (2) Tidal-Illumination Venus, (3) Super-Io, (4) Tidal Earth and (5) Earth-like. For these states, a Tidal Venus and a Tidal-Illumination Venus are uninhabitable, while a Super-Io, a Tidal Earth, and an Earth-like moon could be habitable. In the study, a rocky Earth-type exomoon orbiting a giant planet with a mass 13 times that of Jupiter is considered. Besides an Earth-type exomoon, a Super-Ganymede (i.e. a large exomoon with composition similar to Ganymede) is also considered.

At a distance of 1 AU from a Sun-like star, the results from the study show that the combined stellar radiation and thermal radiation on an Earth-type exomoon orbiting at 10 Jupiter-radii around a 13 Jupiter-mass giant planet would keep the Earth-type exomoon above the runaway greenhouse limit and uninhabitable for about 500 million years (Figure 2). For the Super-Ganymede, it would be in a runaway greenhouse state for about 600 million years. In fact, even in the absence of stellar radiation, thermal radiation from the giant planet alone can trigger a runaway greenhouse effect for the first ~200 million years.

Figure 2: The total illumination absorbed by an exomoon (thick black line) is composed of stellar radiation (black dashed line) and thermal radiation from the giant planet (red dashed line). The critical values for an Earth-type exomoon and a Super-Ganymede to enter the runaway greenhouse effect are indicated by dotted lines. Credit: Heller & Barnes (2013).

With the inclusion of tidal heating, the danger for an exomoon to undergo a runaway greenhouse effect increases. Heller & Barnes (2013) illustrate how the distance and orbital eccentricity of an Earth-type exomoon around a 13 Jupiter-mass giant planet determines whether the exomoon is in a Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) or Earth-like (green) state (Figure 3). Here, the giant planet is assumed to have an age of 500 million years. Furthermore, stellar radiation, thermal radiation from the giant planet and tidal heating are all included.

There is a minimum distance around the giant planet in which an Earth-type exomoon would be in a Tidal Venus or Tidal-Illumination Venus state, and hence uninhabitable. This minimum distance is referred to as the “habitable edge”. For a 13 Jupiter-mass giant planet at 1 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 20 and 12 Jupiter-radii respectively. For the same giant planet at 1.738 AU from a Sun-like star, the habitable edges for orbital eccentricities of 0.1 and 0.0001 are 15 and 8 Jupiter-radii respectively. The habitable edge for an older giant planet would be smaller since thermal radiation from a giant planet is expected to decrease over time. As a means of comparison, Io, Europa, Ganymede, and Callisto orbit Jupiter at approximately 6.1, 9.7, 15.5, and 27.2 Jupiter-radii.

Figure 3: The four panels show the possible states for an Earth-type exomoon around a 13 Jupiter-mass host planet that has an age of 500 million years. Distances from the giant planet are shown on a logarithmic scale. In the left two panels, the giant planet orbits at a distance of 1 AU from a Sun-like star. In the right two panels, the giant planet orbits at a distance of 1.738 AU. In the upper two panels, the orbit of the exomoon around the giant planet has an eccentricity of 0.1. In the lower two panels, the eccentricity is 0.0001. Starting from the giant planet in the centre, the white circle visualizes the Roche radius (i.e. within this region, an Earth-type exomoon would be tidally disrupted), and the exomoon types correspond to Tidal Venus (red), Tidal-Illumination Venus (orange), Super-Io (yellow), Tidal Earth (blue) and Earth-like (green) states. Dark green depicts the extent of orbits for Earth-like exomoons in prograde orbits (i.e. orbits in the same direction as the giant planet’s spin) and light green depicts the extent of orbits for Earth-like exomoons in retrograde orbits (i.e. orbits in the opposite direction to the giant planet’s spin). Credit: Heller & Barnes (2013).

Reference:

Heller & Barnes (2013), “Runaway greenhouse effect on exomoons due to irradiation from hot, young giant planets”, arXiv:1311.0292 [astro-ph.EP]

Heat Redistribution on a Strongly Irradiated Brown Dwarf

KELT-1b, a brown dwarf with 27 times the mass of Jupiter, circles around an F-type star in a close-in 1.2-day orbit. The tight orbit places KELT-1b in a highly irradiated environment, where the incident radiation it receives from its parent star is 5,800 times more intense than what Earth gets from the Sun. Although the radiation environment of KELT-1b is similar to that for hot Jupiters, KELT-1b is different due to it large mass which places it in the brown dwarf regime. With several Jupiter masses packed into a volume that is only slightly larger than Jupiter’s, the surface gravity on KELT-1b is a whopping 115 times the surface gravity on Earth. In a way, KELT-1b can be perceived as a “hot Jupiter” with a very high surface gravity.

Artist’s Impression of a hot Jupiter. Credit: NASA.

Observations of KELT-1b using the Spitzer space telescope show that the amount of heat redistribution from its day side to its night side is very low. This is because KELT-1b quickly radiates the energy it receives from its parent star back into space before it is transported to the night side. As a consequence, KELT-1b has a very hot day night and a much cooler night side. The day side is estimated to have temperatures as high as 3,100 K. As a brown dwarf, KELT-1b is unusual due to the huge amount of insolation it receives from its parent star. If KELT-1b were an isolated brown dwarf, it would have a temperature of about 700 K.

The day side of KELT-1b is so hot that it is above the ~2,000 K condensation temperature of titanium oxide (TiO). This can cause a day-night cold trap for TiO since the night side of KELT-1b is cool enough for TiO to condense and settle out of the atmosphere. In fact, the lack of a strong TiO signal indicates that a day-night cold trap may exist in KELT-1b’s atmosphere. Because gaseous TiO is a strong absorber of optical radiation, its presence in an atmosphere can cause a temperature inversion (i.e. temperature increases with altitude). Therefore, the depletion of TiO due to a day-night cold trap inhibits the presence of a temperature inversion.

KELT-1b was discovered using the using the Kilodegree Extremely Little Telescope (KELT) in southern Arizona. KELT is a small telescope optimized for imaging bright stars. The telescope images of tens of thousands of stars every night in an attempt to detect planets that happen to pass in front of the star that they are orbiting. The discovery of KELT-1b was announced in a paper published in June 2012.

Reference:

- Beatty et al. (2013), “Spitzer and z' Secondary Eclipse Observations of the Highly Irradiated Transiting Brown Dwarf KELT-1b”, arXiv:1310.7585 [astro-ph.EP]

- Siverd et al. (2012), “KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star”, arXiv:1206.1635 [astro-ph.EP]

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