Jet Streams on Uranus and Neptune

Eight from the Sun to see in orbit.

But no one mentions Neptune for that odyssey.

What is it about it.

So vast it is...

Sixty Earths could sit within its grasp.

- Lawrence S. Pertillar, “Neptune”, stanza 3 line 1-5.

Figure 1: An illustration of Neptune’s interior.

In the Solar System, the known planets can be classified into three categories - terrestrial planets (Mercury, Venus, Earth and Mars), gas giant planets (Jupiter and Saturn) and ice giant planets (Uranus and Neptune). Compared to the other planets in the Solar System, very little is known about Uranus and Neptune because the only spacecraft to have ever visited them was NASA’s Voyager 2 which flew by Uranus in 1986 and Neptune in 1989. The bulk composition of an ice giant planet is very different from a gas giant planet such as Jupiter or Saturn. An ice giant planet consists of a rocky core, an icy mantle and an outer gaseous hydrogen-helium envelop. The icy mantle comprises the bulk of the planet’s mass and is what defines an ice giant planet. In contrast, a gas giant planet is almost entirely made of hydrogen and helium.

Figure 2: Zonal winds on Uranus and Neptune. The data points correspond to atmospheric wind speed measurements of Uranus and Neptune by the Voyager 2 spacecraft (circles) and the Hubble Space Telescope (squares). The solid lines for Uranus and Neptune are empirical fits to the data, constrained to zero at the poles. Credit: Yohai Kaspi et al. (2013).

Both Uranus and Neptune have fast atmospheric jets that flow westward near the Equator and flow eastward at higher latitudes. Neptune in particular, has the fastest planetary winds anywhere in the Solar System. A paper by Yohai Kaspi et al. (2013) show that these atmospheric jets extend to depths of not more than ~1100 km for both planets. On Uranus, this depth corresponds to a pressure of ~2000 bar or the outermost 0.15 percent of the planet’s mass. For Neptune, this depth corresponds to a pressure of ~4000 bar or the outermost 0.20 percent of the planet’s mass. When one considers that Uranus and Neptune have mean diameters of 50,700 km and 49,200 km respectively, a depth of not more than ~1100 km implies that the atmospheric dynamics on Uranus and Neptune do not extend deeply into the planetary interiors. As a result, the dynamics of these atmospheric jets are probably driven by shallow processes.

Figure 3: Artist’s impression of a Uranus/Neptune-like ice giant planet.

In the upper atmosphere of Uranus and Neptune, the temperature is cool enough for methane to condense to form clouds. Deeper down, at pressures of up to a few bars, are clouds of ammonia and hydrogen sulphide. Yet deeper, at tens of bars or more, water condenses into clouds. A plausible mechanism that can drive the atmospheric jets on Uranus and Neptune is by latent heat release from moist convection. Deep in the atmosphere at pressures of ~300 bars, the condensation of water to form clouds can release sufficient latent heating to drive the atmospheric jets. Moist convection on Uranus and Neptune cannot be powered by energy from sunlight because sunlight only penetrates to pressures of a few bars. Instead, moist convection on Uranus and Neptune can only be driven by internal heat radiating out from the planetary interior. The internal flux to solar flux ratios of Uranus and Neptune are 0.06 and 1.6 respectively. For Neptune, such a high ratio means that the internal heat radiating out from the planet’s interior is greater than the energy the planet receives from the Sun. As a consequence, Neptune’s high internal heat flux drives a more vigorous moist convection and probably explains why it has the fastest winds in the Solar System.

Reference:

Yohai Kaspi et al., “Atmospheric confinement of jet streams on Uranus and Neptune”, Nature 497, 344-347 (16 May 2013)

Radiation-Blasted Exoplanets

Figure 1: Artist’s impression of CoRoT-2b - a hot-Jupiter being blasted by intense radiation from its parent star. Credit: NASA/CXC/M.Weiss.

Hot-Jupiters are a class of Jupiter-sized exoplanets that orbit very close to their parent stars and receive intense amounts of stellar radiation. A typical hot-Jupiter orbits its parent star at ~1/20th the Sun-Earth distance and receives ~10,000 times more radiation that Jupiter does from the Sun. Being so close to its parent star, a hot-Jupiter is expected to be tidally-locked whereby one side continuously faces it parent star while the other side points away into the darkness of space. A study done by Daniel Fabrycky (2008) shows that reradiated thermal radiation from a hot-Jupiter carries away momentum and this can gradually change the star-planet separation distance by ~1 percent over the planet’s lifetime. For more extreme cases, this change can be as much as a few percent.

Radiation is comprised of photons which carry momentum. As a result, a force is exerted on a planet when radiation is reflected, absorbed or reradiated by the planet. Radiation that is reflected or absorbed by a hot-Jupiter simply pushes it away from its parent star. However, the direction of force exerted on a hot-Jupiter by reradiated thermal radiation is more complicated. Three-dimensional models suggest that the hottest region on a hot-Jupiter is unlikely to lie at the planet’s substellar point. Instead, the hottest region is displaced eastward by a superrotating wind. This causes reradiated thermal radiation from a hot-Jupiter to be emitted in a preferential direction and thus acts as a radiative thruster which adds angular momentum to the planet, pushing it into a wider orbit around its parent star.

Figure 2: The force vectors due to radiation asymmetry as a function of orbital position for the HAT-P-2b - a hot-Jupiter in an eccentric 5.63-day orbit around a star that is slightly bigger and hotter than the Sun. Credit: Daniel Fabrycky (2008).

The radiative thruster effect is amplified for hot-Jupiters that orbit closer than typical to their parent stars or for hot-Jupiters with inflated cross-sectional areas. For example, OGLE-TR-56b is a hot-Jupiter which orbits in a very tight 1.21-day orbit around its parent star. As a consequence, the stupendous amount of stellar radiation received by OGLE-TR-56b amplifies the radiative thruster effect. The result is an estimated increase in the star-planet separation distance of OGLE-TR-56b by ~5 percent over the planet’s lifetime.

This study shows that the intense radiation received and reradiated by a close-in exoplanet can change the planet’s orbit over its lifetime. Although the study done by Daniel Fabrycky (2008) considered only hot-Jupiters, the radiative thruster effect is expected to be more pronounced for Earth-sized rocky planets in close-in orbits around their parent stars. A number of such exoplanets have already been discovered, and they include planets such as Kepler-10b and KIC 8435766b. These terrestrial-sized planets are likely to have a smaller cross-section per unit mass than hot-Jupiters and are likely to be more influenced by the radiative thruster effect.

Reference:

Daniel Fabrycky (2008), “Radiative Thrusters on Close-in Extrasolar Planets”, arXiv:0803.1839 [astro-ph]

Jet-Black Exoplanet

TrES-2b is a Jupiter-sized exoplanet discovered in 2006 by the Trans-Atlantic Exoplanet Survey (TrES). It happens to be in the field-of-view of NASA’s Kepler space telescope and observations by Kepler found that it reflects less than one percent of the sunlight falling on it. This makes TrES-2b one of the darkest exoplanets currently known. TrES-2b orbits just five million kilometres from its parent star and has an orbital period of only 2.47 days. It is in a category of planets known as hot-Jupiters. Being so close to its parent star, TrES-2b is superheated to 1000 degrees Centigrade.

Figure 1: Artist’s conception of TrES-2b with a hypothetical moon in the foreground.

Figure 2: Orbital photometric phase variations of TrES-2b showing a contrast of 6.5 parts per million between the planet’s day-side and night-side. Credit: David M. Kipping & David S. Spiegel (2011).

Since Kepler is a planet-hunting telescope designed to measure the tiny dip in brightness when an Earth-sized planet crosses in front of its parent star, its exquisite photometric precision allows the reflectivity of TrES-2b to be measured. This is done by measuring the combined brightness of the star-planet system as the planet’s day-side rotates in and out of view. Measurements by Kepler showed that the contrast between the day-side and night-side photon flux of TrES-2b is only 6.5 parts per million. Such a tiny day-night contrast indicates that TrES-2b is exceptionally dark because a more reflective planet would have shown larger brightness variations as its day-side rotates in and out of view. TrES-2b appears to have an incredibly low reflectivity of less than one percent. In fact, best fit models show that its reflectivity is a mere 0.04 percent.

Jupiter reflects more than one-third of the sunlight that reaches it due to the presence of bright reflective clouds in its atmosphere. Unlike Jupiter, TrES-2b lacks reflective clouds. However, that alone is far from sufficient to explain the planet’s extremely low reflectivity. The presence of light-absorbing chemicals such as gaseous sodium and potassium in the hot atmosphere of TrES-2b may help explain the planet’s jet-black appearance. Nevertheless, the cause for the extremely low reflectivity of TrES-2b still remains unknown, although it may be explained by the presence of yet unknown atmospheric constituents.

“It’s darker than the blackest lump of coal, than dark acrylic paint you might paint with. It’s bizarre how this huge planet became so absorbent of all the light that hits it”, says astronomer David Spiegel of Princeton University. “However, it’s not completely pitch black. It’s so hot that it emits a faint red glow, much like a burning ember or the coils on an electric stove.”

Reference:

David M. Kipping & David S. Spiegel (2011), “Detection of Visible Light from the Darkest World”, arXiv:1108.2297 [astro-ph.EP]

Trojan Earths

In a protoplanetary disk around a young star, it is believed that regions with higher gas densities tend to concentrate solid material. If these high gas density regions are sufficiently long-lived, the aggregates of solid material within them can gravitationally collapse to form planets. The presence of a gas giant planet like Jupiter can create such long-lived, high gas density regions; especially around the gas giant planet’s leading (L4) and trailing (L5) Lagrangian points. The high gas density regions around L4 and L5 tend to concentrate solid material in a process known as Lagrangian trapping. A paper by Lyra et al. (2008) show that Lagrangian trapping can lead to gravitational collapse of aggregated solid material and form objects with masses in the regime of terrestrial planets (~ 0.1 to 10 Earth masses).

Figure 1: A contour plot showing the 5 Lagrangian points of the Sun-Earth system. The same layout of the 5 Lagrangian points also applies for the Sun-Jupiter system. The regions around the leading (L4) and trailing (L5) Lagrangian points are “islands of stability”.

As a gas giant planet orbits around its infant star, it clears out a wide gap in the protoplanetary disk. The environment within the gap is depleted of gas and other materials. Nevertheless, the regions around L4 and L5 serve as “islands of stability” where high gas densities are retained. The high gas densities create drag and damp the motion of solid particles. This allows Lagrangian trapping to occur where solid particles become trapped in the high gas density regions around L4 and L5. Simulations were conducted by Lyra et al. (2008) to determine the effectiveness of Lagrangian trapping in forming terrestrial planets with masses ranging from sub-Earth-mass to super-Earth-mass. A planet formed at L4 or L5 is known as a Trojan planet or Trojan Earth (if it has ~ 1 Earth mass). The word “Trojan” comes from the Trojan asteroids that exist around Jupiter’s L4 and L5 Lagrangian points.

Lyra et al. (2008) investigated the formation of Trojan planets by running simulations using a gas giant planet with the mass of Jupiter and solid particles with diameters of 1 cm, 10 cm, 30 cm and 1 m. The 10 cm and 30 cm solid particles readily underwent gravitational collapse at the Lagrangian points. For the 1 cm solid particles, they were so well coupled to the gas that gravity is not the main factor driving their collapse. Instead, the 1 cm solid particles collapse as the high gas density regions around L4 and L5 gradually shrunk. For the large 1 m solid particles, they remained unbound because they were too heavy to be captured by gas drag into the high gas density regions.

Each simulation by Lyra et al. (2008) was run using only one particle size. Gravitational collapse was more efficient for 10 cm solid particles than for 30 cm ones. The 10 cm solid particles collapsed to form a 0.6 Earth-mass planet at L4 and a 2.6 Earth-mass planet at L5. For the case involving 30 cm solid particles, a 0.1 Earth-mass planet was formed at L4, but the solid particles remained unbound at L5. These planet masses are only applicable for the particular set of assumptions used in the simulations by Lyra et al. (2008).

Figure 2: Artist’s conception of an Earth-size planet with vegetation covering most of the planet’s surface from pole-to-pole. Credit: Goran Licanin.

Figure 3: Artist’s conception of a habitable Earth-size planet with a large ice sheet on one of the planet’s poles.

Lagrangian trapping can produce a wide range of planetary masses depending on many conditions such as mass of gas giant planet, density of protoplanetary disk, size distribution of solid particles, etc. As a result, a huge diversity of Trojan planets is expected. Trojan planets formed at the Lagrangian points of a gas giant planet orbiting within the habitable zone of its parent star can potentially be Earth-like and habitable. Such “Trojan Earths” can appear very similar to the Earth. For a gas giant planet orbiting further from its parent star, Trojan planets formed at its Lagrangian points can acquire a significant fraction of icy material if temperatures are cool enough for ices to exist in the protoplanetary disk. Such a Trojan planet is expected to have a surface ice shell overlying a vast global ocean of liquid water.

If Trojan objects with masses in the regime of terrestrial planets can form, it is worth considering why Trojan planets are absent in the Solar System. One reason could be that the initial Trojan objects that formed at Jupiter’s L4 and L5 Lagrangian points were de-stabilized when Jupiter and Saturn crossed the 2:1 mean motion resonance. Following that, Jupiter acquired a new population of Trojan objects. However, without gas drag, the new population of Trojan objects could not assembly into planets and hence left behind what is now observed as the Trojan asteroids at Jupiter’s Lagrangian points. As a result, Trojan planets are probably more common in planetary systems with only one gas giant planet or in planetary systems where the gas giant planets did not undergo resonance crossing.

Reference:

Lyra et al. (2008), “Standing on the shoulders of giants: Trojan Earths and vortex trapping in low mass self-gravitating protoplanetary disks of gas and solids”, arXiv:0810.3192 [astro-ph]

Black Holes on the Outskirts

Figure 1: An illustration of the Milky Way with the galactic halo and Sun’s position indicated. The Milky Way is a barred spiral galaxy measuring over 100,000 light-years across and contains a few hundred billion stars. Credit: Pearson Education Inc.

A study done by Rashkov & Madau (2013) show that there may be as many as 2000 to as few as 70 intermediate-mass black holes (IMBHs) lingering in the halo of the Milky Way. Unlike the supermassive black hole (~ 4 million solar-mass) that sits in the heart of the Milky Way, IMBHs have masses ranging from a few 100 to a few 100,000 solar-mass. These IMBHs were once surrounded by subhalos of stars and matter, which were the subgalactic building blocks of present-day massive galaxies. When these subhalos merged in the past to form the present-day Milky Way, a relic population of IMBHs is left behind in the Milky Way’s halo.

The relic population of IMBHs can be divided into two main subpopulations - “naked” IMBHs and “clothed” IMBHs. “Naked” IMBHs dominate in the inner region of the Milky Way’s halo, but become increasingly rarer at larger distances where “clothed” IMBHs dominate. This is consistant with the fact that subhalos orbiting in the denser inner regions of the Milky Way’s halo experience strong disruption which strip off all stars and matter, leaving the IMBHs exposed. In the rarefied outer region of the Milky Way’s halo, subhalos experience weaker disruption and results in “clothed” IMBHs that still hold on to surviving clouds of stars and matter around them.

Figure 2: Artist’s impression of an accretion disk around a black hole.

An IMBH lurking in the Milky Way’s halo can occasionally flare-up if it happens to pass through denser regions of the Milky Way and accrete from the interstellar medium. Such flare-ups can be observed across intergalactic distances. Another way to search for IMBHs in the Milky Way’s halo is to look for stars that may accompany an IMBH. Even a “naked” IMBH will posses some stars in a tight cluster around it. Due to its compactness, the cluster of stars around an IMBH may appear point-like, especially so for a more distant IMBH. IMBHs in the Milky Way’s halo can have tangential velocities of up to a few 100 km/s which translate to proper motions of up to a few milli-arcseconds per year. This motion is detectable using the current Hubble Space Telescope and other future space-based telescopes.

Reference:

Valery Rashkov and Piero Madau (2013), “A Population of Relic Intermediate-Mass Black Holes in the Halo of the Milky Way”, arXiv:1303.3929 [astro-ph.CO]

Jupiter Devoured a Super-Earth

Jupiter and Saturn are gas giant planets with 318 and 95 Earth masses respectively. A gas giant forms when a solid core of rock and ice material with ~10 Earth masses starts accreting hydrogen and helium gas from the protoplanetary disk of material surrounding a young star. The end result is a massive hydrogen-helium (H-He) envelop surrounding a small rocky core. Observations of Jupiter and Saturn have revealed two puzzling properties. Firstly, Saturn seems to have a more massive core than Jupiter even though Saturn is only one-third Jupiter’s mass. Jupiter’s core is less than 10 Earth masses while Saturn core is between 15 to 30 Earth masses. Secondly, there is an enhancement of heavy elements in the H-He envelops of Jupiter and Saturn.

In a previous article, I mentioned how the low mass of Jupiter’s core can be explained by the planet’s higher internal temperature which makes rock material within the planet’s core more dissolvable. This article will cover another mechanism involving the collisions of planetary-mass objects with gas giants, and how such events can also account for the heftier core of Saturn and the enhancement of heavy elements in both Jupiter and Saturn. An impacting planetary object can range from sub-Earth-mass to super-Earth-mass (~10 Earth masses). When a planetary object collides into a gas giant, the outcome largely depends on the object’s mass, but also on the object’s speed and angle of impact.

Figure 1: Artist’s impression of a super-Earth colliding into a gas giant planet. Credit: Jamie Murchison

In a head-on collision event, the impacting object needs to be sufficiently massive to penetrate deep enough to reach the gas giant’s core. This is because ablative disintegration of the impactor occurs as it plows through the H-He envelop of the gas giant. As a result, an impactor needs to have at least a few Earth masses or more to survive ablative disintegration and reach the gas giant’s core. The dissipation of impact energy into and around the gas giant’s core can erode the core and mix the core material with the overlying H-He envelope. Core erosion and ablative disintegration of the impactor enhances the abundance of heavy elements in the gas giant’s H-He envelope. An energetic head-on collision involving a massive impactor (~10 Earth masses) probably occurred for Jupiter, and resulted in Jupiter’s low mass core and enhancement of heavy elements in its atmosphere.

Less massive impacting objects with less than a few Earth masses are expected to disintegrate completely in a gas giant’s H-He envelop and not reach the gas giant’s core at all. Some fraction of the debris being deposited into the gas giant’s H-He envelop by ablative disintegration of the impactor eventually settles onto the gas giant’s core. As a result, low mass impactors promote growth of the gas giant’s core. Saturn’s large core may have grown to its current size in such a manner from the sedimentation of ablated debris from a number of low mass impactors. Ablation of these low mass impactors can also account for the observed enhancement of heavy elements in Saturn’s atmosphere.

Giant impacts involving the collision of sub-Earth-mass to super-Earth-mass objects into gas giant planets are likely to increase their luminosities and puff up their diameters. Such impacts can significantly modify the core-envelope structure and atmospheric composition of gas giants.

Reference:

Shulin Li et al. (2010), “Embryo impacts and gas giant mergers I: Dichotomy of Jupiter and Saturn’s core mass”, arXiv:1007.4722 [astro-ph.EP]

Dissolving Heart of Jupiter

At the planet’s very heart lies a solid rocky core, at least five times larger than Earth, seething with the appalling heat generated by the inexorable contraction of the stupendous mass of material pressing down to its centre. For more than four billion years Jupiter’s immense gravitational power has been squeezing the planet slowly, relentlessly, steadily, converting gravitational energy into heat, raising the temperature of that rocky core to thirty thousand degrees, spawning the heat flow that warms the planet from within. That hot, rocky core is the original protoplanet seed from the solar system’s primeval time, the nucleus around which those awesome layers of hydrogen and helium and ammonia, methane, sulphur compounds-and water-have wrapped themselves.

- Ben Bova, Jupiter (2000)

 Figure 1: Artist’s depiction of Jupiter and the 4 Galilean moons - Io, Europa, Ganymede and Callisto.

Gas giant planets such as Jupiter and Saturn are believed to have formed from the rapid accretion of hydrogen and helium gas around an initial solid core of rock and ice material. Such a protoplanetary core is expected to containing approximately 10 times the mass of Earth. So much hydrogen and helium is eventually accreted that the core of rock and ice material only forms a small fraction of the gas giant planet’s total mass. For instance, Jupiter and Saturn have respectively 318 and 95 times the mass of Earth. Following the formation of a gas giant planet, the icy component of the planet’s core is expected to dissolve into the overlying layers of hydrogen and helium. However, the fate of the rocky component of the planet’s core is less understood. Surrounding the core of a gas giant planet like Jupiter is a layer of metallic hydrogen. This is a state of hydrogen that is formed when hydrogen is crushed by the titanic gravitational compression in the planet’s deep interior.

Calculations have shown that the intense temperatures and pressures at the core of a gas giant planet can cause the rocky component of the planet’s core to partially or fully dissolve into the overlying layer of metallic hydrogen. For example, magnesium oxide is a major constituent of Jupiter’s core and it is soluble in metallic hydrogen at the intense temperatures and pressures found in the heart of Jupiter. One can imagine the core of Jupiter dissolving like an antacid tablet plopped into a glass of water. Over time, the dissolved rock material is expected to be redistributed throughout the entire bulk of the planet. The redistribution of dissolved rock material is consistant with the observed enhancement of heavy elements in the atmosphere of Jupiter.

 Figure 2: Artist impression of a gas giant planet.

The solubility of rock material increases with temperature. More massive gas giant planets have higher interior temperatures and are expected to have higher solubility. This may explain why Saturn, with only 30 percent the mass of Jupiter, seems to have a heftier core than Jupiter. Saturn’s core is either not dissolving at all or is dissolving a lot slower because conditions inside Saturn are not as extreme as inside Jupiter. A gas giant planet more massive than Jupiter can have its core completely dissolved and redistributed throughout the planet. “I suspect that for very large ‘super-Jupiters’, you would have no core at all,” says Hugh Wilson, one of the researchers involved in the study. “If so, this should boost the concentration of heavy elements in their atmospheres, which future telescopes might be able to detect.”

Reference:

Hugh F. Wilson and Burkhard Militzer (2013), “Rocky Core Solubility in Jupiter and Giant Exoplanets”, arXiv:1111.6309 [astro-ph.EP]

Water World or Diamond Planet

Figure 1: Earth and 55 Cancri e shown to scale.

55 Cancri e is a planet orbiting a Sun-like star 41 light years from Earth. This planet takes just 17 hours 41 minutes to orbit its host star and its distance from its host star is only 1/20th the distance of Mercury from the Sun. Being so close to its parent star, the dayside of 55 Cancri e is scorched to a temperature of over 2000 °C. 55 Cancri e is in a class of exoplanets known as “super-Earths”. It has about 8 times the mass of Earth and a diameter just over twice that of Earth. Knowing both the size and mass of 55 Cancri e allows the planet’s internal composition to be predicted. When 55 Cancri e is being plotted onto a mass-radius relationship diagram, it seems that the planet is too large for its mass to be made up of just rock material.

A study done by M. Gillon et al. (2012) suggests that 55 Cancri e has a rocky interior with a thick overlying envelop of water comprising ~30 percent of the planet’s total mass. Such a layer of water is expected to be a few thousand kilometres thick. Because of the stupendous temperatures and pressures, this ocean of water is neither liquid nor gas. Instead, the water is in a supercritical phase where distinct liquid and gas phases do not exist. Beginning from the edge-of-space, a descend down into the depths of this ocean will be unusual because an “ocean surface” will not be crossed. The density of supercritical water will simply increase with depth, from a gas-like density until it exceeds the density of liquid water on Earth, down within the great depths of the ocean. As a result, there will be no clear boundary between sea and sky for this ferociously hot ocean of supercritical water on 55 Cancri e.

Figure 2: 55 Cancri e as plotted in a mass-radius relationship diagram. Credit: M. Gillon et al. (2012).

Another study done by M. Nikku et al. (2012) reports that the mass and diameter of 55 Cancri e can be explained by a carbon-rich interior comprised of iron, carbon, silicon carbide and other silicates. Such a carbon-rich interior will not require the planet to have a thick water envelop to account for its size. The high pressures in the planet’s interior can crush carbon into diamond and this makes 55 Cancri e a candidate for a “diamond planet”. “This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” says lead researcher Nikku Madhusudhan, a postdoctoral researcher at Yale University.

Figure 3: An illustration of 55 Cancri e shows a surface of mostly graphite surrounding a thick layer of diamond. Credit: Haven Giguere, Yale University.

References:

1. M. Gillon et al. (2012), “Improved precision on the radius of the nearby super-Earth 55 Cnc e”, arXiv:1110.4783 [astro-ph.EP]

2. M. Nikku et al. (2012), “A Possible Carbon-rich Interior in Super-Earth 55 Cancri e”, arXiv:1210.2720 [astro-ph.EP]

Cooling a Venus Rover

An ocean flowed on Venus eons past

Before a body blow reversed her spin

And now alas, unlike her earthly twin

Her waters to the heavens have been cast.

Tectonic plates, unoiled, locking fast

And no sure passage frees the heat within

The skin and core are thermally akin

No dynamo protects from cosmic blast.

And lighter gas is swept away by rays

Ten miles deep, from pole to pole she's wrapped

In densest greenhouse gas, her body steeps.

Each blistered night, a hundred plus earth days

In Vulcan's ashy forge forever trapped

And with sulphuric acid tears she weeps.

- Diane Hine, Sister Planet (10 March 2012)

Figure 1: Artist’s impression of the Venusian surface with lightning in the background. Credit: Greg S. Prichard.

With a surface temperature of 450 °C and a surface atmospheric pressure of 92 bars (equivalent to the pressure a kilometre under the Earth’s ocean), the surface of Venus is a hostile environment. Although sending a rover to explore the surface of Venus is expected to yield results of great scientific value, the high surface temperature on Venus will wreak havoc on any electronic components. The longest-lasting lander on the surface of Venus was the Russian Venera 13 lander which touched down on 1 March 1982 and survived for 127 minutes. In addition, the thick cloud layers in Venus’ atmosphere allow only 2 percent of the sunlight to reach the planet’s surface and this severely limits the potential of solar energy to power surface operations. The light level on the Venusian surface is comparable to a rainy day on Earth.

Figure 2: Atmosphere of Venus. Credit: Pearson Education Inc.

Due to the extreme and unique surface conditions on Venus, a rover designed for long-duration surface operations on Venus will have to tackle challengers not faced by Martian or Lunar rovers. A study conducted by two researchers at NASA’s John Glenn Research Centre in Cleveland, Ohio, investigates the power and cooling systems for a rover designed to last more than 50 days on the surface of Venus. The study evaluated a nuclear-powered rover than derives its energy from the decay of radioactive plutonium-238 that is encapsulated in the form of seven general purpose heat source (GPHS) modules. Each GPHS module provides 250 W of thermal energy and weighs approximately 1.5 kg.

Heat from the seven GPHS modules are used to power a Stirling engine. One side of the Stirling engine is in contact with the GHPS modules and serves as the hot-sink with a temperature of 1200 °C. The other side end of the Stirling engine is exposed to the ambient environment on Venus and it is at a temperature of 500 °C. Using helium as a working fluid, the temperature difference drives a pressure difference between two chambers to produce a total mechanical power output of 480 W. To accommodate some level of uncertainty, 400 watts of mechanical power is assumed to be available for use. From the 400 W, 100 W of electrical power is generated to drive the rover and power the electronics, while 280 W of mechanical power is available to actively cool the electronics.

The rover’s electronics are enclosed within a 10 cm spherical vault that is surrounded by a 5 cm thick ceramic-based insulator. Of the total heat load on the electronics vault, 77 W comes from the high temperature environment on Venus and 10 W comes from heat being generated from electronics and sensors. Based on this heat load, the rounded up estimated heat rejection requirement is 100 W. To do that, a Stirling cooler is used to provide active cooling of the rover’s electronics vault. Using 240 W of mechanical power for active cooling, the Stirling cooler can keep the temperature within the electronics vault under 300 °C. This temperature is sufficiently cool for high temperature electronics to operate for long durations.

Figure 3: A rendering of Venus without its atmosphere.

“Understanding the atmosphere, climate, geology, and history of Venus could shed considerable light on our understanding of our own home planet. Yet the surface of Venus is the most hostile operating environment of any of the solid-surface planets in the solar system,” wrote Dr. Geoffrey Landis of NASA’s John Glenn Research Centre who was one of two researchers involved in the study. “Putting a long-lived rover on the surface of Venus could revolutionise our understanding of the planet, helping to answer such questions as why Venus ended up so different from Earth,” says Mark Bullock of the Southwest Research Institute in Boulder, Colorado. “Many scientists suspect Venus was much cooler in the past and was perhaps even covered with oceans of liquid water where conditions could have been friendly to life.”

Reference:

G.A. Landis and K.C. Mellott, “Venus Surface Power and Cooling Systems”, Acta Astronautica 61 (2007) 995-1001

A River on Titan

If viewed in black and white, it could be Earth,

with river deltas, shores and sculpted rock.

But sands in endless dunes round half its girth,

are ever-frozen grains of ice which flock,

enslaved by Saturn’s tidal-driven winds.

Revealed in filtered amber twilight haze,

the similarity to Earth rescinds.

- Diane Hine, On Saturn’s Moon (16 April 2012)

Titan is the only other world in the solar system that has stable bodies of liquid on its surface. With a surface temperature of minus 180 degrees Centigrade, Titan is so cold that water is frozen solid and as hard as a rock. Instead, the liquid that fills its lakes and flows in its rivers is a mixture of ethane and methane. It is a challenge to image Titan because its surface is shrouded under a thick and opaque atmosphere. As a result, NASA’s Cassini spacecraft uses radar to map Titan’s surface by bouncing pulses of microwave energy off the surface of Titan and measuring the time it takes for the pulses to return to the spacecraft. Essentially, the radar instrument on Cassini “sees” the surface of Titan using microwaves instead of visible light.

Figure 1: The colourful globe of Titan passes in front of Saturn and its rings in this true colour snapshot from NASA’s Cassini spacecraft. The image was obtained with the Cassini spacecraft narrow-angle camera on 21 May 2011, at a distance of approximately 2.3 million kilometres from Titan. Credit: NASA/JPL-Caltech/Space Science Institute.

On 26 September 2012, a swat of radar imagery of Titan’s surface was acquired by Cassini. The image shows a river system which stretches more than 400 kilometres in length and empties into a large sea known as Ligeia Mare in Titan’s north polar region. It appears dark along its entire length and this indicates a smooth surface, consistant with it being filled by some form of liquid. The liquid is probably a mixture of ethane and methane. This radar image is a good snapshot of a “hydrological” cycle on another world. Here on Titan, liquid ethane and methane falls as rain, and rivers transport the liquid into lakes and seas where evaporation kicks off the cycle all over again.

Figure 2: Radar image of a river on Saturn’s moon Titan. This “Nile-like” river stretches more than 400 km from its ‘headwaters’ into Ligeia Mare - one of the three great seas in the high northern latitudes of the moon. The image was acquired on 26 September 2012, on Cassini’s 87th close flyby of Titan. Credit: NASA/JPL-Caltech/ASI.

The formation of this river system probably involved some faulting process since part of the river flows along fault lines. A fault line is basically a fracture or discontinuity in a volume of rock. “Though there are some short, local meanders, the relative straightness of the river valley suggests it follows the trace of at least one fault, similar to other large rivers running into the southern margin of this same Titan sea,” says Jani Radebaugh, a Cassini radar team associate at Brigham Young University, Providence, Utah.