Friday, August 31, 2012

Paraterraforming: Creating Habitable Worlds

It’s really incumbent upon us as life’s agents to extend life to another planet. I think that being a multi-planet species will significantly increase the richness and scope of the human experience.
- Elon Musk, founder of SpaceX, interview in Ad Astra, 2006

The quality of a civilization is measured not by what it has to do, but by what it wants to do.
- Bruce Murray, research scientist, Exploring Space, 1991

Terraforming is the process of modifying a planet, moon or any other suitable object in order to make it habitable for humans. The word terraforming literally means “Earth-shaping”. Mars is often regarded as the first candidate for terraforming because it is the most Earth-like planet in the Solar System. Early in its history, Mars is believed to be a lot more like Earth with a significantly thicker atmosphere and abundant liquid water on its surface. Today, transforming Mars into an Earth-like world through terraforming will require thickening its atmosphere, warming up the planet and keeping the atmospheric constituents from escaping into space. Such an endeavour will require numerous technological breakthroughs and demand huge economic resources. Furthermore, terraforming Mars is a gradual process which is expected to occur over a timescale that is likely to exceed a human lifespan and the lack of gratification in return for investment will deter initial investors. For these reasons, paraterraforming serves as an attractive intermediate step before full terraforming is achieved.

Figure 1: Artist’s impression of a terraformed Mars.

 Figure 2: Artist’s impression of a terraformed Venus.

Paraterraforming involves the construction of a pressurised habitat in the form of an air-tight roof over a particular area on a planet where an Earth-like environment is completely enclosed within the habitat. A modular, “pay-as-you-go” approach of expansion is possible for paraterraforming and this allows it to appropriately meet increasing demand. As the population increases, more pressurised habitats can be constructed over the surface of a planet until most of the planet’s surface is covered or until full terraforming of the planet is eventually achieved.

A typical habitat can consists of an ultra-strong membrane that is primarily supported by the air pressure contained within it. The membrane acts as a roof over the surface of the planet, allowing sunlight in and preventing the atmosphere from escaping. At regular intervals, tension cables or support towers can anchor the membrane to the surface of the planet. To support a more Earth-like hydrological cycle, the membrane needs to be at least a few kilometres above the planet’s surface so as to provide sufficient altitude for clouds to form. A higher membrane height is also advantages for aerial transport within the habitat. To deal with the occasional meteor strike, the membrane should be designed to be capable of self-repairing.

An object that is somewhat less massive than Mars will be unsuitable for terraforming since its gravity will be too weak to hold on to an Earth-like atmosphere for long. Therefore, one key advantage of paraterraforming over terraforming is that paraterraforming can be done on objects much less massive than Mars since the atmosphere is contained by the overlying membrane rather than by gravity. As a result, even small objects such as asteroids can be paraterraformed since an Earth-like atmosphere can be contained around an asteroid by a membrane which completely envelops the asteroid. This creates a habitable environment which fully encompasses the asteroid. A paraterraformed asteroid will be rather interesting as a habitat due to its very low gravity environment.

Paraterraforming also allows the creation of Earth-like habitable environments on worlds that are too cold to support life. Such places include asteroids beyond the orbit of Mars, the satellites of the gas giant planets, comets and Kuiper Belt Objects (KBOs). The paraterraforming membranes that are used to envelope such objects can be designed to provide the necessary super-greenhouse effect to create Earth-like habitable environments at large distances from the Sun. Many of these objects are small enough that a single membrane is sufficient to completely envelope such an object. A paraterraformed comet completely enveloped within a membrane that provides a strong greenhouse effect will melt and eventually settle as a sphere of water with denser rocky material gravitationally settling in its core.

Thursday, August 30, 2012

Failed-Detonation Supernova

Type Ia supernovae are known to originate from the thermonuclear explosions of carbon-oxygen (C-O) white dwarf stars. A common pathway for the production of a type Ia supernova begins with a C-O white dwarf accreting matter from a nearby companion star. As the mass of the white dwarf grows towards the Chandrasekhar mass-limit, unstable thermonuclear burning will eventually ignite at some position within the white dwarf. This sets off a buoyancy-driven deflagration flame which rises and burns its way towards the surface of the white dwarf.

In a conventional type Ia supernova, the deflagration flame will transition to a detonation flame as it enters the lower density outer layers of the white dwarf. The detonation flame then consumes the entire white dwarf and the energy release from thermonuclear burning causes the whole star to explode violently as a type Ia supernova. The difference between a deflagration flame and a detonation flame is that the latter propagates faster than the local speed of sound in the white dwarf. As a result, a detonation flame is able to consume the entire white dwarf since the material in front of the detonation flame is unable to “see” the approaching flame front.

However, it is possible to have a failed-detonation scenario where the deflagration flame fails to transition to a detonation flame. This may explain a peculiar subset of type Ia supernovae that are characterised by low ejecta velocities, low luminosities and low ejecta masses. A failed-detonation type Ia supernova occurs when enough mass is burnt during the deflagration phase such that the conditions necessary for the deflagration flame to transition to a detonation flame cannot be achieved and the white dwarf fails to detonate. In this scenario, thermonuclear burning during the deflagration phase delivers energy to the white dwarf, causing the star to expand and then contract. Because too much energy is delivered to the white dwarf, it is unable to attain high enough densities and temperatures to launch a detonation flame during maximum contraction.

For a failed-detonation type Ia supernova, the white dwarf will remain intact as the deflagration is too weak to completely unbind it. However, the white dwarf will now have a lower mass as the failed-detonation event is expected to produce a few tenths of a solar mass of ejecta. The thermonuclear fusion processes occurring within the deflagration flame results in ejecta that is rich in intermediate-mass elements (magnesium, silicon and sulphur) and iron-group elements (iron, cobalt and nickel). A significant proportion of the heavy elements are expected to fall back to the white dwarf and gravitationally settle to form an iron/heavy-core at its centre. The end result is an iron/heavy-core C-O white dwarf.

Due to the highly asymmetric nature of the outburst, the white dwarf will receive a kick velocity of a few 100 km/s. Even so, the large orbital velocities found in most binary star systems suggest that even a kick velocity of a few 100 km/s may be insufficient to unbind the binary. However, for binary systems consisting of a white dwarf accreting matter from an evolved star such as a red giant, the natal kick velocity is likely to unbind the system because of the large binary separation between the white dwarf and the red giant. It is also possible for the natal kick from the asymmetric outburst to launch the white dwarf towards its companion star and this should produce very interesting results.

George Jordan IV, et al., 2012, “Failed-Detonation Supernovae: Sub-Luminous Low-Velocity Ia Supernovae and Their Remnant-Kicked Iron-Core White Dwarfs”, arXiv:1208.5069v1 [astro-ph.HE]

Tuesday, August 28, 2012

Plutonium-238 for Deep Space Exploration

A radioisotope thermoelectric generator (RTG) is a type of power generator which converts heat produced from the decay of a suitable radioactive material into electricity. Plutonium-238 is normally employed in RTGs because it has a long half-life of 87.7 years and is a very powerful alpha emitter that does not emit other forms of more penetrating radiation. Alpha radiation can be easily blocked by something as thin as a sheet of paper. RTGs are commonly used to power spacecraft that travel to places in the Solar System where solar cells are not practical and where the mission duration is too long for batteries or fuel cells to be used. One kilogram of plutonium-238 produces 560 watts of power in the form of heat. Examples of spacecraft powered by RTGs include the Cassini spacecraft in orbit around Saturn, the Curiosity rover on Mars and the New Horizons spacecraft on its way to Pluto and beyond. All these missions are made possible by the availability of Plutonium-238.

Figure 1: Artist’s impression of NASA’s Curiosity rover on the surface of Mars.

Figure 2: Artist’s impression of NASA’s Cassini spacecraft in orbit around Saturn.

The United States ceased producing plutonium-238 in 1988 and since 1993, all plutonium-238 used to power spacecraft for deep space exploration were purchased from Russia whose own supply is already running low. Production of plutonium-238 needs to be restarted soon in order to produce sufficient quantities to support future deep space exploration missions. In the past, plutonium-238 is produced from neptunium-237 extracted from spent nuclear fuel taken out from uranium-fuelled light water reactors (LWRs). When neptunium-237 is extracted, it is bombarded with neutrons to get neptunium-238 which beta decays into plutonium-238. Plutonium-238 cannot be directly extracted from spent nuclear fuel because the presence of uranium-238 in LWRs also leads to the production of other isotopes of plutonium from neutron absorptions. Spent nuclear fuel from LWRs typically contains slightly over 1 percent of plutonium-238 out of the total amount of plutonium produced. Since isotopes are chemically identical, it is almost impossible to separate out plutonium-238 and this makes it necessary to extract neptunium-237 out of the spent fuel to produce plutonium-238.

Figure 3: The series of neutron absorptions and beta decays leading to the production of plutonium-238 in a LFTR.

In a previous article titled “NuclearPower for Lunar Settlements”, a radically different type of nuclear reactor is described. This type of reactor is known as a liquid fluoride thorium reactor (LFTR) and it is basically a reactor where the nuclear fuel is in the form of a fluoride-based molten salt mixture. The operation of a LFTR is attractive for the production of plutonium-238 because almost all of the plutonium it produces is plutonium-238 and allows for the direct chemical extraction of plutonium-238. This is because uranium-238 is not present to produce other isotopes of plutonium. Since the nuclear fuel in a LFTR is fluid in nature, the plutonium-238 can be extracted using a small adjacent chemical plant while the LFTR continues running with no downtime incurred. In a LFTR, fissile uranium-232 that is bred from thorium-232 is used to generate energy through the fission process. For every 1000 kg of naturally occurring thorium that is fed into a LFTR, 15 kg of plutonium-238 is produced as the end product. In comparison, NASA’s Curiosity rover uses 4.8 kg of plutonium-238 while the Cassini spacecraft uses 33 kg of plutonium-238.

Monday, August 27, 2012

On Chariots of Fire

there was a time
we roared to the heavens on fire
and reached for another world
on a voyage of mythological proportions

slipping the bounds of Mother Earth
we leapt far enough
and saw ourselves for the first time
“My God!” we cried

we landed our vessels
and left footprints on an alien world
our ambitions were clear
as we sought new destinations

but the burdens of our world
held down the triumphs we had
the decades
they came and went

but the words echoed through time
that one small step!
that giant leap!
kept the dream alive

poised for flight
we dare once again
to reach for the heavens
on chariots of fire

Written by Xuan Yang Koh on Sunday, 26 August 2012, and this is dedicated to Neil A. Armstrong (1930 - 2012) and the Apollo Program.

Sunday, August 26, 2012

Nuclear Power for Lunar Settlements

The Moon is often regarded as the next logical step in the expansion of human activities into space and it also contains resources which can be exploited for such purposes. Energy is required for these activities and to sustain human settlements on the lunar surface. A lunar settlement will have the same basic needs as any community on Earth, but it will have a number of unique constraints. The absence of coal, natural gas, petroleum, an atmosphere and any lakes or rivers severely limits the number of options available to provide power for a lunar settlement. Solar energy will be a tough option because a night on the Moon lasts 2 weeks and storing 2 weeks worth of energy will be a problem. Only lunar settlements at the poles of the Moon can benefit from solar energy as collectors can be erected on top of strategic mountain peaks at the poles where the Sun rarely sets.

It seems that nuclear energy is the only feasible option to power lunar settlements and to support the expansion of activities on the Moon. However, almost all commercial nuclear reactors used around the world today are uranium-fuelled light water reactors (LWRs) and the numerous disadvantages associated with such reactors make them unsuitable to power lunar settlements. As a result, a different type of reactor known as a liquid fluoride thorium reactor (LFTR) comes in as an attractive choice as it does not have the problems associated with uranium-fuelled LWRs.

In LWRs, U235 is the primary fissile material that is burnt to produce energy. LWRs use solid fuel rods that are arranged into fuel assemblies within the reactor core. The uranium in the fuel rods is enriched with 3 percent U235 and the rest is U238. Some fission energy is also generated from the fissioning of Pu239. Pu239 is produced when U238 absorbs a neutron. LWRs use ordinary water as both the coolant and moderator in the reactor core. Water boils at 100 degrees Centigrade at atmospheric pressure and this is insufficient to carry away the heat that is generated from the fission process in the reactor core. Therefore, water in a LWR needs to be pressurised up to over 150 times atmospheric pressure in order to bring up its boiling temperature for it to become an effective coolant. As a result, a LWR has to be designed as a pressure vessel and it has to be placed within a massive containment building to keep the high pressure steam from escaping in the event of an accident.

Named after the Norse god of thunder, thorium is a silvery-white metal that is slightly denser than lead. It is about 4 times more abundant than uranium in the Earth’s crust and it frequently occurs as a by-product from the mining of rare earth metals. All thorium in nature is found as Th232 which alpha decays with a very long half-life of 14.05 billion years. Within the Earth, the decay of radioactive uranium (U235 and U238), thorium (Th232) and potassium (K40) is responsible for generating most of Earth’s internal heat. Like on Earth, the Moon also contains abundant surface deposits of thorium which can be exploited to power LFTRs.

Figure 1: Global map of elemental thorium on the Moon. Credit: NASA.

A LFTR is a type of molten salt reactor (MSR) where the nuclear fuel is in the form of a fluoride-based molten salt mixture. In such a reactor, U233 is the fissile material while Th232 is the fertile material. The production of nuclear energy originates from the fissioning of U233. When a U233 nuclei absorbs a neutron, it fissions and produces an average of just over 2 neutrons. One neutron continues the chain reaction by causing another U233 nucleus to fission while the excess neutrons are used to create more U233 from Th232. U233 is created by exposing Th232 to neutrons. In this process, Th232 absorbs a neutron to become Th233 and after a couple of beta decays, U233 is produced. In such a fuel cycle, slightly more fissionable U233 is produced than consumed. Therefore, in the operation of a LFTR, all Th232 can be converted into fissionable U233 to produce energy.

A typical design for a LFTR consists of a core which contains fissile U233 and an outer blanket which contains fertile Th232. In the outer blanket, Th232 absorbs neutrons produced from the fissioning of U233 in the core and transforms into U233. The U233 that is produced in the outer blanket can be chemically separated continuously using a small adjacent chemical plant and then fed into the core as fission fuel. Since molten salts are used, a LFTR can operate at atmospheric pressure or lower. The heat produced during the fissioning of U233 in the reactor core mostly comes from the kinetic energy of the resulting fission fragments. The heated molten salt mixture is pumped from the core to a primary heat exchanger. Here, heat is transferred to a second loop of molten salt mixture which is pumped through an intermediary heat exchanger where it heats a working fluid. A typical working fluid is water which is heated to drive a turbogenerator to generate electricity.

Figure 2: Layout of a molten salt reactor (MSR). Credit: Generation IV International Forum (GIF).

To get a LFTR running, an initial load of fissile material will be required. Besides U233, U235 can also be used as the initial start-up material. Since a LFTR breeds slightly more U233 than it consumes, the excess U233 can be used to start-up new LFTRs. The technologies required to construct a LFTR were largely addressed successfully during the 1960s and 1970s. In fact, most of the technologies were tested in the Molten-Salt Reactor Experiment (MSRE) led by American physicist Alvin Weinberg at Oak Ridge National Laboratory (ORNL). The centrepiece of the MSRE was a fluoride-based molten salt reactor which employed U233 as the fissile material. The reactor went critical in 1965 and it operated until 1969 which at that time set the record for the longest continuous operation of a nuclear reactor.

Figure 3: Energy extraction comparison between a uranium-fuelled LWR and a LFTR.

Advantages of LFTRs over LWRs:
  • For LFTRs, no reprocessing of naturally occurring Th232 is required since all of the Th232 can be converted into U233 and be burnt in the reactor to generate energy. Whereas for LWRs, fissile U235 makes up only 0.71 percent of naturally occurring uranium and it has to be enriched to about 3 percent through a complex process of isotope separation before being used as nuclear fuel. In LFTRs, all of the U233 can be burnt to generate energy. However, in LWRs, only a small fraction of the fuel in the fuel rods is burnt before the fuel rods become spent and must be replaced. As a result, LFTRs can produce up to a factor of three hundred times as much electrical power per unit mass of raw fuel ore than LWRs.
  • Since LFTRs are basically tubs of molten salt, fuel fabrication is not needed at all. In the case for LWRs, the enriched uranium fuel needs to be fabricated into solid fuel rods before being inserted into the reactor. This is an expensive and lengthy process which imposes a much higher operational cost for LWRs. The simplicity of LFTRs is a huge plus point for powering lunar settlements since the facilities for enrichment and fuel fabrication are entirely unnecessary.
  • Comparatively, LFTRs produce many times less radioactive fission products than LWRs. Furthermore, the fission products from LFTRs decay to background levels in less than 300 years but those from LWRs take over 10,000 years. This makes it much easier to have a repository to store nuclear waste from LFTRs. However, a lot of the “nuclear waste” from LFTRs have novel applications and are likely to be extracted for use rather then be tucked away in a repository.
  • LFTRs offer much greater resistance to proliferation than LWRs. Although U233 in LFTRs is a fissile material, it is not an attractive bomb-making material since it contains small amounts of U232 which decays into products that emit highly energetic gamma radiation. Also, virtually all of the plutonium produced in LFTRs is Pu238 which is not a fissile material and cannot be employed in bomb-making. In comparison, the technology involved in the enrichment of U235 for LWRs can be extended to produce highly enriched weapons grade U235 for bomb-making. Additionally, fissionable Pu239 produced in LWRs from the absorption of fast neutrons by U238 is also a conventional bomb-making material.
  • Unlike LWRs, LFTRs are not pressurized and do not need to be designed as a pressure vessel. This allows LFTRs to take on a much lighter design which makes them more feasible for space applications as it is a lot less costly to deliver a lighter reactor. Since LFTRs are not pressurized, they cannot explode or fail from overpressure which is a huge safety advantage over LWRs.
  • During any fission process, large amounts of xenon and krypton gases are produced. In LWRs, these gases build up to high pressures within the cladding of the solid fuel rods and it can pose a serious problem during a heating transient or an accident. In LFTRs, these gases are continuously removed from the molten salt mixture and there are no confine spaces where these gases can build up to high pressures.
  • The fluoride-based molten salt mixture employed in LFTRs is chemically stable and impervious to radiation. In LWRs, an overheating anomaly can dissociate water to produce combustible hydrogen gas which can accumulate and lead to an explosion as seen during the Fukushima-Daiichi nuclear accident. Since water is not present in the core of a LFTR, a hydrogen explosion is impossible for such a nuclear reactor. Finally, a fluoride-based molten salt mixture has a slightly higher volumetric heat capacity than water and this allows it to absorb more heat during heating transients.
  • LFTRs can operate with overall thermal to electrical efficiencies that exceed 50 percent. In comparison, LWRs have overall efficiencies of only 30 to 35 percent.
  • LFTRs do not experience downtime during refuelling since the nuclear fuel is in the form of a fluoride-based molten salt mixture and new fuel can be continuously fed into the reactor. This allows LFTR to produce power continuously. In comparison, LWRs will experience downtime during refuelling since the reactor must be shut down before the spent fuel rods can be taken out and replaced by new ones.
  • The reactor core of a LFTR is fail safe since it contains a freeze plug at the bottom which has to be actively cooled using a small electric fan. If the cooling fails because of a power outage or an emergency, the freeze plug melts and the fuel gravitationally drains from the reactor core into a passively cooled storage facility which rapidly shuts down the reactor. Since the drained fuel does not require active cooling to keep it from overheating, an incident like the Fukushima-Daiichi nuclear accident is impossible to occur for a LFTR. Once the power outage or emergency is over, the drained fuel can be fed back to the reactor core and it is business as usual for the LFTR.
  • Unlike a LWR, it is impossible for a LFTR to experience a nuclear meltdown since the fuel in the reactor core is already molten in normal operation.

With a LFTR, a lunar settlement can be entirely self-sufficient. Energy produced from a LFTR can be used to power a wide range of activities which include dissociating water to produce rocket fuel, growing food on the Moon even during the 2 week lunar night, processing lunar material, HVAC (heating, ventilation and air conditioning), life support systems, lighting, communications and the recycling of water, air and waste products. In fact, to power any settlement on any planet or moon in the Solar System, nuclear power generation systems, especially LFTRs, will be well suited for such purposes. This is because nuclear systems can provide power during the night, are not affected by the Sun’s proximity or orientation, can operate in dusty environments, are compact, have a high specific power, can be scaled to very high power levels, can potentially have very long lifetimes and can serve as a source of heat in addition to electricity generation.

Figure 4: This is a split image of Shackleton with elevation map (left) and shaded-relief image (right). Shackleton is a 21 kilometre diameter crater located adjacent to the lunar South Pole. Its interior is permanently shadowed and large deposits of frozen water are known to exist within it. Credit: NASA/Zuber, M.T. et al., Nature, 2012.

If the LFTR is such an attractive means of provide power to lunar settlements, they should also be very useful here on Earth as a cheap, clean, safe and reliable means of energy generation. In July 2001, the Generation IV International Forum which consists of a dozen or so governments was established to explore the feasibility and performance capabilities of the next generation nuclear energy systems. Listed are a number of competing technologies. Most of them are advances to existing technologies and only the molten salt reactor (MSR) is truly different from the rest. The LFTR is a type of MSR and its huge benefits have fuelled a renewed interest worldwide. There is sufficient easily accessible thorium on Earth to provide carbon-free energy to meet the world’s energy needs for many thousands of years. To sum up, LFTRs can deliver what fusion promises but without the numerous difficulties that plague conventional uranium-fuelled reactors.

Saturday, August 25, 2012

Winds on Hot Jupiters

Hot Jupiters are a class of Jupiter-mass exoplanets that are characterised by high surface temperatures as they orbit very close to their parent stars. Most hot Jupiters have near-circular orbits with near-zero orbital eccentricities. A perfectly circular orbit is one where the orbital eccentricity is zero. There exist a fraction of hot Jupiters that have non-circular orbits with orbital eccentricities exceeding 0.1. One of the most extreme cases is the exoplanet HD80606b which has an orbital eccentricity of 0.93. Due to its large orbital eccentricity, the amount of flux HD80606b receives from its parent star varies by a factor of over 800.

Superrotation is a common phenomenon in atmospheric circulation models of hot Jupiters. Planetary scale waves in the atmosphere of a hot Jupiters converges angular momentum from the mid-latitudes towards the equator to generate an equatorial superrotating jet. The presence of an equatorial superrotating jet causes an eastward displacement of the hottest spot on the planet from the substellar point. Hot Jupiters in near-circular orbits are expected to be synchronously rotating where the same hemisphere of the planet perpetually facing its parent star like how the same hemisphere of the Moon always faces the Earth. This is due to the fact that the rotational and orbital periods of a synchronously rotating planet are equal. Things are different for hot Jupiters on non-circular eccentric orbits since they are expected to be pseudo-synchronously rotating where the same hemisphere of the planet does not perpetually face its parent star because the planet’s rotational and orbital periods are not equal. Instead, the same hemisphere of an eccentric hot Jupiter approximately faces its parent star only at closest approach during each orbit.

Figure I: Simulations of a hot Jupiter with an orbit-averaged stellar flux of 185691 W/m2 where the orbital eccentricity is increased from zero (top row) to 0.75 (bottom row). Illustrated here are plots of the orbit-averaged zonal wind speeds (left column) and the wind and temperature profiles at 30 millibars in the atmosphere during closest approach (right column). The vertical bars in the right column denote the substellar longitude. (Credit: Tiffany Kataria, et al., 2012)

Models of atmospheric circulation of eccentric hot Jupiters have shown that at closest approach to their parent stars, dayside temperatures, day-night temperature differences and wind speeds all increase with increasing orbital eccentricity. In Figure I shown above, an orbit-averaged stellar flux of 185691 W/m2 is employed for a hot Jupiter to model the effects of increasing orbital eccentricity on atmospheric circulation. As orbital eccentricity varies from 0 to 0.75, the peak temperatures at closest approach increase from 1000 to 1300 degrees Kelvin at the 30 millibar level in the atmosphere. Since the day-night temperature difference increases with increasing orbital eccentricity, the peak wind speed within the equatorial superrotating jet strengthens from 2500 m/s for a circular orbit to 5000 m/s for an orbital eccentricity of 0.75.

A larger orbital eccentricity also results in a shorter rotational period for the hot Jupiter which translates to an increase in rotational rate and a smaller Rossby radius of deformation. This is because the Rossby radius is inversely proportional to the square root of rotational rate. As a result, a hot Jupiter with a larger orbital eccentricity is expected to have a narrower equatorial superrotating jet since the width of the superrotating jet is directly proportional to the Rossby radius of deformation. In Figure I shown above, an orbital eccentricity of zero gives an average jet width of 100 degrees latitude while an orbital eccentricity of 0.75 gives an average jet width of 40 degrees latitude. Apart from the effects of orbital eccentricity, an increase in the orbit-averaged stellar flux shown below in Figure II also leads to strengthening and narrowing of the equatorial superrotating jet. A planet with a higher orbit-averaged stellar flux orbits its parent star at a smaller distance, resulting in a faster rotation rate and a narrower equatorial superrotating jet.

Figure II: Simulations of a hot Jupiter with an orbital eccentricity of 0.25. From top to bottom, the rows illustrate an increase in the orbit-averaged stellar flux. Shown here are plots of the orbit-averaged zonal wind speeds (left column) and the wind and temperature profiles at 30 millibars in the atmosphere during closest approach (right column). The vertical bars in the right column denote the substellar longitude. (Credit: Tiffany Kataria, et al., 2012)

Tiffany Kataria, et al., 2012, “Three-dimensional atmospheric circulation of hot Jupiters on highly eccentric orbits”, arXiv:1208.3795v1 [astro-ph.EP]

Friday, August 24, 2012

Electroweak Stars

A neutron star is a type of stellar remnant that is left behind after the supernova explosion of a massive star and it consists almost entirely of neutrons. With roughly the mass of the Sun packed into an object measuring just several kilometres across, a neutron star is so dense that a cubic centimetre of its material contains an average mass of a few hundred million metric tons. Such a star is supported against further gravitational collapse by quantum degeneracy pressure where no two neutrons can occupy the same quantum state simultaneously. Between a neutron star and a black hole, another possible stable state known as a quark star can exist. A quark star is even denser than a neutron star and it is made up of quarks instead of neutrons. Similar to a neutron star, quantum degeneracy pressure prevents a quark star from gravitationally collapsing into a black hole. Above roughly 2 to 3 times the mass of the Sun, gravity eventually prevails and a neutron star or quark star is expected to collapse completely to form a back hole.

This image illustrates the size of a typical neutron star in comparison with the size of Manhattan Island.

During the gravitational collapse of a compact star, it is possible that the ever increasing densities and temperatures will eventually cause the distinction between electromagnetic and weak nuclear forces to break down. When this happens, quarks are able to convert into leptons in a process known as electroweak burning which is estimated to last for several million years. The energy produced during electroweak burning can be sufficient to stall the gravitational collapse of the compact star. Throughout this period of electroweak burning, the compact star is known as an electroweak star. Electroweak burning occurs within the core of the star, in a small and incredible dense volume measuring just several centimetres across and containing about twice the mass of Earth. Within this volume, the burning of quarks produces neutrinos which flow out of the central core by diffusion. Neutrinos cannot flow freely out of the electroweak star because the density within the core is so high that matter is opaque even to neutrinos and the mean free paths of all particles are small in relation to the size of the star.

As the neutrinos travel away from the core of the electroweak star, both the local matter density and the energy of the neutrinos will decrease, causing the mean free path of the neutrino particles to increase. The decrease in neutrino energy as a neutrino travel towards the surface of the star is due to gravitational redshift and the opacity of the high density medium through which the neutrino is travelling through. At a certain distance from the centre of the electroweak star, the neutrino’s mean free path will exceed the thickness of the star’s overlying matter. This distance denotes the position of the neutrinosphere and neutrinos crossing this boundary will freely leave the star. As such, there is no backward flow of neutrinos beyond the neutrinosphere. The radial position of the neutrinosphere from the centre of the electroweak star is directly proportional to the initial energy of the neutrinos that are produced from electroweak burning in the star’s core.

A model by De-Chang Dai et al. (2011) consists of an electroweak star with 1.3 times the mass of the Sun and a radius of 8.2 kilometres. If the initial neutrino energy is 300GeV, the radius of the neutrinosphere will be 8.1 kilometres, which places it not far under the surface of the star. This is consistent with the electroweak burning process since it produces neutrinos with energies around 300GeV. For the modelled electroweak star, its minimum lifespan is estimated to be on the order of 10 million years. Electroweak stars are an interesting new class of exotic astrophysical bodies. However, a lot more investigation is still needed to see if such objects can indeed be created from the natural processes of stellar evolution and if they can burn stably for extended periods of time.

Monday, August 20, 2012

Ice Caves on Mars

There is direct evidence for the existence of caves on Mars. These subterranean cavities can prove useful for habitats on Mars since they offer natural protection from radiation, insulation from temperature cycling and sealability to contain breathable atmospheres. Many of these Martian caves can also serve as repositories with perennial water ice deposits. Such caves are termed ice caves and besides the advantages that normal caves offer, ice caves provide direct access to water which make them particularly attractive. Water ice in ice caves can accumulate via a number of mechanisms which include freezing of accumulated water, wind-blown snow and deposited frost.

A large ‘skylight’ entrance into a cave on the slopes of the Pavonis Mons volcano. The entrance is about 35m in diameter and 20m deep, and the floor of the cave is illuminated. (Credit: NASA/JPL/University of Arizona)

In order for an ice cave to exist, it is not necessary that the external ambient air temperature must be below freezing for most of the year. This is possible as long as the geometry of the cave allows it to function as a “cold trap”. A typical geometry for an ice cave is a subterranean cavity with a single entrance leading up to the surface. In winter, air in the cave tends to be warmer than the outside air. Since cold air is denser than warm air, cold air settles into the cave and displaces the warmer air inside. In summer, air in the cave tends to be cooler than the outside air and so continues to remain in the cave. The ceiling thickness of the cave also needs to be sufficient to suppress the seasonal temperature variations that occur on the Martian surface. In this case, a cave ceiling thickness of a metre or more will be enough to thermally isolate the cave.

Since air is only exchanged when the outside air is cooler than air in the cave, the “cold-trapping” effect of an ice cave allows for the accumulation and preservation of water ice over long timescales. Water ice in an ice cave is loss through a process of ice sublimation as the atmospheric pressure on Mars is too low for water ice to melt. Ice sublimation in an ice cave is expected to be a slow process. Additionally, the condensation of carbon dioxide ice over water ice and the deposition of frost produced from the cooling of air that is humidified through ice sublimation can further slow the ice sublimation process. Even in the absence of further accumulation, water ice in an ice cave can persist for tens of thousands of years. It is possible for ice caves on Mars to contain water ice from a pervious epoch in Mars’ recent history about 100 thousand years ago. Back then, the planet had a much greater axial tilt which supported a different global climate that favoured the widespread accumulation of water ice at mid-latitudes.

1. K.E. Williams, et al., “Do ice caves exist on Mars?” Icarus 209 (2010) 358-368
2. D.L. Murphy, et al., “Human utilization of subsurface extraterrestrial environments,” Gravitational and Space Biology Bulletin 16(2), pg 121-131, June 2003

Lunar Underground

High resolution images from NASA’s Lunar Reconnaissance Orbiter (LRO) have revealed the existence of steep-walled pits on a number of locations on the surface of the Moon. These pits are believed to be surface entrances into subterranean voids of yet unknown lateral extent. Caverns on the Moon are particularly attractive for any long-term human presence on since they offer protection from surface hazards such as temperature cycling, radiation and micrometeoroids.

The absence of raised crater rims and ejecta fields rules out the possibility that these pits are created solely by objects impacting onto the lunar surface. Furthermore, the steep wall slopes and the high depth-to-diameter ratios of these pits are inconsistent with an impact origin. Nevertheless, it is possible that the origin of such a pit may have been initiated by an impact event which caused the surface to collapse into an underlying cavern.

Figure 1: Mare Tranquillitatis Pit. The maximum and minimum pit diameters are 100m and 86m respectively, and the maximum depth of the pit floor below the surface is 105m. (Location: 8.34N 33.22E) (A: M126710873R; B: M155016845R; C: M175057326R; D: M152662021R; E: M155023632R; F: M144395745L; Credit: NASA/GSFC/Arizona State University; M.S. Robinson, et al., 2012)

Figure 2: Marius Hills Pit. The maximum and minimum pit diameters are 57m and 48m respectively, and the maximum depth of the pit floor below the surface is 45m. (Location: 14.09N 303.23E) (A: M122584310L; B: M155607349R; C: M137929856R; D: M155614137R; Credit: NASA/GSFC/Arizona State University; M.S. Robinson, et al., 2012)

Sunday, August 19, 2012

Reionization by Massive Runaway Stars

Between 150 million and one billion years after the Big Bang, the universe underwent a phase of reionization where neutral hydrogen gas was transformed into ionized plasma. Energetic sources likely to be responsible for the reionization of the early universe include quasars, population III stars and quark-novae. Another possible mechanism for reionization in the early universe is massive runaway stars that travel sufficiently far from their birthplaces, allowing most of their ionizing radiation to ionize the neutral hydrogen comprising the intergalactic medium.

NGC 4449 - An irregular galaxy located about 12 million light-years away.

In the Milky Way galaxy, a significant fraction of massive stars travel at velocities exceeding 30 km/s. Their high speeds allow them to travel up to thousands of lights years away from their birthplaces before exploding as supernovae at the end of their several million year lifespans. Such a massive runaway star can be produced through dynamical ejection from a densely packed star cluster or by the explosion of a companion star in a binary star system. In the early universe, galaxies were much smaller than they are today. This enabled runaway massive stars to venture out of their host galaxies into the low-density outer regions. Here, the ionizing radiation they produce can ionize the intergalactic medium without being attenuated by the dense interstellar medium permeating the inner regions of their host galaxies.

The influence of massive runaway stars on the reionization of the universe can be directly tested by observing galaxies in the early universe. If massive runaway stars are common, they should dominate the stellar emission at the outer regions of these galaxies. That will manifest itself as an increase in the proportion of ultraviolet flux at the outskirts of such galaxies. Additionally, observational evidence for a high occurrence rate of supernovae far from the inner regions of these early galaxies will support the prevalence of massive runaway stars.

Wednesday, August 15, 2012

Snows of Tharsis Montes

On Earth, tropical glaciers are a rarity and they are only found near the Equator on the mountains of the Indonesian province of Papua, East Africa and in the Andean mountains of South America. The total area of these tropical glaciers represents only 4 percent of the combined area of Earth’s mountain glaciers. Tropical glaciers are unique because they experience very minimal seasonal variations and without a colder winter season where ice can accumulate, these glaciers are more susceptible to climate change. The only point exactly on the Earth’s Equator with permanent snow cover is found on the south slope of Mount Cayambe in Ecuador and at 4690 metres, this is also the highest point in the world crossed by the Equator.

On Mars, huge fan-shaped deposits of glacial origin are known to cover the northwest flanks of the massive Tharsis Montes volcanoes located within the tropics. The Tharsis Montes are three enormous shield volcanoes and Ascraeus Mons is the tallest of them, with a summit elevation of over 18 km above the Martian datum. Based on the sizes of these fan-shaped deposits, each glacier once covered an area of over 100,000 square kilometres. Climate models of Mars have shown that during periods when the axial tilt of Mars was much larger, the climate would have been conducive for the formation of such tropical glaciers. Like the Earth, Mars also experiences Milankovitch cycles which causes the axial tilt of Mars to vary over a 120,000 year cycle. However, Mars’ axial tilt has a larger variation than the Earth’s since Mars does not have a large moon like the Earth to provide a stabilizing influence. The axial tilt of Mars can exceed 45 degrees. In comparison, the current axial tilt of Mars is 25.2 degrees, which is almost the same as the Earth’s.

Fan-shaped deposits (yellow) on the northwest flanks of the Tharsis Montes interpreted to represent tropical ice-age glaciation (Head and Marchant, 2003) (top, Ascraeus Mons; middle, Pavonis Mons; and bottom, Arsia Mons).

With a much larger axial tilt, the climate of Mars would have been quite different as water will sublimate at the polar ice caps and be transported to the tropics. During such periods in Mars’ history, the presence of prevailing winds blowing west to east over the Tharsis Montes volcanoes would have generated strong upwelling and cooling of moist air as the air mass is forced up the slopes of these enormous mountains. This allows the moisture to precipitate as snow on the western flanks of the Tharsis Montes volcanoes to form extensive tropical mountain glaciers. In climate models, the volume of water ice currently in the polar ice caps of Mars served as the source of moisture for the development of these glaciers. On Mars, the extremely cold and hyper-arid environment means that these tropical mountain glaciers are quite different compared to glaciers on Earth. For these Martian glaciers, sublimation serves as the dominant mechanism for mass removal, with buoyancy-driven sublimation and turbulence-driven sublimation being the two sublimation mechanisms. On Earth, melting is the primary mechanism through which a glacier loses mass.

In Mars’ carbon dioxide dominated atmosphere, buoyancy-driven sublimation depends on the density difference between water vapour and carbon dioxide while turbulence-driven sublimation depends on the amount of air blowing over the surface of the glacier. The ability for a glacier to develop and grow on Mars depends on the sublimation and accumulation rates of ice. There must be net accumulation of ice for a glacier to grow. As temperature declines, the sublimation and accumulation rates also decline. Just as on Earth, temperature decreases with altitude on Mars and this means that sublimation and accumulation rates decline with increase in altitude. However, sublimation rates decline less rapidly than accumulation rates and this lead to a negative net accumulation of ice at higher elevations. As a result, contrary to what is seen on Earth, snow-capped mountains are not expected since ice is more likely to accumulate on the flanks than on the summits of the towering Tharsis Montes volcanoes.

At the lowest elevations, increase in turbulence-driven sublimation and possible melting can also lead to a negative net accumulation of ice. Therefore, two equilibrium lines can exist on Mars where one equilibrium line denotes the lowest altitude and the other denotes the highest altitude where ice can accumulate. Estimations of the total ice volume of the tropical mountain glaciers through simulations have shown it to be comparable to the volume of the current north polar ice cap on Mars, which is estimated to be about 1.6 million cubic kilometres. In comparison, the ice sheet covering Greenland on Earth has a volume of 2.85 million cubic kilometres. As such, the inventory of water ice locked in the current polar ice caps on Mars is sufficient to produce the large tropical mountain glaciers during epochs of high axial tilt in Mars’ history.

1. James L. Fastook, et al., “Tropical Mountain Glaciers on Mars: Altitude-dependence of Ice Accumulation, Accumulation Conditions, Formation Times, Glacier Dynamics, and Implications for Planetary Spin-axis/Orbital History”, Icarus 198 (2008) 305-317
2. Seth J. Kadish, et al., “The Ascraeus Mons Fan-shaped Deposit: Volcano-Ice Interactions and the Climatic Implications of Cold-based Tropical Mountain Glaciation”, Icarus 197 (2008) 84-109

Wednesday, August 8, 2012

Triton’s Subsurface Ocean

Triton is the only large moon in orbit around the planet Neptune and it is also the seventh largest moon in the Solar System. Measuring 2700 km across and with an average density of 2065 kg/m3, the internal structure of Triton is consistent with an icy mantle surrounding a large rocky core. With a mean surface temperature of minus 235 degrees Centigrade, the surface of Triton is predominantly water ice with trace amounts of frozen nitrogen, carbon monoxide, carbon dioxide and methane. Triton orbits Neptune in a direction that is opposite to the planet’s spin, making it the only large moon in the Solar System with such an orbit. As a result, Triton could not have formed in situ around Neptune. The leading hypothesis suggests that Triton was once part of a binary system which came too close to Neptune, resulting in Triton being captured into orbit around Neptune.

Following capture, tidal interactions with Neptune over a few hundred million years led to the circularization of Triton’s initial orbit to its current near-circular orbit. This process dissipated a large amount of tidal energy in the form of heat within Triton’s interior. The amount of tidal heating is estimated to be sufficient to melt Triton entirely. Given that Triton has a thick icy mantle surrounding a rocky core; it is worth investigating if an ocean formed during this phase of tidal dissipation could be sustained for billions of years until the present time. As Triton cools, an ice shell will form over an underlying global ocean of liquid water. The rate at which this ice shell thickens depends on the amount of tidal dissipation in the ice shell and the amount of heat flux from the decay of radioactive nuclei within Triton’s large rocky core.

After the circularization of Triton’s initial high eccentricity orbit, Triton is still expected to maintain a small but non-zero orbital eccentricity. For this reason, the small variation in the distance of Triton from Neptune over each orbit causes the ice shell of Triton to be stretched differentially. This allows tidal dissipation to occur within the ice shell which tends to preferentially heat the base of the ice shell. A thin ice shell creates a larger amount of tidal dissipation since a thin shell is more easily deformed by tides. Tidal dissipation within the ice shell decreases as the ice shell thickens. For an ocean that is completely frozen, tidal dissipation decreases sharply as the ice shell becomes locked with the rocky core as it is no longer mechanically decoupled from the rocky core by an intervening layer of liquid water.

The basal heating of the ice shell due to tidal dissipation slows the thickening of the ice shell. Since tidal dissipation is directly proportional to the square of orbital eccentricity, a larger non-zero orbital eccentricity for Triton will generate a larger amount of tidal dissipation. The depth of Triton’s ocean measured from the moon’s surface to the rocky core is estimated to be about 400 km. In this study, a non-zero orbital eccentricity of just 0.00005 is sufficient to leave behind a 130 km thick ocean under a 270 km thick ice shell after 4.5 billion years. Even an orbital eccentricity of 0.00004 is just sufficient to prevent the ocean from completely freezing over 4.5 billion years.

Compared with the heat generated from the decay of radioactive elements, the heat produced from tidal dissipation is orders of magnitude less. Even so, tidal dissipation plays a fundamental role in determining the rate of thickening of the ice shell. Tidal dissipation within the ice shell heats the base of the ice shell and better insulates the rest of the underlying ocean from freezing. This is because warming the base of the ice shell reduces the thermal gradient which lowers the heat flux escaping through the ice shell from the underlying ocean. With an orbital eccentricity of only 0.00010, the thickening of the ice shell is stalled completely, allowing Triton to continuously maintain a 100 km thick ice shell overlying a 300 km thick global ocean of liquid water up to the present time. Such an ocean will contain over three times more water than all oceans on Earth combined.

J. Gaeman et al., “Sustainability of a Subsurface Ocean within Triton’s Interior”, Icarus 220 (2012) 339–347

Tuesday, August 7, 2012


Dark matter constitutes 84 percent of the matter in the universe and unlike normal matter, dark matter neither emits nor absorbs electromagnetic radiation and its existence can only be observed from its gravitational effects. A class of exotic bodies known as compact ultra dense objects (CUDOs) can serve as a potential candidate for dark matter. CUDOs are incredibly dense gravitationally bound objects with typical densities several orders of magnitude greater than nuclear density. At the upper mass limits, a CUDO with the mass of planet Mars is expected to measure just 20 cm across. However, a typical CUDO is likely to be a lot less massive.

Rocky bodies in the Solar System such as the Earth, Mercury, Venus, Mars, the Moon and the large asteroid Vesta can be used to detect the presence of CUDOs since the impact signature of a CUDO with a rocky body is likely to persist over geological timescales. The very high surface gravity of a CUDO keeps its stable and intact during an impact event. A CUDO entering the Solar System is likely to be moving at 30 to 50 km/s. As such, an impacting CUDO travelling through the Earth’s mantle will be going supersonic since the speed of sound in the mantle is only around 8 km/s.

Image: South Pole of Vesta (Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

During its passage through the Earth, the strong gravitational pull from the CUDO is expected to entrain a fair amount of terrestrial material along its trajectory. This is expected to slow the CUDO as it transfers some of its kinetic energy to the entrained material. Although the loss of kinetic energy is likely to be small enough such that the CUDO will pass completely through the Earth, the loss may still be sufficient to cause the CUDO and its entrained material to be captured into an orbit around the Sun. The supersonic shock wave generated by the passage of the CUDO through the Earth should disrupt surface geology over a wide area as the shock wave reaches and interacts with the Earth’s crust.

Siderophile elements are “iron-loving” transition metals which dissolve readily in iron and they are very rare in the Earth’s crust since most of them are concentrated within the Earth’s iron core. The ability for a CUDO to pull material in the direction of its passage creates a transport mechanism which allows material to be dragged up from the Earth’s interior. As a result, deposits of siderophile elements such as gold and platinum in the Earth’s crust could be created by the dragging up of core material from the passage of CUDOs through the Earth. Furthermore, the exit of a CUDO after its passage through the Earth can deliver a large amount of material into the Earth’s upper atmosphere as the CUDO sheds some of its entrained material. The effects from such an event can be catastrophic for life on Earth. Interestingly, a CUDO exiting the Earth from an ocean can entrain a mantle of water which allows it to disguise itself as a comet.

Isolated instances of volcanism on the Earth’s crust can be produced from the exit of a CUDO as its trajectory through the Earth pulls up material and creates a mantle plume beneath the crust. One possible example is the hot spot responsible for the Hawaiian Islands in the middle of the Pacific Ocean, far from any plate boundary. Additionally, solid bodies such as Mercury, Mars, the Moon and Vesta are generally considered to be volcanically inactive. As a result, geological features indicated of recent volcanic activities on these bodies could be attributed to the passage of CUDOs through them.

A CUDO that happens to be captured into an orbit around the Sun after impacting and exiting a planet can disguise itself as either a comet or an asteroid, depending on the type of material it entrains. Such an object can give itself away as it is expected to have a higher apparent density. In the unlikely but nevertheless interesting possibility of a CUDO being gravitationally captured by its target rocky planet, multiple entries and exits can be produced as the CUDO sheds its kinetic energy through the bulk of the planet. An event like this can create surface geological features that are indicative of an odd number of nearly coincidental impacts.

1. Johann Rafelski, et al. (2012), “Solar System Signatures of Impacts by Compact Ultra Dense Objects”, arXiv:1104.4572v2 [astro-ph.EP]
2. Lance Labun, et al., “Properties of Gravitationally Bound Dark Compact Ultra Dense Objects”, Physics Letters B 709 (2012) 123–127