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.

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.

References:

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)

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.

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.

References:

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

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

CUDOs

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.

References:

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

New Worlds from Existing Data

Kepler is a planet-hunting telescope that searches for planets by precisely measuring the dip in a star’s brightness when a planet happens to pass in front of its host star in an event known as a transit. The primary goal of Kepler is to determine the frequency of Earth-like planets around Sun-like stars. To date, over 2000 planet candidates have been found by Kepler with a trend towards an increasing number of Earth-size planet candidates at larger distances from their host stars as such planet candidates are harder and take longer to sniff out. An independent project to reanalyse the existing public Kepler dataset revealed 84 new transit signals on 64 star systems. Each transit signal represents a planet candidate and I shall describe some of the more notable ones.

Image Credit: NASA/JPL-Caltech

KOI 435

This system was known to contain two transit signatures with the first having a period of 20.55 days and the other yet to be determined. The four new transit signatures found in this reanalysis have periods of 3.93 days, 33.04 days, 66.30 days and 9.92 days. Previously, the only know star with six transiting planets was the Kepler-11 system.

KOI 1574

A transiting planet candidate with a period of 114.74 days is known to exist around KOI 1574. In this reanalysis, a second signal with a period of 191.51 days was detected. The outer planet candidate seems to be in a 3:5 orbital resonance with the inner one. What is more interesting is that the outer planet candidate is about twice the Earth’s diameter and its equilibrium temperature is estimated to be 281 degrees Kelvin. Given that the average surface temperature of the Earth is 287 degrees Kelvin, the outer planet candidate of KOI 1574 is a promising Earth-like planet.

KOI 277 (Kepler-36)

Previously known to have a single transiting planet candidate with a period of 13.84 days, this reanalysis found a second transit signal with a period of 16.24 days. The two planet candidates around KOI 277 are remarkable in the sense that their orbits bring them within 5 Earth-Moon distances from one another. On the last day of work on the paper detailing this reanalysis of the Kepler dataset, the Kepler team announced their discovery of the two planets around KOI 277 and named the system Kepler-36. The inner planet (Kepler-36b) is a super-Earth and the outer planet (Kepler-36c) is a mini-Neptune. These planets are twenty times more closely spaced than any adjacent pair of planets in the Solar System. At closest approach, an observer on Kepler-36b will see Kepler-36c appear 2.5 times larger than the Moon as seen from Earth.

KOI 1843

With two planet candidates already known with periods of 4.19 days and 6.36 days, a third transit signal with an incredibly short period of 0.176 days (4.25 hours) was revealed in this reanalysis. For each day on Earth, 5.65 years would have elapsed on this planet candidate. KOI 1843 is a cool and small star with an effective temperature of 3673 degrees Kelvin and 52 percent the Sun’s diameter. The third transit signal has a very small transit depth of 120 parts-per-million which means that this planet candidate is just 68 percent the diameter of the Earth, making it one of the smallest exoplanet candidates known to date.

Reference: Aviv Ofir and Stefan Dreizler (2012), “An Independent Planet Search in the Kepler Dataset. I. A hundred new candidates and revised KOIs”, arXiv:1206.5347v1 [astro-ph.EP]

Venusian Snow

Although Venus has a similar size, mass, gravity and bulk composition as the Earth, the conditions on its surface are unlike anything on Earth. Venus is characterized by a massive carbon dioxide atmosphere which gives a surface pressure that is over 90 times the sea-level pressure here on Earth and a hellish average surface temperature of 740 degrees Kelvin. Near the surface of Venus, the temperature is above the melting points of metals such as lead, tin and zinc. However, at an altitude of 50 kilometres up in the Venusian atmosphere, the atmospheric temperature and pressure are similar to those found on the Earth at sea-level.

Radar observations of the surface of Venus have shown a brightening of the radar reflection from higher elevation regions on Venus. It is believed that the substance responsible for the higher radar reflectivity formed from a process that is similar to the formation of snow on Earth, albeit at a far higher temperature. The furnace-like environment of Venus’ lower atmosphere means that water is not a possible candidate material for this highly reflective substance. Instead, the highly reflective substance is likely to be a heavy metal frost consisting of one or more types of volatile compounds. In this case, the Venusian highlands serve as areas where the temperature is cool enough for these heavy metal compounds to condense and be deposited as frost. The source of these heavy metal compounds is likely to be volcanic in nature. On Earth, these heavy metal compounds are stable solids but the high temperatures on Venus allow many of these compounds to become volatile.

This is a radar image from NASA’s Magellan spacecraft centred along the eastern edge of Lakshmi Planum and the western edge of Maxwell Montes. The highlands on the right are covered in bright “snow” and are 5 kilometres above the above the adjacent plains in Lakshmi Planum. Credit: NASA/JPL

Standing 11 kilometres high, Maxwell Montes is the tallest mountain on Venus and with a temperature of 650 degrees Kelvin at its summit, the top of Maxwell Montes is the coolest location on the surface of Venus. Radar observations of Maxwell Montes show that most of the mountain is covered a layer of highly reflective substance. For this reason, Maxwell Montes serves as a good example of a cool highland region that is covered by a layer of heavy metal frost and as a “snow-capped mountain” on Earth’s scorching planetary neighbour.

Ice Floats on Titan’s Lakes

Seas and lakes of liquid hydrocarbons are known to exist on the surface of Titan beneath a thick atmosphere of nitrogen and methane. It is a common assumption that ice on a liquid hydrocarbon lake is negatively buoyant, causing any ice which forms on the lake’s surface to sink towards the bottom. This is because methane ice is denser than liquid methane and this results in a behaviour that is opposite of water where water ice is less dense than liquid water. A paper by Roe and Grundy (2012) titled “Buoyancy of ice in the CH4-N2 system” suggests that contrary to common assumption, the conditions that exist on Titan can allow ice to float on a liquid hydrocarbon lake.

Cassini delivers this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene. Credit: NASA/JPL/Space Science Institute

Given that nitrogen is the primary constituent of Titan’s atmosphere and nitrogen is soluble in a hydrocarbon mixture, a substantial presence of dissolved nitrogen is expected in the hydrocarbon lakes on Titan. In this study, heavier hydrocarbons are ignored and a liquid solution with a mole fraction abundance of 30.7 percent nitrogen and 69.3 percent methane at a temperature of 88.2 degrees Kelvin is used. In order for ice to start forming, the liquid solution has to be cooled to 78.1 degrees Kelvin where the ice that is formed will have a mole fraction abundance of 16.5 percent nitrogen and 83.5 percent methane. At 78.1 degrees Kelvin, the liquid solution has a density of 0.574 grams per cubic centimetre.

Two methods are used to estimate the density of the ice that is formed. The first method assumes an ideal solution where the weighted mean of the densities of nitrogen and methane are summed up to give a density of 0.564 grams per cubic centimetre. The second method uses a lattice replacement assumption where the lattice structure of the methane ice remains unchanged while a nitrogen molecule replaces a methane molecule and this gives a density of 0.549 grams per cubic centimetre. In either case, the density of the ice is less than the density of the liquid solution and the ice will float. Hence, the decrease in density from the decrease in nitrogen abundance of the ice is larger than the increase in density from the freezing of the liquid solution.

A liquid solution of nitrogen and methane is a simplification since Titan’s lakes contain a mixture of other hydrocarbons and a more accurate study will need to consider these other hydrocarbons. The seasonal variation in surface temperature on Titan is unlikely to be sufficient to allow ice to form on Titan’s lakes. However, it is still possible for ice to be present on the surface of a lake on Titan after a hail storm or a torrential downpour of methane. A hail storm that is large enough can even provide sufficient cooling to freeze the surface of the lake. Raindrops of methane arriving at the surface of a lake from a torrential downpour are expected to be cooler than the local atmosphere and contain some amount of dissolved nitrogen. As the lake consists of a mixture of heavier hydrocarbons, this newly precipitated liquid solution of nitrogen and methane is expected to be less dense and will form a floating layer on the lake’s surface. The volume of this layer can be greatly boosted by drainage from surrounding terrain. Cooling delivered by winds can cause ice to form on this cooler and less dense surface layer of the lake.

When ice floats on the surface of a lake, it isolates the rest of the lake from the atmosphere and allows it to remain liquid beneath. Floating ice also affects the rate of evaporation of methane back to the atmosphere and this can have huge effects on the climate and atmosphere of Titan if lake evaporation is a significant contributor to the atmospheric abundance of methane.