Smallest Hydrogen-Burning Star Yet Discovered

COROT is a space telescope and the first of its kind designed to search for transiting extrasolar planets. It detects planets by measuring the slight dimming caused when a planet happens to cross in front of its parent star. Besides discovering extrasolar planets, COROT has discovered what could be the smallest known hydrogen-burning star. C4780 was initially identified as a transiting planet candidate, but follow-up observations reveal that this object is an eclipsing binary system consisting of 2 stars. The primary star is an F-type star while the secondary star is a low mass red dwarf star which whizzes around the primary on a 20.7 day orbit. Each time the red dwarf star passes in front of the primary star, it causes the primary star to dim by a small amount and allows the size of the red dwarf star to be estimated at one-tenth the Sun’s diameter. This makes the red dwarf star similar in size to the planet Jupiter and slightly smaller than the next smallest red dwarf star with a measured diameter.

As the red dwarf star orbits the primary F-type star, the gravitational pull from the red dwarf star causes a measurable wobbling of the primary star. By measuring this wobbling, the mass of the red dwarf star is estimated to be less than one-tenth the Sun’s mass. This places the red dwarf star close to the minimum mass for a hydrogen-burning star. With its size (0.104 Sun’s diameter) and mass (0.096 Sun’s mass) known, the mean density of the red dwarf star is estimated to be 240 tons per cubic meter, or about 30 times the density of solid iron. Although similar in size to the planet Jupiter, this red dwarf star contains about 100 times the mass of Jupiter. Having such a high density, this red dwarf star is probably the densest hydrogen-burning star discovered so far. On the red dwarf star’s surface, the gravity will be crushing - about 250 times stronger than the surface gravity on Earth. For very low mass stars with masses less than 1/5 the Sun’s mass, only a handful have their masses and diameters measured. A larger sample of such stars with measured masses and diameters will certainly improve the understanding of stars in the lower end of the stellar mass spectrum.

Reference:

Lev Tal-Or, et al. (2013), “CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star”, arXiv:1302.5830 [astro-ph.EP]

Capturing an Ultra-Massive Black Hole

Ultra-massive black holes (UMBH) are known to exist in the cores of giant elliptical galaxies. These black holes have masses exceeding 10 billion times the mass of our Sun. Located approximately 220 million light years from the Milky Way, a galaxy known as NGC 1277 is known to harbour an unusually massive black hole with 17 billion times the mass of our Sun. This is odd because the galaxy NGC 1277 is far too small to be hosting a black hole with such an enormous mass. Typical supermassive black holes do not exceed 0.1 percent of the mass of their host galaxies but the mass of the black hole in NGC 1277 is equivalent to 14 percent of the total stellar mass of the galaxy. NGC 1277 is a modest sized lenticular galaxy that is a member of the Perseus Cluster of galaxies.

Figure: The Perseus Cluster of galaxies.

One possible scenario for the origin of the UMBH in the heart of NGC 1277 is that it did not form at where it currently resides. Instead, such an UMBH is expected to form during the merger of two giant elliptical galaxies, with each galaxy having its own supermassive black hole. Following the merger of the two galaxies, the final in-spiral and merger of the two supermassive black holes can generate sufficient gravitational radiation recoil to eject the resultant UMBH from the post-merger galaxy.

Not far from NGC 1277 is a giant elliptical galaxy known as NGC 1275. This galaxy is the dominant central galaxy of the Perseus Cluster of galaxies and it is likely to be the post-merger galaxy from which the UMBH was ejected. The ejected UMBH probably wandered in the core of the Perseus Cluster of galaxies before it was eventually captured by NGC 1277 during a close encounter. In order for the UMBH to be ejected from NGC 1275, the gravitational radiation recoil following the merger of the two supermassive black holes must be large enough to provide the necessary “natal kick” to escape the galaxy. Assuming the escape velocity from the nucleus of NGC 1275 is 1250 km/s, a “natal kick” of 1800 km/s will give the ejected UMBH a terminal velocity of 1300 km/s.

As the UMBH orbits within the core of the Perseus Cluster of galaxies, it collided with NGC 1277 and became gravitationally captured by the galaxy. Once captured into orbit around NGC 1277, dynamical friction with stars and matter in the galaxy causes the UMBH to gradually in-spiral towards the centre of the galaxy within a timescale of a few hundred million years. If the capture happened more than a billion years ago, that should give NGC 1277 ample time to settle down and regain its current symmetrical appearance. Furthermore, the capture of an UMBH is likely to impart a substantial amount of angular momentum to NGC 1277 and contribute significantly to the current rotation of the galaxy.

The presence of an UMBH in a modest sized galaxy like NGC 1277 shows that migration of black holes between galaxies is possible. In this case, a very massive black hole was ejected from its galaxy (NGC 1275) and got captured by a much smaller galaxy (NGC 1277). In fact, the present day appearance of NCG 1275 is consistent with such an event since the black hole currently residing in its core is too small for a galaxy of its size. The black hole in NGC 1275 is half a billion times the mass of our Sun and it is likely to be a moderate mass black hole that was regenerated after the ejection of the UMBH.

References:

1. van den Bosch and Remco C. E. et al. (29 Nov 2012), “An over-massive black hole in the compact lenticular galaxy NGC 1277”, Nature 491 (7426): 729-731.

2. G. A. Shields and E. W. Bonning (18 Feb 2013), “A Captured Runaway Black Hole in NGC 1277?”, arXiv:1302.4458 [astro-ph.CO]

A Tiny Planet Called Kepler-37b

With an ever increasing number of small exoplanets being discovered, the record holder for the smallest known exoplanet gets replaced fairly quickly. Recently, NASA’s Kepler mission has found a planetary system that is home to the smallest planet yet discovered around a Sun-like star. The discovery was published in Nature in a paper titled “A sub-Mercury-sized Exoplanet”. This tiny planet is named Kepler-37b and it belongs to a planetary system consisting of two other known planets - Kepler-37c and Kepler-37d. Kepler is a space telescope which finds planets by measuring tiny dips in a star’s brightness when a planet happens to pass in front of the star. When the planet Kepler-37b transits in front of its host star, it causes the star’s brightness to dim by a mere 20 parts-per-million. This gives Kepler-37b an estimated diameter of 3850 kilometres, making it smaller than Mercury (4880 km diameter) and only barely larger than the Moon (3475 km diameter).

Figure: The line up compares artist's concepts of the planets in the Kepler-37 system to the moon and planets in the solar system. The smallest planet, Kepler-37b, is slightly larger than our moon, measuring about one-third the size of Earth. Kepler-37c, the second planet, is slightly smaller than Venus, measuring almost three-quarters the size of Earth. Kepler-37d, the third planet, is twice the size of Earth. Image credit: NASA/Ames/JPL-Caltech

	Kepler-37b	Kepler-37c	Kepler-37d
Orbital Period (days):	13.4	21.3	39.8
Orbital Distance (million km):	15.0	20.5	31.1
Planet Diameter (Earth = 1.0):	0.303	0.742	1.99
Planet Diameter (km):	3850	9450	25400

Kepler-37 is the host star of the planetary system that is home to Kepler-37b and it is located at a distance of 210 light-years from Earth in the constellation Lyra. This is a star that is slightly cooler than the Sun with 0.770 times the Sun’s diameter and 0.802 times the Sun’s mass. Kepler-37b is the innermost of the three known planets around Kepler 37. At a distance of just 15 million kilometres from its host star, Kepler-37b orbits one every 13.4 days. In fact, all three known planets around Kepler 37 orbit closer than Mercury orbits the Sun, suggesting that these planets are too hot to be hospitable for life. Kepler-37b is close enough to its host star that it is very likely a scorched and airless world. Any form of volatiles would have long since vanished and its weak gravity prevents it from holding on to any appreciable atmosphere.

Reference:

Thomas Barclay, et al., “A sub-Mercury-sized Exoplanet”, Nature (2013), doi:10.1038/nature11914

When Giant Planets Collide

Giant planets like Jupiter are expected to exist around a significant fraction of stars in the galaxy. In fact, stars containing multiple giant planets in orbit around them are likely to be fairly common as well. Examples include our Sun with 2 giant planets (Jupiter and Saturn) and HR 8799 with 4 known giant planets. The process of planetary system formation around stars is a chaotic one where planets can merge with one another, fall into their host stars, get thrown out, etc. In planetary systems containing multiple giant planets, collisions between giant planets can occasionally occur. It is estimated that a few giant planet collisions are expected to happen in the galaxy each year.

Figure 1: An artistic illustration of the giant planet HR 8799 b. Credit: NASA, ESA and G. Bacon (STScI).

The outcome of a collision event depends on how two giant planets approach each other, their internal structure and the ratio of planet masses. There are 3 possible outcomes following the collision of two giant planets: (i) merger into a single object, (ii) grazing contact or (iii) total destruction of the planets. For the 3rd outcome to occur, the relative approach velocity during a collision event needs to be larger than the surface escape velocity of the giant planet. This is only valid for a class of giant planets called hot-Jupiters which orbit very close to their host stars. The orbital velocity of a hot-Jupiter around its host star is much larger than its surface escape velocity and this provides the kinetic energy needed for a total destruction following a sufficiently “head-on” collision. Giant planets found further from their host stars are moving too slow for total destruction of the planets to occur in a collision event.

Jupiter’s internal structure is believed to consist of a dense core that is surrounded by a thick layer of liquid metallic hydrogen and an outer layer of molecular hydrogen. Such an internal structure is probably typical for giant planets. The most interesting outcome following the collision of two giant planets is the 2nd outcome which involves a grazing collision. After colliding, the giant planets do not merge but continue moving away from each other. During the collision process, some fraction of material will be ejected. The ejected material will be heated to tens of thousands of degrees as kinetic energy gets converted into heat. If the geometric intersection of the giant planets is deep enough, liquid metallic hydrogen can get ejected. Since liquid metallic hydrogen is stable under high pressure, its ejection into the zero pressure environment of space may transform the metallic phase back into the dielectric phase with an energy release of 290 MJ/kg in the process. As a result, a collision event between giant planets is expected to release a prodigious amount of optical and near UV radiation.

Figure 2: The graph here shows a solid curve which represents a giant planet with an internal structure similar to Jupiter’s. The vertical axis represents the relative mass loss ΔM/M and the horizontal axis represents the penetration radius, where r/R = 1.0 denotes the surface of the giant planet. To the left of where ΔM/M ~ 1/80 (horizontal dotted line) intersects the solid curve, the grazing collision is sufficiently deep to penetrate the region of liquid metallic hydrogen. To the left of where ΔM/M ~ 1/20 (horizontal dotted line) intersects the solid curve, the collision is sufficiently “head-on” for the collided giant planets to merge. Between ΔM/M ~ 1/80 and ΔM/M ~ 1/20, liquid metallic hydrogen can be ejected from the collision event. For comparison, the dashed curve represents a giant planet with an internal structure similar to Saturn’s.

Giant planets have strong magnetic fields which gradually become weaker with age. Since giant planet collisions tend to occur in young planetary systems, the giant planets involved are expected to have magnetic fields that are many times stronger than giant planets that are billions of years old. As giant planets collide, the destruction of their powerful magnetospheres produce intense bursts of radio waves that are large enough to be detected by modern radio telescopes.

It should be realized that the properties of metallic hydrogen are still largely unknown because metallic hydrogen can only form under extreme pressure and has yet to be reliably produced in laboratory experiments on Earth. Nevertheless, it is hypothesized that metallic hydrogen may be metastable over billions of years, similar to diamond - a metastable form of carbon. If a metastable form of metallic hydrogen exists, fragments from possible giant planet collisions during the formative period of our solar system may still linger around. Additionally, ejected fragments of metastable metallic hydrogen from other planetary systems can occasionally enter our solar system. The existence of these fragments may be determined by looking out for unusual types of meteorites.

References:

1. V. I. Dokuchaev and Yu. N. Eroshenko (2012), “Collisional Destructions of Giant Planets and Rare Types of Meteorites”, arXiv:1202.5920 [astro-ph.EP]

2. V. I. Dokuchaev and Yu. N. Eroshenko (2013), “Observational Signatures of the Giant Planets Collisions”, http://dx.doi.org/10.1016/j.pss.2013.01.007

Habitable Tidally Heated Exomoons

Direct imagine of exoplanets is a very challenging task. This is especially true for exoplanets that are found within the Habitable Zone of their host stars. The reason for this is that a star is many orders of magnitude brighter than a planet and when observed over interstellar distances, the angular separation between the star and planet is very small. Nonetheless, it does become less difficult to directly image a planet that is located further from its host star because an increase in the star-planet angular separation places the planet further from the glare of its host star. However, such exoplanets tend to be gas giant planets that are located well outside the Habitable Zone of their host stars. Within our solar system, Jupiter and Saturn are typical examples of such planets.

Although gas giant planets are unlikely places for life to exist, they can have systems of moons where life may exist on some of them. Beyond the Habitable Zone, insolation becomes inadequate to warm an exomoon sufficiently for liquid water to exist on its surface. As a result, tidal heating serves as a viable mechanism to provide the extra amount of energy needed to raise the equilibrium temperature of the exomoon. In order for tidal heating to occur, the orbit of an exomoon around a gas giant planet needs to have some amount of eccentricity. Furthermore, for an exomoon to maintain the orbital eccentricity necessary to sustain tidal heating, it needs to be in resonance with at least one or more exomoons that are orbiting the same gas giant planet. A classic example in our solar system is the satellite system of Jupiter where the moons Io, Europa and Ganymede are in a 1:2:4 orbital period resonance. This sustains Io’s tidal heating by keeping its orbit from ever circularizing and makes Io the most volcanically active world in our solar system.

An exomoon with one-tenth Earth’s mass is probably the minimum mass for a potentially habitable tidally heated exomoon. This is because a less massive exomoon will be unable to hold onto an atmosphere if it is tidally heated to an average surface temperature that is sufficient to sustain liquid water. However, such massive moons do not exist in our solar system and even the largest moons are no more than a few percent of Earth’s mass. Regardless of that, it is not unrealistic to consider the existence of much more massive exomoons given the unexpected diversity of exoplanets that have already been discovered.

Consider an Earth-sized exomoon with an Earth-like density and an orbital eccentricity of just 0.005. This Earth-sized exomoon will have to orbit a Jupiter-mass gas giant planet at a distance of roughly 1.1 million kilometres in order for tidal heating to be sufficient to sustain an average surface temperature of 300 K. At this temperature, liquid water can exist on the surface of the exomoon. If the planet-moon distance were halved, tidal heating will be so effective that the surface temperature of the Earth-sized exomoon will be a blistering 1000 K, making it uninhabitable. In comparison, Jupiter’s moon Io orbits Jupiter at an average distance of 422,000 kilometres, with an orbital eccentricity of 0.004. Observing at an infrared wavelength of ~14 micrometres, a 300 K tidally heated Earth-sized exomoon will appear as bright as the Jupiter-mass gas giant planet it is orbiting.

Direct imaging of tidally heated exomoons can be a lot less challenging than the direct imaging of similar sized Habitable Zone exoplanets. This is because a tidally heated exomoon can exist at an arbitrarily large distance from its host star since its mean surface temperature is largely determined by tidal heating rather than insolation from its host star. In comparison, a Habitable Zone exoplanet around a Sun-like star is restricted to distances of ~1 AU from its host star (1 AU is the average Earth-Sun distance) since its mean surface temperature is largely determined by the amount of insolation it receives from its host star. As a result, a tidally heated exomoon can be a lot easier to image directly because it can be much further away from the overwhelming glare of its host star. To conclude, the direct imaging of habitable exomoons can happened long before it is technological possible to directly image habitable exoplanets.

Reference:

Mary Anne Peters and Edwin L. Turner (2012), “On the Direct Imaging of Tidally Heated Exomoons”, arXiv:1209.4418 [astro-ph.EP]

The Universe’s Last Stars

At present, the universe is in the stelliferous era where conventional star formation is an ongoing process and nuclear fusion in stars account for most of the energy generation in the universe.  However, star formation is not a perfectly efficient process and matter is continuously being lost due to incorporation into objects such as brown dwarfs, extremely long-lived red dwarfs and inert stellar remnants (white dwarfs, neutron stars and black holes). As a result, the stelliferous era is expected to end when the universe is around 10¹⁴ years old, about 10³ times its current age. The universe then enters the degenerate era which is expected to last until the universe is around 10³⁷ years old. This is the era where most of the mass in the universe is in the form of brown dwarfs, white dwarfs, neutron stars and black holes.

Brown dwarfs are objects not massive enough to become stars as their masses are below the hydrogen-burning limit. During the degenerate era, non-conventional star formation remains a possibility through 2 methods. The 1st method is when two brown dwarfs collide and merge into an object with a mass above the hydrogen-burning limit. The 2nd method occurs when a brown dwarf very slowly accretes matter from the rarefied interstellar medium until its mass is eventually pushed above the hydrogen-burning limit. At any given time, it is estimated that at least a few stars in the galaxy were created via the 1st method. As for the 2nd method, it is unobservable in the present universe because the timescale for it to happen is many orders of magnitude larger than the current age of the universe. Nevertheless, the every-increasing timescales in the far future of the universe may make the 2nd method a dominant mode of star formation in an almost starless universe.

The number of brown dwarfs in the galaxy is expected to be comparable to the number of hydrogen-burning stars. Over a very long period of time, a brown dwarf drifting through the rarefied interstellar medium can accrete enough matter to push its mass above the hydrogen-burning limit and become a red dwarf star. A more massive brown dwarf needs to accrete less material than a lower mass brown dwarf to arrive at the hydrogen-burning limit. Assuming the minimum mass necessary for hydrogen-burning is 7.7 percent the Sun’s mass and the interstellar medium has an average density of one proton for every 10 cubic centimetres, a brown dwarf with 5 percent the Sun’s mass needs to accrete for 10¹⁷ years before its mass arrives at the hydrogen-burning limit. After that, the newly born red dwarf star will shine for about 10¹³ years in a very dark universe. In comparison, our Sun has a lifespan of just 10¹⁰ years.

During the degenerate epoch, the accretion of matter from the interstellar medium by objects other than brown dwarfs can lead to some interesting phenomenon although such events are likely to be separated by exceedingly long periods of inactivity. For example, the accretion of matter from the interstellar medium by white dwarfs and neutron stars is expected to cause an occasional supernova via the accretion-induced collapse mechanism. On rare occasions, collisions between helium white dwarfs or between a brown dwarf and a white dwarf can create short-lived helium-burning stars and possibly even stars that burn elements heavier than helium. With fleeting lifespans of no more than a few 10⁸ years, such stars are expected to contribute little to the overall stellar luminosity in comparison to hydrogen-burning red dwarf stars in the degenerate epoch. Nonetheless, if the universe should obey the Copernican Principle, then the current universe has no special place in time and interesting things can still happen in the seemingly inactive universe of the very far future.

References:

1. Cirkovic, M. M. (2005), “Brown Dwarf Accretion: Non-Conventional Star Formation over Very Long Timescales”, Serbian Astronomical Journal, Vol. 171, p. 11-17.

2. Fred C. Adams and Gregory Laughlin (1997), “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, arXiv:astro-ph/9701131

3. Accretion-Induced Collapse (http://beyondearthlyskies.blogspot.sg/2013/01/accretion-induced-collapse.html), January 2013

Testing Life’s Cosmic Ubiquity

Is life common in the universe? The successful detection of a second independent origin of life within our own solar system would be absolute proof that life is common in the universe. Our solar system is just one out of billions of other planetary systems in the galaxy and if life can arise twice in a single planetary system, then the galaxy is expected to be teeming with billions of living worlds. Mars, Europa and Titan are undoubtedly the best places in the solar system to search for the existence of life. Of these three worlds, Titan may prove to be the best place to look for a second independent origin of life in the solar system and to test life’s cosmic ubiquity.

Throughout Earth’s history, hypervelocity impacts caused by asteroids and comets crashing into the surface of the Earth are known to be able to throw up rock material into space. These rocks can carry terrestrial life within them through the vacuum of space and a tiny fraction of these rocks do eventually find their way to the surfaces of other planets and moons in the solar system. Mars and Europa have environments just beneath their surface where the conditions are suitable to support terrestrial life and Earthly microbes that have hitched a ride on these rocks can indeed survive there. As a result, if any life is detected on Mars or Europa, it would be difficult to definitively proof if it has an origin that is independent from that of life on Earth unless the type of life turns out to be biochemically distinct from life on Earth.

Titan is the largest moon of Saturn and it has a thick nitrogen-rich atmosphere laden with a wealth of organic molecules. The mean surface temperature on Titan is a frigid -179 degrees Centigrade and this is so cold that any water on Titan is literally rock solid. In fact, frozen water makes up the crust of Titan, creating a geological landscape where features such as mountains, sand dunes and boulders are actually made of frozen water. Instead of liquid water, liquid methane and ethane are the working fluids in Titan’s “hydrological cycle”. The surface of Titan contains widespread fluvial features such as rivers and deltas that were created through the action of liquid methane and ethane. In the high latitudes of Titan, there are lakes and seas of liquid methane and ethane. This makes Titan the only other place in the solar system besides the Earth that has stable bodies of surface liquid.

Since all life on Earth is born of liquid water and sustained by liquid water, the absence of liquid water on Titan creates an environment that is completely inhospitable to terrestrial life. Any terrestrial life that may have hitched a ride to Titan will not survive. However, it would be naive to assume that liquid water is the only solvent suitable for biology just because all life on Earth is water-based due to water being the most common solvent on Earth. Like the Earth, Titan also has the basic requirements for life such as the presence of a fluid environment, a source of energy and abundant organic molecules. If any form of life exists on Titan, it is expected to use the abundant liquid methane and/or ethane as a biosolvent due to the absence of liquid water in such a cryogenic environment. This means that any form of life detected on Titan is expected to be so biochemically distinct from life on Earth that it would be proof for a second independent origin of life in the solar system.

The most common type of star in the universe are not G-dwarf stars like our Sun but the much less massive M-dwarf stars, also known as red dwarfs. M-dwarf stars vastly outnumber G-dwarf stars. A typical M-dwarf star is so much less luminous than the Sun that an Earth-sized habitable planet with surface liquid water can only be sustained at one-tenth the Earth-Sun distance from the star. However, such a close proximity causes the planet to be tidally locked where one hemisphere perpetually faces the M-dwarf star; creating permanent day and night sides. As a result, an Earth-sized habitable planet around an M-dwarf star is unlikely to resemble the Earth. On the contrary, a Titan-like habitable planet could exist at a distance equivalent to the Earth-Sun distance from a typical M-dwarf star. The planet would not be tidally locked and is expected to rotate freely about its axis. If life is indeed found on Titan, it means that Titan-like habitable planets are likely to be very common and may even be more common than Earth-like habitable planets, given that M-dwarf stars vastly outnumber G-dwarf stars.

When considering the exploration of Titan, the only disadvantage is the large distance between Titan and Earth. Apart from that, everything else about Titan such as its low gravity, dense atmosphere, low radiation environment and calm low altitude winds are advantages compared to the exploration of other places in the solar system. For future missions to Titan, the challenges associated with finding an exotic form of life that is biochemically distinct from life on Earth cannot be underestimated. If life is found on Titan, such life would have an origin that is independent from life on Earth and will be proof of life’s cosmic ubiquity.

References:

1. J.I. Lunine (2009), “Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity”, arXiv:0908.0762 [astro-ph.EP]

2. Worlds Like Titan (http://beyondearthlyskies.blogspot.sg/2011/04/worlds-like-titan.html), April 2011

3. Titan Mare Explorer (TiME) (http://beyondearthlyskies.blogspot.sg/2012/02/sailing-titan-seas.html), February 2012

4. Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR) (http://beyondearthlyskies.blogspot.sg/2012/03/titan-airplane-mission-concept.html), March 2012

Mining the Asteroids

Near-Earth asteroids (NEAs) are asteroids that pass near to Earth and they provide attractive targets for the extraction of resources to support the expansion of activities in space. It is estimated that there are about 1000 NEAs larger than 1 km diameter and about 100,000 NEAs larger than 140 m diameter. For as many as 60 percent of the NEAs, the amount of energy required to make a round trip from the Earth to a NEA is less than a round trip from the Earth to the surface of the Moon. A 1 km diameter metallic asteroid is expected to contain a few times more platinum group metals (PGMs) than all that has been mined on Earth.

The large distance between the Earth and an asteroid means that a round-trip time for communications can be several minutes or more and transporting the extracted materials back to Earth can take up to a few years. As a result, one good strategy before mining a selected asteroid is to capture the asteroid and transport it into a stable orbit around the Earth. Having an asteroid in orbit around the Earth makes the resources on the asteroid a lot more accessible and effectively removes the problem of round-trip time for communications. A spacecraft with a solar-electric propulsion system can be used to haul in an asteroid since such a propulsion system contains many times more propulsive energy per unit mass than chemical propulsion systems. The strategy of capturing a NEA is likely to occur in 2 stages. Stage 1 involves transporting the asteroid until it is sufficiently close to the Earth for it to rendezvous with the Earth’s gravitational sphere of influence. Stage 2 involves placing the asteroid into a stable orbit around the Earth. With proper timing, a gravitational flyby with the Moon can be used to slow an incoming asteroid into a stable orbit around the Earth.

Materials extracted from an asteroid can have many useful applications both in space and back on Earth. Volatiles such as water can be extracted for use as rocket fuel and for biological uses. Precious metals such as gold, platinum and PGMs are in demand in the manufacturing of numerous products such as fuel cells, high-end electronics, catalytic converters and advanced medicines. These precious metals are vital to our advancing technological civilization but they are extremely rare on Earth because very little remains in the Earth’s crust as most of them have sank into the Earth’s core. However, asteroids are pristine objects that have undergone very little modifications. For this reason, a typical chunk of asteroid material is expected to have hundreds of times greater concentration of precious metals than in the Earth’s crust and tens of times greater concentration than in the richest mines on Earth. In addition to water and precious metals, other materials such as iron and aluminium can serve as useful materials for constructing structures in space. Even the leftover bulk material of the asteroid can serve as radiation shielding for long duration manned missions.

Mining an asteroid, especially one that has been transported to the vicinity of Earth reduces the need to launch large amount of materials from Earth to support the expansion of activities in space. Precious metals extracted from asteroids can reduce the environmental stresses that result from the mining of these metals on Earth as it creates another avenue of supply. One thing to note regarding the mining of precious metals from asteroids is that the profit margin needs to be high enough because an outside source of precious metals can lower prices sufficiently to doom the asteroid mining venture.

Torrential Rains on Titan

Figure 1: Bright tropospheric clouds on Titan. Credit: NASA/JPL/Space Science Institute.

The thick nitrogen atmosphere of Titan supports a methane hydrological cycle that is akin to Earth’s water cycle. Like the Earth, precipitation and storm activity appear to be quite common on Titan. On Titan’s surface, numerous fluvial features point towards a hydrology based on liquid methane. Much of Titan’s surface is kept wet by a light but persistent drizzle of liquid methane which forms an enduring component of Titan’s methane hydrological cycle. Data collected by the Huygens entry probe during the descend through Titan’s atmosphere in January 2005 suggests the presence of an upper cloud layer of methane ice between 20 km to 30 km and a lower cloud layer of liquid methane-nitrogen between roughly 8 km to 16 km. The upper cloud layer of methane ice is akin to terrestrial cirrostratus clouds while the lower cloud layer of liquid methane-nitrogen is akin to terrestrial stratiform clouds. A gap between the upper and lower cloud layers exists because that region is too cold to sustain liquid clouds, but slightly too dry for pure methane to condense.

Since the upper cloud layer of methane ice is at saturation (100 percent relative humidity), methane ice crystals can grow there and eventually precipitate out. As the methane ice crystals descend down the atmosphere, they warm up and melt before reaching the surface as a light drizzle. The supply of methane for the lower cloud layer upon comes from the melting of falling methane ice crystals. Once melted, the methane droplets also allow nitrogen in the atmosphere to dissolve in them and become droplets of liquid methane with dissolved nitrogen. As the drizzle of methane-nitrogen droplets fall towards the ground, they become less enrich in nitrogen due to the preferential evaporation of dissolved nitrogen. This basic cloud structure is believed to be widespread and represents more than half of Titan’s surface. The low precipitation rate of such a drizzle means that its minuscule erosive potential cannot explain the widespread fluvial features on Titan. However, it can easily account for the generally wet character of the surface material at the Huygens landing site and possibly on most of Titan.

Figure 2: Artist’s impression of a storm on Titan. Credit: Mark A. Garlick.

Precipitation events with strong erosive potentials are required to form the widespread fluvial features on Titan and such intense precipitation events do occur on Titan. Remote observations of Titan have revealed the presence of short-lived tropospheric clouds. These clouds are likely to be convective storms that bring significant amounts of methane precipitation. Three dimensional models show that methane convection storms accompanied by heavy precipitation can occur on Titan under favourable conditions. Such favourable conditions include the presence of aerosol particles that act as cloud condensation nuclei, updrafts provided by the Hadley cell convergence or forced lifting of air masses from topographic features.

When the relatively humidity is over 80 percent, a small temperature perturbation of just 0.5 K can trigger convection, causing methane in the air mass to condense. Latent heat released from the condensation of methane powers strong updrafts with speeds of up to 20 m/s. This establishes a convective cell with cloud tops attaining altitudes of up to 30 km. Within the convective cell, the condensed methane rains out over a period of 4 to 8 hours. During such a torrential downpour, the accumulated precipitation on the surface can be as high as 110 kg/m², comparable to severe storms on Earth. These three dimensional models show that intense downpours of methane can occur on Titan under the right environmental conditions and have the erosive power necessary to create the fluvial features on Titan’s surface. Such intense downpours are expected to leave signatures in the form of temporary liquid deposits and mild surface cooling. More observations of the cloud activity on Titan will provide a better understanding of the role convection plays in Titan’s methane hydrological cycle.

References:

1. Tetsuya Tokano, et al., “Methane drizzle on Titan”, Nature 442, 432-435 (27 July 2006)

2. R. Hueso1 and A. Sánchez-Lavega, “Methane storms on Saturn’s moon Titan”, Nature 442, 428-431 (27 July 2006)

Cloud Decks of Gas Giant Planets

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

Brown dwarfs and gas giant planets such as Jupiter and Saturn have hydrogen-helium dominated atmospheres that are very different from the atmospheres of Earth-like planets. In the hydrogen-helium dominated atmospheres of brown dwarfs and gas giant planets, a wide variety of molecular species can condense to form clouds. This is expected to produce multiple cloud layers in the atmosphere, where each cloud layer is made up of a particular type of condensable species. By comparison, only water clouds produced from the condensation of water exist in the atmospheres of Earth-like planets.

Gas giant planets are known to exist over a wide range of distances from their parent stars, from “star-hugging” hot-Jupiters to isolated Jupiter-mass planets in the frigid depths of interstellar space. Also, depending on its age, a gas giant planet is much hotter when it is young and it will gradually cool over billions of years. As a result, gas giant planets can have a wide range of atmospheric temperatures. The atmospheric temperature of a gas giant planet strongly influences the number of cloud layers it has and the position of its cloud layers. With decreasing atmospheric temperatures, the more refractory cloud layers form at progressively greater depths in the planet’s atmosphere and cloud layers composed of more volatile condensable species become present at the upper portions of the atmosphere.

Figure 2: Illustration of cloud layers in the hydrogen-helium dominated atmospheres of gas giant planets. The three panels correspond roughly to effective temperatures of approximately 120 K (Jupiter-like, left), 600 K (middle) and 1300 K (right).

When a condensable species forms a cloud layer, the condensate is removed from the cooler overlying atmosphere and is no longer available to react with other molecular species higher up in the atmosphere. For example, the detection of hydrogen sulphide (H₂S) in Jupiter’s atmosphere by NASA’s Galileo entry probe indicates that iron (Fe) must be sequestered into a cloud layer much deeper in the planet’s atmosphere because the presence of Fe will lead to the formation of iron sulphide (FeS) instead of H₂S. In another example, the detection of monatomic potassium (K) in the atmospheres of some brown dwarfs suggests that rock-forming elements such as aluminium (Al) and silicon (Si) are removed from the atmosphere due to cloud formation deeper down in the atmosphere. If Al and Si were not removed, the potassium would have condensed into silicate minerals such as orthoclase (KAlSi₃O₈) which would remove the presence of monatomic potassium from the observable atmosphere of the brown dwarf.

In the atmosphere of Jupiter, the highest clouds are composed of a cirrus-like layer of ammonia (NH₃) ice crystals. Further down into the atmosphere are ammonium hydrosulphide (NH₄SH) and water (H₂O) cloud layers. The troposphere of Jupiter’s atmosphere is commonly defined from the 0.1 bar level (approximately 50 km above 1 bar level) down to the 10 bar level (approximately 90 km below 1 bar level). At the top of the troposphere, the temperature is about 110 K and at the bottom of the troposphere, the temperature is about 340 K. The NH₃ (0.6 to 0.9 bar), NH₄SH (1 to 2 bar) and H₂O (3 to 7 bar) cloud layers are all located within the troposphere of Jupiter. Although methane (CH₄) also exists in the atmosphere of Jupiter, the temperatures are too warm for it to condense to form clouds.

Deeper down into Jupiter’s atmosphere are alkali, halide and sulphide cloud layers. These are then followed by silicate and iron cloud layers at increasing atmospheric pressures and temperatures. Finally, the deepest cloud layers consist of refractory species such as perovskite (CaTiO₃) and aluminium oxide (Al₂O₃) crystals. In fact, in the atmospheres of the hottest gas giant planets, only cloud layers consisting of refractory species exist in their atmospheres because the temperatures are too high for other molecular species to condense. As a result, the atmospheres of the hottest gas giant planets resemble the deepest regions of Jupiter’s atmosphere.

References:

1. Visscher et al. (2010), “Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars III. Iron, Magnesium, and Silicon”, arXiv:1001.3639 [astro-ph.EP]

2. Marley et al. (2013), “Clouds and Hazes in Exoplanet Atmospheres”, arXiv:1301.5627 [astro-ph.EP]

3. Loadders et al. (2006), “Chemistry of Low Mass Substellar Objects”, arXiv:astro-ph/0601381