Thursday, September 26, 2013

Red Supergiant in Westerlund 1

Westerlund 1 is the most massive starburst cluster known in the Milky Way galaxy. It contains more than 50 known massive O-type stars, Wolf-Rayet stars and red supergiants (RSGs) at various stages of post-main sequence evolution. Wright et al. (2013) acquired images that show the existence of an ionized circumstellar nebula surrounding the RSG W26 in Westerlund 1. W26 is one of the most luminous and largest RSGs known in the galaxy. It is estimated to have ~320,000 times the Sun’s luminosity and ~1500 times the Sun’s diameter. If placed in the centre of the Solar System, the visible surface of W26 would reach to the orbit of Jupiter and another 260 million kilometres further. The presence of a nebula around W26 suggests it is a highly evolved RSG with extensive mass-loss in its recent history.

An artist’s impression of a RSG from the surface of hypothetical planet. Even though the planet is over a billion kilometres away, the RSG still spans a large breadth of the sky and the planet’s surface scorches under the intense heat. Credit: Scott Cornett.

The ionized circumstellar nebula surrounding W26 consists of a detached circumstellar shell or ring surrounding the star and a triangular nebula located ~0.2 pc from the star. Images taken by the Hubble Space Telescope (HST) reveal filamentary structures in the triangular nebula. The filaments appear to be oriented towards the nearby blue supergiant (BSG) W25 and may be associated with this star instead of W26. However, a thin nebulous strip is shown to connect the triangular nebula with the circumstellar nebula around W26. Such a connection probably suggests that the triangular nebula is associated with W26, although it may be somewhat shaped or photoionized by W25.

The triangular nebula may be some sort of outflow from W26, similar to bipolar outflows observed around other RSGs. Then again, if the triangular nebula originates from a bipolar outflow from W26, the absence of a similar nebula on the other side of W26 is puzzling. Nevertheless, observations of bipolar outflows around a number of RSGs, such as the RSG Sher 25, show one side of the outflow to be significantly brighter than the other. This suggests that a second outflow exists on the other side of W26, but is undetected. Alternatively, outflows exist on both sides of W26, but only one side is being photoionized by the nearby BSG W25. Yet another possibility is the triangular nebular could be a flow of material being blown off the circumstellar nebula by the cumulative cluster wind and radiation field of Westerlund 1.

W26 is unusual for having an ionized nebula surrounding it because RSGs are too cool to produce ionizing photons. As a result, the ionized nebular is probably due to photoionization by the nearby BSG W25 or by the presence of a hot companion star around W26, although the latter possibility is unlikely. Other potential sources of photoionization include the radiation field of Westerlund 1 and shock-induced ionization from the collision of the nebula with the intra-cluster medium. W26 provides a unique opportunity to observe extensive mass-loss from a highly evolved RSG. Further observations with higher resolution spectra can allow the expansion velocity of the nebula around W26 to be measured.

Wright et al. (2013), “The Ionized Nebula surrounding the Red Supergiant W26 in Westerlund 1”, arXiv:1309.4086 [astro-ph.SR]

Wednesday, September 25, 2013

Io Volcano Observer (IVO)

Io is the innermost of the 4 Galilean moons of Jupiter and it is also the most volcanically active world in the Solar System, with hundreds of active volcanoes scattered over its surface. All that volcanic activity is caused by intense tidal heating from friction generated within Io’s interior as it is pulled between Jupiter and the other 3 Galilean moons - Europa, Ganymede and Callisto. The largest volcanic plumes on Io reach a few hundred kilometres above the surface. With almost no impact craters, Io’s surface is extremely young, and is constantly reshaping itself.

Figure 1: Artist’s impression of Io and Jupiter.

Io Volcano Observer (IVO) is a proposed spacecraft that if selected by NASA, will orbit Jupiter and perform repeated flybys of Io to explore Io’s active volcanism and its impact on the Jupiter system as a whole. The science objectives of IVO are to:
- understand the eruption and emplacement of Io’s currently active lavas and plumes;
- determine the melt state of Io’s mantle and map heat flow patterns to distinguish between shallow and deep-mantle tidal heating;
- determine the state of Io’s lithosphere and understand its tectonic processes via observations of mountains and paterae;
- understand Io’s surface-plume-atmosphere compositions and interactions;
- and understand Io’s mass loss, exosphere, and magnetospheric interactions.

To get to Jupiter, IVO will use a Venus-Earth-Earth Gravity Assist (VEEGA) trajectory. Such a trajectory allows launch opportunities to repeat every couple of years on average. Using the VEEGA trajectory, IVO is expected to arrive at Jupiter 5-6 years after launch from Earth. IVO will insert itself into an elongated orbit around Jupiter and encounters with Io will occur when the spacecraft is near periapse (closest to Jupiter). During each encounter, the spacecraft will gather and return about 20 Gb of science data. IVO will perform 7 flybys of Io over its nominal mission duration of ~2 years. The mission is likely to be extended beyond the minimal duration and following that, the mission will terminate by impacting into either Io or Jupiter.

Since the orbit of IVO is inclined ~45° to Jupiter’s orbital plane, the flybys of Io will occur in a nearly north-south fashion. Such a pole-to-pole flyby geometry is favourable for observations of Io and provides excellent views of the polar regions during approach and departure. To accomplish its science objectives, IVO will carry with it a suite of five instruments - (1) Narrow-Angle Camera (NAC); (2) Wide-Angle Camera (WAC); (3) Fluxgate Magnetometer (FGM); (4) Thermal Mapper (ThM); (5) and the INMS/PIA Package (IPP) consisting of dual Ion and Neutral Mass Spectrometers (INMS) and dual Plasma Ion Analyzers (PIA) with shared electronics.

Figure 2: IVO spacecraft with the 2.1 m high-gain antenna (HGA) for scale. (A. McEwen et al., 2014)

Exploration of the outer Solar System is challenging since power is a major constrain at large distances from the Sun. Instead of solar arrays, IVO will be powered by a pair of Advanced Stirling Radioisotope Generators (ASRGs). Each ASRG has a mass of ~20 kg, a design lifetime of 14 years, produces 140 W of electrical power and uses 0.8 kg plutonium-238. The long design lifetime of the ASRGs allows a 6 year extended mission for IVO. During the extended mission, IVO can use Io’s gravity to pump it into a more elongated orbit around Jupiter. This will allow ~8 additional flybys of Io in ~6 years. In addition, a more elongated orbit may allow for a potential flyby of an outer satellite of Jupiter by IVO, providing extra science returns.

A. McEwen et al., “Io Volcano Observer (IVO): Budget travel to the outer Solar System”, Acta Astronautica Volume 93, January 2014, Pages 539-544

Tuesday, September 24, 2013

Habitable Planets around Brown Dwarfs

Brown dwarfs are objects whose masses fall below the limit required for stable hydrogen fusion to occur in their cores. A brown dwarf cools continuously from the gradual release of gravitational potential energy. The timescale over which a brown dwarf cools depends on its mass, with more massive brown dwarfs taking longer to cool. There is good evidence to suggest that terrestrial-mass planets can form around brown dwarfs. One compelling discovery is the object Kepler-42, a very low mass star with a trio of planets measuring 0.7, 0.8 and 0.6 Earth-radii respectively. These planets orbit Kepler-42 with periods no longer than 2 Earth days. The Kepler-42 system very much resembles a scaled up version of Jupiter and its 4 large moons - Io, Europa, Ganymede and Callisto. Since brown dwarfs cover the mass domain between Jupiter-like planets and the lowest mass stars, it is reasonable to expect that the process of forming planets around brown dwarfs is somewhat robust.

Figure 1: This artist’s concept compares the Kepler-42 (KOI-961) planetary system to Jupiter and the largest four of its many moons. The planet and moon orbits are drawn to the same scale. The relative sizes of the stars, planets and moons have been increased for visibility. Credit: NASA/JPL-Caltech.

Because a brown dwarf is so much fainter than the Sun, a terrestrial-mass planet has to orbit much closer in to receive as much warmth as the Earth gets from the Sun. In fact, a habitable planet around a brown dwarf is likely to have an orbital period of not more than a few Earth days. The habitable zone of a brown dwarf is a region of space around a brown dwarf where temperatures are neither too high nor too low for liquid water to exist on the surface of a terrestrial-mass planet. As a brown dwarf cools and fades over time, its habitable zone will likewise shrink inwards. A planet around a brown dwarf may start out too hot to support life. But as the habitable zone shrinks around a cooling brown dwarf, the planet will subsequently find itself within the habitable zone where temperatures are just right. As the habitable zone continues to shrink, the planet will eventually find itself exterior to the habitable zone where temperatures become too cold for surface life.

 Figure 2: An artist’s impression of a habitable planet around a brown dwarf.

The development of life or even complex life on a terrestrial-mass planet around a brown dwarf is expected to depend a lot on the amount of time the planet spends within the habitable zone or the "Goldilocks zone". On Earth, it appears that the development of life required at least 0.5 billion years, while the development of complex multicellular life took perhaps ~3 billion years. As a result, a planet has to reside long enough in the shrinking habitable zone of a brown dwarf in order for life or even advanced lifeforms to develop. Andreeshchev and Scalo (2002) show that a planet in a close-in orbit around a 0.07 solar-mass brown dwarf can reside within the habitable zone for a duration of up to 10 billion years. The duration of habitability decreases for a brown dwarf of lower mass. For instance, a planet around a 0.04 solar-mass brown dwarf can remain habitable for no longer than 4 billion years.

Nevertheless, a planet outside the habitable zone of a brown dwarf does not necessarily mean that it cannot support life. One can imagine, in a close-in orbit around a brown dwarf, a hot Venus-like planet with a think and steaming atmosphere of water vapour. Although the planet's surface is too hot for life, it may be possible for life to develop in the cool layers of the planet's upper atmosphere. Over time, the brown dwarf cools, causing the habitable zone to shrink inwards until the planet eventually finds itself within it. Temperatures now become cool enough for the atmosphere to rain out, forming oceans on the planet's surface and creating a planet that more resembles the Earth. The planet can remain in this state for billions of years.

Slowly but surely, as the brown dwarf continues to cool, the planet will finally find itself exterior to the habitable zone. Temperatures now become sufficiently low for the oceans to freeze. Although the ocean surface freezes, deeper down, the ocean can still be kept liquid due to the feeble input of geothermal heat from the decay of radioisotopes in the planet's interior. Eventually, even the deep ocean will start to freeze as the geothermal heat flux from the decay of radioisotopes dwindles. However, if other planets exist around the brown dwarf, they can provide sufficient perturbation to keep the freezing planet in a slight non-circular orbit around the brown dwarf. This will allow tidal heating to operate for eons, pumping energy into the planet's interior and keeping the oceans liquid under a surface layer of ice. The planet now more resembles Jupiter's moon Europa.

Andreeshchev and Scalo, "Habitability of Brown Dwarf Planets", Bioastronomy 2002: Life Among the Stars. IAU Symposium, Vol. 213, 2004

Monday, September 23, 2013

Einstein’s Planet

Kepler-76b, also known as “Einstein’s planet”, is a hot-Jupiter with twice the mass of Jupiter and orbits its parent star every 1.54 days. Its diameter is 25 percent larger than Jupiter’s. Kepler-76b is tidally locked to its parent star, which means it always presents the same side towards its parent star. As a result, the dayside of Kepler-76b blazes with an estimated temperature as high as 2250 K.

Kepler-76b is the first hot-Jupiter detected by the BEER algorithm - which stands for the relativistic Beaming, Ellipsoidal and Refletion/emission algorithm. The discovery of Kepler-76b was subsequently confirmed by follow-on spectroscopic observations. The BEER algorithm looks for three small effects that occur simultaneously as a planet orbits its star. These three effects are in addition to the dimming of a star as a planet transits in front and the radial velocity wobbling of a star due to gravitational tugging by an orbiting planet.

 Figure 1: Artist’s impression of Kepler-76b transiting its parent star. Credit: David A. Aguilar (CfA)

Figure 2: Light curve of the Kepler-76 system. Phase zero is when the planet Kepler-76b is closest to the observer, while phase 0.5 is when its parent star is closest to the observer, assuming a circular orbit. The secondary eclipse is clearly visible at phase 0.5 of the plot and the green line presents the BEER model. (Faigler et al., 2013)

For Kepler-76b, the “beaming” effect, sometimes also called Doppler boosting, is a relativistic effect that occurs when the star brightens and dims as the planet pulls it towards and away from us. The “beaming” effect created by Kepler-76b has semi-amplitude measuring 15.6 ± 2.2 ppm. Tsevi Mazeh of Tel Aviv University, one of the authors of the paper, mentions: “this is the first time that this aspect of Einstein’s theory of relativity has been used to discover a planet”.

In addition to the “beaming” effect, the team involved in this discovery also looked for the “ellipsoidal” effect whereby the star gets stretched into a football shape by gravitational tides from the orbiting planet. The star appears brighter when viewed from the side due to more visible surface area. Correspondingly, when viewed end-on, the star appears fainter.  The “ellipsoidal” effect induced by Kepler-76b has semi-amplitude measuring 21.5 ± 1.7 ppm. The third effect observed by the BEER algorithm is simply starlight being reflected from Kepler-76b and thermal energy being emitted by the planet. The reflection/emission semi-amplitude of Kepler-76b is 56.0 ± 2.5 ppm.

Interestingly, the team also found that the hottest spot on Kepler-76b is not the planet’s substellar point. Instead, the hottest spot on Kepler-76b is displaced eastwards by about 10 degrees due to the presence of an equatorial super-rotating jet stream within the planet’s atmosphere. The eastward displacement of the hottest spot on the planet shows up as an inflated beaming modulation between phases 0 to 0.5, which is where the hottest spot on the planet is “in view” (Figure 2).

Faigler et al. (2013), “BEER analysis of Kepler and CoRoT light curves: I. Discovery of Kepler-76b: A hot Jupiter with evidence for superrotation”, arXiv:1304.6841[astro-ph.EP]

Friday, September 13, 2013

Superheated by a Pulsar

Using data from NASA’s Fermi gamma ray space telescope, astronomers have found an unusually massive gamma-ray pulsar with a light-weight companion in orbit around it. This heavyweight pulsar is identified as PSR J1311-3430 and it is estimated to weigh no less than twice the mass of our Sun. A pulsar, also known as a pulsating neutron star, is an extraordinarily dense remnant of what was once the core of a massive star. PSR J1311-3430 packs over twice the mass of our Sun in a volume of space measuring just a few kilometres across. A teaspoon of its material would contain a mass amounting to billions of metric tons.

Artist’s impression of a neutron star. Credit: Vadym Sklyaruk

PSR J1311-3430 emits prodigious amounts of gamma-rays and spins at a rate of 390 times per second. Its light-weight companion is a compact object with at least 8 times the mass of Jupiter and is comprised mainly of helium. The companion is believed to be the compact remnant of a star that was cannibalized by the pulsar. Being at a distance of only 1.4 times the Earth-Moon separation distance from the pulsar, the companion whizzes around the pulsar once every 93 minutes. In fact, the companion swings around the pulsar at a terrific speed of 2.8 million km/h.

At such a close proximity to the energetic pulsar, the companion is being blasted at point-blank range by intense gamma radiation and is literally evaporating away. One can imagine the “dayside” of the companion being superheated until it shines blue-white. The cloud of vaporized material emanating from the companion absorbs so much radio wave emissions from the pulsar that the pulsar is invisible to radio telescopes. PSR J1311-3430 is a type of pulsar known as a “black widow,” because like the black widow spider, which kills its partner after mating, the pulsar is expected to eventually vaporize its companion completely.

Romani et al. (2013), “PSR J1311-3430: A Heavyweight Neutron Star with a Flyweight Helium Companion”, arXiv:1210.6884 [astro-ph.HE]

Wednesday, September 11, 2013

A Brown Dwarf on an Eccentric Orbit

Moutou et al. (2013) report the discovery of a long orbital period, highly eccentric transiting brown dwarf around a slightly evolved Sun-like star. This brown dwarf is identified as KOI-415 b and it was observed to transit 7 times in front of its host star by NASA’s Kepler space telescope over a period of 14 quarters. The transit of KOI-415 b has a period of 166.8 days, depth of 0.5 percent and duration of 6 hours. Transiting brown dwarfs are much less common than transiting giant planets and above 20 Jupiter-masses, only a handful of such objects are known.

Figure 1: Artist’s conception of a brown dwarf / gas giant.

 Figure 2: Top panel: phase-folded transit light curve of KOI-415 b. Bottom panel: the residuals between the observations and the best-fit model. (Moutou et al., 2013)

By measuring how much light from its host star gets blocked as KOI-415 b transits in front, the size of KOI-415 b is determined to be 0.79 times the diameter of Jupiter. As for the mass of KOI-415 b, it is determined by measuring the amount of gravitational tugging the brown dwarf exerts on its host star. The gravitational tugging from KOI-415 b causes its host star to wobble with an observed radial velocity semi-amplitude of 3.346 km/s. From this measurement, the mass of KOI-415 b is determined to be 62 times the mass of Jupiter. KOI-415 b is also estimated to be around 10 billion years old.

With a long orbital period of 166.8 days, the position of KOI-415 b on the mass-radius diagram shows that it more resembles an isolated brown dwarf than a heavily irradiated brown dwarf in a close-in orbit around its host star. This is because the radius of KOI-415 b perfectly fits the predicted radius for an isolated brown dwarf with the same age and mass. KOI-415 b is on a highly eccentric orbit around its host star, coming as close as 0.179 AU and swinging out as far as 1.006 AU. This causes the estimated dayside temperature of the brown dwarf to vary by as much as 400 K due to the changing stellar irradiation.

Figure 3: The mass-radius diagram in the domain between giant planets and low-mass stars. Isochrones for objects with ages of 10, 5, 1 and 0.1 billion years are shown for comparison. Colour symbols indicate the mass range: 15-25 Jupiter-mass (green), 37-40 Jupiter-mass (blue), 59-65 Jupiter-mass (black) and 89-97 Jupiter-mass (pink). (Moutou et al., 2013)

Figure 4: The radius as a function of system’s age for objects from 15 to 100 Jupiter-mass. BT-SETTL isochrones are shown for 0.02 to 0.09 solar-mass objects. An object of a given mass cools and contracts as it ages. Colour symbols indicate the mass range: 15-25 Jupiter-mass (green), 37-40 Jupiter-mass (blue), 59-65 Jupiter-mass (black) and 89-97 Jupiter-mass (pink). (Moutou et al., 2013)

Moutou et al. (2013), “SOPHIE velocimetry of Kepler transit candidates IX. KOI-415 b: a long-period, eccentric transiting brown dwarf to an evolved Sun”, arXiv:1309.0905 [astro-ph.EP]

Monday, September 9, 2013

Blue Dwarfs: Stars Yet To Be

M-dwarfs, also known as red dwarfs, are by far the most common stars in the universe. Laughlin et al. (1997) performed stellar evolution calculations for M-dwarfs with masses in the range 0.08 to 0.25 solar mass. Our Sun has a main-sequence lifespan of 10 billion years. In comparison, these M-dwarfs have main-sequence lifespans that are measured in trillions of years. The reason is because unlike our Sun, M-dwarfs have fully convective interiors and this prevents helium produced from the fusion of hydrogen to accumulate at the core, allowing M-dwarfs to burn a larger proportion of their hydrogen before leaving the main sequence. A 0.08 solar mass M-dwarf has a main sequence lifespan of 12 trillion years.

Figure 1: Artist’s depiction of the planetary system around a 0.13 solar mass M-dwarf known as Kepler-42. Credit: NASA/JPL-Caltech.

Figure 2: Evolution in the Hertzsprung-Russell diagram for stars with masses in the range 0.06 to 0.25 solar mass. Stars of less than 0.25 solar mass are not massive enough to evolve into red giants. The inset diagram shows the corresponding main-sequence lifetimes as a function of stellar mass. Note that a 0.08 solar mass M-dwarf has a remarkable main-sequence lifetime that exceeds 10 trillion years. (F. C. Adams, P. Bodenheimer, G. Laughlin, 1997)

The current age of the universe is 13.8 billion years. Since M-dwarfs have main-sequence lifespans that are many times longer than the current age of the universe, the post-main-sequence evolution of these stars can only be predicted based on theoretical models. Stars become more luminous as they age and increasingly massive stars will produce progressively larger luminosity increases. Calculations by Laughlin et al. (1997) show that during the course of evolution, a 0.08 solar mass star will increase in luminosity by ~15 folds, a 0.16 solar mass star will increase in luminosity by ~140 folds and a 0.25 solar mass star will increase in luminosity by close to a factor of a thousand. This increase in luminosity is compensated in one of two ways: (1) the star can grow bigger in size and become a “red giant”; (2) or the star can remain at its usual size but increase its temperature and become a “blue dwarf”.

As a star begins to exhaust most of its hydrogen fuel towards the end of its main-sequence life, its luminosity will increase many folds. M-dwarfs of less than 0.25 solar mass do not become red giants in their post-main-sequence phases. Instead, they remain small and grow hotter to become blue dwarfs. Only stars of more than 0.25 solar mass evolve into red giants. This is because for an M-dwarf of less than 0.25 solar mass, its outer layers do not become significantly more opaque with increasing temperature as the star’s luminosity increases. As a consequence, the star does not expand, but increases its radiative rate, causing its surface temperature to increase. The star appears “bluer” and becomes a blue dwarf. In the present-day universe, a blue dwarf is a hypothetical object whose existence is predicted based on theoretical models because the current universe is far too young for any blue dwarfs to have formed.

A 0.1 solar mass M-dwarf has a main-sequence lifespan of more than 6 trillion years. During this stupendously long period of time, the star fuses hydrogen into helium. Slowly but surely, increasing its helium mass fraction. Initially, the star has a surface temperature of 2230 K and 1/2400 of the Sun’s luminosity. After 5.74 trillion years, its surface temperature will increase to 3450 K and its luminosity increasing to 1/350 of the Sun’s luminosity. At this point, the star has burnt most of its hydrogen and it begins to develop a radiative core. Now, the star’s seemingly eternal youth draws to an end. Its evolutionary timescale accelerates and its luminosity increases more quickly. The increasing luminosity does not cause the star to expand physically in size. Instead, it raises the star’s surface temperature, turning it into a blue dwarf. At about 400 billion years after developing a radiative core, the star’s surface temperature will reach a maximum of 5810 K.

Being a blue dwarf does not mean the star will actually appear blue. It is simply the result of a significant increase in the star’s surface temperature, causing a large shift in the star’s light towards the bluer side of the electromagnetic spectrum. In fact, at its maximum surface temperature of 5810 K, the star’s colour will be somewhat similar to the present-day Sun. After those billions of years shinning as a blue dwarf, fusion eventually comes to a halt as the star runs out of hydrogen fuel. As the foregoing life story of a 0.1 solar mass star comes to an end, the star will continue to cool until it eventually becomes a black dwarf.

F. C. Adams, P. Bodenheimer, G. Laughlin (1997), “The End of the Main Sequence”, The Astrophysical Journal 482: 420-432, 1997 June 10

Sunday, September 8, 2013

Drifter in the Dark

The Inner Oort Cloud (IOC) is a region of space that lies beyond the Kuiper Belt. Objects presently known to be part of the IOC have highly elongated orbits that never come close enough to the Sun for their orbits to be dynamically influenced by gravitational interactions with Neptune. These objects have semi-major axis  ≥ 200 AU (average distance from Sun) and perihelion ≥ 44 AU (minimum distance from Sun). Due to their great distance and faintness, only a few IOC objects are presently known - 2000 CR105 (Gladman et al. 2002) with perihelion 44.1 AU and semi-major axis 228.8 AU; Sedna (Brown et al. 2004) with perihelion 76.2 AU and semi-major axis 542.7 AU; and 2004 VN112 (Becker et al. 2008) with perihelion 47.3 AU and semi-major axis 343.3 AU.

These four panels illustrate the orbit of Sedna - an IOC object which lies in the farthest reaches of the Solar System. Each panel, moving clockwise from the upper left, successively zooms out to place Sedna in context with the Solar System.

One or more mechanisms triggered by external perturbations must have produced the large perihelion distances and highly elongated orbits of the IOC objects. Proposed scenarios include interactions with a massive planet-sized body in the outer Oort cloud; the passage of a solar-mass star within a few 100 AUs from the Sun; stellar encounters that took place while the Sun was still a member of its natal star cluster; and the capture of extrasolar planetesimals from low-mass stars during the early evolution of the solar system.

Chen et al. (2013) report the discovery of 2010 GB174, a likely new member of the IOC. This object was discovered as part of the Next Generation Virgo Cluster Survey (NGVS). 2010 GB174 is estimated to have an orbit with perihelion ~ 48 AU and semi-major axis ~350 AU. At the far end of its highly elongated orbit, 2010 GB174 swings out as far as ~655 AU from the Sun. Based on certain assumptions, 2010 GB174 is estimated to be ~300 km in size. Sedna, the largest known IOC object, has an estimated diameter of ~1000 km.

The very fact that 2010 GB174 was detected at all points towards a large IOC population far beyond the Kuiper belt. In fact, based solely on the discovery of Sedna alone, it has been estimated with reasonable certainty that 15 to 90 Sedna-sized or bigger objects on Sedna-like orbits reside in the IOC (Schwamb et al. 2009). For comparison, the total number of objects Sedna-sized or larger in the Kuiper belt is ∼5 to 8, indicating that there may be ~10 times more mass residing in the IOC than in the Kuiper belt.

- Chen et al. (2013), “Discovery of a New Member of the Inner Oort Cloud from The Next Generation Virgo Cluster Survey”, arXiv:1308.6041 [astro-ph.EP]
- Gladman et al. (2002), “Evidence for an Extended Scattered Disk”, Icarus 157 pp. 269-279
- Brown et al. (2004), “Discovery of a candidate inner Oort cloud planetoid”, arXiv:astro-ph/0404456
- Becker et al. (2008), “Exploring the Outer Solar System with the ESSENCE Supernova Survey”, ApJ 682 L53
- Schwamb et al. (2009), “A Search for Distant Solar System Bodies in the Region of Sedna”, arXiv:0901.4173 [astro-ph.EP]

Saturday, September 7, 2013

A Very Dim and Cool Brown Dwarf

In a paper submitted to the astrophysical Journal on 25 August 2013, Kirkpatrick et al. (2013) present the discovery of an interesting nearby brown dwarf using data from NASA’s Wide-field Infrared Survey Explorer (WISE) satellite. This particular brown dwarf is very cold and very low mass; and it is designated as WISE 0647-6232. Observations by the Hubble Space Telescope confirm WISE 0647-6232 to be a Y dwarf. A brown dwarf is an object too small to be a star, but formed as stars are. Brown dwarfs are categorized according to the spectral types - M, L, T and Y, with Y dwarfs being the coldest class. In fact, Y dwarfs can be cold enough for water clouds to condense in their upper atmosphere. For Y dwarfs colder than ~350 K, water clouds may be a dominant atmospheric feature.

An artist’s rendition shows the relative sizes and likely appearance from left to right of the Sun, a very cool star, a warm brown dwarf, a cooler brown dwarf, and finally Jupiter; as how they might appear through an infrared camera. Credit: Robert Hurt.

Combining imaging data from the Spitzer Space Telescope, HST and WISE, the distance of WISE 0647-6232 is estimated to be 8.7±0.9 pc (28.4±2.9 ly). The best fit model for WISE 0647-6232 based on collected observational data indicates an effective temperature of 350 to 400 K and a mass of ~5 to 30 Jupiter-masses. A brown dwarf cools as it ages and the more massive a brown dwarf, the longer it will take to cool down to a given temperature. As a result, for WISE 0647-6232, a mass of ~5 Jupiter-masses suggests an age of ~100 million years, while a mass of 30 Jupiter-masses suggests an age of over 10 billion years.

By observing the motion of WISE 0647-6232 through space, it seems that this brown dwarf may be a member of a group of stars that are moving in the same direction. This group is known as the Columba group and its member stars form in the same star cluster ~30 million years ago. If WISE 0647-6232 is part of the Columba group, then an age of ~30 million years would imply an even lower mass of less than 2 Jupiter-masses.

The discovery of a single Y dwarf such as WISE 0647-6232 is important because only a handful of Y dwarfs are currently known, partly due to the faintness of such cold objects. Ground-based observations of Y dwarfs are very difficult because the Earth’s atmosphere is opaque to the thermal and mid-infrared wavelengths where these objects emit most of their light. As a result, space telescopes such as the Spitzer Space Telescope and HST serve as vital platforms in the study of Y dwarfs because they are not affected by the limitations imposed by the Earth’s atmosphere. Using space-based telescopes to study Y dwarfs can, for instance, search for transiting exoplanets around these brown dwarfs and observe the evolution of cloud features via photometric monitoring.

The discovery of WISE 0647-6232, a Y dwarf at a distance of less than 10 pc, is significant because it helps improve our understanding of the Y dwarf space density (i.e. number of Y dwarfs per volume of space). An accurate Y dwarf space density can reveal the cut-off for the lowest mass objects that can still form in the same way stars do. In addition to WISE 0647-6232, other notable Y dwarfs include WISE 1828+2650 - one of the coldest known brown dwarf or free floating planet with an effective temperature in the range 250 to 400 K (Beichman at al., 2013); and WISE 1541-2250 - a nearby brown dwarf located ~2.8 pc away and has an estimated temperature of ~350 K (Kirkpatrick et al., 2011).

- Kirkpatrick et al. (2013), “Discovery of the Y1 Dwarf WISE J064723.23-623235.5”, arXiv:1308.5372 [astro-ph.SR]
- Beichman at al. (2013), “The Coldest Brown Dwarf (Or Free Floating Planet)?: The Y Dwarf WISE 1828+2650”, arXiv:1301.1669 [astro-ph.SR]
- Kirkpatrick et al. (2011), “The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE)”, arXiv:1108.4677 [astro-ph.SR]

Friday, September 6, 2013

Polar Ice Deposits on Mercury

Despite the blazing heat from the nearby Sun, water ice exists at Mercury’s polar regions. Ice from comets that crashed into the planet has been cached in deep craters near the poles, where sunlight never reaches. Just as on the Moon, ice is an invaluable resource for humans and their machines.
- Ben Bova, Mercury

Figure 1: Artist’s rendering of Mercury.

Mercury is the closest planet to the Sun and the time it takes to complete one orbit around the Sun is 88 Earth days. There is no atmosphere on Mercury; as a result, Mercury’s surface experiences great temperature variations. During the day, temperatures at some equatorial regions on Mercury can reach over 700 K (427 °C), hot enough to melt zinc. Without an atmosphere to retain the day’s heat, night time temperatures can sink below 100 K (-173 °C).

On 18 March 2011, NASA’s MESSENGER spacecraft entered orbit around Mercury and began a mission to study the closest planet to the Sun. Around the planet’s north pole, in areas permanently shielded from the Sun’s rays, NASA’s MESSENGER spacecraft has found vast deposits of water ice and possible organic materials. Previous Earth-based radar observations have already hinted at the existence of water ice deposits at Mercury’s poles. These radar observations revealed areas of high radar backscatter near Mercury’s north and south poles. These areas are postulated to be near-surface deposits of water ice up to several metres thick.

Figure 2: Perspective view of Mercury’s north polar region with areas of high radar backscatter shown in yellow. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 3: A depiction of the MESSENGER spacecraft at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 4: Schematic of an impact crater located at high latitude on Mercury. The crater rim blocks the angled rays of the Sun, resulting in extremely cold temperatures in regions of permanent shadow. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

MESSENGER’s discovery of polar deposits of water ice and possible organic materials on Mercury comes from three independent lines of evidence: (1) measurements of excess hydrogen at Mercury’s north pole with MESSENGER’s Neutron Spectrometer (Lawrence et al., 2013); (2) measurements of the reflectance of Mercury’s polar deposits at near-infrared wavelengths with the Mercury Laser Altimeter (MLA) on MESSENGER (Neumann et al., 2013); (3) and the first detailed models of the surface and near-surface temperatures of Mercury’s north polar regions that utilize the actual topography of Mercury’s surface measured by the MLA (Paige et al., 2013).

When cosmic ray particles strike the surface of Mercury, they liberate neutrons from atomic nuclei in the near-surface material. The neutrons travel up to the surface and escape into space. These neutrons can be detected by the Neutron Spectrometer on the MESSENGER spacecraft. If a layer of water ice happens to be present on the surface, the abundant hydrogen atoms within the ice will stop the neutrons from escaping into space. Hydrogen has a unique ability to stop neutrons because hydrogen atoms and neutrons have the same mass, which allows very efficient momentum transfer between the two.

Measurements by MESSENGER’s Neutron Spectrometer show a decrease in the flux of neutrons from Mercury’s north polar region that is consistent with the presence of water ice in permanently shadowed regions. The decrease in neutron flux towards the north pole is smaller than expected and this indicates that most of the water ice is buried beneath a 10 to 20 cm thick layer of material that is less rich in hydrogen. Based on the measurements by MESSENGER’s Neutron Spectrometer, the total mass of water at Mercury’s poles is estimated to be as much as 1 trillion metric tons. To put it into perspective, that is equal in volume to a cube of liquid water with edges 10 km in length.

Figure 5: Hydrogen atoms within the layer of water ice stop the neutrons from escaping into space. A decrease in the measured flux of neutrons indicates enhanced hydrogen concentrations and, by inference, the presence of water ice. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Figure 6: Neutron Spectrometer measurements of the flux of high-speed neutrons versus latitude (red). Simulated count rates are shown for the cases of no hydrogen (black) and for a thick layer of pure water ice (blue) located at the surface in all regions of high radar backscatter. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

The Mercury Laser Altimeter (MLA) made measurements of the surface reflectance of permanently shadowed areas near Mercury’s north pole and found regions of anomalously dark and bright deposits. MLA measurements show 3 types of reflectance: (1) typical Mercury reflectivity; (2) a much darker subset corresponding to the MLA-dark deposits; (3) and a smaller subset that is substantially brighter, corresponding to the MLA-bright deposits. The MLA-bright regions are consistant with surface water ice deposits, while the MLA-dark regions are consistant with a surface layer of complex organic material. Since both the bright and dark deposits are spatially correlated with regions of high radar backscatter, it indicates that the dark deposits are a radio-transparent surface layer of complex organic material that partly overlies buried water ice (Figure 7).

Figure 7: Schematic showing the distribution of water ice and complex organic material within a permanently shadowed region. In the coldest areas, water ice remains exposed on the surface (white) and corresponds to the MLA-bright regions. The surface layer of organics (black) partly covers and partly surrounds the water ice deposits, and corresponds to the MLA-dark regions. Credit: NASA/UCLA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

When comets or volatile-rich asteroids impact the surface of Mercury, a mixture of water and organic compounds become spread over a wide region. A small fraction migrates to the permanently shadowed areas around Mercury’s poles where they can become cold-trapped. Over time, water ice in warmer regions sublimates and leaves behind a surface layer that is rich in organic material. Exposure to Mercury’s space environment darkens the layer of organic material to produce the dark deposits observed by MLA.

Thermal models for the north polar region of Mercury were created using the topographic measurements made by MLA on MESSENGER (Figure 8). The models show that the regions of high radar backscatter is well matched by the predicted distribution of thermally stable water ice. Within some permanently shadowed regions, the maximum annual temperature never exceeds ~50 K (-223 °C). In fact, large areas of permanently shadowed regions on Mercury’s north polar region never exceed 100 K (-173 °C) and these are the areas where water ice deposits will remain stable over billion-year timescales. The temperature at which a water ice deposit can be considered thermally stable depends on the timescale under consideration. For instance, at a temperature of 102 K (-171 °C) a 1 m thick layer of pure water ice would sublimate in 1 billion years, while at a temperature of 210 K (-63 °C) a 1 m thick layer of pure water ice would sublimate in 35 days.

Figure 8: Maps of calculated surface and subsurface temperatures in the north polar region of Mercury, superposed on a shaded relief map derived from MLA topographic measurements. (A) Biannual maximum surface temperatures. (B) Biannual average temperatures at 2 cm depth. Credit: NASA/UCLA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

Sean Solomon of the Columbia University’s Lamont-Doherty Earth Observatory, principal investigator of the MESSENGER mission mentions: “For more than 20 years the jury has been deliberating on whether the planet closest to the Sun hosts abundant water ice in its permanently shadowed polar regions. MESSENGER has now supplied a unanimous affirmative verdict.” Soloman also adds: “But the new observations have also raised new questions. Do the dark materials in the polar deposits consist mostly of organic compounds? What kind of chemical reactions has that material experienced? Are there any regions on or within Mercury that might have both liquid water and organic compounds? Only with the continued exploration of Mercury can we hope to make progress on these new questions.”

- Lawrence et al., “Evidence for Water Ice Near Mercury’s North Pole from MESSENGER Neutron Spectrometer Measurements”, Science 18 January 2013: Vol. 339 no. 6117 pp. 292-296 DOI: 10.1126/science.1229953
- Neumann et al., “Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles”, Science 18 January 2013: Vol. 339 no. 6117 pp. 296-300 DOI: 10.1126/science.1229764
- Paige et al., “Thermal Stability of Volatiles in the North Polar Region of Mercury”, Science 18 January 2013: Vol. 339 no. 6117 pp. 300-303 DOI: 10.1126/science.1231106

Thursday, September 5, 2013

Snowball Earth: Thawing a Snowball

During the Neoproterozoic era (~1000 to ~540 Mya), the Earth experienced at least two global-scale glaciations at ~740 and ~635 Mya, where glaciers covered most of the Earth’s surface down to the deep tropics (Trindade and Macouin, 2007). Each global-scale glaciation or Snowball Earth event lasted for several million years. Over those millions of years, atmospheric CO2, a greenhouse gas, is expected to accumulate to immense levels due to continuous emission by volcanic activity and greatly reduced weathering on a frozen planet.

Figure 1: CO2 emitted by volcanos accumulates in the atmosphere of a Snowball Earth. The removal of CO2 from the atmosphere is limited by the absence of rainfall. Furthermore, the drawdown of atmospheric CO2 is almost nonexistent because silicate weathering is greatly reduced due to the overlying ice cover and very cold ground temperatures.

It is believed that the Earth thawed from a snowball state when the amount of atmospheric CO2 reached a level high enough to generate a sufficiently strong greenhouse effect. Evidence show that the amount of atmospheric CO2 towards the end of a Snowball Earth event accumulated to ~10 percent concentration; equivalent to ∼0.1 bar partial pressure (Bao et al., 2008). However, most climate models show that ~0.3 bar partial pressure of atmospheric CO2 is required to deglaciate a Snowball Earth with thick tropical ice cover (Pierrehumbert, 2004, 2005; Le Hir et al., 2007). This indicates that the accumulated amount of atmospheric CO2 during a Neoproterozoic snowball event was insufficient to cause deglaciation of a Snowball Earth. Such a discrepancy suggests that another mechanism in addition to the build up of atmospheric CO2 also played an important role in the deglaciation process.

In a study by Abbot and Pierrehumbert (2010), the authors show that the unique climatic conditions during a Snowball Earth event will allow a dust layer to develop over the ice surface in the tropics. This will cause the ice surface to be less reflective, allowing for greater absorption of solar radiation in the tropics and enabling deglaciation to occur with less than ∼100 mbar partial pressure of atmospheric CO2.

The three main sources of dust that can lead to significant deposits on the ice surface during a Snowball Earth event are: continental dust, volcanic dust and cosmic dust. Large non-glaciated continental areas are likely to exist on a Snowball Earth due to a very weak hydrological cycle. Such areas are probably more prevalent near the tropics where there is net ablation (annual average evaporation greater than precipitation). Additionally, palaeogeographic reconstructions indicate that much of the Earth’s continental masses after the break-up of the supercontinent Rodinia at around 800 to 600 Mya happened to be grouped along the tropics. Any non-glaciated continental area on a Snowball Earth is expected to produce a large amount of dust since it will be extremely dry, devoid of vegetation and subjected to huge diurnal temperature cycles that can cause soil fracturing and cryogenic weathering. Accumulation of continental dust is estimated to be on the order of 1 to 10 m/Myr.

For volcanic dust, the estimated rate of accumulation is ~1 m/Myr. Volcanic dust is important because it represents the minimum dust flux even if the continents are all glaciated and not contributing dust. The contribution from cosmic dust is negligible because its accumulation rate is estimated to be only ~0.1 mm/Myr. Given these estimates, the globally averaged dust accumulation rate during a Snowball Earth is reasonably estimated to be around 1 to 10 m per million years.

Simulations performed in this study show that the low latitudes on a Snowball Earth is a zone of net ablation where dust blown off the continents and from volcanic emission will accumulate over the ice surface, forming a tropical dust strip around the planet that is meters thick. Such a tropical dust strip makes the tropics less reflective to incoming solar radiation. As a result, temperatures will be somewhat higher than for a purely ice-covered surface, allowing the Earth to deglaciate from a snowball state with no more than ~0.1 bar partial pressure of atmospheric CO2.

Figure 2: Simulations of a Snowball Earth with surface air temperature in °C for January (left) and for the annual mean (right) as a function of longitude (horizontal axis) and latitude (vertical axis, which is linear and stretches from the South Pole to the North Pole). Here, the partial pressure of atmospheric CO2 is 0.0001 bar (a and b) and 0.1 bar (c and d) (e and f). The continental outline is represented as a thick black line. To simulate a tropical dust strip (e and f), the continental region is extended to include all the area within 15° of the Equator. Deglaciation, starting in the tropics, is likely to occur when the annual mean surface air temperature warms to roughly -10°C or more.

- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Bao et al. (2008), “Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation”, Nature, 453 (7194), 504-506
- Pierrehumbert (2004), “High levels of atmospheric carbon dioxide necessary for the termination of global glaciation, Nature, 429 (6992), 646-649
- Pierrehumbert (2005), “Climate dynamics of a hard Snowball Earth”, Journal of Geophysical Research, 110, D01111, doi:10.1029/2004JD005162
- Le Hir et al. (2007), “Investigating plausible mechanisms to trigger a deglaciation from a hard Snowball Earth”, Comptes Rendus Geoscience 339 (2007) 274-287
- Abbot and Pierrehumbert (2010), “Mudball: Surface dust and Snowball Earth deglaciation”, Journal of Geophysical Research, Vol. 115, D03104. doi:10.1029/2009JD012007

Wednesday, September 4, 2013

Snowball Earth: Hydrological Cycle

The Earth underwent at least two global glaciation events during the Neoproterozoic era - the Sturtian at ~720 Mya and the Marinoan at ~635 Mya (Pierrehumbert et al., 2011). Other estimates place the two events at ~740 and ~635 Mya respectively (Trindade and Macouin, 2007). These global glaciation events are more commonly known as Snowball Earth events where ice covered the Earth right to the Equator. During a Snowball Earth event, the thick global ice cover effectively eliminates the ocean’s thermal inertia. As a result, the low thermal inertia of the global ice cover resulted in large variations in surface temperature.

Figure 1: January surface air temperature for several general circulation model (GCM) simulations of a Snowball Earth with atmospheric CO2 at 2000 ppmv (parts per million by volume). Note that Earth’s pre-industrial atmospheric CO2 concentration is 280 ppmv, January corresponds to winter in the northern hemisphere and 273 K is equal to 0°C. (Pierrehumbert et al., 2011)

A sluggish hydrological cycle is expected on a Snowball Earth due to the low temperatures and ice covered ocean. The basic structure of a Snowball Earth hydrological cycle consists of a net ablation zone near the Equator where the annual mean precipitation minus evaporation (P-E) is negative. GCM simulations show that the existence of such a net ablation zone is robust. On a Snowball Earth, the ocean will be covered by a thick layer of ice, very much like a global version of Antarctica’s Ross Ice Shelf. The thick ice deforms under its own weight and flows as a sea glacier. Since it is colder towards the poles, the ice cover is thicker at the poles and thinner at the Equator. As a result, the ice will tend to flow from Pole to Equator.

Figure 2: Schematic of Antarctica’s Ross Ice Shelf.

Figure 3: Schematic of sea glacier flow in the weak (P - E) limit (lower panel) and strong P-E limit (upper panel). (Pierrehumbert et al., 2011)

There are 2 limits to the hydrological cycle on a Snowball Earth (Figure 3). The first limit is where atmospheric moisture transport is absent (weak P-E case) and the second limit is where atmospheric moisture transport is substantial (strong P-E case). In the weak P-E case, the sea glacier flows from Pole to Equator. This causes the ice at higher latitudes to be thinner than the local equilibrium value, so that new ice forms there by freezing onto the base. Consequently, the ice in the tropics becomes thicker than the local equilibrium value, so that excess ice melts at the base. Melt water from the tropics circulates through the ocean where it refreezes at the base of the ice at higher latitudes, thereby completing the hydrological cycle.

In the strong P-E case, ice freezes onto the base of the ice in the tropics and is brought upward to the surface, where it sublimates into the atmosphere and falls as snow at higher latitudes. The snowfall thickens the ice at higher latitudes to a thickness that exceeds the local equilibrium thickness. To compensate for the extra thickness, both basal melting of ice at higher latitudes and Pole to Equator flow of ice occur, which completes the hydrological cycle. In the strong P-E case, dust deposited together with snowfall at higher latitudes can be transported through the ice to the basal melting region. This delivers nutrient-laden surface dust into the ocean, allowing life to continue to thrive at localities that serve as refugia on a frozen planet (Hoffman and Schrag, 2000; Campbell et al., 2011).

- Pierrehumbert et al., “Climate of the Neoproterozoic”, Annual Review of Earth and Planetary Sciences 39 (2011) 417-60
- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Hoffman and Schrag, “Snowball Earth”, Scientific American (January 2000), Volume 282, pp. 68-75
- Campbell et al., “Refugium for surface life on Snowball Earth in a nearly‐enclosed sea? A first simple model for sea‐glacier invasion”, Geophysical Research Letters Volume 38, Issue 19, October 2011

Tuesday, September 3, 2013

Snowball Earth: Refugia for Life

At around 600 to 800 Mya, during the Neoproterozoic era, the Earth underwent at least two global-scale glaciation events, more commonly known as Snowball Earth events (Trindade and Macouin, 2007). During a Snowball Earth event, the ocean was completely covered by thick ice right down to the tropics. At low latitudes, the equilibrium ice cover over the ocean is estimated to be hundreds of metres thick and only got thicker at higher latitudes. The ice cover over the ocean would tend to flow from higher latitudes towards the Equator in the form of a global sea glacier (Goodman and Pierrehumbert, 2003; Pierrehumbert et al., 2011). Any unprotected area of open ocean surface is likely to be overrun by it.

The existence of photosynthetic eukaryotic algae predates the Snowball Earth events of the Neoproterozoic era. At that time, life has yet to evolve from ocean to land and a completely ice covered ocean poses a problem for the survival of photosynthetic life. Nevertheless, a number of ways have been proposed that allow photosynthetic life to survive through a Snowball Earth event. It has been suggested that small pools of open water can exisit above geothermal hotspots on coastlines of volcanic islands (Hoffman and Schrag, 2000) and can serve as refugia for photosynthetic life on a frozen planet. Photosynthetic life can also survive around deep-sea hydrothermal vents by using the faint trickle of optical photos coming off from hot water to perform photosynthesis, albeit at very low rates (Beatty et al., 2005; Cardenas et al., 2013).

A paper published by Campbell et al. (2011) in the Geophysical Research Letters show that an inland sea analogous to the present-day Red Sea at low latitudes can serve as a refugium for photosynthetic life during the Snowball Earth events in the Neoproterozoic era. The study examines a long narrow inland sea similar to the present-day Red Sea. One end of the inland sea is connected to the ocean while the rest of it is surrounded by non‐glaciated desert land. The reason for non-glaciated desert land is due to net sublimation expected at low latitudes which prevents ice from accumulating. Such an inland sea can be formed by continental rifting, much like how the present-day Red Sea was formed by rifting of the African and Arabian plates.

Figure 1: An image of the present-day Red Sea taken on 22 June 2013 from on board the International Space Station.

Several conditions must be met in order for an inland sea to serve as a refugium for photosynthetic life during Snowball Earth events. The conditions mentioned in the paper are: (1) the inland sea must not be fully penetrated by a sea glacier; (2) the climate on the inland sea must be such as to maintain it either ice‐free or covered by an ice layer sufficiently thin to allow photosynthesis below the ice; (3) the depth of the sea at its entrance, and throughout its length, must be great enough that seawater is able to flow under the sea glacier to replenish water loss from the refugium by evaporation/sublimation, and (4) water circulation in the inland sea must be adequate to allow nutrients to be delivered to organisms living in the bay at the landward end.

A necessary criteria for the inland sea to serve as a refugium for life is that the sea glacier coming in from the ocean must not reach the landward end of the inland sea. On a Snowball Earth, net sublimation is expected to occur at low latitudes and the inflowing sea glacier will lose mass as it penetrates into the inland sea. Net sublimation will cause the sea glacier to shrink till it reaches zero thickness after penetrating some distance into the inland sea. At the low latitudes where net sublimation is expected, the estimated zonally‐averaged mean annual surface temperatures ranges from -50°C to -20°C. Since ice is softer at warmer temperatures, a warmer sea glacier is expected to penetrate further into the inland sea.

Figure 2: The solid contours represent the penetration length to width ratio L/W as a function of net sublimation rate (vertical axis) and surface ice temperature (horizontal axis) for a sea glacier with an initial thickness of 650 m. A warm sea glacier with low net sublimation rate will have a large penetration L/W, while a cold sea glacier with high net sublimation rate will have a small penetration L/W. The dashed line represents the L/W of 6.5 for the Red Sea. Conditions to the left of the dashed line allow a Red Sea analogue to serve as a refugium for life without being overridden by ice cover.

To determine if an inland sea similar to the present-day Red Sea at low latitudes can remain ice free during a snowball event, the penetration length of a sea glacier coming in from the ocean is estimated. The present-day Red Sea is approximated by a rectangle 200 km wide and 1300 km long, a length to width ratio L/W of 6.5. At the mouth of the inland sea, the initial thickness of the sea glacier coming in from the ocean is 650 m. The study shows that the length of the present-day Red Sea can easily exceed the penetration length of the sea glacier (Figure 2). As a result, a Red Sea analogue during a Snowball Earth event can be long enough to remain ice free and serve as a refugium for photosynthetic life on a largely frozen planet.

- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Goodman and Pierrehumbert (2003), “Glacial flow of floating marine ice in Snowball Earth”, Journal of Geophysical Research 108(C10), 3308, doi:10.1029/2002JC001471
- Pierrehumbert et al., “Climate of the Neoproterozoic”, Annual Review of Earth and Planetary Sciences 39 (2011) 417-60
- Hoffman and Schrag, “Snowball Earth”, Scientific American (January 2000), Volume 282, pp. 68-75
- Beatty et al. (2005), “An obligately photosynthetic bacterial anaerobe from a deep-sea hydrothermal vent”, Proceedings of the National Academy of Sciences 102 (26): 9306-10
- Cardenas et al. (2013), “The potential for photosynthesis in hydrothermal vents: a new avenue for life in the Universe?”, arXiv:1304.6127 [astro-ph.EP]
- Campbell et al., “Refugium for surface life on Snowball Earth in a nearly‐enclosed sea? A first simple model for sea‐glacier invasion”, Geophysical Research Letters Volume 38, Issue 19, October 2011

Monday, September 2, 2013

Snowball Earth: A Frozen Planet

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
- Robert Frost, Fire and Ice (1923)

Rodinia is the name of a supercontinent that existed during the early Neoproterozoic era. It formed around 1100 Mya and involved the assembly of virtually all continental masses known to exist on Earth at that time. Rodinia was entirely barren since it existed at a time before life colonized dry land. Nevertheless, the continental margins of Rodinia may have played an important role in the development of life from ocean to land. Continental crust is considerably thicker than oceanic crust and the existence of a supercontinent insulated the underlying mantle. This fostered the development of a mantle superplume beneath Rodinia and eventually led to widespread continental rifting that resulted in the breaking up of Rodinia between 800 to 600 Mya (Meert and Torsvik, 2003).

Figure 1: Palaeogeographic reconstruction of the Rodinia supercontinent. The model posits two rifting events, one along the present-day western margin of Laurentia between 800 to 700 Mya, and a second along the present-day eastern margin of Laurentia between 600 to 550 Mya. (Meert and Torsvik, 2003)

Figure 2: Palaeogeographic reconstruction of the break-up and dispersal of the Rodinia supercontinent at 720 Mya. (Z.X. Li et al. 2008)

As Rodinia was starting to break-up, most of its continental masses happened to be grouped along the Equator. Such a continental configuration led to the development of a very unique climatic situation on Earth, characterised by intense evaporation and tropical rainfall on these continental areas. The intense rainfall washed out carbon dioxide from the atmosphere and produced carbonic acid which weathered exposed rocks on the continents. As a result, carbon dioxide, a greenhouse gas, was transferred from the atmosphere into the Earth’s crust and the Earth started to cool (Donnadieu et al., 2004). In addition, a tropical distribution of continents helped cool the Earth further because tropical continents are more reflective than open oceans and so absorb less of the Sun’s energy. In comparison, most of the Sun’s energy that is absorbed by present-day Earth occurs over tropical oceans.

As the Earth cooled, ice started to advance beyond the polar regions. When ice advanced to within 30° of the Equator a positive feedback mechanism known as the ice-albedo feedback ensured that the increased reflectivenss due to the formation of ice led to further cooling and the formation of yet more ice. This went on until the whole Earth became ice covered, probably right up to the Equator, creating what is commonly known as a “Snowball Earth”. During the Neoproterozoic era, the Earth may have experienced at least two snowball events at ~740 and ~635 Mya (Trindade and Macouin, 2007).

Figure 3: Artist’s impression of a Snowball Earth. Credit: Walter Myers

During a snowball event, global temperatures fell so low that the Equator may have been as cold as present-day Antarctica. The low temperatures were maintained by the high reflectivity of ice which reflected most of the incoming solar energy back into space. Since the Earth was almost completely ice covered, carbon dioxide could no longer be drawn out of the atmosphere by weathering of rocks. Over several million years, volcanos constantly emitted carbon dioxide which accumulated in the atmosphere. Evidence suggests that when the build-up of carbon dioxide exceeded ~10 percent of the atmosphere, the greenhouse effect became strong enough to thaw the Earth from a snowball state (Bao et al., 2008).

Figure 4: This chart illustrates a hypothetical depiction of a snowball event in terms of global mean surface temperature and ice cover (pale blue) on a palaeogeographic representation of the Earth at ~750 Mya. Note the abrupt onset and termination of glaciation at low-latitudes and the hot aftermath due to high levels of atmospheric carbon dioxide. The gradual temperature rise during the Snowball Earth event was due to the increasing greenhouse effect caused by the build up of atmospheric carbon dioxide from volcanic emissions.

There are pieces of good evidence to support the Snowball Earth hypothesis and that ice cover did extend into the tropics. Glacier deposits from the Neoproterozoic era with estimated palaeolatitudes based on palaeomagnetic data show a global distribution with a large fraction of deposits within 10° of the Equator. When an ice sheet moves over the ocean, rocks carried within the ice can become dislodged and fall onto the sediments on the ocean floor. These rocks become incorporated into the oceanic sediments and are known as dropstones. The presence of dropstones near the Equator indicates sea-level glaciation in the tropics during the Snowball Earth event.

Figure 5: A dropstone of quartzite embedded within sedimentary layers.

After the Snowball Earth event, the Earth’s surface is expected to become very warm due to the huge amount of carbon dioxide still present in the atmosphere. Elevated sea surface temperatures drove torrential rains that dissolve carbon dioxide and washed it out of the atmosphere as a weak carbonic acid. This weathered rocks on the continents and resulted in the release of large amounts of calcium that precipitated to form layers of carbonate sedimentary rocks. In fact, in the geological record, layers of carbonate rocks are indeed found to lie directly on top of glacier deposits. These layers of carbonate rocks are known as cap carbonates. The transition from glacier deposits to cap carbonates is abrupt and points towards a catastrophic collapse of the snowball state where the climate flipped rapidly from very cold to very hot.

Figure 6: Glacial deposits and cap carbonates in the Tillite Group of the East Greenland Caledonides.

Photosynthetic life is already known to exist billions of years before the Snowball Earth events of the Neoproterozoic era. There are a number of ways in which photosynthetic life in the ocean can continue to survive on a completely ice covered planet. One way involves the difference in how ice moves over land and ocean. Ice on land tends to be more locked while ice on the ocean tends to move more freely. Near continental margins, the difference in ice movement creates tension and can cause cracks to develop in the ice, opening up exposed bodies of ocean surface where photosynthetic life can thrive.

After the Snowball Earth event, the huge amount of erosional products being washed off the continents into the ocean by torrential rains fuelled a proliferation of photosynthetic life in the oceans and a corresponding leap in the amount of atmospheric oxygen. The increase in atmospheric oxygen concentration may have led to the rapid emergence of a huge variety of large, multi-cellular life, in what is now known as the Cambrian explosion around 550 Mya.

Figure 7: Artist’s impression of an Earth-like exoplanet. Credit: Scott Richard

The study of Earth-like exoplanets may help shed light on Earth’s geological past. For example, by studying a sample of Earth-like exoplanets in snowball states, it can show whether the grouping of continental masses along the Earth’s Equator during the Neoproterozoic era played a key role in kicking the Earth into a snowball state. However, mapping the continental distribution of an ice covered exoplanet is challenging and determining if one is indeed an Earth-like planet in a snowball state is going to be tricky. If the build up of atmospheric carbon dioxide created a sufficiently strong greenhouse effect to warm the Earth out of a snowball state, then Earth-like exoplanets in snowball states can be expected to show elevated levels of atmospheric carbon dioxide.

- Meert and Torsvik, “The making and unmaking of a supercontinent: Rodinia revisited”, Tectonophysics 375 (2003) 261-288
- Z.X. Li et al., “Assembly, configuration, and break-up history of Rodinia: A synthesis”, Precambrian Research 160 (2008) 179-210
- Donnadieu et al., “A ‘snowball Earth’ climate triggered by continental break-up through changes in runoff”, Nature 428, 303-306 (18 March 2004)
- Trindade and Macouin, “Palaeolatitude of glacial deposits and palaeogeography of Neoproterozoic ice ages”, Comptes Rendus Geoscience 339 (2007) 200-211
- Bao et al. (2008), “Triple oxygen isotope evidence for elevated CO2 levels after a Neoproterozoic glaciation”, Nature, 453 (7194), 504-506