Friday, December 31, 2010

Olympus Mons

Olympus Mons is a large shield volcano mountain that is located on the planet Mars and it has a morphology similar to the large volcanoes that make up the Hawaiian Islands. Rising to a height of 22 kilometres above the surrounding plains or 21 kilometres above the standard topographic datum of Mars, Olympus Mons is the tallest mountain known in the Solar System. This makes Olympus Mons stand at just under three times the height of Mount Everest. The base of Olympus Mons measures over 600 kilometres across and the outer edge of the mountain is rimmed by an immense cliff which rises up to 8 kilometres above the surrounding terrain.

Due to the sheer size of Olympus Mons and from the fact that the average slope of the volcano’s flank is only 5 degrees, the entire vertical profile of Olympus Mons will not be visible to an observer who is standing at a great distance away on the surrounding plains as the curvature of the planet Mars would obscure the mountain’s summit. Similarly, an observer standing on the summit of Olympus Mons will be unable to view the surrounding plains as the slopes of the volcano would extend well beyond the horizon. However, the immense cliffs which surround almost the entire base of Olympus Mons will definitely make an impressive sight.

Olympus Mons is located on the northwestern edge of the Tharsis Bulge which also has some of the largest volcano mountains known in the Solar System. To the southeast of Olympus Mons are the mountains Arsia Mons, Pavonis Mons and Ascraeus Mons. Like Olympus Mons, these mountains are also immense shield volcanoes that rise to impressive heights, greatly dwarfing even the prominence of Mount Everest. The Tharsis Bulge, on which these colossal mountains are located, covers millions of square kilometres in area and the height of Everest’s summit is merely comparable to the surface elevation of the massive plateau above the standard topographic datum of Mars.

The extraordinary size of Olympus Mons is due to the fact that unlike the Earth, Mars does not have plate tectonics and this enables the crust of Mars to remain stationary over a hotspot. By doing so, magma coming out of the hotspot continuously builds the volcano in the same location and allows Olympus Mons to become so large. A unique observational aspect of Olympus Mons is that it is sufficiently high enough to penetrate above the frequent Martian dust storms that can occasionally be large enough to engulf the entire planet. This was the first observational hint of the incredible height of Olympus Mons, long before the first spacecraft arrived in orbit around Mars.

The atmospheric pressure on the top of Olympus Mons is about 70 Pascal and this is about 11 to 12 percent of the atmospheric pressure at the standard topographic datum of Mars which has a value of 610 Pascal. In comparison, the atmospheric pressure on the top of Mount Everest is about 31400 Pascal while the atmospheric pressure at sea level on the Earth is 101325 Pascal. To put this into an Earthly perspective, the atmospheric pressure on the top of Olympus Mons is like being at an altitude of 50.5 kilometres above sea level while the atmospheric pressure at the standard topographic datum of Mars is like being at an altitude of 34.5 kilometres above sea level.

Orographic clouds that are made up of particles of water ice have long been known to be associated with Olympus Mons and with the other great volcano mountains on Mars. These clouds form when air masses are forced from a lower elevation to a higher elevation as they move up the slopes of these great mountains. The air masses cool as they rise and the moisture content carried within them condenses into particles of water ice, forming orographic clouds.

Monday, December 27, 2010

Magnetar Flare

A neutron star is a type of compact star that is formed from the gravitational collapse of the core of a massive star during a supernova explosion. A typical neutron star has a diameter of around 20 kilometres and a mass that generally exceeds the mass of our Sun. In comparison, our Sun has a diameter of 1.392 million kilometres. This incredibly compact configuration for a neutron star means that just a single cubic centimetre of its material packs a mass of around a billion metric tons! The extreme compactness of a typical neutron star also gives it a surface gravity that is over 100 billion times the surface gravity on the Earth and an escape velocity of around one third the speed of light!

A magnetar is an exceedingly rare type of neutron star which possesses an extremely powerfully magnetic field. In fact, the magnetic fields of magnetars are the strongest known in the universe as these magnetic fields have intensities on the order of between a billion to a trillion teslas. For comparison, the strength of the Earth’s magnetic field is about 30 microteslas while the strongest permanent magnets can generate magnetic fields of up 5 teslas. Magnetars are so rare that less than 15 of them are known. These exotic stars give rise to occasional burst of X-rays and gamma-rays, thus manifesting themselves as either Soft Gamma-ray Repeaters (SGRs) or Anomalous X-ray Pulsars (AXPs). SGRs are generally more energetic than AXPs and the bursting/flaring events from magnetars can be roughly classified into 3 types – short bursts, intermediate flares and giant flares. Giant flares are far more energetic than the short bursts and intermediate flares, and only 3 giant flares have been recorded in the decades of monitoring high energy astrophysical events since the 1970s.

SGR 1806-20 is a magnetar that is located around 50 thousand light years away, on the other side of the of the Milky Way galaxy. At this distance, it takes light 50 thousand years to travel from SGR 1806-20 to the Earth. The stellar neighbourhood of SGR 1806-20 contains some highly unusual stars, including one of the most massive and luminous star known in the Milky Way galaxy. What makes SGR 1806-20 unique is that this magnetar has the strongest magnetic field ever discovered for any object in the universe and this magnetar is also the progenitor for one of the 3 giant flares recorded so far. The strength of the magnetic field of SGR 1806-20 is estimated to be on the order of a whopping one trillion teslas!

On Monday 27 December 2004, an extremely energetic giant flare was detected from SGR 1806-20. This giant flare was so energetic that it saturated all but the least sensitive particle detectors regardless of where the detectors were pointed and this event became the brightest blast of gamma-rays ever detected from an astrophysical source. The giant flare from SGR 1806-20 is estimated to have released more than 2000 trillion trillion trillion joules of energy in the form of X-rays and gamma-rays.  Almost all of the energy released from the giant flare was concentrated in an initial hard spike that lasted for around 0.2 seconds. This initial hard spike was then followed by a gradually decaying pulsating tail which shows about 50 cycles of high-amplitude pulsations over the duration of around 600 seconds. The high-amplitude pulsations show a period of 7.5 seconds and this period matches the rotational period of SGR 1806-20.

To place the amount of energy generated by the giant flare from SGR 1806-20 into perspective, the amount of power produced during the initial hard spike which lasted for around 0.2 seconds is on the order of a thousand times the combined luminosity of all the hundred of billions of stars in the Milky Way galaxy! In fact, the amount of energy produced during the 0.2 seconds of the initial hard spike is greater than the total amount of energy generated by our Sun over a period of 100 thousand years! Already, the amount of energy produced by our Sun in a single second is almost a million times the total worldwide energy consumption in 2009! The giant flare from SGR 1806-20 was so bright that even its echo off our Moon was detectable. Interestingly, if all the energy were converted into visible light, the giant flare would have been brighter than the full Moon during the 0.2 seconds duration of the initial hard spike! If the giant flare from SGR 1806-20 had occurred at a distance of 10 light years from the Earth, it will be similar to standing at a distance of 7.5 kilometres from a 15 kiloton nuclear explosion.

The giant flare detected on 27 December 2004 from SGR 1806-20 is hundreds of times more energetic than the two other known giant flare events. The release of such an immense amount of energy within such a short period of time managed to eject a significant amount of matter from the magnetar. The highly energetic ejecta formed an outflow which interacted with the external interstellar medium and produced a radio afterglow this is at least 500 times more luminous than the only other radio afterglow detected from a giant flare. Finally, it may be possible for ultra-high energy cosmic rays from the giant flare to be detected years after the event form the direction of SGR 1806-20, provided that the deflection of the ultra-high energy cosmic rays by galactic magnetic fields is not too large.

Friday, December 24, 2010

Dark Galaxy

VIRGOHI 21 is the name given to an intriguing object that is located approximately 50 million light years away in the Virgo Cluster. The Virgo Cluster is a cluster consisting of between one to two thousand member galaxies. VIRGOHI 21 was discovered through radio telescope observations of the 21 centimeter wavelength radio emissions from its neutral hydrogen content. The total mass of hydrogen in VIRGOHI 21 is estimated to be around 100 million times the mass of our Sun. Observations of the motion of hydrogen gas within VIRGOHI 21 shows that the hydrogen gas is moving far too rapidly to be explained by the gravity from just the mass of the detected hydrogen alone. In fact, the total mass of VIRGOHI 21 is inferred to be as large as 100 billion times the mass of our Sun!

Deep observations by the Hubble Space Telescope revealed no optical counterpart to VIRGOHI 21 and this makes VIRGOHI 21 an excellent candidate for a dark galaxy since it has a mass of a galaxy but is entirely devoid of stars. Almost all of the mass which makes up VIRGOHI 21 is expected to be in the form of dark matter and less than a fraction of a percent of its mass is ordinary matter. Dark matter is basically matter whose existence can only be inferred from its gravitational effects due to the fact that dark matter does not scatter nor emit electromagnetic radiation. Interestingly, a paper entitled “Tidal Debris from High-Velocity Collisions as Fake Dark Galaxies: A Numerical Model of VIRGOHI 21” suggests that VIRGOHI 21 may not be a genuine dark galaxy and instead, it could be the result of a high-speed collision between two large galaxies.

Located half a million light years from VIRGOHI 21 is a large spiral galaxy called NGC 4254 and a filamentary structure of hydrogen gas connects VIRGOHI 21 with NGC 4254. This trail of hydrogen gas is almost devoid of stars and its velocity distribution is coherent with the outer disk of the spiral galaxy NGC 4254 to which it is morphologically connected. Furthermore, a tidal origin for this trail of hydrogen gas is unlikely since a counter trail is nonexistent in the opposite direction from the spiral galaxy NGC 4254. Instead, such a feature is consistent with a high speed collision between the spiral galaxy NGC 4254 and another galaxy since an event like this will cause little disturbance to the stars in the main disk of the spiral galaxy NGC 4254, resulting in the lack of stars in the trail of hydrogen gas that connects VIRGOHI 21 with NGC 4254. A high speed collision with another galaxy will also create a counter trail of hydrogen gas that is much fainter and shorter than the main trail. This counter trail will quickly fall back into the disk of the parent spiral galaxy and in a few hundred million years after the collision, a galaxy with just one trail of hydrogen gas will be observed.

The interloper galaxy which collided with the spiral galaxy NGC 4254 is probably a few million light years away by now since the collision is expected to occur at a velocity on the order of a thousand kilometers per second and it is estimated that a couple of billion years would have already elapsed since the collision. VIRGOHI 21 is located along the trail of hydrogen gas and the velocity distribution within VIRGOHI 21 differs remarkably from the rest of the trail. This can occur when denser parts of the trail contract and become self-gravitating. Eventually, a region like this can become an independent object with the mass of a dwarf galaxy, resulting in an object like VIRGOHI 21.

Observing the composition of the filamentary structure which connects VIRGOHI 21 to the spiral galaxy NGC 4254 can provide further evidence to proof if VIRGOHI 21 is a genuine dark galaxy or if it originated from a high speed collision between two galaxies. This is due to the assumption that genuine dark galaxies will be made up of pristine metal-poor gases as there will be no stars to fuse the hydrogen and helium into heavier elements. On the other hand, if VIRGOHI 21 formed out of matter spewed out from the spiral galaxy NGC 4254 after a high speed collision with another galaxy, VIRGOHI 21 will be observed to be enriched with elements heavier than hydrogen and helium from the many episodes of stellar fusion prevalent in the main stellar disk of NGC 4254. In conclusion, a high speed collision could provide an explanation for the origin of putative dark galaxies such as VIRGOHI 21.

Thursday, December 16, 2010

Consuming Planets

Red giant stars have diameters of around tens to hundreds of times larger than that of the Sun and they occur when stars like the Sun eventually exhaust the supply of hydrogen in their cores and switched to fusing hydrogen in a shell external to the core. The increased temperatures and reaction rates causes the star to expand into a red giant and as the star expands, it spins down due to the conservation of angular momentum. Therefore, red giant stars are expected to rotate much more slowly about their spin axis as compared to stars like the Sun.

An unusual class of red giant stars known as red giant rapid rotators are basically red giant stars that are known to spin much faster than what is predicted for them. Ordinary red giant stars have equatorial velocities of around 2 kilometres per second while red giant rapid rotators have equatorial velocities of around 10 kilometres per second or more. In a paper by Joleen Carlberg, et al. (2010) entitled “The Fate of Exoplanets and the Red Giant Rapid Rotator Connection”, it is suggested that as a red giant star expands; it can consume and accrete planets that happen to be orbiting in close vicinity. Planets accreted in this way can dump sufficient angular momentum into the red giant star and cause the star to spin up to become a red giant rapid rotator.

This mechanism of accreting planets only works for planets whose orbital periods are shorter than the rotational period of their host stars. In other words, the time it takes for the planet to orbit once around its star has to be shorter than the times it takes for the star to complete one rotation about its spin axis. In such a configuration, the tidal bulge raised on the star by the orbiting planet will always be trailing the planet and this allows angular momentum to be transferred from the orbiting planet to the rotation of the star. This causes the planet to lose orbital angular momentum, fall closer towards its host star and eventually getting accreted by the star.

The amount of angular momentum that is dumped into a red giant star by an accreted planet can be many times greater than the angular momentum of the star itself. In our solar system, the Sun holds less than 2 percent of the total angular momentum while the planet Jupiter holds 60 percent of the total angular momentum. However, the orbit of Jupiter is too distant for it to get consumed by the Sun when the Sun expands into a red giant star billions of years from now.

Most of the 500 or so extrasolar planets known to date are Jupiter-like planets which orbit very close to their parent stars, many of which have orbital periods in the range of a few days. These planets are termed hot-Jupiters and they form a large proportion of the currently known planets due to observational biases as these planets are the easiest to detect. Such a hot-Jupiter can dump a huge amount of angular momentum into its host star via accretion when the star expands into a red giant, turning the red giant into a rapid rotator. For example, a Jupiter-mass planet in a Mercury-like orbit around a star that is identical to our Sun will have about 10 times more angular momentum than the star itself.

The lithium abundance of a red giant rapid rotator can also provide further evidence to correlate it with accreted planets. Red giant stars are known to be depleted in lithium due to convective mixing and the accretion of a Jupiter-mass planet can significantly raise the lithium abundance of the red giant star. However, a better understanding of stellar evolution is still required to ensure that any observed lithium abundance or any other observed abundance anomalies are indeed anomalous for a given red giant rapid rotator such that it can be attributed to an accreted planet.

Friday, December 10, 2010

Historic Flight

SpaceX successfully launched its Dragon spacecraft into low-Earth orbit atop a Falcon 9 rocket on Wednesday, 8 December 2010 at 10:43 AM EST (4:43 PM UTC) from Launch Complex 40 at Cape Canaveral Air Force Station in Florida. The Falcon 9 rocket inserted the Dragon spacecraft into an orbit with a low point of 288 kilometers, a high point of 301 kilometers and an orbital inclination of 34.53 degrees. This orbit is remarkably close to the targeted orbit which called for an almost circular orbit 300 kilometers above the Earth’s surface with an orbital inclination of 34.5 degrees. Traveling at a velocity of nearly 28000 kilometers per hour, the Dragon spacecraft made almost two orbits around the Earth before reentering the Earth’s atmosphere and eventually landing on the surface of the Pacific Ocean at 3 hours and 19 minutes after liftoff.

This launch event marks the first time in history a commercial company has successfully recovered a spacecraft reentering from low-Earth orbit. Such a feat has been performed by only six nations or government agencies: the United States, Russia, China, Japan, India and the European Space Agency. Wednesday’s launch of the Dragon spacecraft marks a historic first for the future of space travel. No one was on onboard the Dragon spacecraft on its maiden flight even though the spacecraft has enough room for 7 astronauts. The entire mission from launch to splashdown in the Pacific Ocean was flawless and if there had been people in the Dragon spacecraft, they would have enjoyed the whole ride.

Timeline of Events:
T+00:00:00 – Liftoff
T+0:02:58 - 1st Stage Shut Down (Main Engine Cut Off)
T+0:03:02 - 1st Stage Separates
T+0:03:09 - 2nd Stage Engine Start
T+0:09:00 - 2nd Stage Engine Cutoff
T+0:09:35 - Dragon Spacecraft Separates from Falcon 9
T+0:13 - On-Orbit Operations
T+2:32 - Deorbit Burn Begins
T+2:38 - Deorbit Burn Ends
T+2:58 - Reentry Phase Begins (Entry Interface)
T+3:09 - Drogue Chute Deploys
T+3:10 - Main Chute Deploys
T+3:19 - Water Landing

As the Dragon spacecraft reenters the Earth’s atmosphere at a velocity of over 7 kilometers per second, the spacecraft experiences temperatures of around 2000 degrees Centigrade. To keep the interior of the spacecraft at room temperatures, against the ferocious heating during reentry, SpaceX worked with NASA to create a phenolic impregnated carbon ablator (PICA) heat shield called PICA-X. This heat shield is probably the most advanced heat shield ever to fly as it can be reused hundreds of times with little degradation, somewhat like an “on steroids” version of a Formula One racing car’s carbon brake pads.

Dragon is a reusable spacecraft that was developed by SpaceX under NASA’s Commercial Orbital Transportation Services (COTS) program and it was initially conceptualized by SpaceX in 2005. The Dragon spacecraft is made up of a pressurized capsule and an unpressurized trunk for the transportation of pressurized cargo, unpressurized cargo and/or crew members to low-Earth orbit. Basically, the Dragon spacecraft has 10 cubic meters of pressurized volume, 14 cubic meters of unpressurized volume and it can support up to 7 passengers in crew configuration. The crew and cargo versions of the Dragon spacecraft are designed to be nearly identical to facilitate a rapid transition between cargo and crew.

SpaceX’s Falcon 9 rocket was used to launch the Dragon spacecraft into space on this historic voyage. The Falcon 9 rocket is basically a two stage launch vehicle that is powered by liquid oxygen and rocket grade kerosene (RP-1). The first stage rocket booster is powered by nine Merlin 1C rocket engines which generate a combined thrust of 5 million Newton at liftoff, while the second stage rocket booster is powered by a single Merlin Vacuum rocket engine which generates a thrust of 411 thousand Newton in a vacuum. The Falcon 9 rocket can launch over 10 metric tons into low-Earth orbit and a yet to be launched heavy variant called the Falcon 9 Heavy can launch almost 30 tons into low-Earth orbit.

SpaceX is developing a family of launch vehicles and spacecraft that will increase reliability and performance of space transportation, while ultimately reducing costs by a factor of ten. Next year, the Falcon 9 rocket and the Dragon spacecraft will start delivering cargo, including live plants and animals to and from the International Space Station for NASA. Both the Falcon 9 rocket and the Dragon spacecraft were developed to one day carry astronauts.

SpaceX has also revealed plans for future rocket designs, namely the Falcon X, Falcon X Heavy and Falcon XX. All these launch vehicle designs are in the heavy-lift to super heavy-lift range. The Falcon X can deliver up to 38 metric tons to low-Earth orbit while the Falcon X Heavy can deliver up to 125 metric tons to low-Earth orbit. Finally, the Falcon XX is a behemoth which can deliver up to 140 metric tons to low-Earth orbit. If developed, SpaceX’s Falcon X Heavy and Falcon XX will be among the largest and most powerful rockets ever built, with the long retired legendary Saturn V rocket being the closest rival. For comparison, NASA’s Space Shuttle can deliver 24 metric tons to low-Earth orbit while the Saturn V rocket can deliver a massive 120 metric tons to low-Earth orbit.

Saturday, December 4, 2010

Dark Matter

The existence of dark matter in the universe can only be inferred from its gravitational effects on ordinary matter and on electromagnetic radiation. This is so because dark matter can neither emit nor scatter electromagnetic radiation. Dark matter constitutes 80 percent of the matter in the universe while ordinary matter makes up the remaining 20 percent. Ordinary matter is basically everything which makes up the Earth, the planets, the stars and the vast quantities of gas and dust across interstellar and intergalactic space.

Although ordinary matter can account for a tiny proportion of dark matter, the vast majority of dark matter is made up of something else entirely. While the properties of dark matter can be somewhat constrained, the particle constituents of dark matter continue to elude detection. Dark matter is not made up of atoms and it does not interact with ordinary matter via electromagnetic forces. Hence, the study of dark matter has so far been largely based on the observable gravitational effects that dark matter imposes on ordinary matter and on electromagnetic radiation.

There are a number of independent sources of evidence for the existence of dark matter. Stars are known to orbit around the centre of galaxies and their orbital speeds do not decrease with increasing distance from the galactic centre. This is rather unexpected because the galaxy must have much more mass than can be attributed to ordinary matter alone; otherwise the orbital speeds of stars should decrease with increasing distance from the galactic centre. Thus, dark matter is responsible for the additional mass that can’t be attributed to ordinary matter alone.

Gravitational lensing is another independent piece of evidence for the existence of dark matter and this phenomenon occurs when light from a background object gets deflected by the gravitational field of a foreground object. This can distort the image of the background object and also change its observed brightness. A more massive foreground object will create a more pronounced gravitational lensing effect. Observations of gravitational lensing by foreground galaxies have shown that the amount of mass required by the galaxy to generate the observed lensing far exceeds the combined mass of all its stars and ordinary matter.

A paper entitled “Planet-Bound Dark Matter and the Internal Heat of Uranus, Neptune, and Hot-Jupiter Exoplanets” by Stephen L. Adler from the Institute for Advanced Study at Princeton explores the possibility that the accretion of planet-bound dark matter by gas giant planets could significantly contribute to their internal heat.

The Milky Way galaxy is situated at the centre of a vast halo of dark matter. The dark matter in the vicinity of the solar system is believed to be distributed in a way such that a cubic volume of space measuring 10000 by 10000 by 10000 kilometres contains around 500 grams of dark matter. Although this may seem sparse, scaling up the volume of space to one cubic light year will give a mass of dark matter this is around 80 times the mass of the Earth. One light year is the distance light travels in a period of one year. The dark matter in the vicinity of the solar system orbits around the galactic centre of mass along with the solar system.

It is not known if there is dark matter that is gravitationally bound to the Sun or to the planets in the solar system. Gravitational conglomerations such as stars and planets can accrete the ambient galactic dark matter over time such that Sun-bound and planet-bound dark matter can have densities that are many orders of magnitude greater than the ambient density of dark matter.

Planet-bound dark matter can contribute to internal heating of the planet by depositing energy inside the planet as the dark matter particles lose orbital energy by interacting with the particles of ordinary matter that make up the planet. Planet-bound dark matter can also contribute to internal heating if the particles that comprise them are self-annihilating. When self-annihilating dark matter particles meet, they annihilate each other and convert their mass into energy which can be deposited within the planet as internal heating.

Interestingly, the contribution to internal heating of a planet by the accretion of dark matter can explain the anomalously low rate of internal heat production for Uranus as compared to Neptune. Uranus has an almost identical internal structure and composition as Neptune and it should be producing the same amount of internal heating as Neptune. However, one key difference between Uranus and Neptune is that Uranus is tiled 98 degrees with respect to the plane of the solar system, whereas Neptune is only tiled 28 degrees. For comparison, the Earth has an axial tilt of 23.4 degrees.

The large axial tilt of Uranus is believed to be caused by a massive impact event, whereby an object around the mass of the Earth slammed into Uranus. This impact event could have pushed Uranus out of its accreted planet-bound cloud of dark matter and leave it with a much lower rate of internal heat production than Neptune. Before the impact event, Uranus would have a similar rate of internal heat production as Neptune.