ASOW 2012/05/06: Neptune & Vulcan: Dark Matter in Solar Syst

Seminars presented by astronomers and astrophysicists on a wide variety of astronomy topics.
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ASOW 2012/05/06: Neptune & Vulcan: Dark Matter in Solar Syst

Post by owlice » Sat May 05, 2012 10:00 pm

Neptune and Vulcan: Dark Matter in the Solar System
Dr. Rob Knop
Professor of Physics, Quest University, Canada

Audio (MP3, 57Mb)

The cosmological Dark Matter is one example of a more general thing that we might call "dark matter" with lowercase letters-- that is, something that has not been observed directly, but whose existence we infer because of its gravitational effects. In the 19th century, there was dark matter in the Solar System: Uranus was showing deviations in its orbit that could not be explained by the action of the Sun and the other known planets. That led to the discovery of Neptune. Similarly, deviations in the orbit of Mercury led to the postulated planet Vulcan inside Mercury's orbit. However, it turned out that Vulcan didn't exist at all, and that the answer to the question of Mercury's orbit was a modification of our theory of gravity.
First presented in Second Life in April, 2009.

Dr. Knop will answer a selection of questions posted by May 20, 2012.

A PDF of the slides can be downloaded here (21Mb)

Files also available here: ... and_Vulcan
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Re: ASOW 2012/05/06: Neptune & Vulcan: Dark Matter in Solar

Post by neufer » Sun May 06, 2012 4:21 am wrote: <<The vulcanoids are a hypothetical population of asteroids that may orbit the Sun in a dynamically stable zone inside the orbit of the planet Mercury. They are named after the hypothetical planet Vulcan, whose existence was disproven in 1915. The vulcanoids are thought to exist in a gravitationally stable band inside the orbit of Mercury, at distances of 0.06–0.21 AU from the Sun. All other similarly stable regions in the Solar System have been found to contain objects, although non-gravitational forces such as radiation pressure, Poynting–Robertson drag and the Yarkovsky effect may have depleted the vulcanoid area of its original contents. The outer edge of the vulcanoid zone is approximately 0.21 AU from the Sun. More distant objects are unstable due to the gravitational influence of Mercury and would be perturbed into Mercury-crossing orbits on timescales of the order of 100 million years. The inner edge is not sharply defined: objects closer than 0.06 AU are highly susceptible to Poynting–Robertson drag and the Yarkovsky effect, and even out to 0.09 AU vulcanoids would have temperatures of 1,000 K or more, which is hot enough for evaporation of rocks to be the limiting factor in their lifetime.

In 1998, astronomers analysed data from the SOHO spacecraft's LASCO instrument, which is a set of three coronagraphs. The data taken between January and May of that year did not show any vulcanoids brighter than magnitude 7. This corresponds to a diameter of about 60 kilometres, assuming the asteroids have an albedo similar to that of Mercury. In 2000, planetary scientist Alan Stern performed surveys of the vulcanoid zone using a Lockheed U-2 spy plane. The flights were conducted at a height of 21,300 metres during twilight. In 2002, he and Dan Durda performed similar observations on an F-18 fighter jet. They made three flights over the Mojave desert at an altitude of 15,000 metres and made observations with the Southwest Universal Imaging System—Airborne (SWUIS-A). The MESSENGER space probe may provide evidence regarding vulcanoids. Its opportunities will be limited because its instruments need to be pointed away from the Sun at all times to avoid damage. The spacecraft has already taken a few of a planned series of images of the outer regions of the vulcanoid zone.

The volume of the vulcanoid zone is very small compared to that of the asteroid belt. Collisions between objects in the vulcanoid zone would be frequent and highly energetic, tending to lead to the destruction of the objects. The most favourable location for vulcanoids is probably in circular orbits near the outer edge of the vulcanoid zone. Vulcanoids are unlikely to have inclinations of more than about 10° to the ecliptic. Mercury trojans, asteroids trapped in Mercury's Lagrange points, are also possible.

Any vulcanoids that exist must be relatively small. Previous searches, particularly from the SOHO spacecraft, rule out asteroids larger than 60 kilometres in diameter. The minimum size is about 100 metres; particles smaller than 0.2 μm are strongly repulsed by radiation pressure, and objects smaller than 70 m would be drawn into the Sun by Poynting–Robertson drag. Between these upper and lower limits, a population of asteroids between 1 kilometre and 25 kilometres in diameter is thought to be possible. They would be almost hot enough to glow red hot.

There is evidence that Mercury was struck by a large object relatively late in its development, a collision which stripped away much of Mercury's crust and mantle, and explaining the thinness of Mercury's mantle compared to the mantles of the other terrestrial planets. If such an impact occurred, much of the resulting debris might still be orbiting the Sun in the vulcanoid zone.

Vulcanoids, being an entirely new class of celestial bodies, would be interesting in their own right, but discovering whether or not they exist would yield insights into the formation and evolution of the Solar System. If they exist they might contain material left over from the earliest period of planet formation, and help determine the conditions under which the terrestrial planets, particularly Mercury, formed. In particular, if vulcanoids exist or did exist in the past, they would represent an additional population of impactors that have affected no other planet but Mercury, making that planet's surface appear older than it actually is. If vulcanoids are found not to exist, this would place different constraints on planet formation and suggest that other processes have been at work in the inner Solar System, such as planetary migration clearing out the area.>> wrote: <<The Vulcan homeworld, also named Vulcan, was mentioned in the original series, episode "The City on the Edge of Forever" (1967) to be orbiting the far left star of Orion's belt, i.e. Alnitak, and in the novel Star Trek 2, author James Blish put the planet in orbit around the star 40 Eridani A, 16 light years from Earth, an identification later adopted by Roddenberry. Vulcan is a reddish Minshara-Class planet. Its inhabitants were originally called Vulcanians; a name used by Spock in the Original Series episode A Taste of Armageddon, by Federation colonists in This Side of Paradise and by Harry Mudd in Mudd's Women. The planet is said to have no moons.

Much of its surface consists of deserts and mountain ranges, and large areas are set aside as wilderness preserves. It is much hotter, it has a stronger surface gravity, and its atmosphere is thinner than that of Earth. As a result of these factors, humans tend to tire out more quickly than native Vulcans.

In the alternate timeline of the 2009 film, Nero destroyed Vulcan by creating a singularity in its core as part of his quest to avenge the destruction of Romulus. The resulting implosion destroyed the planet, killing most of its six billion inhabitants. Only around 10,000 managed to escape, including Spock and some of the Elders. At the end of the film, Spock Prime tells the younger Spock a suitable planet had been located to establish a colony for the surviving Vulcans, so it can be assured that this planet will be the new Vulcan.

An alternative to the 40 Eridani identification is also alluded to in the 2009 film. Spock Prime is marooned on Delta Vega, presumably the fourth planet orbiting Vega. Delta Vega is shown to be a Class M planet exhibiting Arctic like conditions. From Delta Vega, Spock Prime clearly observes the destruction of Vulcan. This would place Vulcan in the same system and in close proximity to Delta Vega. Comparing the two respective climates, it would appear that Vulcan is identified as Gamma Vega.>>
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