Moon's scarred surface bears evidence to giant plant migrations...

About 4 billion years ago, the inner Solar System was not a very hospitable place. The planets inward of giant Jupiter - including Earth and the Moon - were pounded by incredible numbers of large objects over a period of some 100 million years. Although short in terms of geologic time, this pounding, called the Late Heavy Bombardment by astronomers, left permanent scars - the craters of the Moon, Mars, and Mercury - that remain to this day. But why was the Late Heavy Bombardment so intense? Where did the meteoroids and asteroids come from? A large part of the answers to these questions may be found in a recent paper published by a team of astronomers in the journal Science.

Researchers at the University of Arizona - Robert Strom, Renu Malhotra, and David Kring - and their Japanese colleagues Takashi Ito and Fumi Yoshida began formulating their picture of the Late Heavy Bombardment by first looking at the available data concerning the ancient impactors. "We were looking at the fingerprints of the event - the data - and worried about explaining it later," said Kring. And it was these fingerprints - geochemical and geophysical - that pointed the investigators straight to the source of material for the Bombardment. And it wasn't a place expected by some astronomers.

In May of this year, Hal Levison, an astronomer at the SouthWest Research Institute in Boulder, had argued on dynamical grounds that the migration of the giant planets - particularly Uranus and Neptune - had gravitationally disrupted the Kuiper Belt, flinging comets and asteroids into the inner Solar System. In this scenario, the Late Heavy Bombardment is produced by these comet-like objects, and Levison was able to show that it would last about 100 million years. However, when the Arizona and Japanese astronomers derived the size distribution of the impactors from the cratering record left on the Moon and elsewhere, it did not match that of the current Kuiper Belt population.

It matched - almost exactly - the size distribution of asteroids in the asteroid belt, located between Mars and Jupiter. "An astonishing fit," said Strom.

Kring points out that this is not too surprising, given that the size distribution of the objects producing craters on the icy moons of Saturn and other outer planets is different from that of the Moon, Mercury, and Mars. The reason is simple - The icy moons of the outer Solar System were hit by objects from the Kuiper Belt, not from the asteroid belt. His geochemical analyses of the abundances of elements found in lunar material - iridium, gold, and germanium in particular - indicate that the lunar impactors possessed a composition similar to the asteroids in the asteroid belt. Taken together, the evidence points to less than 15% of the Late Heavy Bombardment impactors originating in the Kuiper Belt.

Malhotra has an answer for the "Why" of the Late Heavy Bombardment. She argues that the migrations of the giant planets, particularly Jupiter's inward migration, caused gravitaional resonances to sweep across the asteroid belt, disrupting orbits and flinging asteroids towards the inner planets. The devastation on Earth was severe - an estimated 20,000 asteroid impacts would have vaporized any primeval oceans and pulverized the surface. There was, however, a positive consequence - subsurface hydrothermal systems, which some think are critical to the origin of life, would have been created.

The Late Heavy Bombardment was the last great cratering period in Solar System history. Once the giant planets reached their current positions, it ended and impacts on planets in the inner Solar System became infrequent events. This stability is a good thing, for it allowed life to develop and evolve on Earth.

An Icy Interior for Ceres?

HST observations indicate that the largest main belt asteroid may have an icy mantle beneath its surface.

Located near the middle of the main belt of asteroids between Mars and Jupiter, Ceres is the largest, being about 592 miles ( 952 km) across. Despite its size, this asteroid has had a rather bland reputation among astronomers - its low density, low reflectivity (albedo), and relatively featureless spectrum led to the conclusion that asteroid's interior was homogeneous (uniform) in composition, with little structure. Not nearly as interesting as the second largest asteroid, Vesta, which, according to measurements, is differientiated, i.e., it possesses a crust, mantle, and core. However, a paper published recently in the journal Nature provides strong evidence that Ceres may be far more interesting than previously thought.

A team of astronomers led by Peter Thomas of Cornell University used the Hubble Space Telescope to obtain images of Ceres during December of 2003 and January, 2004. Measurements of the shape of the asteroid revealed that Ceres is rotationally symmetric - in other words, the distance from the surface to the center of the asteroid is the same regardless of longitude. This is an important result, as it indicates that the shape of the asteroid is determined by hydrostatic equilibrium, in which the pressure at any point within the body is determined by the weight of the material above it. Stars like the Sun are in global hydrostatic equilibrium, else their size would not remain constant. So are planets - gas giants like Jupiter, and big rocky bodies like the Earth. Some astronomers, notably Alan Stern of the Southwest Research Institute and a member of the team sudying Ceres, think that a shape determined by global hydrostatic equilibrium is the separation point between asteroids and planets. So, if we accept their arguments, Ceres is a planet.

But there's more.

Measurements of the asteroid's shape also provides information about its interior. Any body with a decent rate of rotation is going to have its equatorial regions bulge outwards, causing the poles to shrink inward. Instead of a perfect sphere, the star or planet (or asteroid, in the case of Ceres), will look "flattened." By measuring the distances from the center to the equator and poles, we can determine the amount of this flattening, which then sets constraints on the possible structure of the interior. Ceres has a mean density of about 2.077 grams per cubic centimeter (roughly twice that of water), and a homogeneous body of this density should have a polar radius about 39.7 km smaller than the equatorial radius. Ceres' polar radius is only 32.6 km smaller, strong evidence that its interior is not homogeneous, as had previously been thought. Indeed, the smaller amount of flattening indicates the presence of a mantle and a core. Hardly uniform.

The team developed computer models of Ceres' interior and used the available data on the asteroid to impose realistic constraints on the results. Their findings - Ceres has a rocky core and could have a mantle of ice as thick as 77 miles (124 km), amounting to about one quarter of the asteroids mass. The lack of a water signature in the spectrum does not present a problem, as any water ice on the surface of the asteroid would be unstable and soon lost to space. Going out on a limb, the astronomers make a prediction in their Nature paper - they assert that the Dawn spacecraft, upon its arrival at Ceres, will find a "globally relaxed and differientiated object, but which should retain a visible cratering record, and possible tectonic features..."

We will have to wait until 2015 to find out if they are right.

Speedy Neutron Star May Also Be Strange

A recent discovery by radio astronomers points to the existence of two types of neutron stars

A team of radio astronomers studying the radio pulsar B1508+55 have obtained a somewhat surprising result - the star is heading out of the Milky Way galaxy at the phenomenal velocity of 1100 kilometers per second (670 miles per second). At this great speed, the gravitational pull of the galaxy is not enough to keep the pulsar within its confines - it has for all practical purposes, been "kicked" out of the galaxy.

The measurement of the speed of the pulsar is a story unto itself, as the radio astronomers used the National Science Foundation's Very Long Baseline Array (VLBA) to measure both the star's motion and the increadibly small wobble in its position caused by the Earth's motion around the Sun. This wobble, called parallax, enabled the group to determine the pulsar's distance as 7700 light years, and its measurement was quite a feat - imagine trying to measure the length of a baseball bat at the distance of the Moon! In announcing the findings, one of the astronomers, Shami Chatterjee of the National Radio Astronomy Observatory (NRAO) and the Harvard-Smithsonian Center for Astrophysics, stated "We know that supernova explosions can give a kick to the resulting neutron star, but the tremendous speed of this object pushes the limits of our current understanding."

Pushes, definitely. However, an understanding of this intergalatic speed demon is still within our reach - or so run the claims of several astrophysicists, experts in the theory of pulsars.

A set of models published in 2002 by Z. Arzoumanian of Goddard Space Flight Center and D. Chernoff and J. Cordes, both of Cornell University, suggests that the speed distribution of pulsars may have have two humps, rather than one. In their scenario, the first hump has a maximum at around 90 kilometers per second, whereas the second - and smaller - hump peaks at around 700 kilometers per second. B1508+55 would belong to the second hump, with only about 2% of all pulsars sharing its speed.

But why should there be two humps in the speed distribution? Why not one, which is the most common case in nature? A second set of astrophysicists, I. Bombaci of the University of Pisa, and S. Popov, of the Sternberg Astronomical Institute in Russia, think they may have an answer.

Pulsars are neutron stars - stars that have collapsed inward upon themselves after going through the cataclysmic supernova explosion marking the end of their normal existence. Only a few kilometers (miles) across, these stars are greatly compressed - in simple terms, the nuclei of the atoms are crammed together, and many (about 40%) travel through space at a few tens of kilometers per second. Belonging to the first hump in the speed distribution, the speed of these stars is due to the "kick" imparted to them by the supernova explosion.

But imagine another class of neutron stars, one in which the matter is so compressed that the atoms are broken down into the basic building blocks of matter - quarks. Forming these "quark stars" requires more energy, and, if Bombaci and Popov are right, will receive more of a "kick" when they are formed. Another possible scenario has quark stars forming from the "normal", or hadronic, neutron stars by instability or accretion of mass, in which case the star will get a second kick, greatly adding to its speed. Whether caused by a big initial kick or an additional one, these stars produce the second hump in the speed distribution. It is therefore possible that B1508+55 may be a quark star - very, very compressed and very fast.

And since quark stars can be comprised of up, down, and strange quarks, it may be a little on the strange side as well.

2004 MN4 Is Named

July was a significant month for the asteroid formerly known as 2004 MN4 – What were probably the last optical images before 2007 were made by Dave Tholen, Fabrizio Bernardi, and Roy Tucker on July 9-11 using the 90” Bok Telescope at Kitt Peak. At about the same time, the JPL Near Earth Object Office (NEO), incorporating a new set of observational data, slightly raised the odds of potential impact in the 2030’s to roughly 1 in 8000. Small, but not ignorable, given the asteroid’s 850 megaton striking power – over 4 times the energy released in the 1883 Krakatoa eruption, which kicked up 6 cubic miles of rock and ash into the atmosphere.

But the big news was that, as of July 19th, the asteroid acquired a name and a number, losing its provisional designation of 2004 MN4. The number, the largest yet assigned to an asteroid, is 99942. The name – which will delight Egyptologists and fans of the Stargate SG-1 television series – is Apophis.

For those of you not familiar with the mythology of ancient Egypt, Apophis is the name given by the ancient Greeks to the Egyptian demon Apep, the personification of evil, the enemy of light and order. Depicted as a giant snake, Apophis or Apep (Apep is thought to translate as “he who was spat out” ) attacked the Sun god Ra as he made his way through the Egyptian Underworld during the evening hours. Eclipses were thought to be caused by Apep swallowing Ra. Fortunately, the Sun god always succeeded in cutting his way out of the snake’s belly. As the enemy of Ma’at, the ancient Egyptian concept of order and law, Apep represented chaos.

A very appropriate name for the asteroid that continues to cause much discussion and controversy in scientific circles.

99942 Apophis will pass very near Earth on Friday, the 13th of April, 2029, and may even impact sometime in the following decade. A 1 in 8000 chance, the most likely impact scenario occurs in 2036, off the Pacific coast of North America. A tsunami topping over 30 feet could strike southern California, comparable to the one which lashed Indonesia last December.

Again, the most likely scenario is that Apophis will not strike the Earth. One thing, though, is for sure – the asteroid will not be the same after its close encounter with our planet in 2029. Earth’s gravity and the tidal forces exerted on Apophis will change it in several ways – a cosmic makeover.

First, the orbit of the asteroid will be significantly altered. As things currently stand, Apophis belongs to the Aten group of asteroids, which approach Earth from the inside of its orbit, “out of the Sun”, if you will. After April 13, 2029, the orbit of Apophis will be changed enough to move it into the Apollo class of asteroids, which actually cross the Earth’s orbit.

Second, the asteroid’s rotation (spin) rate will be altered significantly. Recent calculations indicate that tidal forces caused by the Earth’s gravitational pull could result in a change in the length of the 30-hour “day” on Apophis by as much as 27 hours. The 2029 encounter will provide astronomers with their first chance to watch an asteroid’s spin state being disrupted.

Finally, the tidal forces will cause tremendous strain with the asteroid, causing “localized shifts” on the surface and within its interior (in other words, asteroid quakes). A less likely scenario occurs if the asteroid’s density is low, less than half that of water. In this case, Apophis could actually be reshaped or disrupted.

Stargate fans know Apophis as a powerful, militaristic member of the parasitical Goa’uld race, the nemesis of his brother Ra. In the TV show, Apophis launches a major space-based attack against Earth, which is foiled by the SG-1 team. Despite numerous attempts to kill him, he always manages to escape, until his ship collides with a planet. Even then, Jack O’Neil, hero of the series, remarks that he is "100 percent sure... 99 percent sure that Apophis is dead." One of the discoverers of the asteroid Apophis, Dave Tholen, is known to be a Stargate SG-1 fan.

Whether named after the Egyptian demon of evil and chaos, or the fictional TV character, the asteroid formerly known as 2004 MN4 lives up to its name – nothing about it seems certain.

Planet X is discovered... Planet XI as well?

Man, when it rains it pours... Today, the Solar System has two more major bodies, both trans-Neptunians or Kuiper Belt Objects.

The first of these, 2003 EL61, has a very eccentric orbit - perihelion is close to Neptune's distance, and aphelion is way out at 51.5 AU's - about 1.5 times Pluto's average distance from the Sun. Only about 100 km smaller than Pluto, 2003 EL61 also has a moon, which orbits once every 49 days.

Some might consider this a planet.

Late this afternoon, JPL held a press conference announcing the 10th planet (name chosen and applied for, but not announced). This body, located at a distance of 97 AU's, even if it is a perfectly reflective mirror (which it ain't), would still be bigger than Pluto. But it has to be smaller than 2000 miles in diameter, or the new Spitzer Space Telescope would have detected it at infra-red wavelengths. So, this new "planet", provisionally known as 2003UB313, is somewhere between 2274 and 3218 km in diameter.

I think that Clyde Tombaugh, were he still around, would be very happy this day. Planet X has been found - and, depending on who you ask, Planet XI as well.

Links: http://neo.jpl.nasa.gov/orbits/2003el61.html