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Astronomy

Closer to the asteroids

One of the largest surveys of the smallest bodies of the Solar Systemwill help to understand its formation

Dawn was breaking one day last September in the far south of the Chilean desert of Atacama when astrophysicist Daniela Lazzaro took her eyes away from the computer screen connected to the telescope and let out a sigh of relief, after having spent the night watching asteroids hundreds of millions of kilometers away. It was the final moment in 1,300 hours and 135 nights in five years of observations, which produced one of the most wide-ranging studies on the composition of the asteroids and will help to understand the formation and evolution of the Solar System.

“At a meeting of the Brazilian Astronomy Society in August”, says Daniela, “I presented the nearly finished results of this great survey of the composition of the asteroids, which has been under way since 1996. There was also a presentation of the works of my students who worked on the data from this survey. We have plenty of results”. And she gives an example: “Among the important results achieved so far, I could mention: the discovery of some objects with a very rare composition, one of them the theme of an article published in the Science magazine of June 2000; the analysis of the distribution of compositions in some groups and families of asteroids, proving a separate origin; and the analysis of the total distribution of compositions in the asteroid belt, showing that it is different from how it had been regarded hitherto.

With around 800 asteroids observed, our survey is now the second largest in the world, in terms of the number of objects.” The work was done on the 1.52 meter telescope of the European Southern Observatory (ESO), in La Silla, and is part of an agreement with the National Observatory (ON) of Rio de Janeiro, where Daniela works. Under the agreement, the ON uses half of the time of the telescope. From December onwards, the survey and all the data will be available on the Internet, at www.on.br. The method of analysis was reflex spectroscopy, which makes it possible to look into the chemical and mineralogical composition of the surface of an asteroid, based on the principle that different materials reflect light in a different way. The main minerals found were pyroxene, olivine, nickel-iron, feldspars and carbon enriched and organic minerals.

Witnesses of the beginning
Asteroids are blocks of stone of various sizes, located predominantly in the Asteroid Belt, a band positioned between Mars and Jupiter, at a distance of between 1.5 and 5 astronomical units (AUs) from the Earth. Each AU is the equivalent of 150 million kilometers, the average distance between the Earth and the Sun. As they are far smaller than the planets, asteroids preserve materials from the final stages of the formation of the Solar System, without undergoing any significant thermal, chemical and tectonic evolution, after a process of agglomeration called accretion.

Daniela explains the process: “All the bodies of the Solar System were formed from the accretion of small bodies called planetesimals, of a size between a few centimeters and a few meters. The larger the body, the more planetesimals were agglomerated. Accretion in itself already generates internal heat: hence, the larger the body, the greater the ‘primordial’ internal heat – the heat arising from the process of accretion – this body will have. Greater heat leads to greater chemical and mineralogical transformations. Furthermore: the greater the body, the greater the pressures that are created. These pressures also lead to chemical and mineralogical transformations.

On the other hand, a small body will have low primordial heat, as well as small internal pressures: hence, the materials that originally underwent accretion will remain unaltered, or little altered”. That is why the asteroids are called primordial objects. There are 30,000 of them cataloged. “They are fundamental for understanding the mechanisms that gave origin to the Solar System that we observe today”, Daniela stresses.

The study that analyzed the mineral composition of the surface of 830 asteroids only falls short, in number, of the survey released this year by the Massachusetts Institute of Technology (MIT), which covered 1,200 asteroids. “Our study and the MIT’s are very similar, but they concentrate on different regions of the Asteroid Belt – which means that, to a great extent, they are complementary. The majority of the objects observed by us were not observed by them, and vice-versa, since the beginning the cooperation between the two teams was good.”

The observations in Chile allowed several conclusions. It was possible to confirm that the “families” of asteroids, parts of a larger body that suffered a strong collision, show what the researchers call a genetic relationship between its members. Three large families were analyzed: Flora, Themis and Eunomia. In Flora and Eunomia, the main mineral found was pyroxene. “From the mineralogical point of view, the members of the same family have the same profile amongst themselves, and a different one from the other families. That is to say, each large family was the result of a different collision”, says Daniela.

The same cannot be said of the “dynamic groups”, agglomerations of asteroids located at specific points of the Belt, which fall under the gravitational influence of Jupiter. Detailed research into two of these groups, Phocaea and Hungaria, showed that they do not have similar compositions and that each asteroid reveals a separate origin. “In this case, there was no large body that broke up. Each asteroid came from a different place and is where it is because of dynamic processes, such as the gravitational force of Jupiter”, the astrophysicist explains.

Radioactive effect
The model based on the influence of the Sun in the mineralogical composition of the asteroids from the Belt is contested by the data generated through the research project. According to the model, the asteroids arrived at their current composition according to the degree of heating that they suffered from the Sun. Asteroids closest to the star, for example, would have been heated up and gone through transformations that determined their mineralogical compositions. Whereas the most distant from the Sun would not have felt the effects of heating and would therefore be more primitive.

The observations in La Silla dispute this separation of the composition of the asteroids from the Belt according to their distance from the Sun. Rather than this, one of the hypotheses under consideration is the possible presence of radioactive materials in the asteroids. Different quantities of these materials would lead to different degrees of heating. “The Sun shouldn’t be the only to blame for the greater heating that some asteroids have undergone”, explains Daniela. “Actually, what happens is that radioactive materials decay and .generate heat in this process. The more of these materials that an asteroid has collected in the course of its formation, the more heat will be generated later in the process of radioactive decay, which will lead to chemical/mineralogical modifications.”

Basaltic rock
The study also led to the discovery of three asteroids with a rare composition. They have a surface of basaltic rock, created by the eruption of volcanoes and previously only found in six bodies of the Solar System – the Earth, the Moon, Mars, Venus, Io (one of Jupiter’s moons) and in the asteroid Vesta, which is 500 kilometers in diameter. Until then, it was considered that celestial bodies with traces of volcanic activity were rare.

The team from the ON found that Magnya, an asteroid of 30 kilometers that is probably a fragment of a larger body, has a basaltic composition. This finding, published in an article in Science , showed that this asteroid of only 30 kilometers in diameter, may be what was left over larger celestial body, which may have undergone intense volcanic activity. “The theories about the formation of basaltic bodies in the Belt will have to be revised. The composition of Magnya implies that at least a second basaltic asteroid was formed, besides Vesta”, Daniela emphasizes. She thinks that the basaltic meteorites that fall on Earth may be other pieces from the fragmentation of a larger body that gave origin to Magnya.

In 2001, the team from the ON found another two asteroids besides Magnya with a basaltic composition. They are Vesta’s largest and most distant fragments from the asteroid yet discovered. “This proves that the Vesta family extends far beyond its current limits”, says Daniela. Another part of the study shows also that the asteroids with surfaces of pyroxene and olivine are the most abundant in the Belt. It was previously believed that those of the carbonaceous kind predominated.

The main findings of the observation mission are to be published in the Icarus astronomy magazine, of the United States. “This research gives Brazil a prominent position in the area of astrophysics of the Solar System”, says Daniela. The researcher thinks that mineral resources originating from asteroids, like titanium and iron, may in future be used on Earth, and the research by the Observatory will serve as a point of reference for this work. “This data is fundamental for choosing the asteroids to be visited by space missions”, she observes.

With the data, it was possible to obtain the precise distribution of the physical/chemical compositions in the Asteroid Belt as a whole. “This work will be fundamental for understanding the formation and evolution of the Belt and of Solar System”, says Daniela, who worked jointly with researchers from the Astronomical and Geophysical Institute of the University of São Paulo (IAG-USP), from MIT, and from the Jet Propulsion Laboratory of NASA, the space agency of the United States.

From the great Ceres to gravel

Also called “lesser planets”, for orbiting the Sun like planets, they are concentrated in a band between Jupiter and Mars, called the Asteroid Belt. But there are many others scattered about, from the orbit of the Earth – with which they may sometimes crash – to way beyond the orbit of Saturn. The largest asteroid is Ceres, with 1,000 kilometers (km) in diameter, discovered in 1801. There are another 15 with over 240 km in diameter, and many are no larger than gravel. All together, they would not add up to half the size of the Moon. The direct observations of asteroids are carried out basically from the Earth, like the exhaustive Brazilian study from the observatory in La Silla.

The first high resolution image of an asteroid taken from space, though, was obtained of Gaspra in 1991, by the American space probe Galileo. Two years later, the same spaceship came very close to the asteroid Ida. In 1997, the close proximity of the Near probe to the asteroid Mathilde made it possible to discover that it is rich in carbon. Images of the great Vesta, with its 500 km or so in diameter, were supplied by the Hubble space telescope.

There are asteroids that are located on a collision course with the Earth. Daniela Lazzaro distinguished three kinds of them: “a meteoroid is a small asteroid that may enter the Earth’s atmosphere. A meteor is the one that actually enters the atmosphere, and, due to attrition with it, even comes to be incandescent as it crosses the sky, and then disintegrates completely: it is was is popularly called a “shooting star”. And a meteorite is a small asteroid that enters the atmosphere, but does not disintegrate completely when it passes through it, and even reaches the surface. A piece of it is left on the ground, or a crater is even formed”. This is very important for astrophysical studies, and, whenever one is found, it is analyzed in a laboratory. “As we cannot have an asteroid in a laboratory, a least we can have a little bit of one”, says Daniela.

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