Sampling Rocks From The Most Remote Localities

The English astronomer William Herschel proposed the term asteroids, from the Greek for starlike, for Ceres and its siblings. Although long sought for as the possible missing planet between Mars and Jupiter, Ceres did not entirely fit with theoretical expectations, raising questions about its true nature. As a result, it is the only object in the Solar System that has been classified as a planet, comet, asteroid, and now, according to the International Astronomical Union, a dwarf planet. It is astonishing to think that all this restless motion of countless objects was occurring above our heads for millennia, a fact of which we remained entirely unaware. Ancient civilizations worldwide paid a tremendous amount of attention to celestial phenomena, producing sophisticated theories about the planets’ motion. Yet all the complexity of the Solar System beyond the canonical six planets, the Moon, and the occasional bright comet, remains invisible to the naked eye. Today, more than 200 years after the discovery of Ceres, we know of more than 800,000 asteroids between Mars and Jupiter, the smallest of which is currently about 100 m across. And the number of known objects is constantly rising. It is estimated that a million asteroids larger than 1 km may exist in the main belt, and a far greater number of smaller ones. Asteroids are not confined to the main belt between Mars and Jupiter. This rich collection of rocky objects on constantly evolving erratic orbits has important consequences for the evolution of the Earth and other terrestrial planets. The orbits of these objects are not known sufficiently precisely to evaluate accurately if a collision is possible, and typically it is found that they do not pose a threat as more detailed observations of their orbits become available.

The Happiest Days Of Our Lives

But there is more to asteroids than meets the eye. The details of the dynamical perturbations are rather different for these two regions, but if an asteroid finds itself in their proximity, it will be pushed, on a timescale of just 100,000 years, into a highly eccentric orbit. The perturbed objects move like a pinball, receiving substantial gravitational kicks when they come close to the planets, and most importantly Jupiter. This complex web of dynamical perturbations has two interesting consequences. A small fraction of these objects collide with the Earth, landing as meteorites. But main belt asteroids nevertheless face a risk of collisions. The orbits of main belt asteroids can cross so they have a small but nonzero probability of colliding. Because of such collisions, small main belt asteroids have a limited lifetime. Scientists estimate that, on average, a 100 m asteroid survives for only about 100 million years, and therefore they must be being replenished over time. The source is larger main belt asteroids from which the small ones break off during collisions. This process, of larger asteroids spawning smaller ones, is called collisional cascade and it is considered a fundamental evolutionary process for asteroids. In a way, we can think of the collisional cascade as the reverse of planetary accretion.

What Happens Tomorrow?

Scientists think that asteroids smaller than 100 km or so may be fragments generated in this way. This also means that small rocks in space have not been around since the dawn of the Solar System, but have been produced by a number of collisions suffered by larger objects. This redistribution of objects is the ultimate reason why the terrestrial planets, in the current architecture of the Solar System, experience collisions. Certainly, we are aware of and rightly concerned about the devastating effects that large collisions could have on Earth and on ourselves. The cosmic conveyor belt has, however, a far more positive role than one may think. Beyond the Hollywood disaster movie indulgence in doomsday collisions, the importance of small collisions is little appreciated. When a cosmic object enters our atmosphere, it is strongly heated and subject to high pressures due to the compression of the air ahead of the body. Our atmosphere acts as a natural defense system against extraterrestrial intruders. Fragments or larger objects, however, do manage to reach the ground as meteorites. Meteorite falls are often accompanied by amazing stories. Today, meteorites are extremely valuable to scientists as they allow us to undertake detailed analysis of extraterrestrial materials. Just as geologists scour the surface of the Earth and avidly sample rocks from the most remote localities to reconstruct the history of our planet, meteorites offer the chance to sample the material of celestial objects, and what’s more, they travel to us.

How Many More Times?

Indeed, this is a great opportunity, given the tremendous distances that separate us from other celestial objects. Alas, we generally lack contextual information to fully exploit their true scientific potential. This object may seem large to us, but meteorites are small fragments compared to the size of their progenitor asteroids. Planetary geologists face the challenge of reconstructing the history of a large asteroid just by studying a few small fragments. Approximately 670 tons of meteorites have been recovered and cataloged, and while this certainly is a small fraction of the extraterrestrial matter that has landed on Earth throughout its history, nevertheless these rocks have given us a great deal of information about the early Solar System, and most importantly, they are samples of objects with radically different life histories, including evidence for ancient collisions. Some meteorites, called chondrites, are formed of minerals that have undergone little alteration, meaning they were never subjected to high temperatures and pressures. These rocks are thought to have formed very early in the Solar System, and a subgroup called carbonaceous chondrites are thought to be the most pristine extraterrestrial samples in our possession. These represent the first mineral condensates in the protoplanetary disk. As such, these little inclusions effectively provide us with the oldest solid material in our Solar System. What’s more, as we shall see, these inclusions also contain radiogenic elements such as lead, which provide a natural clock that can be used to date when they condensed. The quest for the oldest inclusion is still ongoing, and recent analyses of other inclusions have returned a slightly older age of 4.5682 billion years ago. These meteorites also have considerable archeological interest.