Houston: Focus on Vesta and the HEDs
By Paolo Farinella

Planetary science is an historical science, more akin to evolutionary biology than to "pure physics". The so-called laws appearing in it have almost always an empirical character, resulting from history rather than from basic physical principles, so that peculiar cases and exceptions abound even when some predictive power is granted. The best-known example is perhaps the Titius-Bode "law", a simple formula expressing the quasi-geometric progression of the sizes of planetary orbits from Mercury to Uranus (including the asteroids but not, alas, Neptune and Pluto). Another similar case is the "law" relating the evidence for endogenic activity on a planet's (or satellite's) solid surface to its size: according to this "law", relatively small bodies, such as the Moon or Mercury, have ancient surfaces saturated by craters, whereas larger planets (Venus, the Earth) show the markings of extensive volcanic and tectonic activity (with intermediate-sized bodies like Mars fitting in between). The reason? Well, if the primordial (radioactive or accretionary) internal heat of a planet is its dominant energy source, such a reservoir must be roughly proportional to the body's mass (and volume), whereas the energy subsequently radiated away per unit time is proportional to the available surface area. Therefore, the time needed to cool down significantly is roughly proportional to volume/surface area = size.

The Voyager exploration of the outer solar system has provided ample evidence that the "if" preceding the above explanation is very important: a number of satellites, from the three inner Galilean moons of Jupiter to the tiny Enceladus and Miranda, have been clearly resurfaced by long-lived endogenic processes, fed by external energy sources such as tidal dissipation or giant impacts. This has been often presented as one of the great scientific discoveries resulting from the Voyager saga, and certainly it is. But... for meteorite and asteroid scientists, it should not have been a surprise. The alleged "law" had been disproved before.

Meteorite experts have known for several decades that about 5% of the meteorites falling onto the Earth's surface (the so-called HEDs, namely howardites, eucrites and diogenites) are igneous rocks, formed by solidification of magma on the surface of their parent body. Now, almost all the meteorites (including certainly the HEDs) come from the asteroid belt, where the largest object (asteroid 1 Ceres) is less than 1000 km in diameter, and has a mass of about 1% that of the Moon. Actually, since the early 70s most meteoriticists and asteroid experts believe they know which asteroid is the source of the HEDs: the favorite candidate is 4 Vesta, a body about 500 km across, which has a reflectance spectrum and other surface properties which match very well the expectations for a differentiated object with a basaltic crust (1). A few years ago, both dynamical and spectroscopic studies (2) have shown that sizable fragments have been expelled by cratering impacts from Vesta's basaltic crust and have become minor members of a "dynamical family" of asteroids associated to Vesta. The family extends up to the resonant dynamical channels (3) which allow the delivery of asteroid fragments to Earth _ where they are collected as meteorites _ including the HEDs.

So far, so good. But why is Vesta an exception to the endogenous activity versus size correlation? Tidal dissipation is not available as an energy source in Vesta's case, and impacts may have excavated fragments from its crust, but not have caused the whole asteroid to melt down, lest the same should have occurred to other asteroids of similar size, such as 2 Pallas and 10 Hygiea, which are also the largest members of dynamical families but have "primitive", undifferentiated surfaces. On the other hand, in the main asteroid belt there are at least several tens of M-type objects whose surfaces have been inferred (from spectroscopic and radar data) to be rich in nickel-iron metals: these asteroids are presumably the parent bodies of the iron meteorites, and have been widely interpreted as the metal-rich cores of differentiated parents, later disrupted by catastrophic impacts. But if this is true, how could Vesta escape a similar fate, and survive over the age of the solar system with its basaltic crust almost intact (apart from the few large craters needed to explain the origin of the smaller family members)?

These puzzling questions have been the subject of a very lively scientific workshop held at the Houston Lunar and Planetary Institute on October 16-18, 1996 (4). Despite intense research efforts, no consensus has emerged so far on the likely solutions. On the other hand, few doubts have remained on the starting premise of a genetic relationship between Vesta and the HEDs. This was shown by the outcome of an instant poll among all the attendees, made after an initial debate between R. Binzel, speaking for the "orthodox" view, and J. Wasson, defending the alternative view that HEDs might come instead from a number of smaller parent asteroids (having undergone differentiation and impact disruption independently of Vesta). Some 80% (including this writer) were in favor of Binzel, with almost all the others abstaining.

During the workshop, both physical studies of Vesta itself (including recent observations by the Hubble Space Telescope) and detailed analyses of the HED meteorites were presented and discussed in detail. The surface of Vesta appears to be geologically complex, with extensive magmatic flows and large impact basins similar to the lunar maria, where mantle material may have emerged to the surface. This scenario is consistent with both the formation of the family and the compositional and petrologic differences among eucrites, diogenites and howardites (which may sample different original depths below Vesta's surface). However, several problems remain to be explained.

First, simulations of large-scale cratering events could not reproduce the relatively large (5 to 10 km in diameter) and fast (0.3 to 1 km/s) ejected fragments required to form the observed family (and, in the case of the speeds, to allow meteorites to reach the resonant channels). Second, the cosmic-ray exposure ages of HEDs are typically one order of magnitude longer than the dynamical lifetimes (1 to 10 Myr) of fictitious particles starting in the resonances and studied by numerical integration of their orbits. Third, it is not clear how Vesta's crust could escape being shattered and entirely removed by collisions, while other bodies of similar size (such as the parent body of 16 Psyche, a 260-km diameter M-type asteroid which might be just a big remnant core) were thoroughly disrupted and stripped of both their mantles and crusts. Finally, the primordial heating mechanism which provided the energy to melt and differentiate Vesta is still a mystery. The two candidates which have been proposed, radioactive decay of short-lived nuclides (such as 26Al) and electromagnetic induction during episodes of enhanced solar magnetic activity, are both viable in a general sense, but nobody knows yet why some asteroids (including Vesta) should have melted and differentiated, while others seem to have escaped any strong heating and stayed "primitive", just like most meteorites (the chondrites). Neither size nor location in the asteroid belt seem to work as a dominant discriminating criterion.

As remarked before, these difficulties did not lead most scientists attending the Houston workshop to reject the orthodox model of the Vesta-HED relationship. It was generally felt that all the open problems may be amenable to specific solutions consistent with the model, once further work will be done in several different areas. The three main fields (all well represented at the workshop) where such work is in progress are: (i) astronomical observations of Vesta and other igneous asteroids; (ii) laboratory studies of HEDs; and (iii) theoretical work on modeling collisional and dynamical evolution processes, as well as heating and differentiation mechanisms. But the workshop's final discussion emphasized that perhaps our best hopes should be deferred to a dedicated space mission to Vesta, especially if an orbiter and/or sample return scheme such as planned for the forthcoming NEAR and Rosetta missions will be possible (and affordable). Apart from understanding the history of Vesta and the HEDs, such a mission would allow us to see for the first time big volcanoes, calderas and lava flows on a worldlet smaller than New Zealand (or Italy)... can we resist?

Notes

1. For more details, here are a few references: McCord et al. 1970, Science 168, 1445; Drake 1979, in Asteroids, pp. 765-782, Univ. of Arizona Press, Tucson; Cellino et al. 1987, Icarus 70, 546.

2. Zappalà et al. 1990, Astron. J. 100, 2030; Binzel and Xu 1993, Science 260, 186.

3. See Farinella, Meteorite!, May 1996, pp. 8-10.

4. The workshop program and abstracts can be accessed on line at http://cass.jsc.nasa.gov/lpi.html

A dedicated issue of the journal Meteoritics and Planetary Science, collecting many of the scientific papers presented at the workshop, will be published in late 1997.

University of Pisa, Italy