How did they get here?
For nearly 40 years HED meteorites have been linked to asteroid 4 Vesta because of the very close match in IR and visible spectra between the meteorites and the asteroid (Figure 11; McCord et al., 1970; Larsen and Fink, 1975; Drake, 1979; Sunshine et al., 2004; Gaffey, 1997; Mayne et al., 2009; Binzel and Xu, 1993).
Figure 11: IR spectrum of a eucrites compared to that of asteroid 4 Vesta from the original paper of McCord et al. (1970).
This spectral link was strengthened with the discovery of a dynamic physical link between 4 Vesta and a family of asteroids that can be connected to Earth's orbit via resonances, such as the 3:1 mean motion resonance with Jupiter and the v6 secular resonance between Jupiter and Saturn (Marzari et al. 1996; Migliorini et al. 1997). This discovery led to a large number of studies trying to strengthen and develop this connection (Moscovitz et al., 2008). This connection was dramatically strengthened by Hubble Space telescope images that revealed a large 400 km crater in the south pole of 4 Vesta (Zellner et al., 1997; Figure 12).
Figure 12: Images of asteroid 4 Vesta taken with the wide field planetary camera of the Hubble Space Telescope (from Zellner et al., 1997).
This enormous crater has been suggested to be the source of the many HEDs that have made their way to Earth. Cosmic ray exposure age dating of diogenites and eucrites has revealed two age speaks at 22 and 39 Ma (Welten et al., 1997), suggesting that about half of the HEDs could be accounted for in impact events of these ages (Fig. 13). The remaining HEDs must have been derived from separate impact events, indicating that there are multiple events required to get all of the HEDs to Earth (Welten et al., 1997). These impact events may have been on Vesta or on vestoids that had previously been disrupted from Vesta.
Figure 13: Histogram of cosmic ray exposure ages for HEDs from the paper of Welten et al. (1997). There are clearly two clustering groups of~ 20 Ma and 39 Ma.
There remain a number of asteroids with basalt-like spectra that are not easily connected to asteroid 4 Vesta, suggesting multiple parent bodies. For example Magnya 1459 is a 35 to 40 km basaltic asteroid that appears to be distinct from (i.e., not previously part of) 4 Vesta (Lazzaro et al., 2000). The Yarkovsky effect has been recognized to perturb the orbits of materials in the asteroid belt and some of the material that has migrated within the belt can be attributed to this process (Farinella & Vokrouhlicky 1999).
Much information about the origin of meteorites can be gained from observations of falls of meteorites, which can lead to knowledge of the orbit of the material. Two recent examples have improved our understanding of where HED meteorites can come from. The Australian eucrite Bunburra Rockhole (Bland et al., 2009) was caught upon Earth entry by a camera network in the Australian desert (Bland et al., 2009). These observations, coupled with detailed petrographic, compositional studies, and its distinct O isotopic signature, have led to the conclusion that Bunburra Rockhole came from a portion of the asteroid belt that is not common for an HED meteorite (Figure 14). This sample may represent a piece of a basaltic asteroid that is distinct from 4 Vesta (see discussion below). In 2007, the Puerto Lapice (Trigo-Rodriguez et al., 2009) eucrite was observed to fall, and reconstruction of the orbit indicates that it came from a portion of the asteroid belt where many HEDs have been proposed to originate.
Figure 14: Summary of oxygen isotope data for eucrites showing the multiple possible parent bodies, as well as the anomalous Bunburra Rockhole results (from Bland et al., 2009).
Detailed studies of HED meteorites in world collections, combined with more recent observations such as Bunburra Rockhole and Puerto Lapice, have led to the recognition that there may be as many as six to eight different parent bodies for the basaltic meteorites. In addition to the main group eucrites, there are separate bodies that have been defined by high resolution O isotope analyses. Ibitira, Pasamonte and PCA 91007, NWA 011, A-881394, and NWA 1240 could all be from different bodies based on their O isotope values that are distinct from main group eucrites (Figure 14; Scott et al., 2009). Also, O isotope analyses had initially tied to pallasites to IIIAB irons (Clayton and Mayeda, 1983), but this idea has been revised since high resolution O isotope analysis has revealed a difference between pallasites and main group eucrites and HEDs (Greenwood et al., 2005). So, there could be even additional bodies that have differentiated and produced basaltic crusts like 4 Vesta.