前言：行星探测的发展是天文观测到地质化学分析进步的缩影
The progression from astronomical observation to geochemical analysis epitomizes advancements in planetary exploration.
引自，http://www.sciencemag.org/content/338/6104/203.short
A Golden Spike for Planetary Science
Richard P. Binzel
+ Author Affiliations
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
E-mail: rpb@mit.edu
Vesta is the second most massive asteroid in our solar system and provides the opportunity to study the early Earth as a developing planet while it was still barely one-sixth the size of the Moon. Add to that what appear to be samples from this once molten protoplanet and you have the decades of case building and motivation for the spacecraft exploration of Vesta—the Dawn mission (1). Results of this mission, reported by Prettyman et al. (2) on page 242 and Denevi et al. (3) on page 246 in this issue, not only tell us about the birth and evolution of terrestrial planets, but also offer new insights into the distribution of water in our solar system.
Prettyman et al. link conclusively the elemental chemistry of Vesta to the howardite, eucrite, and diogenite (HED) classes of basaltic meteorites and their detailed record for the earliest phases of planetary evolution. Although this connection has been long suspected (4, 5), Vesta now solidifies its place as the fourth planetary body from which we have definitive samples in our laboratories [Earth, Moon, and Mars constitute the first three; Japan's successful asteroid sample return (6) adds one more]. As often happens in planetary exploration, confirming the expected also uncovers the unforeseen: Prettyman et al. find an abundance of hydrogen within the oldest regions of Vesta. Supporting the logical interpretation that the detected H is bound as H2O, volatile degassing of water is a natural explanation for pitted geologic structures examined by Denevi et al.
Planetary exploration begins with discovery, often in the course of surveys. Physical measurements, with increasing precision, bring to focus particularly tantalizing objects. Science questions become refined through both observation and theory, often to their limit. Spacecraft exploration capable of delivering consequential breakthroughs forms the capstone.
As interesting as Dawn's particular findings for Vesta are, lost in the details is the perspective of embracing how much of what we already knew about Vesta was confirmed by the in situ measurements. No mission wants to be viewed as having done incremental science, but when that progression hews both a capstone and a deeper scientific context, the value is undiminished. It is inherent that planetary exploration must advance itself literally from the ground up, historically beginning with Earth-based telescopes and now increasingly from space. Surveys to find planets (within our solar system or beyond), dwarf planets, Kuiper belt objects, comets, and asteroids, or to reveal particles and fields, present space agencies with numerous destinations, vastly eclipsing available resources. Such breadth of possibilities is like the base of a pyramid (see the figure). Physical investigations to discern basic properties (for example, mass, spin, size, shape, configuration, density, and composition) logically lead up to the next level. Invariably, some objects attain a still higher level by being especially remarkable, inexplicable, quintessential, or tantalizing. Prioritizing which objects to study is a difficult task, but the fundamental depth of the scientific questions that an object raises and the consequences of the answers usually prevail among all missions within feasible technology and cost.
Vesta was recorded at the base of the pyramid with its 1807 discovery as the fourth asteroid known, fostering the modern view that the “missing planet” between Mars and Jupiter is one that never completely coalesced. Physical observations first reported in 1929 (7) were astonishingly correct in revealing that Vesta has a variegated surface. Modern Vesta science began in the 1970s with the finding that its surface colors matched those of the HED meteorites (4). As the only distinctive asteroid-meteorite match among all large asteroids (5), Vesta raised consternation over how samples from the asteroid belt might ever reach Earth (8). Controversy continued until the finding of a Vesta-like family of asteroid debris in heliocentric orbits emanating from Vesta (9) and Hubble images (10) that revealed the “smoking gun” of a huge impact basin (named Rheasylvia) as their source. These Vesta chips, leaving a trail all the way to where Jovian resonances could deliver them to Earth (11, 12), completed the pathway.
With the confidence that we have pieces of Vesta in our laboratories, increasingly detailed maps (13, 14) showed distributions of pyroxene-rich surface basalts, more deeply excavated plutonic rocks near the basin, and widespread mixtures of the two. These three components fit exactly to the laboratory mineralogy for HED meteorites. Their measured ages, several hundred million years older than lunar basalts (15), branded Vesta as the most ancient terrestrial world (16). With Vesta so enticingly unveiled and scientists at the ready, the Dawn mission was born.
Dawn's crowning success at Vesta was the delivery of instruments that require proximity, exemplified by the gamma-ray and neutron measurements of Prettyman et al. yielding elemental abundances that unequivocally confirm the meteorite links. While pyroxene mineral mapping from orbit (17) contends as clinching that link, these maps instead provide geologic context to the mineralogy match known for decades (4, 13). The cacheing of volatiles at Vesta reported by Denevi et al. accentuates the pervasiveness of water throughout the solar system. Yet, Dawn fell short of fulfilling the apex of planetary exploration's pyramid through decisions forced by budgetary constraints and cost overruns. Geophysics was sacrificed at Vesta's altar as two instruments with transformative prospects, a magnetometer and laser topography mapper, were removed to avert Dawn's cancellation. Key planetary questions on early dynamos and protoplanet core-mantle-crust differentiated structure, addressable only in situ at Vesta, remain preserved there.
Finding Vesta not to be as predicted, a terrestrial-like protoplanet (1, 16), would have been a complete surprise. Happily, the geologic context resolved for this terrestrial world now confirmed as predating the Moon will intrigue scientists for decades. Dawn's direct affirmation that a complex formation story and meteorite link could be determined correctly from Earth, roughly 200 million km away, is a triumph for how the pyramid of planetary exploration is built. The underpinning foundations that can be mortared together only by a spacecraft's confirming ground truth data, in turn, support other inferences for planetary places unlikely to be visited in the near future.
