Clément Suavet

Micrometeorites from the Transantarctic Mountains

We report the discovery of large accumulations of micrometeorites on the Myr-old, glacially eroded granitic summits of several isolated nunataks in the Victoria Land Transantarctic Mountains. The number (>3,500) of large (>400 μm and up to 2 mm in size) melted and unmelted particles is orders of magnitudes greater than other Antarctic collections. Flux estimates, bedrock exposure ages and the presence of ≈0.8-Myr-old microtektites suggest that extraterrestrial dust collection occurred over the last 1 Myr, taking up to 500 kyr to accumulate based on 2 investigated find sites. The size distribution and frequency by type of cosmic spherules in the >200-μm size fraction collected at Frontier Mountain (investigated in detail in this report) are similar to those of the most representative known micrometeorite populations (e.g., South Pole Water Well). This and the identification of unusual types in terms of composition (i.e., chondritic micrometeorites and spherulitic aggregates similar to the ≈480-kyr-old ones recently found in Antarctic ice cores) and size suggest that the Transantarctic Mountain micrometeorites constitute a unique and essentially unbiased collection that greatly extends the micrometeorite inventory and provides material for studies on micrometeorite fluxes over the recent (≈1 Myr) geological past.

An ancient core dynamo in asteroid Vesta.

Vesta to the Core Vesta is one of the largest bodies in the main asteroid belt. Unlike most other asteroids, which are fragments of once larger bodies, Vesta is thought to have survived as a protoplanet since its formation at the beginning of the solar system (see the Perspective by Binzel, published online 20 September). Based on data obtained with the Gamma Ray and Neutron Detector aboard the Dawn spacecraft, Prettyman et al. (p. 242, published online 20 September) show that Vestas reputed volatile-poor regolith contains substantial amounts of hydrogen delivered by carbonaceous chondrite impactors. Observations of pitted terrain on Vesta obtained by Dawns Framing Camera and analyzed by Denevi et al. (p. 246, published online 20 September), provide evidence for degassing of volatiles and hence the presence of hydrated materials. Finally, paleomagnetic studies by Fu et al. (p. 238) on a meteorite originating from Vesta suggest that magnetic fields existed on the surface of the asteroid 3.7 billion years ago, supporting the past existence of a magnetic core dynamo. Paleomagnetic studies of a meteorite from asteroid Vesta reveal remanent magnetization produced by an ancient core dynamo. The asteroid Vesta is the smallest known planetary body that has experienced large-scale igneous differentiation. However, it has been previously uncertain whether Vesta and similarly sized planetesimals formed advecting metallic cores and dynamo magnetic fields. Here we show that remanent magnetization in the eucrite meteorite Allan Hills A81001 formed during cooling on Vesta 3.69 billion years ago in a surface magnetic field of at least 2 microteslas. This field most likely originated from crustal remanence produced by an earlier dynamo, suggesting that Vesta formed an advecting liquid metallic core. Furthermore, the inferred present-day crustal fields can account for the lack of solar wind ion-generated space weathering effects on Vesta.

Solar nebula magnetic fields recorded in the Semarkona meteorite

Magnetic fields are proposed to have played a critical role in some of the most enigmatic processes of planetary formation by mediating the rapid accretion of disk material onto the central star and the formation of the first solids. However, there have been no experimental constraints on the intensity of these fields. Here we show that dusty olivine-bearing chondrules from the Semarkona meteorite were magnetized in a nebular field of 54 ± 21 microteslas. This intensity supports chondrule formation by nebular shocks or planetesimal collisions rather than by electric currents, the x-wind, or other mechanisms near the Sun. This implies that background magnetic fields in the terrestrial planet-forming region were likely 5 to 54 microteslas, which is sufficient to account for measured rates of mass and angular momentum transport in protoplanetary disks. Magnetic field strength in the early solar system is recorded in chondrules within a meteorite born of the asteroid Vesta. Magnetic moments in planetary history To know the magnetic history of the solar nebula in the age of planet formation, researchers turn to the most primitive meteorites. Samples such as the Semarkona chondrite are composed partly of chondrules, which reflect the strength of the ambient magnetic field when this material was last molten. Fu et al. used a SQUID microscope to measure the remnant magnetization in a section of Semarkona. The findings reveal secrets about what goes on inside protoplanetary disks. Science, this issue p. 1089

Persistence and origin of the lunar core dynamo

The lifetime of the ancient lunar core dynamo has implications for its power source and the mechanism of field generation. Here, we report analyses of two 3.56-Gy-old mare basalts demonstrating that they were magnetized in a stable and surprisingly intense dynamo magnetic field of at least ∼13 μT. These data extend the known lifetime of the lunar dynamo by ∼160 My and indicate that the field was likely continuously active until well after the final large basin-forming impact. This likely excludes impact-driven changes in rotation rate as the source of the dynamo at this time in lunar history. Rather, our results require a persistent power source like precession of the lunar mantle or a compositional convection dynamo.

Preservation and detectability of shock-induced magnetization

Author(s): Tikoo, Sonia M; Gattacceca, Jerome; Swanson-Hysell, Nicholas L; Weiss, Benjamin P; Suavet, Clement; CournA¨de, Cecile

Constraining the terrestrial age of micrometeorites using their record of the Earth's magnetic field polarity

We propose a new nondestructive method that uses the paleomagnetic record of micrometeorites in Earth9s polar regions to constrain the age of their fall. During atmospheric entry, melted micrometeorites acquire a thermal remanent magnetization and record the polar subvertical geomagnetic field. When the fall vector can be determined, due to the location of bubbles, iron-nickel droplets, or grain-size gradients, it is possible to ascribe the fall to a normal or reverse polarity interval of the geomagnetic field. We tested this concept on a set of eight melted micrometeorites from the Transantarctic Mountains (Antarctica). Two micrometeorites have magnetization directions consistent with a normal polarity of the Earth9s magnetic field, whereas four others have recorded a reverse polarity, and therefore fell to Earth at least 0.78 m.y. ago. One micrometeorite has a magnetization that is seemingly unrelated to the inferred entry direction. The fall direction could not be determined with certainty for one micrometeorite. These results provide new evidence suggesting that the Transantarctic Mountains micrometeorite traps are 1–2 m.y. old, and confirm that they contain the oldest non-fossil micrometeorites available.

Controlled‐atmosphere thermal demagnetization and paleointensity analyses of extraterrestrial rocks

We describe a system for conducting thermal demagnetization of extraterrestrial rocks in a controlled atmosphere appropriate for a wide range of oxygen fugacities within the stability domain of iron. Thermal demagnetization and Thellier-Thellier paleointensity experiments on lunar basalt synthetic analogs show that the controlled atmosphere prevents oxidation of magnetic carriers. When combined with multidomain paleointensity techniques, this opens the possibility of highly accurate thermal demagnetization and paleointensity measurements on rocks from the Moon and asteroids.

Lifetime of the solar nebula constrained by meteorite paleomagnetism

Meteorite magnetism in the early solar system The young solar system contained a disc of gas and dust within which planet formation occurred. The disc eventually dissipated after the Sun ignited and the planets formed, but exactly when that happened has been difficult to determine. Wang et al. measured tiny magnetic fields preserved in angrites, an ancient type of meteorite. They interpret a drop in magnetic field strength about 4 million years after the solar system formed as a sign that the gas had cleared—along with the magnetic field that it carried. The results will enhance our understanding of planet formation, both in our solar system and around other Sun-like stars. Science, this issue p. 623 Magnetic fields in meteorites show how long it took for the gas in the protosolar disk to clear. A key stage in planet formation is the evolution of a gaseous and magnetized solar nebula. However, the lifetime of the nebular magnetic field and nebula are poorly constrained. We present paleomagnetic analyses of volcanic angrites demonstrating that they formed in a near-zero magnetic field (<0.6 microtesla) at 4563.5 ± 0.1 million years ago, ~3.8 million years after solar system formation. This indicates that the solar nebula field, and likely the nebular gas, had dispersed by this time. This sets the time scale for formation of the gas giants and planet migration. Furthermore, it supports formation of chondrules after 4563.5 million years ago by non-nebular processes like planetesimal collisions. The core dynamo on the angrite parent body did not initiate until about 4 to 11 million years after solar system formation.

A two-billion-year history for the lunar dynamo

Paleomagnetic evidence suggests the lunar dynamo persisted beyond 2.5 Ga, requiring an exceptionally long-lived power source. Magnetic studies of lunar rocks indicate that the Moon generated a core dynamo with surface field intensities of ~20 to 110 μT between at least 4.25 and 3.56 billion years ago (Ga). The field subsequently declined to <~4 μT by 3.19 Ga, but it has been unclear whether the dynamo had terminated by this time or just greatly weakened in intensity. We present analyses that demonstrate that the melt glass matrix of a young regolith breccia was magnetized in a ~5 ± 2 μT dynamo field at ~1 to ~2.5 Ga. These data extend the known lifetime of the lunar dynamo by at least 1 billion years. Such a protracted history requires an extraordinarily long-lived power source like core crystallization or precession. No single dynamo mechanism proposed thus far can explain the strong fields inferred for the period before 3.56 Ga while also allowing the dynamo to persist in such a weakened state beyond ~2.5 Ga. Therefore, our results suggest that the dynamo was powered by at least two distinct mechanisms operating during early and late lunar history.