Magnetic Properties of Asteroid (162173) Ryugu

Abstract

Observations of the magnetization state of asteroids indicate diverse properties. Values between 1.9 × 10 −6 Am 2 /kg (Eros) and 10 −2 Am 2 /kg (Braille) have been reported. A more detailed understanding of asteroidal magnetic properties allows far-reaching conclusions of the magnetization mechanism as well as the strength of the magnetic field of the solar system regions the asteroid formed in. The Hayabusa2 mission with its lander Mobile Asteroid Surface Scout is equipped with a magnetometer experiment, MasMag. MasMag is a state-of-the-art three-axis fluxgate magnetometer, successfully operated also on Philae, the Rosetta mission lander. MasMag has enabled, after Eros for the second time ever, to determine the magnetic field of an asteroid during descent and on-surface operations. The new observations show that Ryugu, a low-albedo C-type asteroid, has no detectable global magnetization, and any local magnetization is either small (< 10 −6 Am 2 /kg) or on very small (subcentimeter) scales. This implies, for example, that energetic solar wind particles could reach and alter the surface unimpeded by strong asteroidal magnetic fields, such as minimagnetospheres in case of the Moon.Plain Language Summary Magnetic measurements in space near and on solar system bodies such as asteroids can provide important information about their formation history and their material properties. Hayabusa2, a Japanese mission, visited asteroid Ryugu, a type of asteroid (carbon rich) not visited before. Ryugu is a rubble pile, that is, an agglomeration of rocks and boulders. It is not expected to have any global magnetic field, but it can be magnetized on smaller scales (boulders or pebbles). Hayabusa2 carried therefore a small lander called Mobile Asteroid Surface Scout equipped also with an instrument for magnetic field measurement (magnetometer). In this study, we present observations from the magnetometer that were collected during Mobile Asteroid Surface Scout's descent and landing on the surface of Ryugu. The magnetic measurements show that Ryugu is not magnetized on boulder (greater than centimeter) scales. This gives us indication on its origin and evolution. In particular, it shows that Ryugu and the bodies it was created from did not possess any magnetic field generation mechanism and that they were not created in an environment with strong background magnetic field. The results are important inputs for theories about the solar system evolution that work with magnetic field as one of the drivers for dust accretion and planetary formation.

Hayabusa2 extended mission: New voyage to rendezvous with a small asteroid rotating with a short period

“…5. Close observations on the surface revealed the detailed characteristics of boulders and regolith, which are consistent with those seen in carbonaceous chondrite meteorites, as well as the magnetic environment on the surface (Grott et al, 2019;Hercik et al, 2020;Jaumann et al, 2019;Scholten et al, 2019). 6.…”

History of the solar nebula from meteorite paleomagnetism

“…Moreover, several investigations placed stringent upper limits on the mean asteroid NRM that are at the lower range of the values measured for centimeter-sized samples of meteoritic materials (<2 × 10 −6 emu g −1 ). However, because these missions only constrained the magnetization averaged over spatial scales ranging from 10 to 20 cm [as inferred from recent Hayabusa2 measurements across the surface of asteroid (162173) Ryugu ( 167 )] to 10 5 km 3 [as inferred from asteroid (21) Lutetia ( 169 )], the intensity of the finer-scale NRM in these asteroids may be well above these limits. Furthermore, with the possible exception of the comet measurements, the timing of the magnetization records in these asteroids is unknown because of the lack of radiometric and other ages for their constituents.…”

“…In this regard, nebular field constraints from spacecraft measurements of small bodies like comet 67P are especially valuable because of the additional constraints on the parent body’s formation location provided by the geologic context of in situ measurements. Hence, future sample return missions, like those in progress to carbonaceous asteroids [e.g., ( 167 )], are even more exciting, because they will combine the advantage of geologic context with the sensitivity and precision of laboratory measurements.…”

“…Spacecraft have measured the magnetic field at the surface of two asteroids, during flybys of five other asteroids, and at the surface of a comet (166)(167)(168). Although two of the asteroid flybys have tentatively identified remanent magnetic fields, these detections are ambiguous.…”

The MASCOT separation mechanism

“…The rotation period could be measured by analyzing the harmonic wave of a rotating magnetometer sensor in a constant ambient field and has been determined to 139 s (± 1 s) with high precision. Detailed results are presented in [18]. However, due to the short landing period (about 6 min, which corresponds to ∼ 2.5 full rotations only), the determination of the direction of the rotation axis was less precise.…”