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"Michel, Patrick"
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The Influence of Central Body Tides on Catastrophic Disruptions of Close-in Planetary Satellites
We model the outcomes of catastrophic disruptions on small, gravity-dominated natural satellites, accounting for the tidal potential of the central body, which is neglected in classical disruption scaling laws. We introduce the concept of QTD⋆ , the specific energy required to disperse half of the total mass involved in a collision, accounting for the tidal potential of a central body. We derive a simple scaling relation for QTD⋆ and demonstrate that for close-in planetary or asteroidal satellites, the tides from the central body can significantly reduce their catastrophic disruption threshold. We show that many satellites in the solar system are in such a regime, where their disruption threshold should be much lower than that predicted by classical scaling laws that neglect tidal effects. Some notable examples include Mars’s Phobos, Jupiter’s Metis and Adrastea, Saturn’s ring moons, Uranus’s Ophelia, and Neptune’s Naiad and Thalassa, among others. We argue that traditional impact scaling laws should be modified to account for tides when modeling the formation and evolution of these close-in satellites. Our derivation for QTD⋆ can easily be used in existing N-body and collisional evolution codes.
Journal Article
Bee Venom and Its Component Apamin as Neuroprotective Agents in a Parkinson Disease Mouse Model
Bee venom has recently been suggested to possess beneficial effects in the treatment of Parkinson disease (PD). For instance, it has been observed that bilateral acupoint stimulation of lower hind limbs with bee venom was protective in the acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD. In particular, a specific component of bee venom, apamin, has previously been shown to have protective effects on dopaminergic neurons in vitro. However, no information regarding a potential protective action of apamin in animal models of PD is available to date. The specific goals of the present study were to (i) establish that the protective effect of bee venom for dopaminergic neurons is not restricted to acupoint stimulation, but can also be observed using a more conventional mode of administration and to (ii) demonstrate that apamin can mimic the protective effects of a bee venom treatment on dopaminergic neurons. Using the chronic mouse model of MPTP/probenecid, we show that bee venom provides sustained protection in an animal model that mimics the chronic degenerative process of PD. Apamin, however, reproduced these protective effects only partially, suggesting that other components of bee venom enhance the protective action of the peptide.
Journal Article
Constraining surface properties of asteroid (162173) Ryugu from numerical simulations of Hayabusa2 mission impact experiment
The Hayabusa2 mission impact experiment on asteroid Ryugu created an unexpectedly large crater. The associated regime of low-gravity, low-strength cratering remained largely unexplored so far, because these impact conditions cannot be re-created in laboratory experiments on Earth. Here we show that the target cohesion may be very low and the impact probably occurred in the transitional cratering regime, between strength and gravity. For such conditions, our numerical simulations are able to reproduce the outcome of the impact on Ryugu, including the effects of boulders originally located near the impact point. Consistent with most recent analysis of Ryugu and Bennu, cratering scaling-laws derived from our results suggest that surfaces of small asteroids must be very young. However, our results also show that the cratering efficiency can be strongly affected by the presence of a very small amount of cohesion. Consequently, the varying ages of different geological surface units on Ryugu may be due to the influence of cohesion.
Hayabusa2 mission impact experiment on asteroid Ryugu formed a crater larger than expected. Here, the authors show numerical impact simulations and find that the target cohesion may be very low, indicating the Hayabusa2 impact experiment probably occurred in the transitional cratering regime.
Journal Article
Diverse evolutionary pathways of spheroidal asteroids driven by rotation rate
Asteroids preserve a continuous record of evolutionary processes since the early solar system. They can take various shapes that represent the cumulative results of their evolution. However, for those showing common characteristics, this does not mean that they followed the same evolutionary path. Here, we show that (101955) Bennu and (162173) Ryugu, two near-Earth asteroids with spheroidal shapes, have evolved through distinct pathways despite their similar shapes. Using high-resolution imagery from NASA’s OSIRIS-REx and JAXA’s Hayabusa2 spacecraft, we map ~ 200,000 boulders and find latitudinal particle size sorting on both bodies. This represents opposite directions of surface material movements driven by their different rotation periods (4.3 h for Bennu and 7.6 h for Ryugu): toward the equator on Bennu and toward the poles on Ryugu. Furthermore, the spatial distribution of large boulders on Bennu suggests a prior slower rotation (> 5 h), implying a past shape evolution similar to that of Ryugu. Our findings demonstrate that small variations in rotation period, on the scale of a few hours, can drastically change the gravitational field on an asteroid, sometimes even reversing local gravity direction. This drives complex and diverse evolutionary pathways of asteroids, resulting in top-shaped bodies and binary systems observed today.
Journal Article
Parkinson’s disease-derived α-synuclein assemblies combined with chronic-type inflammatory cues promote a neurotoxic microglial phenotype
Parkinson’s disease (PD) is a common age-related neurodegenerative disorder characterized by the aggregation of α-Synuclein (αSYN) building up intraneuronal inclusions termed Lewy pathology. Mounting evidence suggests that neuron-released αSYN aggregates could be central to microglial activation, which in turn mounts and orchestrates neuroinflammatory processes potentially harmful to neurons. Therefore, understanding the mechanisms that drive microglial cell activation, polarization and function in PD might have important therapeutic implications. Here, using primary microglia, we investigated the inflammatory potential of pure αSYN fibrils derived from PD patients. We further explored and characterized microglial cell responses to a chronic-type inflammatory stimulation combining PD patient-derived αSYN fibrils (F
PD
), Tumor necrosis factor-α (TNFα) and prostaglandin E
2
(PGE
2
) (TPF
PD
). We showed that F
PD
hold stronger inflammatory potency than pure αSYN fibrils generated de novo. When combined with TNFα and PGE
2
, F
PD
polarizes microglia toward a particular functional phenotype departing from F
PD
-treated cells and featuring lower inflammatory cytokine and higher glutamate release. Whereas metabolomic studies showed that TPF
PD
-exposed microglia were closely related to classically activated M1 proinflammatory cells, notably with similar tricarboxylic acid cycle disruption, transcriptomic analysis revealed that TPF
PD
-activated microglia assume a unique molecular signature highlighting upregulation of genes involved in glutathione and iron metabolisms. In particular, TPF
PD
-specific upregulation of
Slc7a11
(which encodes the cystine-glutamate antiporter xCT) was consistent with the increased glutamate response and cytotoxic activity of these cells toward midbrain dopaminergic neurons in vitro. Together, these data further extend the structure–pathological relationship of αSYN fibrillar polymorphs to their innate immune properties and demonstrate that PD-derived αSYN fibrils, TNFα and PGE
2
act in concert to drive microglial cell activation toward a specific and highly neurotoxic chronic-type inflammatory phenotype characterized by robust glutamate release and iron retention.
Journal Article
Thermal fatigue as the origin of regolith on small asteroids
Thermal fatigue resulting from diurnal temperature variations is shown to be the dominant means of rock fragmentation and, consequently, regolith formation on small asteroids.
Heat cycles shape asteroidal surfaces
The surfaces of small asteroids are covered by a dust- or gravel-like layer known as the regolith. The standard view of its formation has been that it is largely debris produced by micrometeorite impacts, but this has come into question with the suggestion that such impacts would be of sufficient force to throw much of the debris away from, rather than back down to, the asteroid surface. In this study, Marco Delbo
et al
. demonstrate that thermal fatigue is a more likely explanation. Experiments in which centimetre-scale samples of the Murchison (CM2) and Sahara 97210 meteorites were exposed to a series of temperature cycles suggest that such rocks would break up more rapidly through thermal fragmentation induced by diurnal temperature variations than as a result of micrometeoroid impacts.
Space missions
1
,
2
and thermal infrared observations
3
have shown that small asteroids (kilometre-sized or smaller) are covered by a layer of centimetre-sized or smaller particles, which constitute the regolith. Regolith generation has traditionally been attributed to the fall back of impact ejecta and by the break-up of boulders by micrometeoroid impact
4
,
5
. Laboratory experiments
6
and impact models
4
, however, show that crater ejecta velocities are typically greater than several tens of centimetres per second, which corresponds to the gravitational escape velocity of kilometre-sized asteroids. Therefore, impact debris cannot be the main source of regolith on small asteroids
4
. Here we report that thermal fatigue
7
,
8
,
9
, a mechanism of rock weathering and fragmentation with no subsequent ejection, is the dominant process governing regolith generation on small asteroids. We find that thermal fragmentation induced by the diurnal temperature variations breaks up rocks larger than a few centimetres more quickly than do micrometeoroid impacts. Because thermal fragmentation is independent of asteroid size, this process can also contribute to regolith production on larger asteroids. Production of fresh regolith originating in thermal fatigue fragmentation may be an important process for the rejuvenation of the surfaces of near-Earth asteroids, and may explain the observed lack of low-perihelion, carbonaceous, near-Earth asteroids
10
.
Journal Article