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Variability in the Martian upper atmosphere is strongly linked to the lower atmosphere and much of it can be attributed to vertical wave propagation. Atmospheric tides in particular are a well-known phenomenon in the Martian atmosphere that play a key role in the transport of energy as they propagate to higher altitudes. Previous theoretical predictions and observations suggest that tides producing wavenumber-2 and wavenumber-3 patterns are strongest in a fixed local time at high altitudes, however, the energy they carry and the region of deposition are not well characterized. Given the availability of atmospheric observations from several spacecraft at the same time, in this paper, the nature and behavior of tides are studied concurrently at several altitudes. Here, six intervals are identified focused at fixed low latitudes utilizing simultaneous observations of the middle and upper atmosphere from in situ and remote sensing instruments on different spacecraft. In the middle atmosphere, strong wavenumber-2 signatures are identified in the intervals north of the equator whereas, in the south, wavenumber-3 signatures are strongest. Wave signatures observed in the upper atmosphere seem to be dominated by a mix of wavenumbers-2 and -3. Seasonal variation is observed in the northern intervals, with very little interannual variability in all intervals considered. Estimates of energy based on dominant wavenumber amplitude suggest that most of the energy dissipates below ∼90 km. Furthermore, model sampled output captures the dominant wavenumbers observed in the middle atmosphere as well as the energy dissipation characteristics.

The current rate of small impacts on Mars is informed by more than one thousand impact sites formed in the last twenty years, detected in images of the martian surface. More than half of these impacts produced a cluster of small craters formed by fragmentation of the meteoroid in the martian atmosphere. The spatial distributions, number and sizes of craters in these clusters provide valuable constraints on the properties of the impacting meteoroid population as well as the meteoroid fragmentation process. In this paper, we use a recently compiled database of crater cluster observations to calibrate a model of meteoroid fragmentation in Mars’ atmosphere and constrain key model parameters, including the lift coefficient and fragment separation velocity, as well as meteoroid property distributions. The model distribution of dynamic meteoroid strength that produces the best match to observations has a minimum strength of 10–90 kPa, a maximum strength of 3–6 MPa and a median strength of 0.2–0.5 MPa. An important feature of the model is that individual fragmentation events are able to produce fragments with a wide range of dynamic strengths as much as ten times stronger or weaker than the parent fragment. The calibrated model suggests that the rate of small impacts on Mars is 1.5–4 times higher than recent observation-based estimates. It also shows how impactor properties relevant to seismic wave generation, such as the total impact momentum, can be inferred from cluster characteristics.

This paper provides an overview of the Curiosity rover's exploration at Vera Rubin ridge (VRR) and summarizes the science results. VRR is a distinct geomorphic feature on lower Aeolis Mons (informally known as Mount Sharp) that was identified in orbital data based on its distinct texture, topographic expression, and association with a hematite spectral signature. Curiosity conducted extensive remote sensing observations, acquired data on dozens of contact science targets, and drilled three outcrop samples from the ridge, as well as one outcrop sample immediately below the ridge. Our observations indicate that strata composing VRR were deposited in a predominantly lacustrine setting and are part of the Murray formation. The rocks within the ridge are chemically in family with underlying Murray formation strata. Red hematite is dispersed throughout much of the VRR bedrock, and this is the source of the orbital spectral detection. Gray hematite is also present in isolated, gray‐colored patches concentrated toward the upper elevations of VRR, and these gray patches also contain small, dark Fe‐rich nodules. We propose that VRR formed when diagenetic event(s) preferentially hardened rocks, which were subsequently eroded into a ridge by wind. Diagenesis also led to enhanced crystallization and/or cementation that deepened the ferric‐related spectral absorptions on the ridge, which helped make them readily distinguishable from orbit. Results add to existing evidence of protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record., Key Points We summarize Curiosity's campaign at Vera Rubin ridge (Sols 1726–2302) and the high‐level results from articles in this special issueVera Rubin ridge formed when diagenesis hardened rocks along the base of Aeolis Mons; wind subsequently etched the feature into a ridgeResults add evidence for protracted aqueous environments at Gale crater and give new insight into how diagenesis shaped Mars' rock record

This paper explores the implications of the observed Bennu particle ejection events for that asteroid's spin rate and orbit evolution, which could complicate interpretation of the Yarkovsky‐O'Keefe‐Radzievskii‐Paddack (YORP) and Yarkovsky effects on this body's spin rate and orbital evolution. Based on current estimates of particle ejection rates, we find that the overall contribution to Bennu's spin and orbital drift is small or negligible as compared to the Yarkovsky and YORP effects. However, if there is a large unseen component of smaller mass ejections or a strong directionality in the ejection events, it could constitute a significant contribution that could mask the overall YORP effect. This means that the YORP effect may be stronger than currently assumed. The analysis is generalized so that the particle ejection effect can be assessed for other bodies that may be subject to similar mass loss events. Further, our model can be modified to address different potential mechanisms of particle ejection, which are a topic of ongoing study., Key Points The net contribution of particle ejection from Bennu's surface on its spin rate and orbit evolution is minimal under some assumptionsIf there are systematics in ejection geometry, there may be meaningful contributions to rotational accelerationOur model can be tuned to address different potential mechanisms of particle ejection

The relationship between frontal/flushing dust storms and northern hemisphere synoptic period transient eddies in Mars year 24 is examined in this paper. Frontal dust storms are observed roughly continuously during the presolstice (early/middle fall) and postsolstice (middle/late winter) time periods, but flushing dust storms that cross the equator are confined to shorter seasonal windows on both sides of the solsticial pause. In the lower atmosphere, the timing of cross-equatorial flushing dust storms correlates better with eddy temperature than with eddy meridional wind; in the middle atmosphere, it correlates better with eddy meridional wind than with eddy temperature. This is because both the lower atmosphere eddy temperature and the middle atmosphere eddy meridional wind are dominated by zonal wave number m = 3 eastward traveling waves during the cross-equatorial flushing dust storm periods. Frontal dust storms do not seem to be limited to any particular wave mode, but cross-equatorial flushing dust storms appear to be closely related to m = 3 eastward traveling waves, at least in Mars year 24. The effectiveness of m = 3 waves in this regard is partially due to their amplitudes but more importantly due to their seasonal distributions and latitudinal positions. During the time periods when m = 3 waves are strong, the m = 3 waves are also located at lower latitudes, closer in distance to the fairly strong southward mean meridional wind in the low latitudes. Dust in frontal dust storms at high latitudes can be easily entrained into the low-latitude circulation and be efficiently transported southward.

[1] The impact velocities of asteroids on Earth and other terrestrial planets can be greater than 10 km/s, and impacts at these high velocities can produce significant effects on the planetary surfaces. However, since macroscopic (>∼0.1 mm) projectiles with an aspect ratio of ∼1 are not easily accelerated to more than 10 km/s in laboratories, there are few detailed experimental studies. In this paper, we demonstrate that impact velocities greater than 10 km/s can be achieved with glass and aluminum projectiles of 0.1–0.3 mm in diameter using a high-power laser, GEKKO XII-HIPER at Institute of Laser Engineering, Osaka University. The velocity of the projectiles is estimated based on the images taken by high-speed X-ray streak and framing cameras. Projectiles collide into copper or LiF plate targets. The copper plates are recovered for analysis. The sizes of craters on the copper plates are not far from the extrapolations from previous work with lower velocities. A tantalum witness plate placed near the copper plates records a large number of secondary craters from each impact. In the case of the impacts of the LiF plates, we observe two emission lines of Li gas using a spectrometer with a streak camera. Thus, we can simulate the hypervelocity impacts with velocities higher than 10 km/s in laboratories.