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The use of impact crater densities to estimate the ages of planetary surfaces began in the 1960s. Some predictive successes have been confirmed with radiometric dating of sites on the Moon and Mars. The method is highly dependent on our understanding of the rate of crater formation on different worlds, and, more importantly, on the history of that rate, starting with intense cratering during planetary formation 4.5 Ga ago. The system is thus calibrated by obtaining radiometric dates from samples of relatively homogeneous geologic units on various worlds. Crater chronometry is still in its infancy. Future sample-returns and in situ measurements, obtained by missions from collaborating nations to various worlds, will provide ever-increasing improvements in the system in coming decades. Such data can lead to at least two-significant-figure measurements, not only of the ages of broad geologic provinces on solar system worlds, but of the characteristic survival times of various-sized smaller craters. Such data, in turn, clarify the rates of turnover of surface materials and the production rates of gravel-like regolith and megaregolith in the surface layers. Better measurements of the impact rate at various times, in turn, support better modeling of the accretion and fragmentation processes among early planetesimals as well as contemporary asteroids, in various parts of the solar system. Once the crater chronometry system is calibrated for various planetary bodies, important chronological information about those various planetary bodies can be obtained by orbital missions, without the need for expensive sample-return or lander missions on each individual surface.

Impact rates in the first 500 Myr of the solar system are critical to an understanding of lunar geological history, but they have been controversial. The widely accepted, post-Apollo paradigm of early lunar impact cratering (ca. 1975-2014) proposed very low or negligible impact cratering in the period from accretion (>4.4 Ga) to about 4.0 Ga ago, followed by a 170-million-year-long spike of cataclysmic cratering, during which most prominent multi-ring impact basins formed at age of about 3.9 Ga. More recent dynamical models suggest very early intense impact rates, declining throughout the period from accretion until an age of about 3.0 Ga. These models remove the basin-forming spike. This shift has important consequences on megaregolith evolution and properties of rock samples that can be collected on the lunar surface today. We adopt the Morbidelli et al. (2018) "accretion tail" model of early intense bombardment, declining as a function of time. We find effects differing from the previous models: early crater saturation and supersaturation; disturbance of magma ocean solidification; deep early megaregolith; and erosive destruction of the earliest multi-ring basins, their impact melts, and their ejecta blankets. Our results explain observations such as differences in numbers of early lunar impact melts vs. numbers of early igneous crustal rocks, highland breccias containing impact melts as old as 4.35 Ga, absence of a 170 Myr-long spike in impact melt ages at 3.9 Ga among lunar and asteroidal meteorites, and GRAIL observations of lunar crustal structure., in press in Meteoritics and Planetary Science (MAPS)

New information has been obtained in recent years regarding formation rates and the production size-frequency distribution (PSFD) of decameter-scale primary Martian craters formed during recent orbiter missions. Here we compare the PSFD of the currently forming small primaries (P) with new data on the PSFD of the total small crater population that includes primaries and field secondaries (P + fS), which represents an average over longer time periods. The two data sets, if used in a combined manner, have extraordinary potential for clarifying not only the evolutionary history and resurfacing episodes of small Martian geological formations (as small as one or few km2) but also possible episodes of recent climatic change. In response to recent discussions of statistical methodologies, we point out that crater counts do not produce idealized statistics, and that inherent uncertainties limit improvements that can be made by more sophisticated statistical analyses. We propose three mutually supportive procedures for interpreting crater counts of small craters in this context. Applications of these procedures support suggestions that topographic features in upper meters of mid-latitude ice-rich areas date only from the last few periods of extreme Martian obliquity, and associated predicted climate excursions.

Recent concerns about the giant impact hypothesis for the origin of the Moon, and an associated “isotope crisis” may be assuaged if the impactor was a local object that formed near Earth. We investigated a scenario that may meet this criterion, with protoplanets assumed to originate in 1:1 co-orbital resonance with Earth. Using N -body numerical simulations we explored the dynamical consequences of placing Mars-mass companions in various co-orbital configurations with a proto-Earth of 0.9 Earth-masses (M ⊕ ). We modeled 162 different configurations, some with just the four terrestrial planets and others that included the four giant planets. In both the 4- and 8-planet models we found that a single Mars-mass companion typically remained a stable co-orbital of Earth for the entire 250 million year (Myr) duration of our simulations (59 of 68 unique simulations). In an effort to destabilize such a system we carried out an additional 94 simulations that included a second Mars-mass co-orbital companion. Even with two Mars-mass companions sharing Earth's orbit about two-thirds of these models (66) also remained stable for the entire 250 Myr duration of the simulations. Of the 28 2-companion models that eventually became unstable 24 impacts were observed between Earth and an escaping co-orbital companion. The average delay we observed for an impact of a Mars-mass companion with Earth was 102 Myr, and the longest delay was 221 Myr. In 40% of the 8-planet models that became unstable (10 out of 25) Earth collided with the nearly equal mass Venus to form a super-Earth (loosely defined here as mass ≥1.7 M ⊕ ). These impacts were typically the final giant impact in the system and often occurred after Earth and/or Venus has accreted one or more of the other large objects. Several of the stable configurations involved unusual 3-planet hierarchical co-orbital systems.

A Coronado-era name, variously spelled as “Chichiltecally,” “Chichilticale,” etc., was applied in 1540 to a mountain range, a ruin, and the surrounding area in southeast Arizona. Today it is usually described as a Nahuatl word assigned to a red-colored pueblo ruin on the Coronado route to Cibola. We note, however that the chronicler Jaramillo, with the lead portion of the Coronado expedition, assigned “Chichiltecally” solely to a mountain range and associated pass. Recent archaeological finds indicate that the ruin was at the west base of the Chiricahua Mountains and south of Apache Pass. We show that the name recorded by Jaramillo was an attempt to render an Opata term connoting “Mountain Range of Many Turkeys,” and that this name evolved through various spellings to “Chiricahua,” which essentially means “Turkey Mountain.” Application of a similar Nahuatl term to the ruin may have arisen from a misunderstanding or adaptation of the Opata term by Nahuatl speakers on the Coronado expedition, or it may have...

Two words used by Marcos de Niza in the first European report on travel north through Sonora (1539) ‘“Sayota” and “Vacapa”’ appear to be traceable to the Hiaki (Yaqui) language. Also, the Coronado expedition's assignment (1540) of the name “Yaquimi” to what is now called the Rio Yaqui, in its middle stretch near Onavas, indicates the presence of Hiaki/Yaqui-speakers in that region. These results are used to constrain the likely region of Hiaki/Yaqui-speaking peoples in 1539–42 and also the poorly known movements of Marcos de Niza and the Coronado expedition in Sonora at that time.

The well-recorded Chelyabinsk event, the Tunguska event, and the re-entry of the Zond IV vehicle offer opportunities to compare reactions of modern eyewitnesses to eyewitness accounts of possible ancient fireball events. The first-century book, Acts of the Apostles, gives three separate descriptions of a bright light "from heaven," which occurred probably in the 30s (C.E.) near Damascus, Syria. The details offer a strikingly good match to a Chelyabinsk-class or Tunguska-class fireball. Among the most impressive, unexpected consistencies with modern knowledge is the first-century description of symptoms of temporary blindness caused by exposure to intense radiation, matching a condition now known as photokeratitis. An analysis of the re-entry of debris from the Russian Zond IV over the eastern United States in 1968 shows how actual perceived phenomena in an unfamiliar natural celestial apparition are often conceived by the observer in terms of current cultural conceptions, and it is suggested that this happened also in the first-century case.

Asteroid 2008 TC3 (approximately 4 m diameter) was tracked and studied in space for approximately 19 h before it impacted Earth’s atmosphere, shattering at 44–36 km altitude. The recovered samples (>680 individual rocks) comprise the meteorite Almahata Sitta (AhS). Approximately 50–70% of these are ureilites (ultramafic achondrites). The rest are chondrites, mainly enstatite, ordinary, and Rumuruti types. The goal of this work is to understand how fragments of so many different types of parent bodies became mixed in the same asteroid. Almahata Sitta has been classified as a polymict ureilite with an anomalously high component of foreign clasts. However, we calculate that the mass of fallen material was ≤0.1% of the pre-atmospheric mass of the asteroid. Based on published data for the reflectance spectrum of the asteroid and laboratory spectra of the samples, we infer that the lost material was mostly ureilitic. Therefore, 2008 TC3 probably contained only a few percent nonureilitic materials, similar to other polymict ureilites except less well consolidated. From available data for the AhS meteorite fragments, we conclude that 2008 TC3 samples essentially the same range of types of ureilitic and nonureilitic materials as other polymict ureilites. We therefore suggest that the immediate parent of 2008 TC3 was the immediate parent of all ureilitic material sampled on Earth. We trace critical stages in the evolution of that material through solar system history. Based on various types of new modeling and re-evaluation of published data, we propose the following scenario. (1) The ureilite parent body (UPB) accreted 0.5–0.6 Ma after formation of calcium-aluminum-rich inclusions (CAI), beyond the ice line (outer asteroid belt). Differentiation began approximately 1 Ma after CAI. (2) The UPB was catastrophically disrupted by a major impact approximately 5 Ma after CAI, with selective subsets of the fragments reassembling into daughter bodies. (3) Either the UPB (before breakup), or one of its daughters (after breakup), migrated to the inner belt due to scattering by massive embryos. (4) One daughter (after forming in or migrating to the inner belt) became the parent of 2008 TC3. It developed a regolith, mostly ≥3.8 Ga ago. Clasts of enstatite, ordinary, and Rumuruti-type chondrites were implanted by low-velocity collisions. (5) Recently, the daughter was disrupted. Fragments were injected or drifted into Earth-crossing orbits. 2008 TC3 comes from outer layers of regolith, other polymict ureilites from deeper regolith, and main group ureilites from the interior of this body. In contrast to other models that have been proposed, this model invokes a stochastic history to explain the unique diversity of foreign materials in 2008 TC3 and other polymict ureilites.

We have assembled data on thirteen cases of meteorite falls with accurate tracking data on atmospheric passage. In all cases we estimate the bulk strength of the object corresponding to its earliest observed or inferred fragmentation in the high atmosphere, and can compare these values with measured strengths of meteorites in the taxonomic class for that fall. In all thirteen cases the strength corresponding to earliest observed or inferred fragmentation is much less than the compressive or tensile strength reported for that class of stony meteorites. We find a more random relation between bulk sample strength and sample mass than is suggested by a commonly used empirical power law. We estimate bulk strengths on entry being characteristically of the order of 10 -1 to 10 -2 times the tensile strengths of recovered samples.

Recent controversies about systems of crater-count dating have been largely resolved, and with continuing refinements, crater counts will offer a fundamental geological tool to interpret not only ages, but also the nature of geological processes altering the surface of Mars. As an example of the latter technique, we present data on two debris aprons east of Hellas. The aprons show much shorter survival times of small craters than do the nearby contiguous plains. The order-of-magnitude depths of layers involved in the loss process can be judged from the depths of the affected craters. We infer that ice-rich layers in the top tens of meters of both aprons have lost crater topography within the last few 10 8 yr, probably due to flow or sublimation of ice-rich materials. Mantling by ice-rich deposits, associated with climate change cycles of obliquity change, has probably also affected both the aprons and the plains. The crater-count tool thus adds chronological and vertical dimensional information to purely morphological studies.

We have identified two classes of crater clusters on Mars. One class is “small clusters” (crater diameter D∼ tens m, spread over few hundred m), fitting our earlier calculations for the breakup of weak stone meteoroids in the martian atmosphere [Popova, O.P., Nemtchinov, I.V., Hartmann, W.K., 2003. Meteorit. Planet. Sci. 38, 905–925]. The second class is “large clusters” (D∼ few hundred m, spread over 2 to 30 km), which do not fit any predictions for breakup of known meteoroid types. We consider a range of possible explanations. The best explanation relates to known, high-speed ejection of large, semi-coherent, fractured rock masses from the surface, as secondary debris from primary impacts. The clusters are probably due to breakup of partly fracture, few-hundred-meter scale weak blocks, especially during ascent (producing moderate lateral spreading velocities among the fragments during sub-orbital flight), and also during descent of the resulting swarm. These conclusions illuminate the launch conditions of martian meteorites, including fragmentation processes, although more work is needed on the lateral separation of fragments (during either atmosphere descent or ascent) due to the effects of volatiles in the projectiles. Martian meteorites probably come from smaller martian craters than the clusters' source craters. The latter probably have D ≳ 85 km , although we have not ruled out diameters as small as 15 km.

The Malin et al. [Malin, M.C., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E., 2006. Science 314, 1573–1577] report of ongoing production of decameter scale primary craters matches within an order of magnitude the crater production curves of Hartmann [Hartmann, W.K., 2005. Icarus 174, 294–320], and appears to contradict models that say our curves grossly overpredict primary production. Nonetheless, such models may still predict correct order-of-magnitude secondary production. The new observations, if correct, require geologically recent martian surface modification in some areas, on timescales comparable to the cycles of obliquity changes, suggesting episodic martian climatic/environmental change driven by the obliquity cycles.

Crater counts at lunar landing sites with measured ages establish a steep decline in cratering rate during the period ∼3.8 to ∼3.1 Gyr ago. Most models of the time dependence suggest a roughly constant impact rate (within factor ∼2) after about 3 Gyr ago, but are based on sparse data. Recent dating of impact melts from lunar meteorites, and Apollo glass spherules, clarifies impact rates from ∼3.2 to ∼2 Gyr ago or less. Taken together, these data suggest a decline with roughly 700 Myr half-life around 3 Gyr ago, and a slower decline after that, dropping by a factor ∼3 from about ∼2.3 Gyr ago until the present. Planetary cratering involved several phases with different time behaviors: (1) rapid sweep-up of most primordial planetesimals into planets in the first hundred Myr, (2) possible later effects of giant planet migration with enhanced cratering, (3) longer term sweep-up of leftover planetesimals, and finally (4) the present long-term “leakage” of asteroids from reservoirs such as the main asteroid belt and Kuiper belt. In addition, at any given point on the Moon, a pattern of “spikes” (sharp maxima of relatively narrow time width) will appear in the production rate of smaller craters (≲500 m?), not only from secondary debris from large primary lunar impacts at various distances from the point in question, but also from asteroid breakups dotted through Solar System history. The pattern of spikes varies according to type of sample being measured (i.e., glass spherules vs impact melts). For example, several data sets show an impact rate spike ∼470 Myr ago associated with the asteroid belt collision that produced the L chondrites (see Section 3.6 below). Such spikes should be less prominent in the production record of craters of D≳ few km. These phenomena affect estimates of planetary surfaces ages from crater counts, as discussed in a companion paper [Quantin, C., Mangold, N., Hartmann, W.K., Allemand, P., 2007. Icarus 186, 1–10]. Fewer impact melts and glass spherules are found at ∼3.8 Gyr than at ∼3.5 Gyr ago, even though the impact rate itself is known to have been higher at 3.8 Gyr ago than 3.5 Gyr. This disproves the assertion by Ryder [Ryder, G., 1990. EOS 71, 313, 322–323] and Cohen et al. [Cohen, B.A., Swindle, T.D., Kring, D.A., 2000. Science 290, 1754–1756] that ancient impact melts are a direct proxy for ancient impact (cf. Section 3.3). This result raises questions about how to interpret cratering history before 3.8 Gyr ago.

Quantin et al. [Quantin, C., Allemand, P., Mangold, N., Delacourt, C., 2004a. Icarus 172, 555–572] tabulated crater count data for 56 landslides along the walls of Valles Marineris. Under the assumption of a constant cratering rate after about 3 Gyr ago, as used in the 1999–2005 iterations of the crater chronology isochron system of Hartmann, and in the Hartmann and Neukum system, these data indicate a regularly increasing rate of landslides, which would be difficult to explain. We suggest that these data may support a decline in inner Solar System cratering rates by about a factor of 3 since 3 Gyr ago, not unlike predictions based on asteroid belt collision models. Such a decline is also supported by our review of data on lunar impact melts and glass spherules in a companion paper [Hartmann, W.K., Quantin, C., Mangold, N., 2007. Icarus 186, 11–23]. Such models produce not only a more uniform rate of landslides over the last 3 Gyr, but also a more uniform rate of resurfacing processes which also had an apparent increase under the assumption of a constant cratering rate.

The native village at Sonoyta, Sonora, was first recorded by Europeans when Father Eusebio Kino visited it in 1698. By 1700, he wrote that this "ranchena ... is the best there is on this coast. It has fertile land, with irrigation ditches for good crops . . . , water which runs all the year, good pasture for cattle, and everything necessary for a good settlement" (Kino 1924, 2:255). Kino was proved right. Sonoyta was a good place to live.

Martian meteorites (MMs) have been launched from an estimated 5-9 sites on Mars within the last 20 Myr. Some 80-89% of these launch sites sampled igneous rock formations from only the last 29% of Martian time. We hypothesize that this imbalance arises not merely from poor statistics, but because the launch processes are dominated by two main phenomena: first, much of the older Martian surface is inefficient in launching rocks during impacts, and second, the volumetrically enormous reservoir of original cumulate crust enhances launch probability for 4.5 Gyr old rocks. There are four lines of evidence for the first point, not all of equal strength. First, impact theory implies that MM launch is favored by surface exposures of near-surface coherent rock (≤10 2 m deep), whereas Noachian surfaces generally should have ≥10 2 m of loose or weakly cemented regolith with high ice content, reducing efficiency of rock launch. Second, similarly, both Mars Exploration Rovers found sedimentary strata, 1-2 orders of magnitude weaker than Martian igneous rocks, favoring low launch efficiency among some fluvial-derived Hesperian and Noachian rocks. Even if launched, such rocks may be unrecognized as meteorites on Earth. Third, statistics of MM formation age versus cosmic-ray exposure (CRE) age weakly suggest that older surfaces may need larger, deeper craters to launch rocks. Fourth, in direct confirmation, one of us (N. G. B.) has found that older surfaces need larger craters to produce secondary impact crater fields (cf. Barlow and Block 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with prominent fields of secondaries have diameters of ~45 km, ~19 km, and ~10 km, respectively. Because 40% of Mars is Noachian, and 74% is either Noachian or Hesperian, the subsurface geologic characteristics of the older areas probably affect statistics of recognized MMs and production rates of secondary crater populations, and the MM and secondary crater statistics may give us clues to those properties.

A survey of craters in the vicinity of Newton Basin, using high-resolution images from Mars Global Surveyor and Mars Odyssey, was conducted to find and analyze examples of gullies and arcuate ridges and assess their implications for impact crater degradation processes. In the Phaethontis Quadrangle (MC-24), we identified 225 craters that contain these features. Of these, 188 had gullies on some portion of their walls, 118 had arcuate ridges at the bases of the crater walls, and 104 contained both features, typically on the same crater wall. A major result is that the pole-facing or equator-facing orientation of these features is latitude dependent. At latitudes >44° S, equator-facing orientations for both ridges and gullies are prevalent, but at latitudes

Ridges that resemble terrestrial moraines are commonly visible at the foot of many mid-latitude crater walls in Mars Global Surveyor Mars Orbiter Camera images. These moraine-like ridges are often associated with hillside gullies, mantling material, and glacier-like flows, and are usually in contact with crater fill, suggesting possible interrelationships. We consider terrestrial glacier systems that may be analogs of martian moraine-like ridges and glacier-like flows and suggest that the formation of some gullies and crater fill is intimately tied to ice deposition, ice flow, and rock-glacier processes. Upper limits on age suggest the possibility that many of these features formed during the last, or last few, high obliquity cycles.

Historians have puzzled over early reports of a 1538 reconnaissance of the SonoraArizona region prior to the 1539 exploration of Sonora and Cibola by Fray Marcos de Niza's expedition. We show that the 1538 expedition is mythical and based on misunderstandings of the travels of Marcos, Governor Francisco Vazquez de Coronado, and associates, who departed Mexico City for the northern frontier in 1538. Contrary to several earlier historians, we conclude that Marcos sent messages back as he traveled north. These messages about prosperous northern lands arrived in Mexico City weeks before Marcos himself, triggering a later myth that Marcos had only confirmed results of a prior expedition. We also shed light on the possible messengers.

We investigate the action of the martian atmosphere on entering meteoroids for present and past atmospheres with various surface pressures to predict the smallest observable craters, and to understand the implications for the size distributions of craters on Mars and meteoroids in space. We assume different strengths appropriate to icy, stone, and iron bodies and test the results against available data on terrestrial bolides. Deceleration, ablation, and fragmentation effects are included. We find that the smallest icy, stone, and iron meteoroids to hit the martian ground at crater forming speeds of ≥500 m/s have diameters of about 2 m, 0.03-0.9 m (depending on strength), and 0.01 m, respectively, in the current atmosphere. For hypothetical denser past atmospheres, the cutoff diameters rise. At a surface pressure of 100 mb, the cutoff diameters are about 24 m, 5-12 m, and 0.14 m for the 3 classes. The weaker stony bodies in the size range of about 1-30 m may explode at altitudes of about 10-20 km above the ground. These figures imply that under the present atmosphere, the smallest craters made by these objects would be as follows: by ice bodies, craters of diameter (D) ~8 m, by stones about 0.5-6 m, and by irons, about 0.3 m. A strong depletion of craters should, thus, occur at diameters below about 0.3 m to 5 m. Predicted fragmentation and ablation effects on weak meteoroids in the present atmosphere may also produce a milder depletion below D ~500 m, relative to the lunar population. But, this effect may be difficult to detect in present data because of additional losses of small craters due to sedimentation, dunes, and other obliteration effects. Craters in strewn fields, caused by meteoroid fragmentation, will be near or below present-day resolution limits, but examples have been found. These phenomena have significant consequences. Under the present atmosphere, the smallest (decimeter-scale) craters in sands and soils could be quickly obliterated but might still be preserved on rock surfaces, as noted by Horz et al. (1999). Ancient crater populations, if preserved, could yield diagnostic signatures of earlier atmospheric conditions. Surfaces formed under past denser atmospheres (few hundred mbar), if preserved by burial and later exposed by exhumation, could show: a) striking depletions of small craters (few meter sizes up to as much as 200 m), relative to modern surfaces; b) more clustered craters due to atmospheric breakup; and c) different distributions of meteorite types, with 4 m to 200 m craters formed primarily by irons instead of by stones as on present-day Mars. Megaregolith gardening of the early crust would be significant but coarser than the gardening of the ancient lunar uplands.

The hypothesis of a lunar cataclysmic cratering episode between 3.8 and 3.9 Gyr ago lacks proof. Its strongest form proposes no cratering before about 4.0 Gyr, followed by catastrophic formation of most lunar craters and basins in

We report observations of Icelandic hillside gully systems that are near duplicates of gullies observed on high-latitude martian hillsides. The best Icelandic analogs involve basaltic talus slopes at the angle of repose, with gully formation by debris flows initiated by ground water saturation, and/or by drainage of water from upslope cliffs. We report not only the existence of Mars analog gullies, but also an erosional sequence of morphologic forms, found both on Mars and in Iceland. The observations support hypotheses calling for creation of martian gullies by aqueous processes. Issues remain whether the water in each case comes only from surficial sources, such as melting of ground ice or snow, or from underground sources such as aquifers that gain surface access in hillsides. Iceland has many examples of the former, but the latter mechanism is not ruled out. Our observations are consistent with the martian debris flow mechanism of F. Costard et al. (2001c , Science 295, 110–113), except that classic debris flows begin at midslope more frequently than on Mars. From morphologic observations, we suggest that some martian hillside gully systems not only involve significant evolution by extended erosive activity, but gully formation may occur in episodes, and the time interval since the last episode is considerably less than the time interval needed to erase the gully through normal martian obliteration processes.

Athabasca and Marte Valles lie on the Cerberus plains, between the young, lava-covered plains of Elysium Planitia and Amazonis Planitia. To test pre-MGS (Mars Global Surveyor) suggestions of extremely young volcanic and fluvial activity, we present the first crater counts from MGS imagery, at resolutions (∼2–20 m/pixel) much higher than previously available. The most striking result, based on morphologic relations as well as crater counts from different stratigraphic units, is to confirm quantitatively that these channel systems are much younger than most other major outflow channels. The general region has an average model age for lava and fluvial surfaces of ≤200 Myr, and has possibly seen localized water releases, interspersed with lava flows, within the past 20 Myr. The youngest lavas may be no more than a few megayears old. Access of lava and liquid brines to the surface may be favored by openings of the Cerberus Fossae fracture system, but, as shown in the new images, the fractures appear to have continued developing more recently than the most recent lavas or fluvial activity. The Cerberus Fossae system may be an analog to an early stage of Valles Marineris, and its youthful activity raises questions about regional tectonic history. Large-volume water delivery to the surface of young lava flows in recent martian history puts significant boundary conditions on the storage and history of water on Mars.

Viking-era researchers concluded that impact craters of diameter D Unlike lunar regolith, martian regolith has been affected, and fines removed, by many processes. Deflation may have been a factor in leaving widespread boulder fields and associated dune fields, observed by the first three landers. Ancient regolith provided a porous medium for water storage, subsurface transport, and massive permafrost formation. Older regolith was probably cemented by evaporites and permafrost, may contain interbedded sediments and lavas, and may have been brecciated by later impacts. Growing evidence suggests recent water mobility, and the existence of duricrust at Viking and Pathfinder sites demonstrates the cementing process. These results affect lander/rover searches for intact ancient deposits. The upper tens of meters of exposed Noachian units cannot survive today in a pristine state. Intact Noachian deposits might best be found in cliffside strata, or in recently exhumed regions. The hematite-rich areas found in Terra Meridiani by the Mars Global Surveyor are probably examples of the latter.

Mars Global Surveyor (MGS) and Viking images allow analysis of chronological and geological relations among lava flows in southern Elysium Planitia, based on crater populations. MGS and Viking images clearly show morphological features of lava flows with some extremely sparsely cratered young flow units atop somewhat older surfaces. There is no evidence of substantial dust mantling on the young flows, and hence we infer that the crater populations date not sediment accumulation but the lava flows themselves. The youngest flows have some of the lowest crater densities that we have seen on Mars, some

Four flybys of Phobos by the Mars Global Surveyor spacecraft in 1998 allowed imaging at resolutions (∼2–7 m) better than obtained for any other satellite or asteroid except Earth's Moon. The images show the interior and vicinity of the large crater Stickney. There are ∼2000 identifiable ejecta blocks, the largest about 85 m across. The great majority of these blocks come from Stickney as part of low-velocity ejecta spread eastward by the influence of Phobos's rapid rotation. They appear to have experienced only a few meters, at most, of burial or modification since emplacement. The images also show materials of different albedos within Stickney that mark downslope motion of regolith. The number of craters on the steep slopes of Stickney that are subject to the downslope motion suggests that 4 km) have been degraded to the point that they are no longer recognizable.

The Mars Orbiter cameras (MOC) on Mars Global Surveyor observed the south polar cap of Mars during its spring recession in 1997. The images acquired by the wide angle cameras reveal a pattern of recession that is qualitatively similar to that observed by Viking in 1977 but that does differ in at least two respects. The 1977 recession in the 0° to 120° longitude sector was accelerated relative to the 1997 observations after L_S = 240°; the Mountains of Mitchel also detached from the main cap earlier in 1997. Comparison of the MOC images with Mars Orbiter Laser Altimeter data shows that the Mountains of Mitchel feature is controlled by local topography. Relatively dark, low albedo regions well within the boundaries of the seasonal cap were observed to have red-to-violet ratios that characterize them as frost units rather than unfrosted or partially frosted ground; this suggests the possibility of regions covered by CO_2 frost having different grain sizes.

— We give a nonmathematical review of recent work regarding the Yarkovsky effect on asteroidal fragments. This effect may play a critical, but underappreciated, role in delivering meteorites to Earth. Two variants of the effect cause drifts in orbital elements, notably semimajor axes. The “classic” or “diurnal” Yarkovsky effect is associated with diurnal rotation at low obliquity. More recently, a “seasonal” effect has also been described, associated with high obliquity. Studies of these Yarkovsky effects are combined with studies of resonance effects to clarify meteorite delivery. If there were no Yarkovsky drift, asteroid fragments could reach a resonance only if produced very near that resonance. However, objects in resonances typically reach Earth-crossing orbits within a few million years, which is inconsistent with stone meteorites' cosmic-ray exposure (CRE) ages (5–50 Ma) and iron meteorites' CRE ages (100–1000 Ma). In the new view, on the other hand, large objects in the asteroid belt are “fixed” in semimajor axis, but bodies up to 100 m in diameter are in a constant state of mixing and flow, especially if the thermal conductivity of their surface layers is low. Thus, small asteroid fragments may reach the resonances after long periods of drift in the main belt. Yarkovsky drift effects, combined with resonance effects, appear to explain many meteorite properties, including: (1) the long CRE ages of iron meteorites (due to extensive drift lifetimes in the belt); (2) iron meteorites' sampling of numerous parent bodies; (3) the shorter CRE ages of most stone meteorites (due to faster drift, coupled with weaker strength and more rapid collisional erosion); and (4) the abundance of falls from discrete impact events near resonances, such as the 8 Ma CRE age of H chondrites. Other consequences include: the delivery of meteorite parent bodies to resonances is enhanced; proportions of stone and iron meteorites delivered to Earth may be different from the proportions at the same sizes left in the belt, which in turn may differ from the ratio produced in asteroidal collisions; Rabinowitz's 10–100 m objects may be preferentially delivered to near-Earth space; and the delivery of C-class fragments from the outer belt may be inhibited, compared to classes in other parts of the belt. Thus, Yarkovsky effects may have important consequences in meteoritics and asteroid science.

— This paper develops a methodology to establish absolute Martian ages by deriving isochrons on a plot of Martian impact crater density vs. crater diameter, calibrated by lunar crater/age data. The isochrons illustrated here are based on a Mars/Moon cratering ratio of 1.6 at constant size, but there is a factor of 2 to 4 uncertainty in this ratio and the consequent model ages. Martian crater diameter distributions are determined in several areas down to diameter D = 16–45 m; the shapes of the curves in young areas are found to be close to that of the predicted isochrons and close to the standard production function found by Neukum. The youngest areas studied here display the lunar-like production function down to D ∼30 m, where saturation equilibrium sets in. Model crater retention ages of several volcanic units are found to be a few hundred million years or less, with estimated uncertainties ranging from a factor of 2 lower to a factor of 4 higher. The results are consistent with Martian meteorite ages. Volcanism on Mars has probably persisted into the last 10 to 15% of the planet's history and is likely ongoing. Because surfaces as young as a few hundred million years have reached crater saturation equilibrium at D < ∼60 to 100 m, Mars is likely to have widespread impact-produced regoliths at least a few meters deep, and this may contribute to the widespread mobile dust and boulder fields of Mars.

— We have detected three unusual, low-relief circular features, 1.2 to 2.1 km in diameter, in the northwest Noachis highlands, which may be craters that have undergone isostatic deformation. They may shed light on the existence, nature, and timing of suspected widespread Martian erosion/obliteration events, and offer clues to a type of Martian terrain softening. In the surrounding area, we find an anomalous deficiency of craters in the 3–11 km diameter range and evidence that larger, older craters have undergone relief softening and infill. We discuss three different hypotheses to explain these features, two of which involve Martian ice. This region may have undergone a transient event in which a near-surface permafrost layer (several hundred meters deep) underwent partial melting or softening. This would allow relaxation of kilometer-scale craters and softening of larger craters. Crater data presented here suggest that this event happened some time in mid-Martian history. Whether the event was regional or related to global-scale events is uncertain, though it may represent a class of events that also happened in other Martian areas.

Impact craters help characterize the age of a planetary surface, because they accumulate with time. They also provide useful constraints on the importance of surface erosion, as such processes will preferentially remove the smaller craters. Earlier studies of martian crater populations revealed that erosion and dust deposition are important processes on Mars. They disagreed, however, on the age of the youngest volcanism. These earlier studies were limited by image resolution to craters larger than a few hundred metres in diameter. Here we report an analysis, using new images obtained by the Mars Global Surveyor spacecraft, of crater populations that extend the size distribution down to about 16 m. Our results indicate a wide range of surface ages, with one region—lava flows within the Arsia Mons caldera—that we estimate to be no older than 40–100 million years. We suggest that volcanism is a continuing process on Mars.

The solar systems second largest natural satellite is shrouded by a thick nitrogen atmosphere, rich in methane, within which sunlight- and cosmic-ray-driven organic chemistry has gone on for some 4.5 billion years. The earliest history of Titans atmosphere, and specifically its origin, remains unclear until the Cassini–Huygens probe measures the ratio of argon to nitrogen and the abundances of other noble gases and isotopes. However, the abundance of deuterated methane in the atmosphere today is consistent with an atmosphere that originated in the chemically-processed sub-nebula around Saturn, rather than in cometary material. Titans overall atmospheric history is driven by the depletion of methane, and the mechanisms by which methane might be resupplied from surface or external sources. Remote sensing data mitigate against a large reservoir of methane at Titans surface, leaving open the possibility that methane is periodically depleted from Titans atmosphere on timescales of 107–108 years; under such conditions Titan might oscillate between thin and thick atmospheric epochs. Titans surface may provide a repository for complex organic molecules that were synthesized during times when liquid water was temporarily available on the surface, such as after impacts or cryo-volcanic eruptions. Such molecules might provide clues to the resolution of some difficult issues associated with the origin of life. Identifying the presence and nature of such molecules is a difficult exploration problem that must be left to missions which follow-on from the Cassini–Huygens exploration of Titan.

The 66-km diameter martian crater, Greg, east of Hellas, hosts various distinctive features, including dendritic valleys filled with chevron-textured masses (south wall), and lobate tongues a few kilometers long (north wall). We analyze these features by various quantitative techniques to illuminate martian geologic and climatic history. Crater retention model ages indicate that Greg is at least 1–3 Gy old, but surface layers of mantles and glacial features are orders of magnitude younger. Properties of the dendritic valleys, combined with climate models, suggest that fluvial activity began under a thicker, warmer atmosphere, soon after the crater’s formation. The oldest exposed fluvial systems have surface crater retention ages of a few hundred My, indicating runoff in recent geologic time. Much of Greg is covered by ice-rich mantle deposits, for which we infer gradual accumulation and depths of order 30–85 m; they mask pre-existing landforms. The lobate tongues are interpreted as glaciers with mean slope of 10.2 ± 2.3° and average thickness of 33 ± 19 m. Our calculations and data suggest that these glaciers were originally ice-rich and that their surface layers have been depleted by volatile loss. The glaciers probably formed when ice-rich mantle deposits reached critical thickness and flowed downhill. The top 5–10 m of the mantle and glaciers show crater survival times of order a few My to ∼15 My, which, remarkably, is the time since the last 1–4 episodes of obliquity >45°. Global climate models affirm that Greg lies in one of two non-polar areas with extremes of ice deposition during high-obliquity epochs. This match with observations supports the use of such models in studies of planetary climate change.

— A realistic computer model has been developed to display images of imaginary cratered surfaces, taking into account empirically measured input size distributions of primary and secondary craters, ejecta blanket morphology including feathering with distance, obliteration due to ejecta from outside the imaged area, lighting effects, etc. The model allows us to track surface evolution of morphology as new craters are added. Using the model as well as lunar photos, we have studied the approach to saturation equilibrium (defined as a condition when no further proportionate increase in crater density occurs as input cratering increases). We find that an identifiable saturation equilibrium occurs close to a level previously identified for this state (Hartmann, 1984), typically fluctuating around a crater density from ∼0.4 to 2 times that level. This result is fairly robust vis-a-vis the range of model parameters we have chosen. Flooding, basin ejecta blankets, and other obliterative effects can introduce structure and oscillations within this range, even after saturation equilibrium is achieved. These findings may constrain or revise certain earlier interpretations of satellite and planet surface evolution and impactor populations, which were predicated on the assumed absence of saturation equilibrium. In our fourth experimental run, we found that suppression of “sandblasting” by subresolution impacts allows the smallest secondaries to rise above the saturation equilibrium line, a result that might be relevant to a similar situation on Gaspra and perhaps some other asteroids.

A means for putting limits on the history of Titan's atmosphere is proposed by considering the breakup of bolides during atmospheric entry and the resulting modification of the crater size-frequency distribution at the surface, which will be determined by radar during the Cassini-Huygens mission. Bolides entering the atmosphere experience forces that eventually exceed the body's strength and disruption occurs. The size of the bolide which can reach the surface unfragmented depends on the atmospheric pressure. Given observation of the position of the turndown in crater numbers and relative surface age (the surface age is determined through the crater size-frequency distribution for diameters exceeding the minimum) a rough determination of the evolutionary trend of atmospheric pressure in Titan's history is achievable. This is a key scientific issue since Titan's atmosphere went through changes of atmospheric pressure in the past. We show that (a) Cassini-Huygens instruments are potentially capable of obtaining the necessary observations, and (b) for an atmosphere with the current density, crater numbers are predicted to begin to turn down at roughly 40-20 km for icy cometary bolides and stony bolides, respectively. Results for plausible early denser or thinner atmospheres are measurably different.

— This paper addresses several current issues related to use of craters in interpreting planetary surface histories. The primary goal is to test the widely adopted hypothesis of multiple populations of impactors at different times or places in the Solar System. New data presented here revise a “lunar highland” crater diameter distribution that has been widely used as evidence of an early distinct population of impactors. This curve, which has a depression of the size distribution at mid-sizes, does not, in fact, represent the lunar highlands generally. I show that it is associated with regions of intercrater plains. The more extensive the obliteration by intercrater plains, the deeper the depression. Modeling indicates that the depression of the curve is caused by the obliteration process itself. The oldest, most cratered regions of lunar highlands do not show the depression. These findings call into question earlier interpretations of multiple populations of impactors in the Solar System and of a distinctive primordial population. The present work is consistent, instead, with (1) a relatively uniform size distribution of interplanetary impactors, of mixed origins, back to 4 Ga ago and throughout the sampled Solar System; (2) fragmentation as the process that produced that size distribution; (3) saturation equilibrium on the most heavily cratered surfaces; and (4) differences in structure in the size distribution caused not by distinct impactor populations but by episodes of endogenic obliteration. If accepted, these results would modify some studies of solar system evolution, including assertions of two to five distinct populations of impactors, assumptions of lack of saturation equilibrium, and identifications of specific heliocentric or planetocentric sources for impactors within outer planet satellite systems.

Attention is given to an experimental investigation of the physical mechanism of comet outbursts which is consistent with the general picture of mantle presence on comets and clarifies the relation of mantles to eruptive activity. The experiment and closeup observation of Comet P/Halley suggest a result different from most mathematical models in that the release of gas pressure does not occur only from uniform gas flow out of the entire surface. In some active comets near perihelion within a few AU of the sun, gas production rates and disturbance of the surface may be so high that the outflow is nearly continuous, with the regolith being entirely stripped away, as in many of the models. The present model provides a cyclic eruption and recharge mechanism which is lacking in most other models.

The study of the near-Earth object 2201 Oljato has resulted in many surprises in the decade in which it has been examined. Its orbital properties have been associated with meteor showers, and its modeled orbital evolution is chaotic, a property which might indicate a history related to comets. Telescopic observations of its visible and near-infrared spectral reflectance, broad-band visible and near-infrared photometry, infrared radiometric measurements, and radar echoes are reported here from two apparitions, 1979 and 1983. A look at all available observational data shows that this asteroid has a high radiometric albedo, a property not associated with comet nuclei. In certain wavelength regimes it is classified as an S-type asteroid, in others, an E-type, but its overall spectral reflectance is not typical of either taxonomic type, and neither type is thought of as cometlike. Unexpectedly high ultraviolet reflectance at the 1979 apparition was suggested to be the result of residual outgassing as in a comet. The UV photometric data are modeled as fluorescent emission from neutral species found in comets. The resulting calculations indicate a plausible value for OH and CN emission at 0.3085 and 0.38 μm relative to the observed range of active comets. Observations to monitor photometric variations and to search for gaseous activity using observational techniques designed specifically for this purpose were planned and executed in October 1992 to verify or refute the interpretations presented here.

International audience; McEwen et al. (McEwen, A.S., Preblich, B.S., Turtle, E.P., Artemieva, N.A., Golombek, M.P., Hurst, M., Kirk, R.L., Burr, D.M., Christensen, P. [2005]. Icarus 176, 351-381) developed a useful test for the internal consistency of crater-count chronometry systems. They argued that certain multi-kilometer, fresh-looking martian craters with prominent rays should be the youngest or near-youngest craters in their size range. The "McEwen et al. test" is that the ages determined from crater densities of the smallest superimposed craters (typically diameter D similar to 5-20 m) should thus be comparable to the expected formation intervals of the host primary. McEwen et al. concluded from MOC data that crater chronometry failed this test by factors of 700-2000. We apply HiRISE and other imagery to eight different young craters in order to re-evaluate their arguments. We use existing crater chronology systems as well as the reported observed production rate of 16 m craters (Malin, MC., Edgett, K., Posiolova, L., McColley, S., Noe Dobrea, E. [2006]. Science 314, 1573-1557; Hartmann, W.K., Quantin, C., Mangold, N. [2007]. Icarus 186, 11-23; Kreslavsky [2007]. Seventh International Conference on Mars, 3325). Every case passes the McEwen et al. test. We conclude that the huge inconsistencies suggested by McEwen et al. are spurious. Many of these craters show evidence of impact into ice-rich material, and appear to have ice-flow features and sublimation pits on their floors. As production rate data improve, decameter-scale craters will provide a valuable way of dating these young martian geological formations and the processes that modify them

The colors of the recently discovered 1992 AD indicate a heretofore unknown spectral class, with colors that are far redder than those of any asteroids and comets thus far studied. It is suggested that the redness may be due to the exposure of organic substances that are purer than those associated with C, P, and D asteroids and comets, and therefore more likely to exhibit diagnostic spectral features in the IR.

Recent telescopic observations have led to the identification of cyanogroup-containing molecules in the dark surface solids of several D-class asteroids, cometary dusts, and the rings of Uranus, as well as the low-albedo atmosphere of Iapetus. The occurrence of the 2.2-micron overtone of C triple-bond N's stretching fundamental mode in all four classes of small solar system bodies is presently suggested to serve as a diagnostic of both exposure duration and degree of modification of surface materials.

An experimental study is conducted of collisions employing artificial aggregates as targets, in order to ascertain the fragmentation characteristics of weakly bonded bodies that consist of stronger constituent particles, such as loosely aggregated planetesimals. It is established that weakly bonded aggregate bodies may exhibit unexpectedly high impact strengths due to constituent particles' energy dissipation. The cumulative size distributions of fragments that are larger than the constituent grains form power-law segments resembling those formed by the fragmentation of homogeneous targets.