Benjamin D. Boatwright

Dendritic valley networks on Mars have been cited as evidence for a warm and wet Noachian Mars, permitting rainfall precipitation and surface runoff. However, the climatic conditions required to sustain rainfall on early Mars remain debated. In particular, climate models that produce significant rainfall commonly require a denser CO ₂ atmosphere, additional greenhouse warming from reducing gases, and substantial surface water reservoirs. In contrast, some model configurations predict very low global mean annual temperatures, snow accumulation, and ice sheet development in the equatorial southern highlands. In this study, we simulate both endmember scenarios for three well‐preserved, highly dendritic valley networks: Warrego Valles, Terra Cimmeria, and Parana Valles. For the ice‐sheet case, we model meltwater from a fixed terminus, representing an oversimplified ice‐sheet scenario for use as a diagnostic benchmark, and a time‐varying terminus, which is consistent with results from ice‐sheet modeling. For the rainfall precipitation‐runoff scenario, we tested uniform and elevation‐dependent precipitation. We then delineate the model‐produced valley networks and quantitatively analyze the results. Our results show that the advancing and receding ice sheet scenarios driven by obliquity cycles can generate the same morphometric signatures as rainfall. These findings address a key geomorphology‐based objection to the ice‐sheet hypothesis, showing that dynamic ice‐sheet melt cannot be excluded based on valley morphology alone. While rainfall remains consistent with the observed valley network forms, our results demonstrate that ice‐sheet melt is also geomorphologically viable under obliquity‐modulated melting regimes.

The Lunar Reconnaissance Orbiter (LRO) has returned a wealth of remotely sensed data of the Moon over the past 15 years. As preparations are under way to return humans to the lunar surface with the Artemis campaign, LRO data have become a cornerstone for the characterization of potential sites of scientific and exploration interest on the Moon's surface. One critical aspect of landing site selection is knowledge of topography, slope, and surface hazards. Digital elevation models derived from the Lunar Orbiter Laser Altimeter (LOLA) and Lunar Reconnaissance Orbiter Camera (LROC) instruments can provide this information at scales of meters to decameters. Shape-from-shading (SfS), or photoclinometry, is a technique for independently deriving surface height information by correlating surface reflectance with incidence angle and can theoretically approach an effective resolution equivalent to the input images themselves, typically better than 1 m per pixel with the LROC Narrow Angle Camera (NAC). We present a high-level, semiautomated pipeline that utilizes preexisting Ames Stereo Pipeline tools along with image alignment and parallel processing routines to generate SfS-refined digital elevation models using LRO data. In addition to the present focus on the lunar south pole with Artemis, we also demonstrate the usefulness of SfS for characterizing meter-scale lunar topography at lower equatorial latitudes.

Inverted fluvial channels are widespread on Mars, and along with valley networks, they provide evidence of flowing liquid water in the early period of the planet's geologic history. We previously described tributary ridge networks within a closed‐source drainage basin crater in the southern highlands, which we interpreted as inverted fluvial channels that were fed by meltwater from the surface of cold‐based glaciers located in crater wall alcoves. Here, we estimate both the paleodischarge within the inverted channels and the necessary melting rates for discharge sources in glacial alcoves. We find that paleodischarges of up to ∼7,000 m³/s were possible with melting rates exceeding 2,000 mm/hr in an extreme warming scenario, with lower rates potentially representing more gradual glacial ice loss. These end‐member estimates can provide key constraints for further early Mars climate and landform evolution modeling.

Described here is a concept for a variable-altitude aerobot mission to Venus developed as part of the 2020 NASA Planetary Science Summer School in collaboration with NASA Jet Propulsion Laboratory. The Venus Air and Land Expedition: a Novel Trailblazer for in situ Exploration (VALENTInE) is a long-duration New Frontiers–class mission to Venus in alignment with the goals recommended by the 2013 Planetary Science Decadal Survey. VALENTInE would have five science objectives: (1) determine the driving force of atmospheric superrotation, (2) determine the source of D/H and noble gas inventory, (3) determine the properties that govern how light is reflected within the lower cloud later, (4) determine whether the tesserae are felsic, and (5) determine whether there is evidence of a recent dynamo preserved in the rock record. The proposed mission concept has a total duration of 15 Earth days and would float at an altitude of 55 km, along with five dips to a lower altitude of 45 km to study Venus’s lower atmosphere. The instrument payload allows for measurements of the atmosphere, surface, and interior of Venus and includes six instruments: an atmospheric weather suite, a mass spectrometer, a multispectral imager, a near-infrared spectrometer, light detection and ranging, and a magnetometer. Principle challenges included a limitation caused by battery lifetime and low technology readiness levels for aerobots that can survive the harsh conditions of Venus’s atmosphere. This preliminary mission was designed to fit within an assumed New Frontiers 5 (based on inflated New Frontiers 4) cost cap.

A body of geologic evidence suggests that Noachian-aged craters on Mars were modified primarily by runoff from rainfall in a warm and wet early Mars climate. Although melting and runoff of frozen water ice have been suggested as plausible alternatives, supporting geomorphic evidence of Noachian glaciation on Mars has been elusive. We previously identified a Noachian-aged crater in the southern highlands that contained evidence of glacially derived fluvial and lacustrine features but was hydrologically disconnected from its surroundings. This closed-source drainage basin (CSDB) crater and proglacial paleolake provided the first candidate evidence of Noachian cold-based glaciation on Mars. Here, we describe a second nearby CSDB crater that contains similar evidence of glacial melting leading to the formation of proglacial fluvial channels and paleolakes on the crater floor. This new evidence suggests that CSDB formation was occurring episodically at regional scales throughout the Late Noachian and into the Early Hesperian. Recurrent episodes of glacial melting are consistent with climate model predictions of a cold and icy early Mars highlands with limited melting of glacial ice during periods of punctuated warming.

A 54 km diameter Noachian-aged crater in the southern highlands of Mars contains unusually well preserved inverted fluvial channel networks and lacustrine deposits, all of which formed completely inside the crater. This “closed-source drainage basin” (CSDB) crater is distinct from previously documented fluvially breached or groundwater-fed crater basin lakes on Mars. We compare our observations to previously established models of crater degradation, fluvial incision, and topographic inversion on Mars to assess the most likely origins of the water that formed the fluvial and lacustrine features. We favor top-down melting of a cold-based glacier as the source of water in the CSDB crater, which would represent the first examples of proglacial fluvial channels and lakes found on Noachian Mars.

Most Noachian-aged craters on Mars have distinctive morphologic characteristics that suggest they were modified by runoff from rainfall in a predominantly warm and wet early Mars climate. However, melting and runoff of frozen water ice (snowmelt) represents a plausible alternative for fluvial erosion in the Noachian. In recent work, we described a "closed-source drainage basin" (CSDB) crater in Terra Sabaea that contained inverted fluvial channel networks and lacustrine deposits. The crater is not breached by fluvial channels and lacks depositional morphologies such as fans or deltas, which sets it apart from previously described open- and closed-basin lakes on Mars that are hydrologically connected to their surroundings. The lack of hydrologic connectivity, along with additional evidence of remnant cold-based glacial morphologies within the crater, led us to hypothesize top-down melting of a cold-based crater wall glacier as the source of runoff and sediment for the fluvial and lacustrine deposits, which produced one or more proglacial lakes within the crater.Here, we describe the results of a follow-on survey of the region within 500 km of the first CSDB crater. We searched for examples of features that could be interpreted as inverted fluvial channels regardless of their location. Of the 42 inverted channel networks we identified, 19 are located within unbreached craters; 17 are within breached craters with at least one inlet but no outlets; and 6 are located in the intercrater plains. The features are not randomly distributed; rather, they form two distinct groupings, one in the southwest of the study area and another in the east, with very few in the north or west. All but one occurs within an elevation range of 0 to +3 km. There are several previously identified closed-basin lakes within the study area, but none contained inverted channels.The 42 inverted channel systems represent a wide variety of geologic and hydrologic settings. The region has distinctly low valley network density, and the few mapped valley networks in the region are clustered around +2 km elevation. If the fluvial regime were controlled primarily by elevation, and assuming no significant sequestration, lower elevations should have greater overall runoff production due to the accumulation of flow from upslope. The difference between breached and unbreached craters could therefore represent glacial melting occurring within craters (higher elevation) as opposed to significantly upslope of them (lower elevation), which would instead promote runoff and breaching of craters by valley networks.We previously described a single CSDB crater that showed evidence for cold-based crater wall glaciation, sedimentation and proglacial lake formation, but this new work adds a significant body of evidence that such processes were operating at much greater regional scales. While runoff from rainfall is usually considered the most likely mechanism of fluvial erosion in the Noachian, the possibility remains that fluvial erosion could have occurred via snowmelt in a subfreezing ambient climate. We have provided compelling evidence that fluvial and lacustrine features could have formed in such a climate and that Noachian Mars may have been colder than previously believed.