Introduction: The Spirit rover traversed across over 7 kilometers of Martian terrain in Gusev Crater [1]. Over the course of its mission, the rover encountered several small circular depressions thought to be formed by impact cratering [2, 3]. Small craters below diameter, D < 5.0 meters are difficult to distinguish with even the best HiRISE imagery, which has a resolution of ~25cm/pixel. Rover imagery provides a unique opportunity to survey the smallest craters that form on the Martian surface. These small impactors are important because they serve as indirect geological evidence of the recent conditions of the Martian atmosphere [4 6]. The obliquity, or tilt, of Mars is theorized to be unstable and to fluctuate semi-periodically [7]. Similar to Earth, these changes in planetary tilt can produce dramatic changes in the atmosphere [8]. When Mars is at a lower obliquity (0-30 degrees), the poles are exposed to less sunlight in the summers and are able to “freeze” out more carbon dioxide from the atmosphere and store it in polar ice. During periods of higher obliquity angle (30-60 degrees), the poles are exposed to sunlight longer and more CO2 sublimates, contributing to the density of the atmosphere. These fluctuations in obliquity cause corresponding changes in atmospheric density which in turn, changes the number of craters being formed at the surface [4, 8]. Cratering Mechanics: When the density of the Martian atmosphere is thicker, it is able to filter out more incoming projectiles from space that could form a small crater at the surface if it is travelling at hypervelocity when it impacts [4, 5, 8]. When the Martian atmosphere is less dense, more incoming projectiles survive ablation and declaration to reach the surface with enough velocity to form a crater [5]. Fragmentation is also a key factor in determining impactor survivability through an atmosphere [9]. However, fragmentation occurs less in smaller, D < 5 m crater forming impactors while being more common in larger, D > 5 m crater forming impactors [8]. Another consideration is the influence of secondary craters in the crater catalog. Secondary craters are craters formed from ejecta emitted from a larger, primary impact on the planet’s surface. Secondary craters are not an accurate reflection of cratering rates as one primary impact can create hundreds or thousands of secondary impacts. There is ongoing debate in the literature of how to account for secondary cratering [10, 11]. For this study, which focuses on D < 6 m craters, the craters are approximately 100 Ma or less with most smaller craters under 1.0 meters in diameter being < 20 Ma, given current erosion rate estimates [12]. Additional evidence of secondary cratering such as rays or clustering would be more apparent given their young ages. It is not obvious if the majority of these small craters are primary craters or distant secondaries, which are secondary craters that travel further from their original location and have a stronger resemblance to primaries. The contamination of distant secondaries in this catalog cannot be ruled out due to substantial evidence of secondary cratering in Gusev Crater. Methods: Crater candidates include craters near the rover with a long axis of less than six meters in length. Documented crater candidates are identified using compellations of the rover’s panoramic and navigation camera’s landscape photos. Midnight Planets, a program that combines images, data, and text from the Mars Rover missions, including Spirit, is used for this identification. After identifying a crater, the crater’s azimuth, distance from rover, short axis lengths, and long axis lengths are measured with the location and ruler tools in Analyst’s Notebook. For crater candidates that are questionable, Analyst’s Notebooks profile tool is used to draw a topographical profile of the crater. If the profile showed the crater to have a circular depression, it is added to the crater data.

On November 5-8, 2019, the "Mars Extant Life: What's Next?" conference was convened in Carlsbad, New Mexico. The conference gathered a community of actively publishing experts in disciplines related to habitability and astrobiology. Primary conclusions are as follows: A significant subset of conference attendees concluded that there is a realistic possibility that Mars hosts indigenous microbial life. A powerful theme that permeated the conference is that the key to the search for martian extant life lies in identifying and exploring refugia ("oases"), where conditions are either permanently or episodically significantly more hospitable than average. Based on our existing knowledge of Mars, conference participants highlighted four potential martian refugium (not listed in priority order): Caves, Deep Subsurface, Ices, and Salts. The conference group did not attempt to reach a consensus prioritization of these candidate environments, but instead felt that a defensible prioritization would require a future competitive process. Within the context of these candidate environments, we identified a variety of geological search strategies that could narrow the search space. Additionally, we summarized a number of measurement techniques that could be used to detect evidence of extant life (if present). Again, it was not within the scope of the conference to prioritize these measurement techniques-that is best left for the competitive process. We specifically note that the number and sensitivity of detection methods that could be implemented if samples were returned to Earth greatly exceed the methodologies that could be used at Mars. Finally, important lessons to guide extant life search processes can be derived both from experiments carried out in terrestrial laboratories and analog field sites and from theoretical modeling.

Introduction: Calcium sulfate veins are remnants of groundwater that once was present in the subsurface of Mars. Hence these veins, and their major, minor, trace chemistry, hydration, and mineralogy are windows into the past subsurface aqueous processes of ancient Mars. The conditions inferred from the study of veins are key to understanding the long-term habitability of the martian subsurface. Veins are ubiquitous in the Murray formation, phyllosilicate rich lacustrine mudstone deposits in Gale crater. Vein morphology and abundance, hydration, minor chemistry, and mineralogy has been well-documented [1–9].

Introduction: The obliquity of Mars has undergone semi-periodic, quasi-chaotic fluctuations throughout the planet’s history that have subsequently resulted in atmospheric density fluctuations [1], [2]. Periods of high obliquity expose the poles to longer amounts of sunlight and shorter amounts at low obliquity. The exposure of the Martian poles allows for greater sublimation of the reservoir of CO2 ice contributing more CO2 to the atmosphere. During periods of higher obliquity, the increase contribution of CO2 increases the atmospheric pressure of Mars and alters the interaction of projectiles encountering the atmosphere which form the smallest craters on Mars. Small craters are a geologic reflection of the atmospheric density [2]. The distribution of the smallest craters on a surface can determine if there have been atmospheric fluctuations over the lifespan of small craters. We are reporting here our improvements to the data since the work of Hoffman et al. [3]. Methods: The objective of this study was to identify and measure all crater candidates near the rover. The primary method for identifying craters was OnSight, an augmented reality of the traverse constructed with images from the mission. OnSight is compatible with a Hololens and has a web-based version. Being able to see the traverse with a 3D headset proved to be the most useful tool to find small craters of D < 1 m. After a crater was identified, it was measured with the ruler tool in OnSight. Candidates that were harder to distinguish as craters we also analyzed in Midnight Mars, a program that collects anaglyphs of images from the mission. If the crater appeared to be a circular, bowl-shaped depression, it was considered to be a more favorable crater candidate. If a crater was still questionable, Analyst Notebook, another tool to review data from the mission, was used to draw topographical profiles of craters to see if there was a circular depression. If a candidate was still questionable after these methods were implemented, it was not included. Crater Mechanics: Objects that encounter planetary atmospheres undergo some degree of deceleration, ablation, and possibly fragmentation as they travel to the planet’s surface [4], [5]. Smaller objects that result in D < 5 m primary impact craters are greatly influenced by deceleration and ablation but are not likely to fragment which would result in a primary impact cluster [5], [6]. Most small objects are slowed down through energy loss to the atmosphere to speeds below hypervelocity or ablate completely before the projectile can impact the surface [5]. Despite the fact that most small primary projectiles vaporize completely or are decelerated below hypervelocity, there are still objects that survive and form the smallest hypervelocity primary craters. Atmospheric affects are dependent on whether it is an iron or stony meteorite. Fragmentation of small primary projectiles is not likely to occur because smaller impactors are more homogenous, contain less fracturing, and have a greater bulk strength [5]. For present day Martian atmospheric conditions, the smallest primary crater theorized to be able to form is D = 25 cm [5], [7], [8]. Crater Catalog: Over the first 2119 sols, a total of 156 craters were found along the traverse using the rover’s imaging capabilities. Of the 156 total craters, 126 were D < 5 m, 26 were D < 1 m, and 5 were D < 0.5 m. The smallest crater measured was D = 0.33 m.