Do Lunar-like Swirls Occur on Mercury?

Introduction: Lunar swirls are unusual, high-albedo markings found in both the maria and the highlands [1, 2]. These sinuous patches sometimes exhibit dark lanes between bright segments. Swirls have no apparent topographic expression and appear to over-print the surfaces on which they lie. Several origins for lunar swirls have been proposed. These include surface effects produced during relatively recent (<1 My) impacts of meteor swarms [3], a comet coma and nucleus [2], or disrupted comet fragments [4]. Alternatively , the association between swirls and crustal magnetic anomalies has led to the hypothesis that the magnetic anomaly protects the surface from solar wind bombardment [e.g., 5]. Lacking solar wind sputtering and implantation, the swirl has not undergone the normal space weathering (soil-darkening) process to which unshielded areas are subjected [5, 6]. Thus it may be that the presence of a magnetic anomaly preserves a high albedo, even though a magnetically shielded surface would still experience micrometeorite impact. A number of magnetic anomalies are correlated with terranes antipodal to a major impact basin [e.g., 7], and the creation of a crustal magnetic anomaly may involve the amplification of existing magnetic fields by the expanding vapor-melt cloud produced in a lunar basin-forming impact [e.g., 8]. Swirls on Mercury: In their classic paper on the comet-impact origin of lunar swirls, Schultz and Srnka [2] noted that swirl-like markings exist on Mercury. The availability of higher-resolution, higher-quality images from the first two MESSENGER flybys of Mercury [9] now permit us to reexamine the features on Mercury that Schultz and Srnka suggested could be swirls as well as search for additional swirl candidates. Schultz and Srnka Candidates: The "bright loops and swirls" recognized by Schultz and Srnka [2] are near the craters Lermontov and Handel. Lermontov has a relatively high-albedo floor that contains bright streaks in Mariner 10 images (Fig. 1). MESSENGER Narrow Angle Camera (NAC) images reveal these markings as irregular depressions and small unresolved bright spots (Fig. 2). The high-reflectance floor has an anomalous red color [10]. Elsewhere on Mercury, similar high-reflectance, red material associated with irregular depressions has been interpreted to be of py-roclastic origin [11]. Therefore, the high-resolution MESSENGER data demonstrate that no features resembling lunar swirls are present at this location.