On September 28, 2015, NASA reported new evidence that seasonal dark streaks on Mars known as recurring slope lineae (RSL) are linked to hydrated salts, a finding widely interpreted as indicating transient liquid water on the modern Martian surface. The announcement was based principally on spectral data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) aboard the Mars Reconnaissance Orbiter (MRO). Background and observations Recurring slope lineae are narrow, dark markings that appear on steep Martian slopes during warm seasons and fade in colder seasons. Since their discovery in orbital images, RSL have been puzzling because they form in conditions where temperatures and atmospheric pressure make stable surface water unlikely. In the 2015 announcement, researchers reported detecting spectral signatures consistent with hydrated perchlorate salts at several RSL sites. Hydrated salts can lower the freezing point of water and produce brines that remain liquid at colder temperatures than pure water, providing a plausible mechanism for transient liquid flows. Data and interpretation The CRISM instrument detected absorption features at wavelengths associated with water bound in salts—specifically signatures interpreted as indicative of hydrated chlorate and perchlorate minerals—at multiple locations where RSL were active. Team members emphasized that the salt detections were localized to the RSL themselves rather than the broader surrounding terrain. The presence of hygroscopic salts suggested that atmospheric water vapor could be absorbed by salts in the soil to form briny films, or that small amounts of subsurface brine could flow briefly to the surface, darkening slopes and creating the observed streaks. Scientific context and caveats NASA presented this result as strong evidence for transient liquid water processes, but the data did not demonstrate standing bodies of water or large-scale flows. The spectral detections are indirect and limited by CRISM’s spatial resolution and signal-to-noise constraints; they identify hydrated minerals consistent with brines but do not directly image liquid water. Subsequent and concurrent research has continued to debate the relative roles of dry granular flows, sublimation-driven processes, and briny liquids in producing RSL. Some researchers emphasize that darkening could also arise from changes in grain size or surface roughness without requiring liquid water. Implications If RSL involve transient brines, the finding has implications for present-day Martian geologic and chemical activity, potential habitats for microbial life (by providing transient liquid water and dissolved nutrients), and planetary protection policies for future missions. Because perchlorate salts are oxidizing and can be toxic to some life forms, their presence complicates assessments of habitability. The discovery also influenced planning for rover operations and landing-site considerations, especially regarding contamination avoidance and the preservation of potentially habitable environments. Reception and follow-up The 2015 announcement generated significant media and scientific interest. NASA and the research teams called for further observations and modeling to clarify the RSL mechanism. Follow-up orbital imaging, laboratory experiments simulating Martian conditions, and expanded spectral analyses have been used to test brine-based and alternative dry-flow hypotheses. The debate over the precise nature of RSL remains active in the planetary science community, with consensus that hydrated salts are present at some RSL sites while the role of liquid water in forming all RSL is not universally settled. Conclusion The September 28, 2015, NASA statement marked a notable step in understanding active surface processes on Mars by linking recurring slope lineae with hydrated salts consistent with transient briny flows. The evidence strengthened the case that liquid-related processes can occur under contemporary Martian conditions, while leaving open questions about frequency, volume, and habitability implications that continue to motivate research.