The Tijuana River’s polluted waters don’t just contaminate Southern California’s beaches – they also release toxic gases and aerosols that travel far beyond the riverbanks, threatening the health of nearby communities, according to a new study. The Tijuana River Valley, straddling the US-Mexico border, faces a severe and worsening pollution crisis as untreated sewage, industrial waste, and toxic runoff flow into the Pacific, causing prolonged beach closures and persistent environmental health risks. While most concern has centered on direct contact with contaminated water, mounting evidence shows pollutants can aerosolize, becoming airborne and dispersing far beyond the riverbanks. This overlooked pathway means communities may face greater exposure through inhalation than through direct contact with contaminated water. With over half the global population living near waterways, understanding the impact of water pollution on air quality is an urgent yet understudied public health priority.

 

Building on previous research that traced airborne bacteria and chemical pollutants near the mouth of the Tijuana River in San Diego, California, and guided by community reports of foul odors and health symptoms of those who live near the river, Benjamin Rico and colleagues identified a turbulent stretch of the river as a likely hotspot for gas and aerosol emissions. This prompted the authors to use a mobile air quality lab to measure hydrogen sulfide (H₂S) – a toxic gas produced by the breakdown of sewage – as an airborne tracer of water pollution. They found that the record high dry-season flows of 2024 led to a significant spike in H₂S emissions, with nighttime peaks reaching 4500 parts per billion (ppb) – thousands of times above typical urban levels (<1 ppb). According to Rico et al., the findings highlight the impact that turbulent portions of contaminated rivers have on regional air quality. Because existing air quality models omit emissions from polluted rivers and estuaries, incorporating these pathways is critical for accurately predicting health impacts, addressing inhalation risks, and guiding mitigation. Moreover, the exceedingly high H₂S concentrations confirmed the validity of long-dismissed community observations, highlighting the disproportionate burden borne by marginalized communities. “Sustained monitoring, coordinated cross-border efforts, and leadership from federal, state, and local authorities are crucial to finally provide the protection and justice long denied to communities affected by this ongoing environmental and public health crisis,” Rico et al. write.

 

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Locked beneath a single-plate crust, Mars’ mantle holds a frozen record of the red planet’s primordial past, according to a new study of Martian seismic data collected by NASA’s InSight mission. The findings reveal a highly heterogenous and disordered mantle, born from ancient impacts and chaotic convection in the planet’s early history. “Whereas Earth’s early geological records remain elusive, the identification of preserved ancient mantle heterogeneity on Mars offers an unprecedented window into the geological history and thermochemical evolution of a terrestrial planet under a stagnant lid, the prevalent tectonic regime in our Solar System,” write the authors. “This evolution holds key implications for understanding the preconditions for habitability of rocky bodies across our Solar System and beyond.” A planet’s mantle – the vast layer that lies sandwiched between its crust and core – preserves crucial evidence about planetary origin and evolution.  Unlike Earth, where active plate tectonics continually stirs the mantle, Mars is a smaller planet with a single-plate surface. As such, Mars’ mantle undergoes far less mixing, meaning it may preserve a record of the planet’s early internal history, which could offer valuable insights into how rocky worlds form and evolve. Using data from NASA’s InSight lander, Constantinos Charalambous and colleagues studied the seismic signatures of marsquakes to better constrain the nature of Mars’ mantle. By analyzing eight well-recorded quakes, including those triggered by meteorite impacts, Charalambous et al. discovered that high-frequency P-wave arrivals were systematically delayed as they traversed the deeper portions of the mantle. According to the authors, these delays reveal subtle, kilometer-scale compositional variations within the planet’s mantle. Because Mars lacks plate tectonics and large-scale recycling, these small-scale irregularities must instead be remnants of its earliest history. The scaling of Mars’ mantle heterogeneity suggests an origin in highly energetic and disruptive processes, including massive impacts early in the planet’s history, which fractured the planet’s interior, mixing both foreign and crustal materials into the mantle at a planetary scale. Moreover, the crystallization of vast magma oceans generated in the aftermath likely introduced additional variations. Instead of being erased, these features became frozen in place as Mars’ crust cooled and mantle convection stalled.