The Mars Ocean Theory suggests that about one-fifth to one-third of the planet’s surface was covered by a primordial sea approximately 4 billion years ago, according to NASA. Now, scientists are delving into the details of this water loss using data from NASA’s Hubble Space Telescope and the Mars Atmosphere and Volatile Evolution (MAVEN) mission. A study published on July 26, 2024, in Science Advances titled “Martian atmospheric hydrogen and deuterium: Seasonal changes and paradigm for escape to space” explores this atmospheric escape mystery.

Study leader John Clarke of the Center for Space Physics at Boston University explained in a press release, “There are only two places water can go. It can freeze into the ground, or the water molecule can break into atoms, and the atoms can escape from the top of the atmosphere into space. To understand how much water there was and what happened to it, we need to understand how the atoms escape into space.”

Evidence of ancient oceans

The presence of water on Mars was first suggested by early telescopic observations. However, it wasn’t until the Mariner 9 mission in 1971 that direct evidence of water-related features was provided, as noted by Wikipedia. Since then, the Mars Ocean Theory has gained popularity, supported by evidence including ancient shoreline features and the chemical properties of the soil and atmosphere.

Recent topographic maps have revealed a 3.5-billion-year-old shoreline with substantial sedimentary accumulation, according to ScienceDaily. However, challenges to this theory remain, including Mars’ low surface gravity and alternative explanations such as wind erosion.

Present-day water on Mars

While the ancient oceans are long gone, evidence of present-day water on Mars continues to emerge. NASA’s Mars Reconnaissance Orbiter found evidence of intermittent liquid water flows, known as recurring slope lineae. These features are thought to involve briny water, as indicated by the presence of hydrated minerals on slopes.

More recently, the InSight lander’s seismic data revealed large subsurface water reservoirs 6-12 miles deep, as ScienceAlert reported. Despite these findings, scientists believe that the volumes of water were likely considerably greater billions of years ago.

A glimpse into the Martian climate’s history

The aforementioned study published in Science Advances provides new insights into Mars’ water loss. By combining data from Hubble and MAVEN, researchers observed dramatic seasonal variations in hydrogen and deuterium escape rates. These rates peak when Mars is closest to the Sun (perihelion) and plummet when it’s farthest away (aphelion), with 5-20 times higher values near perihelion.

These findings offer a glimpse into the evolution of the Martian climate and the mechanisms driving atmospheric escape over geological timescales. The researchers discovered that the source from the lower atmosphere is a more significant factor in the escape fluxes of both hydrogen and deuterium than the details of the escape processes themselves.

Importantly, the study revealed that Mars’ atmosphere is more dynamic and turbulent than previously thought, with conditions changing on timescales as short as hours. The rapid changes in escape rates suggest that additional energy sources, such as solar wind or chemical reactions driven by sunlight, play a role in the escape process.

The paper concludes, “Overall, the results presented here offer strong supporting evidence for a warm and wet period with an abundance of water on early Mars and a large amount of water loss into space over the lifetime of the planet.”

Implications for life

The presence of liquid water, both past and present, highlights possibilities for life on Mars. According to NASA, the discovery of hydrated salts and briny flows supports the potential for past or present microbial life. These findings are guiding the search for life and informing future exploration efforts to utilize water resources.

Looking ahead, the researchers point to areas for future investigation. They emphasize the need for improved models of Martian atmospheric dynamics and escape processes, which must accurately account for the observed rapid hydrogen escape rates and their dramatic seasonal variations. These models may need to incorporate the influence of superthermal processes, a mechanism suspected to drive accelerated atom loss.

The findings could also enhance our understanding of exoplanetary atmospheres, particularly those of Earth-like planets orbiting M-dwarf stars. By applying the knowledge gained from studying Mars’ atmospheric escape, scientists could refine habitability assessments that consider the role of atmospheric loss and inform inquiries into potential signs of ancient biological or geological activity on the Red Planet.