NASA has confirmed the existence of a large liquid water reservoir beneath the surface of Mars, based on radar data from the Mars Reconnaissance Orbiter analyzed over a three-year period. The reservoir, located approximately 1.5km below the surface near the Hellas Planitia basin, is estimated to be at least 20km in diameter and potentially much larger. The finding, published in Science, represents the most significant discovery in Mars exploration since the confirmation of ancient riverbeds — and the most compelling evidence yet that Mars may harbour life today.
How It Was Discovered
The SHARAD (Shallow Radar) instrument aboard the Mars Reconnaissance Orbiter detected anomalous radar reflections consistent with liquid water during 847 orbital passes over the region. The signal pattern — high radar reflectivity at a consistent depth — matches the signature of subglacial lakes on Earth, such as Lake Vostok beneath the Antarctic ice sheet.
The three-year analysis period was necessary to rule out alternative explanations. Radar reflections at depth can be caused by several geological features, including certain types of volcanic rock and CO₂ ice deposits. The research team, led by Dr. Elena Petrov of NASA's Astrobiology Institute, systematically eliminated each alternative through a combination of thermal modelling, comparison with known geological features, and analysis of the reflection's polarisation characteristics. The liquid water interpretation is now considered the only explanation consistent with all the available data.
The discovery builds on earlier work by Italian researchers who identified a similar radar signature near the Martian south pole in 2018. That earlier finding was controversial because the proposed mechanism for keeping water liquid at the south pole — geothermal heating — seemed implausible given what was known about Mars's interior. The Hellas Planitia reservoir is located in a region with stronger evidence of geothermal activity, making the liquid water interpretation more physically plausible.
Why It's Liquid
Mars's surface temperature averages -60°C, far too cold for liquid water. The reservoir is kept liquid by a combination of geothermal heat from the planet's interior and the presence of dissolved salts, which lower the freezing point of water significantly. Similar hypersaline lakes exist on Earth and are known to support microbial life.
The salt concentration required to keep water liquid at Martian subsurface temperatures is high — potentially approaching saturation. This raises an important question about habitability: while hypersaline environments on Earth do support life, the organisms that thrive in them are highly specialised extremophiles. If Martian life exists in the reservoir, it would need to be adapted to conditions more extreme than anything in Earth's biosphere.
The geothermal heat source is also significant beyond its role in keeping water liquid. Geothermal systems on Earth — hydrothermal vents, hot springs — are among the most productive ecosystems on the planet, supporting entire food webs that derive energy from chemical reactions rather than sunlight. A Martian geothermal system could, in principle, support a similar chemosynthetic ecosystem entirely independent of the sun.
Implications for Life
"This is the most significant discovery in Mars exploration since the confirmation of ancient riverbeds," said Dr. Petrov. "Liquid water is the prerequisite for life as we know it. We now have a confirmed location where the conditions for life could exist on Mars today — not in the ancient past, but right now."
The astrobiological implications extend beyond Mars. The discovery strengthens the case for liquid water oceans beneath the icy surfaces of Europa and Enceladus — moons of Jupiter and Saturn respectively — and by extension for the possibility of life in those environments. If liquid water can persist beneath the surface of Mars, a planet that lost its magnetic field and most of its atmosphere billions of years ago, the conditions for subsurface liquid water are clearly more robust than previously assumed.
The discovery also changes the risk calculus for Mars exploration. Any future mission that could potentially contaminate the reservoir — through drilling, for example — would need to meet the most stringent planetary protection standards. NASA's Office of Planetary Protection will need to develop new protocols for missions targeting the Hellas Planitia region.
The Geological Context
Hellas Planitia is the largest confirmed impact crater in the solar system, approximately 2,300km in diameter and 7km deep. The basin's depth means atmospheric pressure at its floor is significantly higher than the Martian average — high enough that liquid water could theoretically exist on the surface under the right temperature conditions. The subsurface reservoir is consistent with a geological history in which Hellas Planitia was once a lake or shallow sea, with water gradually retreating underground as the climate cooled.
The region also shows evidence of relatively recent volcanic activity — within the last 100 million years, geologically recent by planetary standards. This volcanic history is consistent with the geothermal heat source required to maintain liquid water, and suggests the reservoir may have been continuously liquid for an extended period — long enough, potentially, for life to have originated and adapted to the environment.
What Comes Next
NASA's Mars Sample Return mission, currently in development, will not reach the reservoir — it's designed to collect surface samples from Jezero Crater. A dedicated mission to drill to the reservoir and sample the water directly would require a new mission class, likely not launching before 2035. But the discovery fundamentally changes the scientific priority of Mars exploration.
The technical challenges of such a mission are formidable. Drilling 1.5km through Martian rock requires technology that does not yet exist in a form suitable for planetary deployment. The drill would need to operate autonomously, in a vacuum, at temperatures that swing by 100°C between day and night, without any possibility of maintenance. And the samples would need to be returned to Earth without contaminating them — or contaminating Mars with Earth organisms carried by the drilling equipment.
Despite these challenges, the scientific community is already mobilising. A workshop convened by the International Astronomical Union in the weeks following the announcement identified subsurface water access as the top priority for the next decade of Mars exploration. Funding proposals are being developed at NASA, ESA, and JAXA. The question is no longer whether to go — it's how fast we can get there.
