Mars is the only planet we know of that used to be more habitable than it is now. Though NASA is working towards putting humans on Mars in the next 50 years, the planet would have been far more hospitable to human life in the past. Today, red dust storms cover the planet, and its only water rests in ice caps and deep under the surface. But wide rivers once flowed over Mars, scientists revealed in a new paper Wednesday, and they were massive ones — twice as wide than the rivers found on Earth.

Edwin Kite, Ph.D., first author of the Science Advances paper and a professor at the University of Chicago, tells Inverse that it came as a surprise that early Mars was home to massive rivers and lakes. The Sun makes water cycles work and keeps planets warm enough to hold liquid water. Early Mars orbited far from the young Sun so it’s strange that it was such a watery place.

Understanding exactly how Mars went from a water-filled planet to a dry one, Kite says, contributes to the greater understanding of how planets stay habitable.

“Over the past decade, evidence for habitable environments beyond Earth has become unequivocal,” Kite says. The emerging, complex history of Mars is an opportunity to see if there are simple rules that define the evolution of a planet.

The black line follows a preserved river channel. NASA

Mars’ ancient rivers, the team writes, covered the now-dry planet at least 3.4 billion years ago. Their existence at that point in the timeline means that they flowed as Mars started to lose its atmosphere and dry out. But at the same time, climate-driven precipitation continued, keeping water in the ancient channels.

The authors read this story of ancient Mars in the planet’s sedimentary deposits and in the imprints those rivers and lakes left behind. Image data taken by NASA’s Mars Reconnaissance Orbiter of the paleo-river channels and deltas across Mars allowed them to calculate the intensity of ancient river runoff. While some channels are now too eroded for reliable measures, hundreds are well preserved — stuck in time because Mars erodes very slowly now that it’s in its dry era, and it is tectonically inactive.

Because the rivers were wider than the rivers on Earths, they likely had intense runoff, the authors write. But what they didn’t expect was the revelation that the production of runoff persisted late into the period when Mars transitioned from wet to extremely dry. That’s not to say Mars was never dry. Kite explains that although ancient Mars had these huge rivers, they didn’t have perennial lakes — only seasonal playas.

Though it’s much less wet now, the topography of Mars remains largely unchanged. While plate tectonics creates new mountain belts on Earth, the plates are quieter on Mars,. The only big difference between its past and present topography is that some of its volcanoes are taller now.

While this study demonstrates that massive rivers once existed — and likely persisted during a time when scientists didn’t think rivers could flow — nobody is sure what happened to Mars’ reserve of water. Kite says it might have escaped in the form of atmospheric carbon dioxide (CO2) into space or became lost because of non-CO2 greenhouse warming. For its part, Mars continues to lose water to space today.

As some scientists seek to understand Mars’ past, others are consumed by its future. But drinking water on Mars won’t be an easy task: As of now, it looks like the best option for colonists is harvesting deposits of ice near the planet’s equator.

Abstract:

Mars is dry today, but numerous precipitation-fed paleo-rivers are found across the planet’s surface. These rivers’ existence is a challenge to models of planetary climate evolution. We report results from a global survey indicating that, for a given catchment area, rivers on Mars were wider than rivers on Earth today. This difference is not the result of Mars’ lower surface gravity. We use the scale (width and wavelength) of Mars paleo-rivers as a proxy for past runoff production. Using multiple methods, we infer that intense runoff production of >3 to 20 kg/m2 per day persisted until <3 billion years (Ga) and, probably, 1 Ga. Our improved history of Mars’ river runoff places new constraints on the unknown mechanism that caused wet climates on Mars