Tsunamis are a recurring theme in disaster movies. The Impossible depicts a family’s struggle to survive in the aftermath of the 2004 Indian Ocean tsunami. Deep Impact, San Andreas, The Day After Tomorrow, and 2012 all showcase enormous tidal waves that threaten to destroy civilization. Exodus: Gods and Kings uses a tsunami as part of God’s judgment on Egypt. Recorded tsunamis on Earth typically have waves a few tens of feet high, but the apocalyptic waves of cinema reach more than a football field in height. Recent studies show that these cataclysmic events actually happened on Earth’s neighbor Mars.
Finding Evidence of Tsunamis on Mars
Elevation maps of Mars reveal an extensive lowland region in the northern hemisphere. Around 3.4 billion years ago, the surface of Mars resembled a cold, dry desert. However, the planet had large subsurface aquifers that contained the bulk of Mars’s water. For unknown reasons, it appears that these aquifers catastrophically burst, spewing massive quantities of water across the planet. Presumably, this water then quickly flowed toward the northern lowlands forming a large ocean. Some questions have been raised about this scenario, though, due to the lack of visible shorelines in all the expected locations.
Figure 1: The boxed region shows the location of recently discovered tsunami deposits. Image credit: NASA/JPL/USGS
Using data from the THEMIS instrument on board Mars Odyssey, scientists found evidence of two different tsunamis that covered the ancient shoreline. Zooming in on the boxed region of the topographic map (see figure 1), scientists observed features characteristic of tsunamis on Earth, only on a much grander scale (see figure 2). The older formations, located farther from the lowlands, have vast debris fields extending hundreds of kilometers inland and containing boulders up to 10 meters across. Furthermore, channels cut through the debris field as expected from the water retreating back to an ocean. Lacking a source of water input, the ocean would recede over time. Evidence of a later tsunami shows massive ice-rich lobes indicative of waves flowing inland and then freezing before they have time to retreat.1 This second tsunami occurred during much colder conditions akin to deep winter environments. The evidence indicates both tsunamis resulted from asteroid or comet impacts.
Figure 2: Images show tsunami deposits as well as regional reconstruction of a receding 3.4-billion-year-old ocean on Mars. Image credit: NASA/Alexis Rodriguez
What Does It All Mean?
The existence of massive tsunamis in Mars’s distant past highlights some important ideas. First, they provide strong evidence for the presence of a large Martian ocean roughly 3.4 billion years ago. The tsunamis could not have happened without an ocean. And scientists now have an explanation for the lack of expected shoreline features—they are covered with the debris fields deposited by the tsunamis.
Second, these types of events would also happen on Earth, but the active geological processes occurring on Earth remove all the requisite evidence. Scientists can measure the rate of large impacts, but any tsunami effects wear away too quickly. Additionally, measured rates from Earth, Mercury, and Mars serve as independent checks of formation and impact models for the solar system.
Third, and perhaps most excitingly, these tsunami deposits provide future exploration sites for determining whether Mars ever hosted life or not. The common theory says that where we find water, life will surely follow. This principle is incredibly difficult to test but plays a significant role in how people talk about the vast number of exoplanets that exist. The capacity to measure existing life on exoplanets is decades away, but we already have the ability (if not the funding) to study our solar system’s planets for both existing and past life. The ancient Martian tsunamis produced fields littered with debris from a prime environment where life might have existed—namely, the bottom of a shallow ocean. Additionally, the debris fields would have protected the shoreline regions from the hostile Martian surface environment.
Tsunamis often wreak havoc and bring destruction. With these discoveries on Mars, they also bring the possibility to answer the question: How rare is Earth’s capacity to support life? Furthermore, tsunamis provide a glimpse of the awesome forces at work in our universe, solar system, and planet. It reminds me that (borrowing a phrase from C. S. Lewis) Earth may not be a safe place, but it is good!