Mars is often referred to as “Earth’s Twin” because of the similarities the two planets have. In fact, Mars is ranked as the second most-habitable planet in the Solar System behind Earth. And yet, ongoing studies have revealed that at one time, our two planets had even more in common. In fact, a recent study showed that at one time, the Gale Crater experienced conditions similar to what Iceland experiences today.
Since 2012, the Curiosity rover has been exploring the Gale Crater in search of clues as to what conditions were like there roughly 3 billion years ago (when Mars was warmer and wetter). After comparing evidence gathered by Curiosity to locations on Earth, a team from Rice University concluded that Iceland’s basaltic terrain and cool temperatures are the closest analog terrain to ancient Mars there is.
The study was led by postdoctoral alumnus Michael Thorpe, a Mars Sample Return Scientist with NASA’s Johnson Space Center (JSC). He was joined by Prof. Kirsten Siebach of Rice University (and member of the Science and Operations Teams for Mars the Perseverance and the Curiosity rovers) Prof. Joel Hurowitz an Associate Professor geology at Stony Brook University and a Research Scientist at NASA’s Jet Propulsion Laboratory.
Curiosity image showing sedimentary rock at Gale Crater that likeley formed more than 3 billion years ago. Credit: NASA
For their study, the team examined data collected by Curiosity since it landed in the Gale Crater in 2012, which provides insight into the chemical and physical states of sedimentary deposits that formed in the presence of water. By comparing the chemistry of these mudstone samples to similar formations on Earth, they were able to reconstruct what conditions were like upstream of the crater, where the sediment erosion took place.
While it is well-established that the crater once contained a standing lake, the climate conditions that led to its formation is still the subject of scholarly debate. Whereas some theorize that Mars was warm and wet roughly 3 billion years ago (and rivers and lakes were common) others think that it was cold and dry and that glaciers and snow were common.
After examining the evidence, Thorpe and his team determined that temperature played the biggest role in how mudstone was formed from sediment deposited by ancient streams and weathered by the climate. As Thorpe explained in a Rice University press release:
“Sedimentary rocks in Gale Crater instead detail a climate that likely falls in between these two scenarios. The ancient climate was likely frigid but also appears to have supported liquid water in lakes for extended periods of time.
“On Earth, the sedimentary rock record does a fantastic job of maturing over time with the help of chemical weathering. However, on Mars we see very young minerals in the mudstones that are older than any sedimentary rocks on Earth, suggesting weathering was limited.”
Artist rendition of how the “lake” at Gale Crater on Mars may have looked millions of years ago. Credit and copyright: Kevin Gill
For comparison, the team conducted direct studies of basaltic formations found in Iceland and Idaho, and consulted studies of similar sediments from a range of climates around the world – from Antarctica to Hawaii – where conditions are known to vary considerably. They then made a comparative analysis using the standard geological tool known as the chemical index of alteration (CIA).
This method allows geologists to infer past climate conditions from chemical and physical weathering of a sample. In the end, they determined that Iceland’s basaltic terrain and cool weather were the best match for the floor of the Gale Crater and Mount Sharp. The similarity between formations that over 3 billion years old on Mars and sediments found in rivers and lakes in Icelandic today was actually surprising to the team.
In effect, the simi;arity is only possible because rocks on Mars have experienced so little weathering since 3 billion years ago (and are therefore so well preserved). Said Siebach, who will be an operator for the Perseverance rover after it touches down in February:
“Earth provided an excellent laboratory for us in this study, where we could use a range of locations to see the effects of different climate variables on weathering, and average annual temperature had the strongest effect for the types of rocks in Gale Crater. The range of climates on Earth allowed us to calibrate our thermometer for measuring the temperature on ancient Mars.”
“As water flows through rocks to erode and weather them, it dissolves the most soluble chemical components of the minerals that form the rocks. On Mars, we saw that only a small fraction of the elements that dissolve the fastest had been lost from the mud relative to volcanic rocks, even though the mud has the smallest grain size and is usually the most weathered.”
A river-fed sedimentary plain in Iceland bears resemblance to what might have fed Mars’ Gale Crater more than 3 billion years ago. Credit: Michael Thorpe
These results place limits on the average annual temperature change on Mars when the Gale Crater was still home to a lake. Had it been warmer, more of the water-soluble elements in the sedimentary deposits would have been flushed away. This stands in stark contrast to conditions in the Gale Crater today, where Curiosity has recorded temperatures that went from ?90 to 0 °C (?130 to 32 °F) in the course of a Martian year (687 Earth days).
The results also indicate that the climate shifted over time as fluvial processes (flowing water) continued to deposit sediments in the crater – going from Antarctic-like conditions to more Icelandic conditions. While this study focused on mudstone deposits in the lowest and most ancient part of the crater, other studies that have looked into other areas have shown similar results.
In short, all of these studies indicate that the Martian climate probably fluctuated and became drier with time. In parallel, says Siebach, climate change (especially in Iceland), may shift where the best places on Earth for studying past and present conditions on Mars are located:
“This study establishes one way to interpret that trend more quantitatively, by comparison to climates and environments we know well on Earth today. Similar techniques could be used by Perseverance to understand ancient climate around its landing site at Jezero Crater.”
Orbital picture of the Jezero crater, showing its fossil river delta. Credit: NASA/JPL/JHUAPL/MSSS/BROWN UNIVERSITY
This research was made possible thanks to support provided the NASA Solar System Workings program – which issued a NASA postdoctoral fellowship to Thorpe – and the David E. King Field Work Award issued by Stony Brook University. The study, “Source?to?Sink Terrestrial Analogs for the Paleoenvironment of Gale Crater, Mars,” recently appeared in JGR Planets, a journal maintained by the American Geological Union (AGU).
Further Reading: Rice University, JGR Planets