Perseverance landing site — ancient riverbed or lava flow?

NASA’s landing of the Perseverance rover right on target in the Jezero impact crater last month is both a remarkable achievement and a welcome diversion from the Covid-19 pandemic. The landing site is also of great interest to exploration geologists, of whom I am one of many. It is often said that when examining a particular geological feature, especially one from early in Earth’s history, each of us will interpret it differently. Perhaps this is inevitable because critical evidence has often been displaced structurally, covered by younger rocks or erased by erosion.

On Mars, evidence of past geological events is also very far away. Moreover, imaging essential details from an orbiting satellite through the dusty atmosphere is difficult. Despite these impediments, there seems to be general agreement that a 3.5 billion-year-old canyon in Jezero Crater is an ancient riverbed, a bird’s-foot shaped structure at its mouth is a delta and the relatively flat rover landing site facing the delta is the bed of the lake in which the delta was deposited.

While an interpreted wet environment long ago on Mars’ now-parched surface supports the mission’s goal of finding clues to possible primitive life on the planet, the river theory on which the landing site is based does have apparent flaws. How, for example, did the inflowing river breach the 60-metre-high rim of the crater? Why is the delta front reported to have steep, dangerous cliffs rather than the gentle, tapering slope of both ancient and modern deltas on Earth? How have the delta sands avoided major erosion by Mars’ notorious windstorms for 3.5 billion years when even relatively young, 10,000-year-old glacial deltas on Earth have extensive dune fields on their down-wind side? Why does the first photo received from the rover show a boulder-strewn landing site typical of a crater rather than the expected silt-clay plain of an ancient lake bed?

These issues suggest that the fluid that produced the observed canyon and delta-like structure may have been komatiitic lava rather than water. Komatiite (pronounced: ko-mat-ee-ite) is a relatively rare volcanic rock that is highly enriched in magnesium and iron relative to common basalt and also contains significant nickel, although not in a useful metallic form. On Earth, komatiite is produced by partial melting, at extremely high temperature, of the upper mantle which is of a similar composition.

For the superheated komatiite melt to ascend rapidly through the Earth’s thick, cool crust without solidifying and then erupt as a lava flow, a deep-penetrating fault must be present. Major meteorite impacts such as the one that produced the 45-km-wide Jezero Crater are capable of producing such faults, most notably as ring structures around the crater’s rim. This could explain why the canyon in the crater seems to originate at its rim.

While basaltic lava, being relatively cool and viscous, flows over bedrock without disturbing it significantly, komatiitic lava is such a hot, thin and corrosive fluid that it “burns” a channel in the bedrock until the lava cools sufficiently to solidify. The distance that the lava can flow before solidifying increases with the steepness of the slope. The considerable, five km length of the Jezero canyon is consistent with its location on the inner slope of a crater.

A komatiitic eruption, like a Hawaiian-type basaltic eruption, may continue for days or weeks, and thus carve a sizeable flow channel. If erupted on a slope, the flow will branch bird’s-foot style into several smaller, topographically controlled flows as it levels out at the toe of the slope. This both decreases the flow rate and increases the cooling rate. The coolest parts of the flow, the top and front (leading edge), then solidify, forming a crust over the lava, but the ongoing arrival of new lava bulges this solid crust upward and rafts it forward to form a steep flow front similar to that of the Jezero “delta”. Being comprised of solid rock rather than unconsolidated sand, the eroded channel and lava delta would be little-affected by wind, the main erosional force on Mars, and thus tend to be well preserved.

The consumption of certain types of bedrock by a komatiitic lava flow as it erodes its channel may convert the otherwise worthless nickel in the lava into heavy sulphide droplets that pool at the bottom of the flow and solidify into valuable ore deposits. The nickel deposits near Timmins, Canada and Kambalda, Australia were formed in this manner 2.7 billion years ago. Time will tell whether the Perseverance mission finds evidence of water and life in Jezero Crater, or only nickel which on Mars is much less valuable than water.

—Stuart Averill is Chairman of Overburden Drilling Management Ltd., a company he founded in 1974 to develop and apply indicator mineral methods for mineral exploration. He has published several influential papers on indicator minerals and received the PDAC’s Distinguished Service Award in 2003 and the international gold medal of the Association of Applied Geochemists in 2017. He holds a BSc. (Hons.) in geology from the University of Manitoba.