You have to be careful what you say to people. When NASA or someone else says that the Perseverance rover will be looking for fossil evidence of ancient life, the uninformed may guffaw loudly. Or worse, they may think that scientists are looking for actual animal skeletons or something.
NASA’s Mars 2020 Rover is heading to Mars soon to look for fossils. The ESA/Roscosmos ExoMars rover is heading to Mars in the same time-frame to carry out its own investigations into Martian habitability. To meet their mission objectives, the scientists working the missions will need to look at a lot of rocks and uncover and understand the clues those rocks hold.
To help those scientists prepare for the daunting task of analyzing and understanding Martian rocks from 160 million km (100 million miles) away, they’ve gone on a field trip to Australia to study stromatolites.
Back in November 2018, NASA announced that the Mars 2020 rover would land in the Jezero Crater. Jezero Crater is a geologically diverse area, with an alluvial fan of sediment deposited by an incoming river. That sediment may contain preserved ancient organic molecules, and the deposit is clearly visible in satellite images of the Crater.
But the crater holds something else that has scientists intrigued, something that doesn’t show up so clearly in visible light images: a “bathtub ring” of carbonates, which scientists think could hold fossils.
The title of Earth’s Earliest Life has been returned to the fossils in the Pilbara region of Australia. The Pilbara fossils had held that title since the 1980s, until researchers studying ancient rocks in Greenland found evidence of ancient life there. But subsequent research questioned the biological nature of the Greenland evidence, which put the whole issue into question again.
Now a new study of the Pilbara fossils has identified the presence of preserved organic matter in those fossils, and handed the ‘Ancient Life’ crown back to them.
Evidence of water and a warmer, wetter climate abound on Mars, but did life ever put its stamp on the Red Planet? Rocks may hold the secret. Knobby protuberances of rock discovered by the Spirit Rover in 2008 near the rock outcrop Home Plate in Gusev Crater caught the attention of scientists back on Earth. They look like cauliflower or coral, but were these strange Martian rocks sculpted by microbes, wind or some other process?
When analyzed by Spirit’s mini-TES (Mini-Thermal Emission Spectrometer), they proved to be made of nearly pure silica (SiO2), a mineral that forms in hot, volcanic environments. Rainwater and snow seep into cracks in the ground and come in contact with rocks heated by magma from below. Heated to hundreds of degrees, the water becomes buoyant and rises back toward the surface, dissolving silica and other minerals along the way before depositing them around a vent or fumarole. Here on Earth, silica precipitated from water leaves a pale border around many Yellowstone National Park’s hot springs.
Both at Yellowstone, the Taupo Volcanic Zonein New Zealand and in Iceland, heat-loving bacteria are intimately involved in creating curious bulbous and branching shapes in silica formations that strongly resemble the Martian cauliflower rocks. New research presented at the American Geophysical Union meeting last month by planetary geologist Steven Ruff and geology professor Jack Farmer, both of Arizona State University, explores the possibility that microbes might have been involved in fashioning the Martian rocks, too.
A sizzling visit to El Tatio’s geysers
The researchers ventured to the remote geyser fields of El Tatio in the Chilean Atacama Desert to study an environment that may have mimicked Gusev Crater billions of years ago when it bubbled with hydrothermal activity. One of the driest places on Earth, the Atacama’s average elevation is 13,000 feet (4 km), exposing it to considerably more UV light from the sun and extreme temperatures ranging from -13°F to 113°F (-10° to 45°C). Outside of parts of Antarctica, it’s about as close to Mars as you’ll find on Earth.
Ruff and Farmer studied silica deposits around hot springs and geysers in El Tatio and discovered forms they call “micro-digitate silica structures” similar in appearance and composition to those on Mars (Here’s a photo). The infrared spectra of the two were also a good match. They’re still analyzing the samples to determine if heat-loving microbes may have played a role in their formation, but hypothesize that the features are “micro-stromatolites” much like those found at Yellowstone and Taupo.
Stromatolites form when a sticky film of bacteria traps and cements mineral grains to create a thin layer. Other layers form atop that one until a laminar mound or column results. The most ancient stromatolites on Earth may be about 3.5 billion years old. If Ruff finds evidence of biology in the El Tatio formations in the punishing Atacama Desert environment, it puts us one step closer to considering the possibility that ancient bacteria may have been at work on Mars.
Silica forms may originate with biology or from non-biological processes like wind, water and other environmental factors. Short of going there and collecting samples, there’s no way to be certain if the cauliflower rocks are imprinted with the signature of past Martian life. But at least we know of a promising place to look during a future sample return mission to the Red Planet. Indeed, according to Ruff, the Columbia Hills inside Gusev Crater he short list of potential sites for the 2020 Mars rover.
Steve Ruff paper comparing El Tatio with an early hot springs environment in Gusev Crater