[You can click that image and enlarge it by more than 2X. Take a close look at those “leopard spots.”]
Biosignatures are signs of current or past biological life forms. In the case of potential extraterrestrial life, those life forms are assumed to be microbial. Higher life forms, like plants and animals, requires very specific conditions to evolve. We have discovered extremophile species on Earth, living on the bottom of the ocean, with no sunlight and little oxygen. They “eat” the chemical energy of compounds provided by hydrothermal vents. Others have been found thriving in hot springs, as well as the arid Atacama Desert. This all informs scientists who are searching for proof of extraterrestrial life.
The photo above was taken by NASA’s Perseverance Mars rover last year on July 18, in an ancient riverbed within the “Bright Angel” region of Jezero Crater on Mars (see graphic below). The image was captured using a camera called WATSON (Wide Angle Topographic Sensor for Operations and eNgineering). Other instruments onboard detected carbon-based molecules that are considered the building blocks of life.
The morphology of the “leopard spots” is curious. Their regular shapes, lighter colors, and darker ring at their periphery, are characteristic of several biological processes observed on Earth. Colonies of microbes typically spread outward symmetrically, altering the chemical makeup (and color) of the substrate. The dark rings on the periphery are harder to explain, but can be caused by chemical reactions that release iron and phosphate — energy sources for microbes.
The presence of olivine, seen as greenish crystals, is also curious. Olivine is typically of volcanic origin, and it makes up a significant portion of Earth’s crust. It can hold a lot of water, up to 8900 parts per million (weight). That water is available to microbes via chemical processes. But the questions of when it formed is a puzzle. Did it form before the leopard spots of after? There is no doubt Jezero Crater once held water.
The only way to distinguish geologic vs. biologic origin would be to return a sample to Earth for exhaustive analysis. That’s not likely to happen in the near future due to mission complexity. At this time, all we have to go on is the appearance of those leopard spots.
A cautionary lesson would be appropriate here. On August 6, 1996, a team of researchers led by NASA scientists including lead author David S. McKay announced that a meteorite found on the Allan Hills Far Western Icefield in Antarctica during the 1984–85 season was a rock from Mars. Isotopic analysis confirmed its martian origin. The rock, officially designated ALH 84001, was likely blasted off the planet by a large meteor impact. It spent perhaps billions of years wandering the solar system before finally falling to Earth.
The team claimed ALH 84001 contained trace evidence of ancient life on Mars. They used a scanning electron microscope to image sections of the rock. Here’s the photo showing their discovery:
The objects that look like worms were from 20–100 nanometers in diameter — smaller than any terrestrial cellular life known at the time of the discovery. But not smaller than theoretical nanobacteria. So scientists were initially excited about the discovery. Further research showed the structure could be a mineral crystallization artifact, and not a fossil. The analysis is still controversial.
Carl Sagan often cautioned that “Extraordinary claims require extraordinary evidence.” That scientific criterion applies to everything from UAPs to Sasquatch. So we can’t yet claim there is life beyond Earth. Still, those leopard spots are compelling to some scientists given where they were found.
It’s worth noting that there are other forms biosignatures can take. Spectroscopic analysis of the upper atmosphere of Venus (where temperatures would be more temperate than the surface) has indicated the presence of phosphine (PH3). On Earth, most of the phosphine found has a biological origin. But there may also be geochemical or photochemical pathways for its production, so again, the presence of life is not proven.
A biosignature could also take the form of an electromagnetic signal that would not be produced by any natural phenomenon — say, a series of pulses counting out the first ten prime numbers. In fact, back in 1967, before pulsars were understood, the pulsar now known as PSR B1919+21 was picked up on a radio telescope. The strength and regularity of the signals prompted the discoverer, Jocelyn Bell Burnell, to suggest it may be a “beacon” from an extraterrestrial civilization. It was nicknamed LGM-1 (Little Green Men). When other pulsars were eventually discovered, the knickname was dropped.
And finally, though today’s telescopes are woefully insufficient, future telescopes might be able to image exoplanets in sufficient detail to reveal clusters of lights from cities on the night side of the exoplanet. Earth’s “night lights” are easily visible from the ISS.
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