I agree that those surface features were always thought to be caused by liquid water. However, science does like to have two independent lines of evidence point to the same conclusion. We have that now. I think we’re safe moving forward saying there has been, in fact, liquid water on the surface of Mars.
Maybe I should say I’m looking for fossils on Mars, and I can’t believe I’m the only one. I might be wildly speculating here, but I think the odds of finding fossils on the surface is better than finding ongoing biology on the surface. I’m not talking about dinosaur bones, I’m talking about any fossil of any kind. Mars may have had a much stronger magnetic field around herself in times past, this would give some protection from the “caustic” radiation from the sun for any surface life. Now the magnetic field is mostly gone and if life developed, it would now be underground.
What we know from recent data isn’t that many surface features show signs of being formed by surface water flows, which we’ve known for a long time, but that there is strong evidence of water flowing in the present and recent past, especially the recurrent slope lineae. We know this both from observations by the Mars Reconnaissance Orbiter (MRO) and direct inspection of the areas around Mount Sharp by Curiosity indicating recent wet brine flows, as well as water-deposited minerals found by the Opportunity and Spirit rovers. The mechanism by which water is stored and released is still unknown but widely assumed to be subsurface brines (a sort of slushy aquifer with enough salt to prevent water from evaporating due to low vapor pressure).
Fossilization is a relatively rare event even on Earth where precipitation falls regularly and the composition of mud and fine silt is well suited to preserving skeletal and soft tissue. For the most part, fossils are not found “on the surface” except where geological substrates are exposed by seismic and hydrological activity (earthquakes and floods or rivers carving new paths). Mars, of coruse, has no tectonic activity (that we’ve been able to detect) and very modest tides from its two small moons, so the only seismic actiivty is a result of residual stresses in the crust and meteorite impact. Mars does have many canyons, including the magestic Valles Marineris, which appear to have formed as part of seismic and hydrogloic activity in the distant past but the landers and rovers that we’ve sent to date have tended to avoid these features because of the hazards they present, preferring to land in relatively low laying areas with large sand dunes. If there were fossils of extinct life on Mars, the locations we’ve inspected so far are the least likely to exhibit them.
But again, the period that we believe the climate of Mars may have ever have been amenable to life as we know it was relatively short (on close order of 500 million years) which is just not viewed by the majority of evolutionary biologists as long enough to develop complex life. This means that any “fossils” are likely to be akin to Archaea of Earth, trace fossils, or biochemical chemofossils, and the seasonal dust storms which often blanket the low to middle latitudes of Mars would likely destroy or muddle surface traces. In order to find those, we would need to drill into sedimentary substrates in many locations and inspect samples microscopically, which is beyond the means of the MSL, and frankly probably of any mission, including a human crewed one, for the foreseeable future. Coming across fossils randomly on the surface is improbable even with the assumption that life was at one point common and left fossil traces.
Lacking either photosynthetic or geothermal supplies of energy, what would power a hypothetical extant Martian biosystem? Chemosynthetic energy is a possible mechanism, but has to be replenished by some mechanism that has not yet been seen, nor has any evidence of waste products been seen.
Life on Earh took about two and a half a billion years to evolve to use oxygen for respiration (allowing much faster growth and deliberate movement), three and a half billion years before it could produce multicellular life, and closing on four billion years before it produced flowering plants and mobile animals. Potential life on Mars had a small fraction of that time–certainly less than a billion years–to evolve before regular standing surface water disappeared and the atmosphere became the extreme near-vacuum condition that it is today. So, if it would have evolved at a similar rate, we may assume that it evolved to no more complex forms than something like Clostridium botulinum. Even if it could survive in the caustic soil and the highly ionic salt brines of occasional flows, the ability to evolve into something complex enough to leave more than chemical traces is unlikely to the extreme, at least from our perspective of what we seen in Earth-borne life.
If life exists elsewhere in the solar system (and I’m optimistically giving even odds for it) it is most likely in one of the subsurface seats of a Galilean moon of Jupiter (Ganymede, Europa, or if you want to consider sulphur based biochemistries, Io) or Saturn’s larger moons (Titan or Enceladus). Here are the most optimal conditions for life as we might know it, including a liquid medium, available hydrocarbon precursors, and energy (tidal) and enough stability to permit it to evolve.
The oldest fossils on Earth are 3.5 billion years old {Cite} … that’s within your window of opportunity. I thought they put a microscope on the rovers {Cite}. If fossils are there in abundance, then we have a good chance of finding them.
The fossils on Earth that are 3.5 Bya are chemofossils (those formed by chemical reactions between the substrate and the organic material as it breaks down) and stromatolites. These are found by breaking part or drilling into suitable rock formations; when found on the surface of rocks, weathering and oxidation quickly reduce them, and because they are the size of small single celled organisms they can only be seen by using scanning electron microscopy (SEM). The visual instruments on board the MSL are the Mast Camera (MastCam), Panoramic Camera (PanCam, the arm-mounted camera that Curiosity uses to take ‘selfies’), the Mars Hand Lens Imager (MaHLI), and the Microscopic Imager (MI), none of which have the resolution to see microscopic fossils. Curiosity does have a number of instruments for chemical analysis including an X-ray spectrometer, but these are used to look for general signs of volatile and organic (as well as inorganic) materials.
This notion that visible fossils will be found just sitting on the frequently wind-blown and dust covered surface of Mars is just not realistic. That isn’t the way we find fossils on Earth (except in places where seismic or hydrological activity has recently revealed strata) and it is even more unrealistic on the harsh surface of Mars. There is not a single respected xenobiologist I’m aware of who would seriously propose that complex multicellular life is likely to have evolved on Mars; the time, conditions, and presumed environment just do not support the evolution of complex life, and if it were to have evolved it would be expected to be in the context of a vast biosphere that we would expect to see clear traces of.
If life exists, or ever existed on Mars, it is very, very likely to be simple, limited in scope, and is powered by the slow thermal gradients through the subsurface layer in which it lives, which would give it very little impetus for differentiation or vigorous competition. Any fossil evidence is almost certainly microscopic and buried below the surface in the middle of sedimentary layers.
I wonder whether panspermia (life coming from comets or meteoroids) or lithopanspermia (life coming from rocks via planetary ejection. It seems likely (though not certain) that anything hitting mars would have also hit earth, at least within say a million years. I see from wiki that while mars had lots of water 3.8 billion years ago, that was well before the Cambrian explosion on Earth. I wonder whether recurring pools of water that may have existed on mars could have been seeded.
How long did it take for eukaryotes (which I’m heroically assuming could have survived the passage to mars) to evolve into multicellular life? Or maybe we’re back to hoping that tardigrades could have hitched a ride.
There is zero evidence for panspemia; it is purely hypothetical, and without need. Even tardigrades could not survive the months exposure of a minimum time transfer from Earth to Mars, much less the decades to centuries that are a more realistic transfer time for planetary ejection, even of they could have survived the extreme heating and shock conditions. The potential for complex life on Mars is enormously unlikely whether indigenous or imported unless we actually bring it there.
I think we’d be able to recognise life, as long as it was life, but that’s a matter of definition (which itself is controversial) - but we’d be looking for entities that reproduce, consume, have some sort of organisation to them, and are self-contained and distinct from their surrounding ‘non-life’ minerals and geology.
I guess we might not spot a form of life that met most of our definition, but was not self-contained - for example if a ‘living organism’ was actually comprised of a collection of processes distributed geographically across a wide area - it might look like a collection of otherwise natural, non-living phenomena.
But I don’t think that’s likely. We might have trouble fitting anatomical terms to an alien form of life, even down to terms like ‘organ’ and ‘cell’ - but I reckon we’ll know it when we see it, because it needs to be something that is distinct from local geology.
And that’s not to say it would be easy to spot - not all life forms here on Earth are easy to spot, due to camouflage, size, or just that they prefer to remain hidden away.
Shifting goalposts (and keeping the thread alive), I see from wiki that Earth had simple life as old as 4.1 billion years ago and Mars had liquid water 3.8 billion years ago. From 3.8-4.1 billion years ago was the Late Heavy Bombardment, involving lots of meteors crashing into the inner planets.
Is it highly likely that some forms of simple (unicellular) prokaryotes could survive planetary ejection and trips between Earth and Mars? If so, past life on Mars is highly likely.
A bit of an aside, but how do you define “minimum time transfer”? It seems to me that one could get the transfer time arbitrarily low, with a sufficiently-high initial speed.
Chronos: True…but really short transits would require bigger and bigger rockets at both ends of the trip. (And that means sending along a lot of deceleration fuel, and that, coupled with our best rocketry, puts a minimum limit on the transit time.)
Amp up with a nuclear rocket, you could do shorter times. But that isn’t going to happen any time soon…