The distinction between (liquid) water and ice is pretty significant for astrobiology. I feel more like we are very quickly reaching the point where we have established that our solar system is barren of any (macroscopic) life[1]. And while we might be able to detect existence of life on other systems (by some chemical signatures), I very much doubt that we are going to see any more direct discoveries of extraterrestrial life during our lifetime simply due the vast interstellar distances involved.
[1] Sure, even micro-organisms would be a major discovery, but ultimately imho of limited interest and impact.
The difference between nothing and micro-organisms is much greater than the difference between micro-organisms and macro-organisms. The discovery of non-terrestrial-origin micro-organisms in our solar system would be massively significant.
Why? It seems to me the requirement is to have a mass with escape velocity from the parent body, and then have it’s orbit intercept another. The delta v required for the latter is symmetrical, from mars to earth. Hence, if a rock reaches escape velocity it would seem to me to be as easy to have pieces of earth rain on mars as it is to have mars fragments rain on earth. The impact on earth to cause ejecta to have escape velocity would probably need to be larger than on mars.
The dV to get off Mars and to interplanetary transfer is about half that from Earth. I would expect the ejection energy of impact debris particles to scale with the square root of the impact energy, so you'd need four times the impact energy on Earth to achieve the same level of debris ejection as on Mars, and that's not counting the effect of Earth's atmosphere on cushioning and ameliorating such impacts.
The dV to then end up on either planet doesn't matter because aerobraking and lithobraking do all the work for you.
> The impact on earth to cause ejecta to have escape velocity would probably need to be larger than on mars.
significantly larger, because it has to get through our atmosphere once on the way in, and then the ejecta has to do it again on the way out, keeping in mind that drag is proportional to velocity squared, and that whatever ejecta is making this trip has to find a way not to get cooked into sterility by impact energy or friction with the air....
Too many extra factors are in play, each of which adds an exponent to the unlikelihood. But that's just my uneducated opinion, I'm certainly open to discussion.
The other answers here so far are philosophical, I'll give you one that's scientific. Most of the biochemistry of life is required just to have life at all. The incremental increases in complexity of the biochemistry for complex or multi-cellular life is relatively modest. Therefore you get most of the bang for your biochemical buck just by having life at all.
Because it poses new interesting questions: If extra-terrestrial life exists, do we have a common ancestor? This is the theory of panspermia. Or does life spontaneously arise when conditions are right, but with variation?
It would be particularly exciting to discover life that is similar to Earth but with an opposite chirality, or a genetic code that encodes amino acids using different base pairs.
I am willing to bet that whatever is eventually found will be remarkably similar to what we are familiar with on Earth, assuming a liquid-water environment, and the constraints that implies. Single-celled life appears to have arisen so rapidly that it predates the end of the Late Heavy Bombardment, after all.
(I'd be thrilled to be wrong about this, of course, just as I would be fascinated with any xenobiological discovery at all. But I do think the 'endless forms most beautiful' are also strongly constrained by simple physics, and thus the solutions found by an evolutionary random walk will resemble what we already know.)
This is what I suspect with microorganisms as well. Macroorganisms are much more complex and they have common features that seem very arbitrary like moving via cylindrical appendages called legs, or having faces that feature 2 eyes, a nose, and a mouth. It makes me think that it's much more likely to see something wildly different between macroorganisms on Earth and elsewhere than between microorganisms. It would be cool to see other forms of movement, other forms senses, other forms of communication, other forms of anatomy, etc.
EDIT: It also makes me wonder how different an ecosystem could be. Imagine one that did not require macroorganisms eating other macroorganisms, like ones that feed on heat or wind.
EDIT 2: Maybe mating requires 3 genders, etc. There seems to be a lot of room for variation.
There's even wild differences between macroorganisms
on Earth.
Invertebrates are much more relaxed about number of eyes etc.
We can go from "humans are the optimal form!", to "well, some kind of mammal", "c'mon, it's got to be a vertebrate", then get into the Cambrian experiment, go back further to different DNA (RNA anyone), then to different metabolic pathways, then to different organic biochemistry, then to non-carbon... and why should life have a chemical basis, anyway? So chemocentric.
How can we tell when he go from too anthropocentric to too anti-anthropocentric? We have guesswork only. We're like a child rebelling against its parents.
It seems to me, that elephants, dolphins, parrots, octipuses, and even spiders could have undergone the rapid brain-size increase that happened to some apes. (We don't even know why it happened to us.) They might still do so, in a few million years.
On the other hand, some physical features have evolved multiple times independently. The eye is one key example. IMO this is a strong indication that creatures that evolved in places other than Earth probably also have something we'd recognize as eyes.
Note that some earth creatures - deep sea and underground - have almost lost eyes. Like [blind mole rats](https://wikipedia.org/wiki/Spalax). They have appropriately alien-like nose-antennae.
The rapid emergence of single-celled life on Earth is not necessarily evidence that it was an easy step for evolution. See the discussion surrounding “conditional on success, all the hard steps, no matter how hard, take about the same time” in this article: http://mason.gmu.edu/~rhanson/greatfilter.html
A lot of scientists are headed in the other direction: the discovery that planets are common, and that there are a couple of places in the Solar system which have liquid water up against rock make it more likely that extra-terrestrial life exists, even in our own solar system.
[1] Sure, even micro-organisms would be a major discovery, but ultimately imho of limited interest and impact.