The most likely scenario is that it would be microbes, but even microbes would be pretty amazing if they turned out to be based on something other than DNA/RNA or proved sufficiently distinct from Earth life to suggest a second abiogenesis event.
The interesting part to detect a second abiogenesis event is the translation table from DNA to amino acids https://en.wikipedia.org/wiki/Genetic_code It's almost blocked, but as far as we know, the blocks are random. And there are even a few small variants here on Earth, that suggest in a second abiogenesis event it would work with a different randomization.
I wrote a comment with more details a long time ago https://news.ycombinator.com/item?id=34229988
Self nitpicking: The DNA could be twisted in the oposite direction. As far as we know it should work if you have the "mirrored" version of all the helper proteins and stuff. This could be a strong evidence of a second abiogenesis event.
On the nucleotide side there are synthetic creations like Hachimoji DNA [1] and xDNA [2] and a grand total of over 500 (!) amino acids have been found in nature but only 22 are encoded in DNA. IIRC it's mostly a question of the initial conditions on Earth, specifically the acidity and temperature that decides the possible chemistry available to evolving life.
(Are long codons a problem? We have codons of length 3 but the last base is somewhat ignored. Is it to hard to have codons of length 4 or 5? With 4 bases it would be a problem because you would need too many tRNA, but with 2 bases the number of tRNA is similar to ours.)
Anyway, even if there is a unknown biology law that says that DNA and proteins are inevitable and that confuses us when we detect a second planet with life, if they have an independent origin there is absolutely no way that they have a "genetic code" that can be confusing.
You just need one rock with live microbes to splash into the Europa ocean, and over 4 billion years, many such rocks must have been knocked off Earth.
> In September last year, a team of scientists launched a squad of tiny animals into space aboard a Russian satellite. Once in orbit, the creatures were shunted into ventilated containers that exposed them to the vacuum of space. In this final frontier, they had no air and they were subjected to extreme dehydration, freezing temperatures, weightlessness and lashings of both cosmic and solar radiation. It’s hard to imagine a more inhospitable environment for life but not only did the critters survive, they managed to reproduce on their return to Earth.
[0] https://www.nationalgeographic.com/science/article/tardigrad...
https://www.esa.int/Enabling_Support/Space_Engineering_Techn...
The radiation around Jupiter is... really crazy. Like, not around-orbit-of-earth crazy, way worse.
On the other hand, landing on the surface of Europa doesn't get you much. There's likely several km of ice and insane intense radiation before there's possibly anything looking like a habitable zone for earth-life.
"Surviving" in harsh environments until they get somewhere nicer is their entire purpose, and they're very good at it.
I think it's extremely likely that we'll not only find life is very common, but that many species found on earth are also found on other planets.
They even made a TV show about this back in the 1980s, called "V".
but sharks have been sharks for 400 to 450 million years.
I'm sure sharks are terrestrial in origin, but that's an intergalactic timescale during which very little evolutionary change occurred, or was necessary, for a particular category of creature.
maybe tardigrades have been tardigrades for 450 million years too.