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It has been speculated that Venus billions of years ago could've had a much different atmosphere with liquid water on its surface and possibly life. Partly thanks to the cooler young Sun. But is it possible for science to find out?

What traces would remain that our probes could measure? AFAIK Venus was completely resurfaced 300-600 million years ago so I suppose there won't be any geological signs of water of the kind found on Mars. And the atmosphere obviously has changed dramatically if Venus was once Earth-like. What should we look for to discover the evolution of Venus? What instruments should we land there, or fly in its atmosphere or orbit?

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My ad-hoc opinion: This wouldn't be the first step of Venus exploration. Geologic in-situ investigation of the resurfacing hypotheses would already be a very challenging mission. Might be, one could find some metamorphic remnants which have survived the last resurfacing, and one could determine the age of rocks. Might be there exist some old layers below the surface, which haven't been molten up completely.

Some kinds of crystals are more resistant to heat than others. They may also contain some records of more ancient epochs. Comparing their isotopic ratios might tells something about the ancient atmosphere, or at least about ancient geology. But conditions on the surface of Venus are very hard for probes.

More feasible would be a detailed isotopic analysis of Venus' atmosphere, as e.g. proposed as VDAP mission concept, or by follow-up balloon missions. A submarine-like ballon diving into the hellish conditions of the lower atmosphere or even to the surface for a short period might be an approach.

A TLS (tunable laser spectrometer) has been suggested to analyse the atmosphere.

May be an additional mass spectrograph would be useful. If short dives close to the surface become feasible, cameras would make sense. A LIBS (laser induced breakdown spectroscope) would make sense only on direct contact with the surface due to the atmospheric absorption. APXS wouldn't probably work due to the heat, unless samples could be taken for analysis higher into the (cooler) atmosphere. IR spectroscopy could possibly work, if cooling can be achieved somehow (e.g. by insulation and adiabatic decompression). If surface samples can be taken during short surface contact, more ways to analyse rocks would become possible. The more instruments, the more weight, the larger the ballon, the larger the probe, the more expensive. But I'm far from elaborating technical or scientific proposals here for space agencies.

Hence VDAP with a small TLS and a QMS (quadrupole mass spectrometer) - as already proposed by hugely experienced scientists - would be a good start.

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Great answer! I just want more... I gather that isotopes of nobel gasses is one way to find out, because they haven't chemically cared much about the changes. Their isotopes basically reveal how much exposure to cosmic radiation they've had i.e. how thick the atmosphere has been. And the highlands which might be much older than most of the surface. High altitude gliders and quick sample returns seem suitable to avoid the hellish environment. Sounds like there're things to be done here, and without 8 year travel times. –  LocalFluff May 8 at 10:26
    
Could a meteorite from ancient Venus contain minerals that must've formed in water? Theoretically, because no venutian meteorite seems to be confirmed and gravity and nearness to the Sun makes it unlikely. Venus' history seems to be a tough case to solve. –  LocalFluff May 10 at 8:26
    
... plus higher escape velocity needs high impact energy making appropriate impacts rare, and asteroids of Venus impacts more than 3 billion years ago should be rarified in the meanwhile, or left the inner solar system. –  Gerald May 10 at 23:59
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