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For Carl, 25 years later
JRehling
post Nov 20 2021, 04:55 PM
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25 years ago this December, Carl Sagan passed away, far too young. For decades, he did more to promote the exploration of space than any other individual, and as new discoveries have been made, I've often thought of how Sagan would have been thrilled by, and enjoyed, them.

Here's a list of ten discoveries that have taken place in the past 25 years that Carl certainly would have loved to have learned about:

1) The prevalence of exoplanets. In 1996, just a few exoplanets had been discovered, and those were mainly very atypical ones. Now we know how prevalent exoplanets of various more common types are, and of the existence of many specific exoplanets of terrestrial size orbiting the nearest stars.

2) Rain, rivers, and seas on Titan. Sagan performed research on just this question, and it is certainly one of the most exciting discoveries on its own merit.

3) A past wetter, warmer climate on Mars. This was speculated about, but not known, in 1996. The detection from orbit of phyllosilicates was probably the most important result clarifying the past history, although of course the MER rovers played a key role as well.

4) The Europan subsurface ocean. This was also a matter of speculation going back far before 1996, but it was the evidence from Galileo, in the years just after that, which provided substantial evidence for it.

5) The Enceladan subsurface ocean. This was not even a matter of speculation, owing perhaps just to the bad lucky that the Saturn system had its southern poles all in long winter night when the Voyagers sailed past.

6) Pluto's diversity. I won't try to do justice to this but it's easily safe to say that Pluto turned out to be more complex and dynamic than one had a right to imagine before we got our one good look from New Horizons.

7) Ice at the poles of Mercury and the Moon. This was first tipped off by radar from Earth, but it's been confirmed since, and is one of – or two of – the discoveries that were possibly too wild to have guessed before we found it.

8) The Kuiper Belt. In 1996, we'd discovered just a few KBOs. We now know of thousands.

9) The accelerating expansion of the universe. This was completely unknown in 1996 and was discovered in the subsequent years thanks to observations from the Hubble Space Telescope. The rate of expansion was known only with very high uncertainty before that time; the uncertainty now is only a couple of percent.

10) So many more. There's no way to cap the list at ten. Gullies on Mars. Temporary moonlets in the rings of Saturn. The depths of the winds of Jupiter. Spikes in the abundance of sulfur dioxide on Venus. The diversity of Mercury's surface. Details of Io's volcanic eruptions, vividly imaged by Galileo. What asteroids and comets look like from the surface.

May the next twenty-five years be just as exciting.
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