# Sagans List and life in the universe **URL:** \ **Category:** Great Debates\ **Created:** [November 12, 2010, 12:10am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372 "2010-11-12T00:10:09Z")\ **Posts on this page:** 5\ **Page:** 3
**Author:** ![wheresmymind](https://avatars.discourse-cdn.com/v4/letter/w/91b2a8/32.png) [@wheresmymind](https://boards.straightdope.com/u/wheresmymind)\ **Post date:** [November 16, 2010, 3:19am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/41 "2010-11-16T03:19:43Z")
> [@Blake](#): > > Unless you are talking about molten metal, any liquid is going to have an appreciable vapor phase. So surface liquid=atmosphere. This is most definitely rue of water. If a planet can’t hold an atmosphere it sure can’t hold liquid water. True, but I’ll counter that surface liquid isn’t necessary. Galilean moons Europa and Ganymede and Saturn’s moon Enceladus are all believed to have liquid water (warmed by tidal heating) beneath their surface. ---
**Author:** ![Blake](https://avatars.discourse-cdn.com/v4/letter/b/a9a28c/32.png) [@Blake](https://boards.straightdope.com/u/Blake)\ **Post date:** [November 16, 2010, 10:36pm UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/42 "2010-11-16T22:36:11Z")
> [@wheresmymind](#): > > True, but I’ll counter that surface liquid isn’t necessary. Galilean moons Europa and Ganymede and Saturn’s moon Enceladus are all believed to have liquid water (warmed by tidal heating) beneath their surface. Correct me if I’m wrong, but aren’t all those bodies large enough to retain an atmosphere? ---
**Author:** ![wheresmymind](https://avatars.discourse-cdn.com/v4/letter/w/91b2a8/32.png) [@wheresmymind](https://boards.straightdope.com/u/wheresmymind)\ **Post date:** [November 17, 2010, 5:29am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/43 "2010-11-17T05:29:25Z")
> [@Blake](#): > > Correct me if I’m wrong, but aren’t all those bodies large enough to retain an atmosphere? Actually, I guess those moons ARE large enough to have an atmosphere, they just don’t for whatever reason. They have trace atmospheres, much like our own moon, but their surface pressures are measured in fractions of a micro Pascal, whereas 1 atm is about 100,000 pascals. So their “atmosphere” is on the order of one trillionth to one hundred billionth as dense as our own - similar to the very hardest laboratory vacuums. Titan is the only moon that has a dense atmosphere. Titan is similar in size to the moons I mentioned, and its atmosphere is more dense and more massive than Earth’s despite Titan’s small size/mass (compared to Earth). So there are factors besides size that play a role in the formation and retention of an atmosphere. ---
**Author:** ![wintertime](https://avatars.discourse-cdn.com/v4/letter/w/c68b51/32.png) [@wintertime](https://boards.straightdope.com/u/wintertime)\ **Post date:** [November 18, 2010, 9:55am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/44 "2010-11-18T09:55:45Z")
> [@wheresmymind](#): > > So there are factors besides size that play a role in the formation and retention of an atmosphere. A pretty good online source concerning these factors is [_Origins of Atmospheres_](http://articles.adsabs.harvard.edu//full/1998ASPC..148..364Z/0000371.000.html) by Kevin Zahnle. A couple of interesting articles by [Catling](http://www.sciencemag.org/content/293/5531/839.abstract), [Kasting](http://www.sciencemag.org/content/293/5531/819.summary) and Zahnle, published in _Science_, deal with the possible influence of life on the depletion of certain gases. In case someone is interested in the new field of “astrobiology” in general, I’ve enjoyed the book [_Planets and Life - The Emerging Science of Astrobiology_](http://www.cambridge.org/gb/knowledge/isbn/item1170633/?site_locale=en_GB), edited by Woodruff T. Sullivan III and John Baross. ---
**Author:** ![wheresmymind](https://avatars.discourse-cdn.com/v4/letter/w/91b2a8/32.png) [@wheresmymind](https://boards.straightdope.com/u/wheresmymind)\ **Post date:** [November 19, 2010, 5:07am UTC](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372/45 "2010-11-19T05:07:59Z")
> [@wintertime](#): > > A pretty good online source concerning these factors is [_Origins of Atmospheres_](http://articles.adsabs.harvard.edu//full/1998ASPC..148..364Z/0000371.000.html) by Kevin Zahnle. A couple of interesting articles by [Catling](http://www.sciencemag.org/content/293/5531/839.abstract), [Kasting](http://www.sciencemag.org/content/293/5531/819.summary) and Zahnle, published in _Science_, deal with the possible influence of life on the depletion of certain gases. > > In case someone is interested in the new field of “astrobiology” in general, I’ve enjoyed the book [_Planets and Life - The Emerging Science of Astrobiology_](http://www.cambridge.org/gb/knowledge/isbn/item1170633/?site_locale=en_GB), edited by Woodruff T. Sullivan III and John Baross. Thanks a lot! It’s a small world; John Baross is actually my grand-advisor (he was my MS advisor’s advisor). I didn’t realize he was so involved in astrobiology, I know him as a marine microbial ecologist. [Previous page](https://boards.straightdope.com/t/sagans-list-and-life-in-the-universe/560372.md?page=2)