Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds. (arXiv:2004.03631v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Glaser_D/0/1/0/all/0/1">Donald M Glaser</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hartnett_H/0/1/0/all/0/1">Hilairy Ellen Hartnett</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Desch_S/0/1/0/all/0/1">Steven J. Desch</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Unterborn_C/0/1/0/all/0/1">Cayman T. Unterborn</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Anbar_A/0/1/0/all/0/1">Ariel Anbar</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Buessecker_S/0/1/0/all/0/1">Steffen Buessecker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Fisher_T/0/1/0/all/0/1">Theresa Fisher</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Glaser_S/0/1/0/all/0/1">Steven Glaser</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Kane_S/0/1/0/all/0/1">Stephen R. Kane</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lisse_C/0/1/0/all/0/1">Carey M. Lisse</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Millsaps_C/0/1/0/all/0/1">Camerian Millsaps</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Neuer_S/0/1/0/all/0/1">Susanne Neuer</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+ORourke_J/0/1/0/all/0/1">Joseph G. ORourke</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Santos_N/0/1/0/all/0/1">Nuno Santos</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Walker_S/0/1/0/all/0/1">Sara Imari Walker</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zolotov_M/0/1/0/all/0/1">Mikhail Zolotov</a>
The search for life on exoplanets is one of the grand scientific challenges
of our time. The strategy to date has been to find (e.g., through transit
surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then
use transmission spectroscopy to measure biosignature gases, especially oxygen,
in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope).
Already there are more such planets than can be observed by JWST, and missions
like the Transiting Exoplanet Survey Satellite and others will find more. A
better understanding of the geochemical cycles relevant to biosignature gases
is needed, to prioritize targets for costly follow-up observations and to help
design future missions. We define a Detectability Index to quantify the
likelihood that a biosignature gas could be assigned a biological vs.
non-biological origin. We apply this index to the case of oxygen gas, O2, on
Earth-like planets with varying water contents. We demonstrate that on
Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed
continents) a reduced flux of bioessential phosphorus limits the export of
photosynthetically produced atmospheric O2 to levels indistinguishable from
geophysical production by photolysis of water plus hydrogen escape. Higher
water contents >1wt% that lead to high-pressure ice mantles further slow
phosphorus cycling. Paradoxically, the maximum water content allowing use of O2
as a biosignature, 0.2wt%, is consistent with no water based on mass and
radius. Thus, the utility of an O2 biosignature likely requires the direct
detection of both water and land on a planet.
The search for life on exoplanets is one of the grand scientific challenges
of our time. The strategy to date has been to find (e.g., through transit
surveys like Kepler) Earth-like exoplanets in their stars habitable zone, then
use transmission spectroscopy to measure biosignature gases, especially oxygen,
in the planets atmospheres (e.g., using JWST, the James Webb Space Telescope).
Already there are more such planets than can be observed by JWST, and missions
like the Transiting Exoplanet Survey Satellite and others will find more. A
better understanding of the geochemical cycles relevant to biosignature gases
is needed, to prioritize targets for costly follow-up observations and to help
design future missions. We define a Detectability Index to quantify the
likelihood that a biosignature gas could be assigned a biological vs.
non-biological origin. We apply this index to the case of oxygen gas, O2, on
Earth-like planets with varying water contents. We demonstrate that on
Earth-like exoplanets with 0.2 weight percent (wt%) water (i.e., no exposed
continents) a reduced flux of bioessential phosphorus limits the export of
photosynthetically produced atmospheric O2 to levels indistinguishable from
geophysical production by photolysis of water plus hydrogen escape. Higher
water contents >1wt% that lead to high-pressure ice mantles further slow
phosphorus cycling. Paradoxically, the maximum water content allowing use of O2
as a biosignature, 0.2wt%, is consistent with no water based on mass and
radius. Thus, the utility of an O2 biosignature likely requires the direct
detection of both water and land on a planet.
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