Table 5.

Boundary conditions for different planetary bodies of astrobiological interest (compared to Earth), split into atmosphere, surface, and subsurface layers.

Planetary	Type	Layer	Temperature	pH	Pressure	Salinity	Geochemistry	References
body			(°C)		(MPa)	(% NaCl)
Earth	Planet	Atmosphere	-100 – 40	Neutral, local acidic conditions possible due to volcanism and human activities	0.0001–0.1	0	78% N2, 21% O2, 9340 ppm Ar, 400 ppm CO2 18.2 ppm Ne, 5.2 ppm He, 1.7 ppm CH4, 1.1 ppm Kr, 0.6 ppm H2, variable H2O	Hans Wedepohl, 1995; McDonough and Sun, 1995; Wayne, 2000
		Surface	-98.6 – 464	-3.6 – 13.3	0.003–112	0–saturation	Soils and sediments of varying lithologies, siliceous crust, ranging from mafic to felsic composition. Extensive ocean (70% planet surface), with 4,000 m average depth, 4°C and 3.5% average temperature and salinity respectively
		Subsurface	3.25–<400	∼1–12.8	<800	0.05–saturation	Soils and sediments of varying lithologies, siliceous crust, ranging from mafic to felsic composition, ultramafic mantle
Venus	Planet	Atmosphere	-40 – 482a	0b	0.1–9.3c	nr	96.5% CO2, 3.5% N2; small quantities of CO, SO2, HCl, HF, HDO, and H2O; H2SO4 condensates	Cockell, 1999; Basilevsky and Head, 2003; Schulze-Makuch et al., 2004; Lang and Hansen, 2006; Bertaux et al., 2007; Airey et al., 2017
		Surface	377–482	nr	4.5–9.3c	nr	Rocks are similar to tholeiitic and alkaline basalts; no liquid water
		Subsurface	nr	nr	nr	nr	Fluid channels; volcanism
Mars	Planet	Atmosphere	-138 – 35d	nr	0.0001–0.0009	nr	95.3% CO2, 2.7% N2, 1.6% Ar, 0.13% O2, 0.08% CO; trace amounts of H2O, NO, Ne, Kr, Xe	Varnes et al., 2003; Fairén et al., 2004; Nicholson and Schuerger, 2005; Hecht et al., 2009; Smith et al., 2009; Johnson et al., 2011; Jones et al., 2011; Michalski et al., 2013; Longstaff, 2014; Wordsworth, 2016; Sinha et al., 2017; NASA, 2018
		Surface	-138 – 30	7.7e	0.0004–0.0009	5.2–5.8	Basaltic, Fe-/Mg-rich phyllosilicates, perchlorate salts, Al-rich clays, sulfates, chlorides, calcite, and silicas; potential cryosphere
		Subsurface	55g	4.96–9.13h	10–303g	Cl-rich brines	Potential groundwater; basalt crust; possible serpentinization
Enceladus	Icy moon	Plume jets	0	∼8.5–9	High velocity jets	>0.5	90–99% H2O, ≤0.61–4.27% N2, 0.3–5.3% CO2, 0.1–1.68% CH4, 0.4–0.9% NH3, 0.4–39% H2, trace amounts of hydrocarbons; high mass organic cations, silicates, sodium, potassium, carbonates	Gioia et al., 2007; Postberg et al., 2009, 2018; Waite et al., 2009; Zolotov et al., 2011; Glein et al., 2015; Holm et al., 2015; Hsu et al., 2015; Taubner et al., 2018
		Icy shell (∼10 km thick)	-233 – -23	nr	nr	May have ammonia brine pockets	May have tectonics
		Subsurface global ocean (∼0–170 km depth)	<90	8.5–12.2k	1–8	0.45–<4	Possible serpentinization
Titan	Icy moon	Atmosphere	-183 – -73j	nr	>0.01–0.15	nr	98.4% N2, 1.4% CH4, 0.2% H2, trace hydrocarbons and organics; ∼50 ppmv CO and ∼15 ppbv CO2; HCN, C2H3CN, and other nitriles; hazes and clouds	Fulchignoni et al., 2005; de Kok et al., 2007; Norman, 2011; Baland et al., 2014; Mastrogiuseppe et al., 2014; Mitri et al., 2014; Sohl et al., 2014; Jennings et al., 2016; McKay, 2016; Mitchell and Lora, 2016; Brassé et al., 2017; Cordier et al., 2017
		Surface	-183 – -179	nr	0.15–0.35i	nr	95% N2, 5% CH4, 0.1% H2; lakes and sea have CH4, C2H4, and dissolved nitrogen; dunes of solid organic material; low-latitude deserts and high-latitude moist climates
		Subsurface	-18	11.8l	50–300m	Likely dense subsurface ocean (≤1,350 kg m-3) suggesting high salinity	CH4 and C2H6
Ceres	Dwarf planet	Atmosphere	nr	nr	nr	nr	Transient atmosphere with possible water vapor	Fanale and Salvail, 1989; Zolotov, 2009, 2017; Küppers et al., 2014; Hayne and Aharonson, 2015; Neveu and Desch, 2015; Hendrix et al., 2016; Villarreal et al., 2017; Vu et al., 2017; Castillo-Rogez et al., 2018; McCord and Castillo-Rogez, 2018; McCord and Zambon, 2019
		Surface	(-157 – -30)n	9.7–11.3n	nr	<10n	Surface clays; (Mg, Ca)-carbonates; (Mg, NH4)-phyllosilicates; Fe-rich clays; salt deposits; chloride salts; water-rock interactions; brucite and magnetite; sulfur species and graphitized carbon; localized Na-carbonates (e.g., Na2CO3), NH4Cl, NH4HCO3
		Subsurface	-143 – -93°	Likely alkaline	<140–200p	Potentially has briny or NH3-rich subsurface liquid	Active water/ice-driven subsurface processes
Europa	Icy moon	Atmosphere (tenuous)	nr	nr	0.1-12–1-12	nr	Ion sputtering of the surface; potential water plumes; O2; trace amounts of sodium and potassium	Spencer et al., 1999; Chyba and Phillips, 2001; Marion et al., 2005; McGrath et al., 2009; Zolotov and Kargel, 2009;
								Travis et al., 2012; Cassidy et al., 2013; Muñoz-Iglesias et al., 2013; Kattenhorn and Prockter, 2014; Soderlund et al., 2014; Hand and Carlson, 2015; Kimura and Kitadai, 2015; Noell et al., 2015; Vance et al., 2016; Teolis et al., 2017; Zhu et al., 2017; Jones et al., 2018; Martin and McMinn, 2018; Pavlov et al., 2018
		Surface (icy shell)	-187 – -141	nr	0.1-12	May be saline, as delivered to the surface from a salty ocean, may have brine or salt inclusions	H2O2, H2SO4, CO2; salts concentrated in cracks; oxidants and simple organics; potentially MgSO4, Na2SO4, Na2CO3, may have gas inclusions; may have tectonics
		Subsurface ocean	Daily inundation of seawater at T = -4 – 0	Potential for wide rangeq	0.1–30r	<3.5	Likely contains Mg2+, SO42-, Na+, Cl-; oxidants and simple organics

The observed or putative geochemistry as well as other potential influences are also listed. ^aThermosphere can be as cold as -173°C (Bertaux et al., 2007); the upper-to-middle cloud layers are between -40 and 60°C (Cockell, 1999). ^bAcid concentration in upper cloud layer is 81%, in lower layers up to 98% (Cockell, 1999). ^cUp to 11 MPa in a deep depression (Basilevsky and Head, 2003). ^dSummer air temperatures on Mars near the equator can reach a maximum of 35°C (Longstaff, 2014). ^eMeasured by the Phoenix Mars Lander Wet Chemistry Laboratory at the northern plains of the Vastitas Borealis (Hecht et al., 2009). ^fLiquid water may have had water activity >0.95 (Fairén et al., 2009). ^gCalculated temperature at a depth of 1–30 km (Jones et al., 2011; Sinha et al., 2017); at a depth ∼310 km, the calculated temperature is <427°C (Jones et al., 2011); the Martian core has temperature 1,527°C (Longstaff, 2014). ^hCalculated groundwater pH (Varnes et al., 2003). ⁱCalculated pressure at Titan’s large sea, Ligeia Mare, is 0.20–0.35 MPa (Cordier et al., 2017). ^jTropospheric temperature can be -193°C; 80% of incident sunlight is absorbed by Titan’s atmosphere, suggesting that there are greenhouse and antigreenhouse effects (Mitchell and Lora, 2016). ^kThe subsurface ocean on Enceladus could also have pH range 10.8–13.5 (Glein et al., 2015). ^lCalculated ocean pH with 5 wt% NH₃ (Brassé et al., 2017). ^mCalculated pressure for the subsurface ocean with thickness 100 km and outer shell thickness 40–170 km (Baland et al., 2014); 800 MPa at the mantle ice shell-core boundary (Sohl et al., 2014). ⁿCalculated surface temperatures, illuminated surfaces can have temperature <-173°C (Hayne and Aharonson, 2015); calculated pH and salinity for bright deposits in Occator crater (Zolotov, 2017); temperature for bright deposits in Occator crater might reach <-0.2°C (Zolotov, 2017). ^oInternal temperature might reach 77°C (McCord and Sotin, 2005). ^pCeres’ center pressure (Zolotov, 2009). ^qAcid brine may result from hydrothermal systems and be enriched with sulfuric acid (Kargel et al., 2000); neutral brine may occur as leachate from chondritic material and be enriched with magnesium sulfate (Kargel et al., 2000; Pasek and Greenberg, 2012); alkaline brine may occur in areas with natron (Na₂CO₃^⋅10H₂O), produced from the venting of CO₂ from aqueous reservoirs (Langmuir, 1971; Millero and Rabindra, 1997). ^rAt the base of a 100 km Europan ocean, the pressure is calculated to be 146 MPa (Marion et al., 2005).