Probable Cold and Alkaline Surface Environment of the Hadean Earth Caused by Impact Ejecta Weathering

In the standard case, ocean pH is constrained to be smaller than the threshold pH of 9.

“However, the ocean pH is constrained to be smaller than the threshold pH of 9 as shown in Figure 3c.”

• AssumptionThe authors state this as an assumption: Seafloor and ejecta weathering decrease if pH is higher than a certain threshold; the nominal threshold pH is 9.

In the control case (without ejecta weathering), surface temperature is moderate or even hot.

“In the control case, the ejecta weathering is absent, and the surface temperature is moderate or even hot (Figure 3b), and pCO 2 is higher than the modern level (Figure 3a) to compensate the faint young Sun.”

The probability that the surface temperature is less than 273.15 K is 67% at 4.3 Ga.

“In particular, the probability that the surface temperature is less than 273.15K is 67% at 4.3 Ga.”

In the control case, pCO2 is higher than the modern level to compensate the faint young Sun.

“In the control case, the ejecta weathering is absent, and the surface temperature is moderate or even hot (Figure 3b), and pCO 2 is higher than the modern level (Figure 3a) to compensate the faint young Sun.”

The relatively low pCO2 in the standard case results in a comparatively high ocean pH.

“The relatively low pCO 2 results in a comparatively high ocean pH (Figure 3c).”

In the standard model, at 4.3 Ga, log10(pCO2) (in bar) is -2.21 +3.01 -2.54 [2σ].

“We find that relatively low pCO 2 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, log 10 (pCO 2 ) (in bar) is -2.21 +3.01 -2.54 [2𝜎] and temperature is 259.2 +84.1 -14.4 [2𝜎] K.”

In the standard case, DIC and ALK tend to be lower than in the control case.

“The DIC and ALK of the standard case also tend to be lower than those of the control case (Figures 3d and 3e).”

In the standard model, at 4.3 Ga, surface temperature is 259.2 +84.1 -14.4 [2σ] K.

“We find that relatively low pCO 2 and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, log 10 (pCO 2 ) (in bar) is -2.21 +3.01 -2.54 [2𝜎] and temperature is 259.2 +84.1 -14.4 [2𝜎] K.”

The probability that the ocean pH is higher than 7 is 70% at 4.3 Ga.

“In contrast, the ocean pH of the standard case is higher than that of the control case, and the probability that the ocean pH is higher than 7 is 70% at 4.3 Ga.”

At 3.7 Ga, the model result for ocean pH is consistent, within uncertainties, with the pH inference from REE patterns in banded iron formations.

“At 3.7 Ga, our model result for ocean pH is consistent, within uncertainties, with this pH inference from data.”

• cites Friend et al., 2008Not checked yet

  Not checked in the free run.

Such a low pCO2 would result in a circumneutral to basic pH of seawater, for example, 7.90 +1.21 -1.69 [2σ] at 4.3 Ga.

“Such a low pCO 2 would result in a circumneutral to basic pH of seawater, for example, 7.90 +1.21 -1.69 [2𝜎] at 4.3 Ga.”

In the control case, the relatively high pCO2 results in an oceanic pH lower than the modern value.

“The relatively high pCO 2 results in an oceanic pH lower than the modern value (Figure 3c).”

If we take the CO2 degassing rate during the Hadean to be roughly 10^-7 to 10^-5 bar per year, the duration of a snowball state is ∼10^4 to 10^6 year.

“If we take the CO 2 degassing rate during the Hadean to be roughly 10 -7 to 10 -5 bar per year, the duration of a snowball state is ∼10 4 to 10 6 year.”

The carbon cycle during the Hadean is mainly controlled by impact ejecta weathering.

“This is because the carbon cycle during the Hadean is mainly controlled by impact ejecta weathering as also shown in previous studies (Sleep & Zahnle, 2001;Zahnle & Sleep, 2002).”

• cites Sleep & Zahnle, 2001Not checked yet

  Not checked in the free run.

• cites Zahnle & Sleep, 2002Not checked yet

  Not checked in the free run.

During the Hadean, the surface temperature mainly depends on the CO2 degassing rate and impact flux, that is, the production rate of impact ejecta.

“During the Hadean, the surface temperature mainly depends on the CO 2 degassing rate and impact flux, that is, the production rate of impact ejecta, as explained in section S3.”

The possibility of T < 260 K is 51% at 4.3 Ga.

“The possibility of T < 260 K is 51% at 4.3 Ga (Figure 4).”

The situation does not differ in the no-land case, where the ocean pH tends to be lower than that of the standard case.

“The situation does not differ in the no-land case, where the ocean pH tends to be lower than that of the standard case (Figure 6b).”

The cold and alkaline ocean due to ejecta weathering is confined in the early Hadean.

“The cold and alkaline ocean due to ejecta weathering is confined in the early Hadean (Figure 3).”

The energy to make open waters large enough for gas exchange is ∼10^17 to 10^18 kJ.

“Hence, the energy to make open waters large enough for gas exchange is ∼10 17 to 10 18 kJ.”

In our model, at 3.5 Ga, the surface temperature of the no-land case is larger by 13.7 +28.9 -12.9 K [2σ] than that of the standard case.

“In our model, at 3.5 Ga, the surface temperature of the no-land case is larger by 13.7 +28.9 -12.9 K [2𝜎] than that of the standard case, which in good agreement with the results of Krissansen-Totton et al. (2018) (see Figure S6).”

In a case excluding continental weathering (a 'no-land' case), our model produces a surface temperature that tends to be larger than that of the standard case.

“In a case excluding continental weathering (a "no-land "case), our model produces a surface temperature that tends to be larger than that of the standard case (Figure S5).”

If we exclude ejecta weathering, the difference at 4.3 Ga increases to 19.2 +58.0 -19.5 K [2σ].

“If we exclude ejecta weathering, the difference at 4.3 Ga increases to 19.2 +58.0 -19.5 K [2𝜎] (Figure S6).”

The ocean pH is relatively high during the LHB.

“In addition, the ocean pH is relatively high during the LHB (Figure 5c).”

An impact making such open waters corresponds to 10^12 to 10^13 kg (equivalent to an asteroid ∼0.9 to 1.9 km across).

“If we assume that 50% of energy is used for the melting of ice sheet, following Sleep et al. (1989), and assume a velocity 20 km/s, an impact making such open waters corresponds to 10 12 to 10 13 kg (equivalent to an asteroid ∼0.9 to 1.9 km across).”

• cites Sleep et al., 1989Not checked yet

  Not checked in the free run.

In the early Hadean, the difference in the surface temperature at 4.3 Ga of a case without land versus that with land is 7.2 +32.6 -14.2 K [2σ].

“In the early Hadean, the difference in the surface temperature at 4.3 Ga of a case without land versus that with land is 7.2 +32.6 -14.2K [2𝜎] (Figure S6).”

The ocean pH is so high that the concentration of Fe2+ under siderite saturation is lower than the threshold of the widespread BIF (i.e., 50 μM Fe2+).

“As shown in Figure 6, the ocean pH is so high that the concentration of Fe 2+ under siderite saturation is lower than the threshold of the widespread BIF (i.e., 50 μMFe 2+ ).”

• AssumptionThe authors state this as an assumption: Siderite saturation controls the concentration of Fe2+.

The flux drops below a median value of ∼10^12 kg/year in a 2-sigma range of time from 4.4 to 4.0 Ga.

“The flux drops below a median value of ∼10 12 kg/year in a 2-sigma range of time from 4.4 to 4.0 Ga.”

We find that the low pCO2 (caused by acid weathering of impact ejecta) results in a neutral to weakly alkaline ocean, for example, 7.90 +1.21 -1.69 at 4.3 Ga.

“We find that the low pCO 2 (caused by acid weathering of impact ejecta) results in a neutral to weakly alkaline ocean, for example, 7.90 +1.21 -1.69 at 4.3 Ga.”

During the Hadean, most model realizations produce low pCO2 and surface temperature with a lower number of runs producing high pCO2 and moderate surface temperatures like those during the Archean.

“In contrast, during the Hadean, most model realizations produce low pCO 2 and surface temperature with a lower number of runs producing high pCO 2 and moderate surface temperatures like those during the Archean.”

The probability of a cold surface environment below freezing point in the early Hadean is high, approximately 70% at 4.3 Ga.

“The probability of a cold surface environment below freezing point in the early Hadean is high, approximately 70% at 4.3 Ga.”

For the standard case, the probability of the ocean pH being above 7 is around 70%.

“Besides, for the standard case, the probability of the surface temperature below 273.15K and the probability of the ocean pH above 7 are around 70%.”

The modeled pCO2 and ocean pH at 3.8 Ga are outside the condition required for banded iron formation.

“The pCO 2 and ocean pH are out of the condition of BIF.”

• AssumptionThe authors state this as an assumption: Siderite is saturated in the ocean (FeCO3 + 2H+ → Fe2+ + CO2 + H2O).

Because ejecta weathering decreases pCO2, the surface temperature of the standard case (with ejecta weathering) is lower than that of the control case (without ejecta weathering).

“Since the ejecta weathering decreases pCO 2 , the surface temperature of the standard case is lower than that of the control case (a).”

The ocean pH of the standard case (with ejecta weathering) is higher than that of the control case (without ejecta weathering).

“And the ocean pH of the standard case is higher than that of the control case (b).”

The coldest environment and most alkaline ocean conditions produced by the model are confined to the early Hadean, when the impact flux and associated ejecta production rate were high.

“The coldest environment and most alkaline ocean conditions produced by our model are confined to the early Hadean when the impact flux and associated ejecta production rate were high.”

In the traditional LHB model, the estimated impact flux is high enough to result in a cold and alkaline ocean in the early Archean (about 267.0 +34.6/-19.5 K and pH 7.53 +1.53/-1.01 at 3.8 Ga).

“In the traditional LHB model, the estimated impact flux is high enough to result in a cold and alkaline ocean in the early Archean (e.g., 267.0 +34.6 -19.5 K and pH7.53 +1.53 -1.01 at 3.8 Ga).”

For the standard case, the probability of the surface temperature being below 273.15 K is around 70%.

“Besides, for the standard case, the probability of the surface temperature below 273.15K and the probability of the ocean pH above 7 are around 70%.”

Reverse weathering was probably unimportant during Hadean.

“Thus, reverse weathering was probably unimportant during Hadean, which is the focus of this paper.”

• cites Bada & Korenaga, 2018Not checked yet

  Not checked in the free run.

The climate at 4.3 Ga was probably cold as a result of the consumption of CO2 by ejecta weathering.

“Thus, the climate at 4.3 Ga was probably cold as a result of the consumption of CO 2 by ejecta weathering.”

The ocean at 4.3 Ga was probably weakly alkaline considering the ejecta weathering.

“Thus, the ocean at 4.3 Ga was probably weakly alkaline considering the ejecta weathering.”

The hot environment from impactors comparable in size to those that produced the Orientale, Imbrium, and South Pole-Aitken craters on the Moon would last only ∼100 year.

“For example, the hot environment from impactors comparable in size to those that produced the Orientale, Imbrium, and South Pole-Aitken craters on the Moon would last only ∼100 year (e.g., Nisbet et al., 2007).”

• cites Nisbet et al., 2007Not checked yet

  Not checked in the free run.

A probably cold and alkaline marine environment is associated with a high impact flux.

“A probably cold and alkaline marine environment is associated with a high impact flux.”

If there was an interval of enhanced impact flux, i.e., Late Heavy Bombardment, similar conditions may have existed in the early Archean.

“Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean.”

The concentration of DSi should be low during the Hadean and early Archean because the influx of DSi to the ocean and sedimentation rates were low due to the likely smaller land mass.

“However, during the Hadean and the early Archean, the concentration of DSi should be low since the influx of DSi to the ocean and sedimentation rates were low because of the likely smaller land mass (Bada & Korenaga, 2018).”

• cites Bada & Korenaga, 2018Not checked yet

  Not checked in the free run.

Seawater pH is potentially important because seafloor weathering is both pH and temperature dependent.

“Seawater pH is potentially important because seafloor weathering is both pH and temperature dependent (Krissansen-Totton & Catling, 2017).”

• cites Krissansen-Totton & Catling, 2017Not checked yet

  Not checked in the free run.

On geological timescales of 10^6 years or more, the overall effect of impacts in the Hadean would be to cool the Earth.

“On geological timescales of 10 6 years or more, comparable to that on which the carbonate-silicate cycle operates, the overall effect of impacts in the Hadean would be to cool the Earth.”

• cites Charnay et al., 2017Not checked yet

  Not checked in the free run.

• cites Sleep & Zahnle, 2001Not checked yet

  Not checked in the free run.

• cites Zahnle & Sleep, 2002Not checked yet

  Not checked in the free run.

The pattern of rare earth elements (REEs) in banded iron formations at ~3.7 Ga is consistent with precipitation from circumneutral to alkaline seawater in which carbonate ions preferentially complexed the heavy REEs.

“At that time, the pattern of rare earth elements (REEs) in banded iron formations is consistent with precipitation from circumneutral to alkaline seawater in which carbonate ions preferentially complexed the heavy REEs (Friend et al., 2008).”

• cites Friend et al., 2008Not checked yet

  Not checked in the free run.

A conservative range for parameters in the model (e.g., for abiotic weathering) does not affect the overall results for the Hadean.

“Hence, a conservative range for parameters in the model (e.g., for abiotic weathering) does not affect the overall results for the Hadean.”

The evolution of the reservoir size of the mantle and/or oceanic crust would affect results less.

“Thus, the evolution of the reservoir size of the mantle and/or oceanic crust would affect results less.”

A surface temperature below 273.15K does not always mean that the Earth is globally ice covered (i.e., a snowball Earth) because the surface temperature only represents the globally averaged temperature.

“However, a surface temperature below 273.15K does not always mean that the Earth is globally ice covered (i.e., a snowball Earth) because the surface temperature only represents the globally averaged temperature.”

The massive CO2 atmosphere would remain until the atmospheric CO2 subducts into the mantle as carbonate, which may take tens of millions of years or longer.

“The massive CO 2 atmosphere would remain until the atmospheric CO 2 subducts into the mantle as carbonate, which may take tens of millions of years or longer.”

• cites Sleep, 2018Not checked yet

  Not checked in the free run.

The dust injected into the stratosphere shields sunlight because of its high albedo, resulting in a decrease in the surface temperature.

“The dust injected into the stratosphere shields sunlight because of its high albedo, resulting in a decrease in the surface temperature (e.g., Brugger et al., 2017).”

• cites Brugger et al., 2017Not checked yet

  Not checked in the free run.

The Hadean Earth could enter a snowball state because of a large amount of impact ejecta.

“Nonetheless, the Hadean Earth could enter a snowball state because of a large amount of impact ejecta, so it is worth considering the stability of a snowball state.”

Critical biomolecules, such as nucleobases, are more stable at lower temperatures.

“critical biomolecules, such as nucleobases, are known to be more stable at lower temperatures (Bada & Lazcano, 2002)”

• cites Bada & Lazcano, 2002Not checked yet

  Not checked in the free run.

A decrease in the surface temperature might trigger the snowball event.

“A decrease in the surface temperature might trigger the snowball event.”

Such aqueous chemistry concentrates critical reactants for prebiotic schemes, such as cyanide and phosphate.

“such aqueous chemistry concentrates critical reactants for prebiotic schemes, such as cyanide (Toner & Catling, 2019) and phosphate (Toner & Catling, 2019)”

• cites Toner & Catling, 2019Not checked yet

  Not checked in the free run.

The duration of a soft snowball state would be longer than that of the hard snowball case (i.e., longer than 10^4 to 10^6 year).

“Hence, the duration of a soft snowball state would be longer than that of the hard snowball case (i.e., longer than 10 4 to 10 6 year).”

Our model results support hypotheses that the origin of life occurred during a cool, quiescent mean climate state of the Hadean that existed between impacts.

“Thus, our model results support hypotheses that the origin of life occurred during a cool, quiescent mean climate state of the Hadean that existed between impacts.”

If the amount of impact ejecta was large enough to result in a snowball state, the Earth might experience the snowball limit cycle.

“Therefore, if the amount of impact ejecta was large enough to result in a snowball state, the Earth might experience the snowball limit cycle.”

Since the duration of a nonsnowball state tends to be much shorter than that of the snowball, the period of the snowball limit cycle during the Hadean is also 10^4 to 10^6 year.

“Since the duration of a nonsnowball state tends to be much shorter than that of the snowball (e.g., Kadoya & Tajika, 2014), the period of the snowball limit cycle during the Hadean is also 10 4 to 10 6 year.”

• cites Kadoya & Tajika, 2014Not checked yet

  Not checked in the free run.

Because of the faint young Sun, the threshold pCO2 to melt the global ice sheet is high (∼0.1 bar) during the Hadean.

“Because of the faint young Sun, the threshold pCO 2 to melt the global ice sheet is high (∼0.1 bar) during the Hadean.”

Open waters of 10^3 km^2 allow CO2 to dissolve efficiently into the ocean.

“Open waters of 10 3 km 2 allow CO 2 to dissolve efficiently into the ocean (Le Hir et al., 2008).”

• cites Le Hir et al., 2008Not checked yet

  Not checked in the free run.

The results of Sleep and Zahnle (2001) and of Krissansen-Totton et al. (2018) are in good agreement with each other.

“Nonetheless, the results of Sleep and Zahnle (2001) and of Krissansen-Totton et al. (2018) are in good agreement with each other according to Krissansen-Totton et al. (2018).”

• cites Krissansen-Totton et al., 2018Not checked yet

  Not checked in the free run.

The evolutionary range of surface environment depends on the range of parameters, such as of CO2 degassing rate, rather than on each evolution of the parameters.

“Hence, the evolutionary range of surface environment depends on the range of parameters, such as of CO 2 degassing rate, rather than on each evolution of the parameters.”

• AssumptionRests on an assumption: The timescale of the carbon cycle is relatively short (<∼10^6 year), so the surface environment is in a steady state for a timescale of >∼10^7 year.

The concentration of cyanide is favored at lower temperature.

“the concentration of cyanide is favored at lower temperature (Toner & Catling, 2019)”

• cites Toner & Catling, 2019Not checked yet

  Not checked in the free run.

The amount of ejecta would be preserved during the snowball state.

“So the amount of ejecta would be preserved during the snowball state.”

The median time is 4.3 Ga, so a 'soft' snowball state, if it existed, would mostly likely cease in the early Hadean.

“The median time is 4.3 Ga, so a "soft" snowball state, if it existed, would mostly likely cease in the early Hadean.”

In such a case, the BIF would indicate a local condition rather than the average condition of the ocean.

“In such a case, the BIF would indicate a local condition rather than the average condition of the ocean.”

Assuming the LHB, such a intense impact flux also results in a cold surface environment at 3.8 Ga.

“Assuming the LHB, such a intense impact flux also results in a cold surface environment at 3.8 Ga (Charnay et al., 2017).”

• cites Charnay et al., 2017Not checked yet

  Not checked in the free run.

The continental size and associated continental weathering rates have less effect on pCO2 and surface temperature.

“Hence, the continental size and associated continental weathering rates have less effect on pCO 2 and surface temperature.”

Since the impact ejecta also decreases via seafloor resurfacing and loss to the mantle, the amount of the impact ejecta would decrease on the timescale of seafloor resurfacing (∼10^8 year).

“Since the impact ejecta also decreases via seafloor resurfacing and loss to the mantle (equation ( 2)), the amount of the impact ejecta would decrease on the timescale of seafloor resurfacing (∼10 8 year).”

The dissolution of CO2 would prevent the Earth from recovering from such a 'soft' snowball state.

“The dissolution of CO 2 would prevent the Earth from recovering from such a "soft" snowball state.”

The average condition of the ocean might not be suitable for the BIF.

“Hence, the average condition of the ocean might not be suitable for the BIF.”

The partial pressure of NH3 would be very small because of photodissociation of NH3.

“However, the partial pressure of NH 3 would be very small because of photodissociation of NH 3 (e.g., Kasting, 1982) though there is still a possibility that organic haze could shield NH 3 from ultraviolet light (Sagan & Chyba, 1997) .”

• cites Kasting, 1982Not checked yet

  Not checked in the free run.

A banded iron formation (BIF) is present around 3.8 Ga in Isua in western Greenland.

“A banded iron formation (BIF) is present around 3.8 Ga in Isua in western Greenland (e.g., Bekker et al., 2014).”

• cites Bekker et al., 2014Not checked yet

  Not checked in the free run.

By 3.5 Ga, the effect of ejecta weathering can be ignored.

“Our model includes impact ejecta (unlike that of Krissansen-Totton et al., 2018), but by 3.5 Ga, the effect of ejecta weathering can be ignored (see also section 3).”

Ejecta weathering suppresses the effect of zero continental size during the early Hadean.

“Thus, ejecta weathering suppresses the effect of zero continental size during the early Hadean.”

The timescale of the effect of the impact ejecta after the LHB does not exceed the ∼10^8 year.

“Therefore, the timescale of the effect of the impact ejecta after the LHB does not exceed the ∼10 8 year.”

If the surface temperature increases by 20 K compared to the standard case, the probability of the surface temperature below 273.15K at 4.3 Ga to ∼30%.

“For example, if the surface temperature increases by 20 K compared to the standard case (i.e., Figure 3b), the probability of the surface temperature below 273.15K at 4.3 Ga to ∼30% (see also Figure 4a).”

The net increase of the surface temperature by the H2-N2 CIA would be suppressed.

“Hence, the net increase of the surface temperature by the H 2 -N 2 CIA would be suppressed.”

The cold and alkaline surface environment would remain likely.

“Therefore, the cold and alkaline surface environment would remain likely.”

The decrease in pCO2 would result in surface waters with basic pH.

“In addition, the decrease in pCO 2 would result in surface waters with basic pH.”

The negative feedback of carbon cycle decreases pCO2 if there is an additional greenhouse effect other than of CO2.

“However, the negative feedback of carbon cycle decreases pCO 2 if there is an additional greenhouse effect other than of CO 2 (e.g., Krissansen-Totton et al., 2018).”

• cites Krissansen-Totton et al., 2018Not checked yet

  Not checked in the free run.

Since the average impact flux is ∼10^12 to 10^13 kg/year during the Hadean, open waters large enough for gas exchange are plausible during the Hadean.

“Since the average impact flux is ∼10 12 to 10 13 kg/year during the Hadean (Sleep & Zahnle, 2001), open waters large enough for gas exchange are plausible during the Hadean.”

• cites Sleep & Zahnle, 2001Not checked yet

  Not checked in the free run.

As pointed out in Charnay et al. (2017), the LHB also results in a relatively low pCO2 and surface temperature.

“As pointed out in Charnay et al. (2017), the LHB also results in a relatively low pCO 2 and surface temperature (Figures 5a and 5b).”

• cites Charnay et al., 2017Not checked yet

  Not checked in the free run.

The amount of atmospheric CH4 would also be small during the prebiotic Hadean because of a lack of biogenic CH4.

“The amount of atmospheric CH 4 would also be small during the prebiotic Hadean because of a lack of biogenic CH 4 (Kasting & Catling, 2003).”

• cites Kasting & Catling, 2003Not checked yet

  Not checked in the free run.

It is possible that the early Hadean Earth got locked into a 'soft' snowball state, at least until the impact flux declined.

“Consequently, it is possible that the early Hadean Earth got locked into a "soft" snowball state, at least until the impact flux declined.”

Since the solubility of Fe2+ depends on a concentration of CO3^2- and pH, the existence of BIF might give a constraint on pCO2 and ocean pH.

“Since the solubility of Fe 2+ depends on a concentration of CO 2- 3 and pH, the existence of BIF might give a constraint on pCO 2 and ocean pH.”

The oldest inference about seawater pH comes from rare earth element (REE) distributions in ∼3.7 Ga banded iron formation units in Isua, West Greenland.

“The oldest inference about seawater pH comes from rare earth element (REE) distributions in ∼3.7 Ga banded iron formation units in Isua, West Greenland.”

• cites Friend et al., 2008Not checked yet

  Not checked in the free run.

Eutectic freezing could result in a high concentration of prebiotic ingredients in an ice matrix.

“Also, eutectic freezing could result in a high concentration of prebiotic ingredients in an ice matrix (e.g., Kitadai & Maruyama, 2018).”

• cites Kitadai & Maruyama, 2018Not checked yet

  Not checked in the free run.

This indicates that the insolation needed at the outer limit of the HZ is higher (i.e., the HZ is narrower) than previous estimates if a snowball planet is common.

“This indicates that the insolation needed at the outer limit of the HZ is higher (i.e., the HZ is narrower) than previous estimates (Kasting et al., 1993;Kopparapu et al., 2013) if a snowball planet is common.”

• cites Kasting et al., 1993Not checked yet

  Not checked in the free run.

• cites Kopparapu et al., 2013Not checked yet

  Not checked in the free run.

Recent studies suggest that under a low insolation even within the HZ limits, such a recovery from a snowball state is prevented owing to the condensation of CO2 ice and/or ice albedo feedback.

“However, recent studies suggest that under a low insolation even within the HZ limits, such a recovery from a snowball state is prevented owing to the condensation of CO 2 ice (Turbet et al., 2017) and/or ice albedo feedback (Kadoya & Tajika, 2019).”

• cites Turbet et al., 2017Not checked yet

  Not checked in the free run.

• cites Kadoya & Tajika, 2019Not checked yet

  Not checked in the free run.

This result is due to the weathering of ejecta from intense meteorite impact bombardment.

“This result is due to the weathering of ejecta from intense meteorite impact bombardment.”

The results of this study, showing a high probability of a cold Hadean, may have some advantages for a surface environment for the origin of life.

“The results of this study, showing a high probability of a cold Hadean, may have some advantages for a surface environment for the origin of life.”

Snowball states can affect the width of the habitable zone (HZ) in extrasolar systems.

“Snowball states can affect the width of the habitable zone (HZ) in extrasolar systems.”

Because of the way planets are formed by accretion, an exoplanet would also experience intense meteorite impacts in its early history.

“Because of the way planets are formed by accretion, an exoplanet would also experience intense meteorite impacts in its early history.”

The cold and alkaline surface environment due to ejecta weathering might be supportive of the origin of life.

“Nonetheless, the cold and alkaline surface environment due to ejecta weathering might be supportive of the origin of life.”

Intense meteorite impacts, acting within the context of a carbonate-silicate cycle on a rocky world, might cause the location of the outer limit of the habitable zone to move inward.

“Therefore, intense meteorite impacts, acting within the context of a carbonate-silicate cycle on a rocky world, might cause the location of the outer limit of the habitable zone to move inward.”

Cold, carbonate-rich lakes (with quasi-neutral to basic) concentrate cyanide and phosphate, which are essential reagents for prebiotic schemes.

“In addition, cold, carbonate-rich lakes (with quasi-neutral to basic basic) concentrate cyanide and phosphate (Toner & Catling, 2019;2019), which are essential reagents for prebiotic schemes that have had considerable success in recent years in making biomolecules with relatively high yield and specificity (Islam & Powner, 2017).”

• cites Toner & Catling, 2019Not checked yet

  Not checked in the free run.

• cites Islam & Powner, 2017Not checked yet

  Not checked in the free run.

A cold temperature is favorable for the stability of critical biomolecules, such as RNA, which survives better at low temperatures.

“As mentioned previously, a cold temperature is favorable for the stability of critical biomolecules, such as RNA, which survives better at low temperatures (Bada & Lazcano, 2002).”

• cites Bada & Lazcano, 2002Not checked yet

  Not checked in the free run.

Cold conditions in the Hadean and moderate levels of pCO2 are favorable for hypotheses of an origin of life on land.

“Cold conditions in the Hadean and moderate levels of pCO 2 are favorable for hypotheses of an origin of life on land.”

Thermodynamics favors the concentration of cyanide in low temperature aqueous solutions.

“Thermodynamics favors the concentration of cyanide in low temperature aqueous solutions (Toner & Catling, 2019).”

• cites Toner & Catling, 2019Not checked yet

  Not checked in the free run.

Cyanide is a critical reactant for prebiotic chemistry because it can repeatedly add C-N moieties to organic molecules, allowing the buildup of long biomolecules including amino acids, ribonucleotides and lipid precursors.

“One critical reactant for prebiotic chemistry is cyanide because cyanide can repeatedly add C-N moieties to organic molecules, allowing the buildup of long biomolecules, including amino acids, ribonucleotides, and lipid precursors (Canavelli et al., 2019;Islam & Powner, 2017;Sutherland, 2016).”

• cites Canavelli et al., 2019Not checked yet

  Not checked in the free run.

• cites Islam & Powner, 2017Not checked yet

  Not checked in the free run.

• cites Sutherland, 2016Not checked yet

  Not checked in the free run.

The moderation of climate and associated modulation of pCO2 is a result of negative feedbacks on surface temperature and pCO2 from continental and seafloor weathering in the carbonate-silicate cycle.

“The moderation of climate and associated modulation of pCO 2 is a result of negative feedbacks on surface temperature and pCO 2 from continental and seafloor weathering in the carbonate-silicate cycle.”

The relatively low pCO2 would allow alkaline lakes on land, as today.

“The relatively low pCO 2 (e.g., Figure 3a) would allow alkaline lakes on land, as today.”

The earliest empirical inferences are consistent with our model results, within uncertainty.

“Thus, the earliest empirical inferences are consistent with our model results, within uncertainty.”

Impact craters on the Moon indicate the possibility of an enhanced impact flux around 3.8 Ga, the late heavy bombardment (LHB).

“However, impact craters on the Moon also indicate the possibility of an enhanced impact flux around 3.8 Ga, the late heavy bombardment (LHB) (e.g., Niihara et al., 2019)”

• cites Niihara et al., 2019Not checked yet

  Not checked in the free run.

A concentration of Fe2+ of 50 μM is the threshold for a widespread banded iron formation (BIF).

“The gray dashed line represents the condition that a concentration of Fe 2+ is 50 μM, which is the threshold for a widespread banded iron formation (BIF)(Song et al., 2017).”

• cites Song et al., 2017Not checked yet

  Not checked in the free run.

Biomolecules needed for the origin of life, such as nucleobases, are more stable at low temperatures.

“biomolecules that are needed for the origin of life, such as nucleobases, are more stable at low temperatures (Bada & Lazcano, 2002)”

• cites Bada & Lazcano, 2002Not checked yet

  Not checked in the free run.

There is a possibility that early life in the Archean evolved in a cold and alkaline ocean.

“Hence, there is also be the possibility that early life in the Archean evolved in a cold and alkaline ocean.”