Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth

Roig

et al. 2022

Preprint

The Moon holds important clues to the early history of the Solar System. The large scars evident on its near side are impact basins filled with dark basalt; some 50 lunar basins (crater diameter $D>300$ km) have been recognized overall$^1$. The question of the origin of basin-forming impactors remained unsettled. Here we show that most impactors were rocky planetesimals left behind at $\sim 0.5$--$1.5$ au after the terrestrial planet accretion. The number of basins expected from impacts of leftover planetesimals largely exceeds the number of known lunar basins, suggesting that the first $\sim200$ Myr of impacts is not recorded on the lunar surface$^{2,3}$. The Imbrium basin formation (age$^4$ $\simeq 3.92$~Gyr; impactor diameter$^{5,6}$ $d>100$ km) occurs with a 15--35\% probability in the model. Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schr\"odinger) forming afterwards. The model predicts $\simeq 20$ $d>10$-km impacts on the Earth 2.5--3.5 Gyr ago (Ga), which is comparable to the number of known spherule beds in the late Archean$^7$.

Section: Discussionsupporting

confidence: 82%

“…Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schrödinger) forming afterwards. The model predicts ≃ 20 d > 10-km impacts on the Earth 2.5-3.5 Gyr ago (Ga), which is comparable to the number of known spherule beds in the late Archean 7 .…”

mentioning

confidence: 67%

Formation of Lunar Basins from Impacts of Leftover Planetesimals

Roig

et al. 2022

Preprint

“…As the plume cools down, glassy spherules form and fall back, producing a global layer that can be several millimeters to many centimeters thick. Some ∼16 spherule beds have been found in the late Archean period (T = 2.5-3.5 Ga; Marchi et al 2021), although preservation biases and incomplete sampling may be an issue. At least some of these layers may have been produced by very large, d ∼ 50 km impactors, but most are thought to record d > 10 km impacts (Marchi et al 2010, Johnson et al 2016b).…”

Section: Resultsmentioning

confidence: 99%

Formation of Lunar Basins from Impacts of Leftover Planetesimals

Roig

et al. 2022

ApJL

Self Cite

The Moon holds important clues to the early evolution of the solar system. Some 50 impact basins (crater diameter D > 300 km) have been recognized on the lunar surface, implying that the early impact flux was much higher than it is now. The basin-forming impactors were suspected to be asteroids released from an inner extension of the main belt (1.8–2.0 au). Here we show that most impactors were instead rocky planetesimals left behind at ∼0.5–1.5 au after the terrestrial planet accretion. The number of basins expected from impacts of leftover planetesimals largely exceeds the number of known lunar basins, suggesting that the first ∼200 Myr of impacts are not recorded on the lunar surface. The Imbrium basin formation (age ≃3.92 Gyr; impactor diameter d ≳ 100 km) occurs with a 15%–35% probability in our model. Imbrium must have formed unusually late to have only two smaller basins (Orientale and Schrödinger) forming afterwards. The model predicts ≃20 d > 10 km impacts on the Earth 2.5–3.5 Gyr ago (Ga), which is comparable to the number of known spherule beds in the late Archean.

“…Reducing gases in general (e.g., CH 4 , H 2 ) can impact O 2 and O 3 levels, whether produced biologically or through volcanic outgassing (e.g., Hu et al 2012;Black et al 2014;Gregory et al 2021;Cooke et al 2022). O 3 can also be depleted by cometary impacts (e.g., Marchi et al 2021) and through solar flares (e.g., Pettit et al 2018). In addition, O 3 can vary throughout different seasons on modern Earth (Olson et al 2018).…”

Section: Factors That Impact the O 2 -O 3 Relationshipmentioning

confidence: 99%

Is ozone a reliable proxy for molecular oxygen?

Kozakis

Mendonça

Buchhave

2022

A&A

Molecular oxygen (O 2 ) paired with a reducing gas is regarded as a promising biosignature pair for the atmospheric characterization of terrestrial exoplanets. In circumstances when O 2 may not be detectable in a planetary atmosphere (e.g., at mid-IR wavelengths) it has been suggested that ozone (O 3 ), the photochemical product of O 2 , could be used as a proxy to infer the presence of O 2 . However, O 3 production has a nonlinear dependence on O 2 and is strongly influenced by the UV spectrum of the host star. To evaluate the reliability of O 3 as a proxy for O 2 , we used Atmos, a 1D coupled climate and photochemistry code, to study the O 2 -O 3 relationship for "Earth-like" habitable zone planets around a variety of stellar hosts (G0V-M5V) and O 2 abundances. Overall, we found that the O 2 -O 3 relationship differed significantly with stellar hosts and resulted in different trends for hotter stars (G0V-K2V) versus cooler stars (K5V-M5V). Planets orbiting hotter host stars counter-intuitively experience an increase in O 3 when O 2 levels are initially decreased from 100% Earth's present atmospheric level (PAL), with a maximum O 3 abundance occurring at 25-55% PAL O 2 . As O 2 abundance initially decreases, larger amounts of UV photons capable of O 2 photolysis reach the lower (denser) regions of the atmosphere where O 3 production is more efficient, thus resulting in these increased O 3 levels. This effect does not occur for cooler host stars (K5V-M5V), since the weaker incident UV flux does not allow O 3 formation to occur at dense enough regions of the atmosphere where the faster O 3 production can outweigh a smaller source of O 2 from which to create O 3 . Thus, planets experiencing higher amounts of incident UV possessed larger stratospheric temperature inversions, leading to shallower O 3 features in planetary emission spectra. Overall it will be extremely difficult (or impossible) to infer precise O 2 levels from an O 3 measurement, however, with information about the UV spectrum of the host star and context clues, O 3 will provide valuable information about potential surface habitability of an exoplanet.