The Impact Environment of Ancient Mars: Implications for Habitability

The impact bombardment periods that likely affected ancient (Noachian and pre-Noachian) Mars are (i) the post-accretionary elevated impactor flux, commonly known as the "heavy bombardment" and (ii) a putative increase in the number of impact events around 4.0 Ga, commonly known as the "late heavy bombardment" (LHB). These events would have affected the biological potential of Mars by melting and fracturing the crust, depositing large amounts of impact ejecta, generating hydrothermal systems, delivering elements essential for life, and likely increasing the temperature of the Martian atmosphere.

We model two types of post-accretionary bombardments: (i) a classical exponential decay and (ii) a sawtooth timeline, characterized by faster-than-exponential decay and reduced total mass. Likewise, two types of LHB are modeled: (i) a classic "spike," centered at 3.9 Ga and lasting ~100 Myr, and (ii) a "sawtooth" LHB, characterized by a sudden increase in the number of impacts at ~4.1 Ga, overall lower delivered mass, and a longer duration.

Partial results of these simulations are summarized in Table 1. To estimate habitable volumes in hydrothermal environments, surface temperatures of 1 °C and -63 °C were tested. Even in the latter case, life may have persevered in a global aquifer underneath a layer of permafrost termed the cryosphere. Over 10⁵-10⁶of the impact craters that formed during the LHB and post-accretionary bombardment, respectively, would have accessed this global aquifer via cryospheric melt-through, resulting in a subsurface plumbing network between individual impact-induced hydrothermal systems.

Table 1. Summary of impact simulation results.

Bombardment type	Total mass delivered (kg)	Largest impactor (km)	Percent of crust melted	Percent resurfaced
Classic post-accretion	6.5 × 1020	492	8.2%	100%
Classic LHB	1.0 × 1020	246	1.6%	36%
Sawtooth post-accretion	1.6 × 1020	310	2.6%	46%
Sawtooth LHB	2.8 × 1019	196	0.5%	10%

Figure 1. A 3-dimensional model representing the Mars lithosphere at (a) 4.4 Ga and (b) 4.1 Ga in the "classical post-accretionary bombardment" scenario. Dark areas denote crater imprints. Upper surface shows temperatures at a depth of 4 km. Light blue represents the cryosphere, dark blue represents the approximate liquid water stability region and/or the habitable zone.