The 2008 Phoenix Mars lander mission sampled ground ice at 68N latitude. Mission results, considered along with climate modeling studies, suggest that the site is habitable for life during high obliquity periods. The Icebreaker mission has been proposed to the NASA Discovery program to search for biosignatures produced during habitable periods. This paper explores its rationale and approach

Noe Dobrea, E. Z.

Stoker, C. R.

English

NASA Technical Reports Server

WE SHOULD SEARCH FOR LIFE IN MARS N. POLAR GROUND ICE.  C.R. Stoker1 and E.Z. Noe Dobrea, 
1,2, 1NASA Ames Research Center, Moffett Field, CA 94035, carol.stoker@nasa.gov, 2Planetary Science Institute, 
Tucson, AZ, eldar@psi.edu 
 
Introduction: The 2008 Phoenix Mars lander 
mission sampled ground ice at 68o N latitude. Mission 
results, considered along with climate modeling stud-
ies, suggest that the site is habitable for life during high 
obliquity periods.  The Icebreaker mission has been 
proposed to the NASA Discovery program to search 
for biosignatures produced during habitable periods. 
This paper explores its rationale and approach. 
Rationale for a Habitable Environment: The 
Phoenix lander dug into the subsurface with a robotic 
arm equipped with a digging scoop to reveal an ice 
table at 5 cm depth. The case for habitable conditions 
encountered is summarized in [1].  Briefly, 1) beneath 
a cover of dry soil, patches of pure ice was discovered 
that covered 10% of the approximately 1m2 excavated 
by the scoop[2]. The rest of the surface coverage was 
ice cemented soil. Segregated ice forms when liquid 
water films are concentrated during freezing. 2) The 
soil contained pure calcite mineral[3], which forms 
under aqueous conditions. 3) Microscopic imaging 
identified a population of soil grains showing evidence 
of solutional weathering [1]. 4) Energy sources are 
available to support autotrophic surface and chemoau-
totrophic metabolism in the ground ice. Semitranspar-
ent soil grains that could protect microbes from steri-
lizing UV radiation are present[1].  Perchlorate found 
in the soil at ~0.5% [4] is a known chemoautotrophic 
energy source used by microbes from a variety of envi-
ronments including permafrost [5]. 
While current climate conditions are too cold to 
support metabolism in the polar region, climate model-
ing studies [6] show that variations in solar insolation 
associated to obliquity variations cause climate change 
that would permit periods of growth to occur. For the 
past 5 Myr, Mars’ obliquity has oscillated around a 
value of 25° but from 5 Myr to 10 Myr ago, the mean 
obliquity was ∼35° and the maximum obliquity was 
almost 50° [7]. At high obliquities, the maximum inso-
lation is up to 2.5 times the present value, and surface 
temperatures at 68 N latitude can exceed 273 K up to 
100 days per year [8].  At obliquity of 45° the 253K 
isotherm, the measured limit for terrestrial microbial 
growth, extends to ~1 m depth for durations allowing 
microbial growth and population rebound [8].  
Terrestrial permafrost communities are an ex-
ample of possible life in the ice-rich regolith. Studies 
in permafrost have shown that microorganisms can 
function in ice-soil mixtures at temperatures as low as  
-20°C, living in thin films of interfacial water [9]. 
Ground ice can preserve living cells, biological materi-
al, and organic compounds for long periods of time. 
Microbes have been cultured from Antarctic ground 
ice that is up to 8M years old [10]. If life is present N. 
polar ground ice, growing when conditions allow, bi-
omolecular evidence of life should accumulate in the 
soils. 
Surface Age Analysis: Detecting biosignatures 
produced during episodes of high obliquity depends on 
accessing ice that has experienced growth periods so it 
is important to understand the erosional and deposi-
tional processes in the landing region. Questions rele-
vant to a sampling mission include: What has been the 
net level of deposition or erosion over time?  What is 
the thickness of the ice table and how abundant is the 
ice?  The characterization of crater morphology and 
degradation rates can help assess the presence of vola-
tiles in the regolith, while the characterization of the 
ejecta can address whether the site has seen net deposi-
tion that would bury the ejecta blocks and, by exten-
sion, make the most relevant material inaccessible. 
We inspected all HiRISE images acquired to 
date over the area including the Phoenix landing el-
lipse, covering a total contiguous area of about 4000 
km2 with the aim of cataloguing every identifiable 
crater within the ellipse on the basis of size, presence 
of ejecta, and degree of crater degradation [11]. Char-
acterization was facilitated by grouping the craters into 
size bins of 1) less than 100 m, 2)100-300m, 3) 300-
1000m and 4) greater than 1000m. The craters in bins 
(1) and (2) rarely exhibit ejecta blocks. The transition 
to craters with ejecta occurs from crater diameters of 
200 to 350 meters. The lack of ejecta in the small cra-
ters suggests that an incompetent layer of ice-rich ma-
terial is present to a depth of 20 to 35 m. Small craters 
disappear rapidly leaving no trace except a circular 
fracture highlighted by frost in spring.  Crater removal 
rates are substantial: craters less than 100 m lose relief 
within 105 years. Hence, crater infilling in the small 
craters constitutes a sedimentary process in the region, 
and potentially a method by which water ice is trapped 
into the subsurface. In contrast, ejecta blocks detecta-
ble down to 40 to 80 cm size are associated with cra-
ters in bins 3 and 4 except in the most degraded cra-
ters. This and the size frequency distribution for all 
craters larger than 500 m compared to a model crater 
frequency distribution [12] is consistent with a surface 
age of ~1 Gyr with deflation being the dominant pro-
cess. 
Discussion: In contrast to the generally static to 
slow erosional history of the region as a whole, net 
deposition occurs within small craters. The infilling 
process is likely related to annual deposition of dust 
during winter when atmospheric CO2 condenses onto 
the surface at this latitude, and then sublimates in the 
https://ntrs.nasa.gov/search.jsp?R=20200001524 2020-03-28T19:14:31+00:00Z
spring, leaving the water ice and dust it brought with it 
behind. Winter images show bright deposits in the 
small fresh craters that do not fully sublimate away 
through the summer, implying a slow accumulation of 
this material within the craters.  The crater bowls may 
prevent the wind from removing the icy dust. If a suf-
ficiently large part of the landing ellipse has been re-
surfaced by impacts and subsequent crater infilling 
since the latest habitable period, the scientific viability 
of a mission seeking to target the habitable zone would 
be compromised. Since the habitable zone extends to 
no more than 1m depth, we determined the fraction of 
terrain excavated and infilled to at least that depth in 
the last 10 Myr.  Given a crater depth-to diameter ratio 
of 1:5, craters as small as 5 m in diameter forming in 
the last 10 Myr will remove the record of the most 
recent habitable period. We estimate that 1.2% of the 
area surveyed has been resurfaced by craters smaller 
than 100 meters in diameter in 10Ma. 
Icebreaker Mission Concept:  The Mars Ice-
breaker mission [13] proposes to land in high latitude 
ice rich terrain with a payload designed to address the 
following science goals: (1) search for molecular sig-
natures of life; (2) assess the habitability of the ice 
bearing soils in the context of orbital cycles.  The mis-
sion features a 1-m drill to auger subsurface icy mate-
rial to the surface where it is delivered to payload in-
struments that search for organic molecules and have 
the capability to distinguish patterns in amino acids 
and fatty acids to discriminate biotic from abiotic 
sources of organics. By drilling to 1m the full depth of 
the habitable zone can be probed. The Icebreaker pay-
load fits on the same spacecraft/ lander used by Phoe-
nix and can be accomplished within the Discovery 
mission cost profile.  The solar powered mission is 
constrained to accomplish its objectives in less than 
180 sols.  This can be accomplished only with careful 
operational planning and a highly autonomous drilling 
system.  To facilitate its maturity, the life search pay-
load and sampling system has been extensively tested 
in field experiments performed in the Chilean Atacama 
desert performed under the NASA PSTAR pro-
gram[14]. 
 References: [1] Stoker, C.R. et al. (2010) J. 
Geophys. Res. Doi:10.1029/2009JE00342. [2] Mellon, 
M. et al. (2009) J. Geophys. Res. 
Doi:10.1029/2009JE003417. [3] Boynton, W. et al. 
(2009) Science, 325, 61–64, 
Doi:10.1126/science.1172768. [4] Hecht, M. et al. 
(2009) Science 325, 64-67. [5] Coates, J. and 
Achenback, L. (2004). Nat. Rev. Microbol. 2, 596-580, 
Doi:10.1038/nrmicro926. [6] Richardson, M. and 
Michna, M. (2005) J. Geophys. Res. 
Doi:10.1029/2004JE002367. [7] Laskar et al. (2002) 
Nature Doi:10.1038/nature01066. [8] Zent, A. (2008) 
Icarus 196, 385-408. [9] Rivkina, E. et al. (2000) Appl. 
Env. Microbiology 66 (8) 3230-3233. [10] Bidle, K.D. 
et al. (2007)  PNAS 104(33):13455-13460. [11] Noe 
Dobrea, E.Z. et al. (2019) Icarus submitted. [12] 
Hartmann, W. (2005) Icarus 174: 294-320. [13] 
McKay, C.P. et al. (2013) Astrobiology 13, 334-353. 
[14] Stoker, C.R. et al. AbSciCon 2019.  


We Should Search for Life in Mars N. Polar Ground Ice

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