Access to glacial and subglacial environments in the Solar System by melting probe technology

Abstract

A key aspect for understanding the biological and biochemical environment of subglacial waters, on Earth or other planets and moons in the Solar system, is the analysis of material embedded in or underneath icy layers on the surface. In particular the Antarctic lakes (most prominently Lake Vostok) but also the icy crust of Jupiter’s moon Europa or the polar caps of Mars require such investigation. One possible technique to penetrate thick ice layers with small and reliable probes is by melting, which does not require the heavy, complex and expensive equipment of a drilling rig. While melting probes have successfully been used for terrestrial applications e.g. in Antarctic ice, their performance in vacuum is different and theory needs confirmation by tests. Thus, a vacuum chamber has been used to perform a series of melting tests in cold (liquid nitrogen cooled) water ice samples. The feasibility of the method was demonstrated and the energy demand for a space mission could be estimated. Due to the high energy demand in case of extraterrestrial application (e.g. Europa or polar caps of Mars), only heating with radioactive isotopes seems feasible for reaching greater depths. The necessary power is driven by the desired penetration velocity (approximately linearly) and the dimensions of the probe (proportional to the cross section). In comparison to traditional drilling techniques the application of a melting probe for exploration of Antarctic lakes offers the advantage that biological contamination is minimized, since the Probe can be sterilized and the melting channel freezes immediately after the probe’s passage, inhibiting exchange with the surface layers and the atmosphere. In order to understand the physical and chemical nature of the ice layers, as well as for analysing the underlying water body, a melting probe needs to be equipped with a suite of scientific instruments that are capable of e.g. determining the chemical and isotopic composition of the embedded or dissolved materials.

Appendices

Appendix: Thermophysical properties of ices

1.1 Overview

Property	Symbol, Unit	H2O	CO2	CO	CH4
Molar mass	M, g/mol	18.01526 8	44.0098 ±  0.0016	28.01	16.043
Triple point temperature	Tt,K	273.16 (exact)	216.592 ±  0.001 (ITS-90 secondary ref. point)	68.05 ± 0.05	90.694(1) (ITS-90 secondary ref. point)
				68.13 ± 0.05 [CC]
Triple point pressure	Pt, Pa	611.655	(0.517950 ±  0.00010)E6	0.01537(3)	1.169(6) E4
Critical point temperature	Tc, K	647.096	304.128 ±  0.015	132.9	190.6
				134.45 ± 0.4
Critical point pressure	Pc, Pa	2.2064 E6	(7.3773 ±  0.0030) E6	(3.499 ± 0.03) E6	4.592 E6
Critical point density	ρc, kg/m3	322	467.6 ± 0.6	301	162.0(2)
Normal melting point	Tm, K	273.1525	(Triple point)	68.05	90.7
				68.08
Normal boiling point	Tb, K	373.124	194.686* (*sublimation pressure = 1 atm, ITS-90 secondary ref. point)	81.60	111.67 (ITS-90 secondary ref. point)
				81.65
Density of solid at triple point/melting point	ρs, kg/m3	916.700 ±  0.026	1541	919.8	489.8 (490 – 530 over the whole range)
Enthalpy of sublimation	Hsub, kJ/kg	2834.359	573.31 at Tb	261.3 [CC]	600(23) for 53–90K
Enthalpy of fusion	Hmelt, kJ/kg	333.44	196.65 at Tt	29.86(3)	58.5(2)
Enthalpy of vaporization	Hevap, kJ/kg	2500.5 (0°C) 2255.5 (100°C)		217	510–584
Specific heat capacity of solid at Tt	cp(s), kJ/kg/K	2.2	1.383	1.9	2.73
Thermal conductivity of solid	λs, W/m/K	2.1		0.303 (50 K)	0.4(1) at Tt(extrapolated)
Comments		Melting temperature under pressure p: t F /°C = 0 – 0.00076 p – 1.32E-6 p 2 with p (bar) for 0 <p < 2000 bar		Additional transition at 61.55 K, enthalpy change 22.62(14) kJ/kg	Rotational transition in solid at 20.48 K (ITS-90 secondary ref. point) with λ-peak in cp, enthalpy change 5.8 (1) kJ/kg

1.2 Temperature dependence of selected quantities

1.2.1 Density

With sufficient accuracy from 0 to 273 K:

$$\rho =933.31+0.037978T-3.6274\cdot 10^{-4}T^2[\hbox{kgm}^{-3}]. $$

1.2.2 Thermal conductivity

After (Slack 1980), best estimates of λ of Ih H₂O ice between 10 K and the melting point at atmospheric pressure:

$$ \lambda =619.2/T+58646/T^3+3.237\cdot 10^{-3}T-1.382\cdot 10^{-5}T^2 \hbox{in W/m/K} $$

(0.3)

This equation gives about 12% higher values than the (Klinger 1980) equation,

$$ \lambda =\frac{567}{T}\hbox{W/m/K} ({T}>50\hbox{ K}) $$

(0.4)

1.2.3 Specific heat capacity at constant pressure

$$ C_{\rm p}=x^3\frac{c_1+c_2x^2+c_3x^6}{1+c_4x^2+c_5x^4+c_6x^8}\hbox{[J/kg/K]} $$

x = T/T _t ,T _t  = 273.16 K and c₁ = 1.843· 10⁵, c₂ = 1.6357· 10⁸, c₃ = 3.5519· 10⁹, c₄ = 1.667· 10², c₅ = 6.465· 10⁴, c₆ = 1.6935· 10⁶ after Haida et al. (1974), Flubacher et al. (1960) and Giauque and Stout (1936).

Note that the pressure dependence of Cp is only about dCp/dP =  − 8.5e − 10^*T [J/K/kg/Pa] for 0.273.15 K at 1 bar.

1.2.4 Latent heat of sublimation

After Feistel (2006) for 0.273.15 K, approximated by a quadratic polynomial,

$$ L_{\rm s}=2636.77+1.65924T-0.0034135T^2\hbox{[kJ/kg]} $$

1.2.5 Viscosity

After Kestin et al. (1978), Hallet (1963) and IAPWS (2003); correlation for temperatures from − 24°C [supercooled] to 373 °C, at saturation pressure, ± 5%:

$$ \begin{aligned} &\eta (T)=A\left({\frac{T}{B}-1} \right)^\alpha \\ &A=1.4147\cdot 10^{-4}[\hbox{Pa s}] \\ &B=226.8[\hbox{K}] \\ &\alpha =-1.5914 \\ \end{aligned} $$

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