Thursday, March 31, 2016

EXOMARS, Trace Gas Orbiter & Schiaparelli

The objectives of ExoMars mission is to look for life beyond earth (exo-biology) and it is performed by sending an orbiter (Trace Gas Orbiter, TGO) and Lander (Schiaparelli) which is already on its way to the Red planet and preparing for a rover in the next available celestial window in 2018. It is a joint venture between ESA and Russian space agency Roscosmos. Instead of giving a routine information on this mission, I try to bring out the niche technologies utilized by the participating European countries in TGO, which are at the forefront of Planetary Exploration. Special emphasis is made to bring out those attractive aspects of the "suite of instruments" which are employed to hunt down the elusive secrets (methane) on Mars.

Courtesy: ESA 

Trace Gas Orbiter, as the name suggests is geared to look for trace gases, these are defined as the species which fall in "less than 1% composition" of a planet. Since the signature of life could be buried with methane, a trace gas, concentrations occurring at parts per billion (10-9) by volume; TGO carries a suite of instruments to monitor this gas primarily and many other trace gases which have been contemplated but never have been recorded at all. There is also an un-resolved issue of how the Martian atmosphere is lost; this could be accomplished by measuring ratios of isotopic species of various gaseous compounds with respect to the normal; example: HDO/H2O. By comparing the ratios with the one appearing earth; one can estimate the loss of lighter species as compared to heavy. In summary, the task of TGO is 1. to look for methane at ppb concentration levels (huge demand on sensitivity of the instruments) and 2. measure the ratios of isotopologus species (very high spectral resolution required) and 3. technology demonstration for landing in a thin Martian atmosphere (Schiaparelli).

A brief introduction on composition of Martian atmosphere:
Major gases [Martian atmospheric pressure ~ 10 torr (Earth's: 760 torr)]
CO2:  95%  (0.95)
N2 : 2.7%  (0.027)
Ar : 1.6% (0.016)

Trace gases
O2:  0.13% or 1.3X10-2
H2O:  2.0X10-4
CH4:  ~ 1.0X10-8 or parts per billion (10-9), (ppb)

For a composition analysing scientist measuring methane at a ppb level concentration is a million dollar ??? (or higher) question. This is where the Trace Gas Orbiter, TGO's journey to the red planet is holding huge expectations from the planetary scientists across the globe (which includes me).

There are 4-gem of instruments gear to break the technology barriers primarily geared in nailing down the elusive gas - Methane.
1. NOMAD (Nadir and Occultation for MArs Discovery)
2. ACS ( Atmospheric Chemistry Suite)
3.CaSSIS (Colour and Stereo Surface Imaging System)
4. FREND – Fine Resolution Epithermal Neutron Detector.

Each instrument's configuration and their scientific goals are nicely outlined here:

Courtesy, J.L. Vago, ESA
Both ACS and NOMAD are a set of spectrometers designed to measure a huge range of gaseous species (including isotopolagous), while CaSSIS is a color camera employed in imaging, FREND searches for H-atoms down to 1-meter looking for traces of buried water.

I spent 2-days in reading the "suite of instruments" NOMAD (Nadir and Occultation for MArs Discovery) carries and almost went MAD in appreciating the abundance amount of technological prowess this suite possesses. After debating on what proportion of "technical-popular" combination; I chose to take a middle path and try to keep the information flow in perspective so that both the casual and serious reader will be interested in reading beyond this paragraph.

Courtesy : ESA
(NOMAD instrument: 1. SO, 2. LNO, 3. UVIS, 4. Electronics)
NOMAD basically covers 0.2 – 4.3µ  spectral region with a set of 3-instruments operating in 3-different modes: 
1. SO: 2.3-4.3µ (Solar Occultation)
2. LNO : in 2.3-3.8µ (Limb, Nadir and Occultation) 
3. UV-VIS channel : 200-650 nm 

The SO mode is to look at the Sun during sunrise and sunset, while Nadir is looking straight down at the planet, LNO is a combination of limb scan /nadir view/Solar occultation. The table below is taken from a very recent publication of Robert et al appearing in Planetary and Space Sciences, outlining the greater details of what all NOMAD can deliver in different modes of observations.

Robert et al., Planet.Sp.Sc., 2016
Another notable point is the ability of NOMAD in detecting methane signal is given in the above said reference. Interestingly the NOMAD in LNO mode is capable of detecting methane even at 0.018 ppb (18 ppt) concentrations; which carries much superior sensitivity than the present day instruments both on Martian surface (CURIOSITY) and in the orbit (MOM: MSM, MENCA).

 Atmospheric Chemistry Suite is a kind of complementary, IR spectrometer again with a suite of 3- built-in instruments,covering a huge spectral region of 2.3 - 17µ  i.e. 1. NIR, 2. MIR and 3. TIRVIM.

Korablev, J.App. Remote Sensing, 2014

Just like NOMAD, ACS too has SO, Nadir and LNO observation modes explained in the above table.

The science goals of various sub-sytems are:

NIR instrument:
a. Monitoring and profiling of trace components, CO, H2O, O2
b. Vertical profiles of atmospheric density
c.  Sensitive search for new OH, O2 and NO night glow

MIR instrument:
a. Vertical profiles of СО2 (atmospheric density and temp.) ; minor species like CH4 , H2O, СО
b. Profiling of isotopic ratios HDO/H2O, 13CO2/CO2, CO18O/CO2

TIRVIM instrument:
a. Search/monitoring of minor constituents
b. Monitoring of atmospheric dust, and condensation clouds
c. Monitoring of the thermal state from the surface.

Most striking aspect of ACS is a huge spectral resolution (resolving power~ 50,000) it offers in the MIR region of 2.3-4.3µ. This would help in detecting the trace gas species first time ever: CH4, C2H2, H2S, HCl.  The TIRVIM region is helpful in:a. detecting trace gases, b. measurement of thermal profiles, c. aerosol properties and d. trace species: NO, N2O measurements.

The color camera's spatial coverage (swath) and resolution details are outlined here:

Courtesy: J.L. Vago, ESA

Artist's concept of the instreumnt and similar kind of observation made are given here:

Courtesy: J.L. Vago, ESA
TGO has added huge expectations among the Planetary scientists who are looking forward to fix the jigsaw puzzle on life beyond earth and in particular signatures of life on our next neighbour.

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