Chapter 8
Future Investigations of Comets
Given the details revealed by space missions in the past 35 years culminating in the
Rosetta mission, it is a valid question to ask whether there is any future for groundbased and Earth-orbiting observations of comets. However, there are several important aspects that space missions cannot answer within financially realistic boundary
conditions. The first of these is diversity. We are now aware that comets are by no
means identical and that there is enormous diversity in composition. Hence, observing many different objects has a clear goal in relating this diversity to origins.
The evolution of the outgassing with heliocentric distance is a related issue. It is a
rule-of-thumb that cometary activity increases from 2.7 AU inwards but this is
simply because this is the approximate distance at which the surface reaches the
free sublimation temperature of water ice. The relationship between the outgassing
of the major species (CO, CO 2 , and H 2 O) is more complex than this, as has been
clearly demonstrated by, for example, the presence of water ice particles in the inner
coma of 103P/Hartley 2 in comparison with 67P or by the evidence for CO 2
outgassing from the nightside of the nucleus of 67P which may or may not be a
universal phenomenon.
Given the timescales for future missions, it is also interesting to look at the
questions that our most recent spacecraft investigations have failed to answer and
at questions that have arisen as a consequence of those observations (see also
Thomas et al. 2019).
While the observations of nuclei have provided hints on the structure of surface
layers and specifically the porosity, the interrelation of volatiles and refractories and
their variation with depth at intermediate scales are still uncertain. Many of the issues
are connected to
– the density of cometary material with depth
– the refractory to volatile ratio with depth
– the volatile composition with depth
These three quantities would provide us with several pieces of information that
are blocking study of cometary activity, namely
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_8
439
Future Investigations of Comets
Given the details revealed by space missions in the past 35 years culminating in the
Rosetta mission, it is a valid question to ask whether there is any future for groundbased and Earth-orbiting observations of comets. However, there are several important aspects that space missions cannot answer within financially realistic boundary
conditions. The first of these is diversity. We are now aware that comets are by no
means identical and that there is enormous diversity in composition. Hence, observing many different objects has a clear goal in relating this diversity to origins.
The evolution of the outgassing with heliocentric distance is a related issue. It is a
rule-of-thumb that cometary activity increases from 2.7 AU inwards but this is
simply because this is the approximate distance at which the surface reaches the
free sublimation temperature of water ice. The relationship between the outgassing
of the major species (CO, CO 2 , and H 2 O) is more complex than this, as has been
clearly demonstrated by, for example, the presence of water ice particles in the inner
coma of 103P/Hartley 2 in comparison with 67P or by the evidence for CO 2
outgassing from the nightside of the nucleus of 67P which may or may not be a
universal phenomenon.
Given the timescales for future missions, it is also interesting to look at the
questions that our most recent spacecraft investigations have failed to answer and
at questions that have arisen as a consequence of those observations (see also
Thomas et al. 2019).
While the observations of nuclei have provided hints on the structure of surface
layers and specifically the porosity, the interrelation of volatiles and refractories and
their variation with depth at intermediate scales are still uncertain. Many of the issues
are connected to
– the density of cometary material with depth
– the refractory to volatile ratio with depth
– the volatile composition with depth
These three quantities would provide us with several pieces of information that
are blocking study of cometary activity, namely
© Springer Nature Switzerland AG 2020
N. Thomas, An Introduction to Comets, Astronomy and Astrophysics Library,
https://doi.org/10.1007/978-3-030-50574-5_8
439