Relation to Step 1 science section. There are no major changes
in the Science section. Responses to review comments are in red sections.
1. SCIENTIFIC GOALS AND OBJECTIVES
1.A Introduction and Mission Summary
The science goals of NASA are to understand the formation, evolution, and present state of the solar system, the galaxy, and the universe. Most planetary missions investigate the present state of planetary objects. By, in effect, going back in time, Genesis addresses questions about the materials and processes involved in the origins of the solar system by providing precise knowledge of solar isotopic and elemental compositions, a cornerstone data set around which theories for materials, processes, events, and time scales in the solar nebula are built, and from which theories about the evolution of planets begin. This is illustrated further in the non-technical summary found in Box 1-1. Moreover, Genesis tests the basic assumption that solar and solar nebula compositions are the same. In order to better communicate these motivations to the public, we have adopted "Genesis" as a mission name in lieu of "Suess-Urey", used previously.
Genesis measures solar composition by collecting solar wind for
analysis in terrestrial laboratories. The solar wind is just a
convenient source of solar matter readily available outside the
terrestrial magnetosphere. Solar wind ions have velocities in
the well-understood ion implantation regime and are quantitatively
retained upon striking passive collectors. This was demonstrated
by the highly successful Apollo solar wind foil experiments [Geiss
et al., 1972]. With 100-times longer exposure and, especially,
with purer collector materials, Genesis provides precise solar
isotopic compositions and greatly improved solar elemental composition
for most of the Periodic Table. (The Apollo foils were only sufficiently
pure for the study of noble gases.)
1.B Need for this Mission; Value to Planetary Science
(a). Planetary Science Requires Higher Precision. Solar
composition is important for astrophysics and solar physics, but
planetary science requires greater elemental coverage and much
higher levels of precision. For example, most theories of stellar
nucleosynthesis are considered successful if solar system isotope
ratios are reproduced within a factor of 2. By contrast, isotopic
measurements of terrestrial, lunar, martian, and meteoritic materials
deal with 0.1% and smaller differences. In atmospheric modeling,
differences of percents are crucial for, e.g., 38Ar/36Ar
and the Xe isotopes.
(b). Sample Return is Required. The sensitivities and accuracies
required for planetary science can be achieved only by analysis
in sophisticated terrestrial laboratories. The major advantages
of planetary sample return missions for understanding planetary
material and objects are summarized in Table 1.1.
(c). Essentially Nothing is Known about Solar Isotopic Compositions.
Solar isotopic compositions should be the reference point for
comparisons with planetary matter. The only practical source of
precise solar isotopic abundances is the solar wind. Omitting
details here, no solar-terrestrial differences can be seen for
C, O, and Mg isotopes, but uncertainties (5-40%) are too large
for planetary science purposes (section a. above). The
Apollo foils provided precise solar wind He and Ne isotope ratios
with a 20Ne/22Ne ratio, surprisingly, 38%
greater than the terrestrial atmosphere. The above three sentences
summarize everything that is known. The magnitude of the Ne variations
are likely to be exceptionally large (Section 1.C.2), thus Genesis
is designed to fill this fundamental knowledge gap by measuring
solar isotopic ratios to a minimum of 1% (2 sigma) and much better
in important cases (Section 2.A.2).
(d). Solar Elemental Abundances Can be Greatly Improved.
The observed diversity in solar system objects is chemical in
origin. Quantitativly, diversity can be defined as the difference
in planet composition from solar compsition, illustrating the
importance of solar elemental abundances. The present best source
of solar abundances comes from analysis of photospheric absorption
lines in the solar spectrum. A small number of elements have quoted
errors of ±10% (one sigma), but overall there are large uncertainties
in these abundances and a significant number of elements cannot
be measured. Thus, compilations of "solar" abundances
for non-volatile elements are currently based on analyses of carbonaceous
(CI) chondrite meteorites. The limitations to this have been discussed
[Web Document B]. Solar abundances
should be based on solar data. It is quite possible that a
new CI fall or Antarctic find would have slightly different abundances
than known CI meteorites, presenting a major challenge to how
well we think we know solar abundances. If solar composition is
based on solar data, we are immune to such a perturbation. The
best hope for major improvement in knowledge of solar abundances
is the solar wind.
(e). Genesis has an Important Legacy. Because planetary
objects are complex and resources are limited, NASA cannot afford
missions that completely characterize planetary objects. Knowledge
must be accumulated incrementally, and it is likely that -- in
fact one should hope that -- by the end of the 21st century the
information obtained with 20th century missions will be relatively
obsolete. In contrast, assuming that Genesis is successful, there
need not be a series of solar wind sample return missions. Genesis
will return a reservoir of solar matter which can be used to meet
presently unforeseen requirements for solar composition. When
more precise data are needed, it is likely that improved analytical
techniques will be developed to meet those requirements using
curated samples acquired by Genesis.
(f). Relation to NASA Science Planning These studies focus
on exploring the present solar system and do not specifically
address solar composition. Nevertheless, Genesis objectives
are totally consistent with NASA Solar System Exploration goals,
as spelled out in the National Research Council's Committee on
Planetary and Lunar Exploration (COMPLEX) report, "An Integrated
Strategy for the Planetary Sciences: 1995-2010". In this
report (p. 3) we find: "The broad scientific goals for solar
system exploration are to: Understand how physical and chemical
processes determine the main characteristics of the planets...;
Learn how planetary systems originate and evolve;...". Under
the primary objectives for understanding origins, COMPLEX includes
"define the conditions and processes active during the evolution
of the solar nebula" (p. 3) and "Construct an internally
consistent, quantitative theory of the formation of our entire
planetary system that contains sufficient detail to permit comparison
with as much observational evidence as possible" (p. 4).
The primary observational evidence about solar nebular processes
is compositional, and Genesis results provide the "enabling
technology" to reach the COMPLEX goals. Moreover, Genesis
makes major contributions to the understanding of planetary atmospheres;
on p. 132 of the report we find that a major objective is to "measure
the isotopic ratios of the reactive elements H, C, N, and O and
of the noble gases to a minimum accuracy of 10% for all substantial
planetary atmospheres to enable meaningful comparisons with elemental
compositions observed in the Sun, in meteorites, and in other
planets." Such comparisons will not be "meaningful"
without precise solar data from Genesis. Finally, one of COMPLEX's
approaches to ranking scientific objectives is to "prioritize
scientific questions of significance to the whole of the planetary
sciences rather than to just localized regions of the solar system"
(p. 25). The broad application of the results of the Genesis mission,
from planetary atmospheres, to lunar soils, to meteorites, and
to the evolution of the Sun and nebula certainly meets that criterion
for high priority. Genesis science goals also relate closely to
those of the Origins program. For example, in
"Mission to
the Solar System: Exploration and Discovery ... Roadmap",
in a campaign entitled "Building Blocks and Our Chemical
Origins" (p. 14), a major purpose is to "determine how
the present solar system evolved from the solar nebula to the
early planets." This is our major science goal, as stated
in the
. The recent Origins program brochure (JPL 400-639;
11/96) has considerable overlap with Box 1-1.
1.C Specific Measurement Objectives: What They Prove.
TABLE 1-2: PRIORITIZED MEASUREMENT OBJECTIVES
1.C.1 Introduction
Flow from Goals to Mission Design. Our science goals
(
Sheet; Section 1.A) lead to the operational
Science Objectives given on the Fact Sheet. From these general
science objectives we derive a well-defined set of 18 prioritized
specific measurement objectives (Table 1-2). In turn, these specific
measurement objectives define the mission science requirements
from which flow the instruments and mission design. The Baseline
Mission presented here meets all of these measurement objectives.
No new instruments are required for objectives 5-18 beyond those
required for 1-4.
(Measurement of bulk solar wind except when noted)
| Proposal Sections | ||
| (1) | O isotopes | Box 1-2 |
| (2) | N isotopes (a) | 1.C.2, 1.C.3 |
| (3) | Noble gas elements and isotopes (a) | 1.C.2 |
| (4) | Noble gas elements and isotopes, individual s.w. regimes | 1.D |
| ****** SCIENCE FLOOR ****** | ||
| (5) | C isotopes (a) | 1.D |
| (6) | C isotopes in individual solar wind regimes | 1.D |
| (7) | Mg,Ca,Ti,Cr,Ba isotopes | 1.C.1, 1.C.4.b |
| (8) | Mass 80-100 and 120-140 elemental abundance patterns | 1.C.4.b |
| (9) | Survey of solar-terrestrial isotopic differences | 1.C.1, 1.C.4.b |
| (10) | Noble gas and N, elements and isotopes for higher energy solar particles | 1.C.5.a |
| (11) | Li, Be, B elemental and isotopic abundances | 1.C.4.c, 1.C.5.b |
| (12) | F abundance | 1.C.5.b |
| (13) | Pt-group elemental abundances | 1.C.1 |
| (14) | Key s-process heavy elements | 1.C.4.a |
| (15) | Heavy-light element comparisons | 1.C.4.c |
| (16) | Solar rare earth element abundance pattern | 1.C.1 |
| (17) | Comparison of solar and chondritic elemental abundances | 1.C.1 |
| (18) | Radioactive nuclei in the solar wind (a) | 1.C.5.b |
Importance of Isotopes. An important distinction is between isotopic and elemental composition measurements. Higher priority (first 7 objectives) is given to isotopes [Web Document A]. Determination of isotopic differences among different parts of the solar system is of enormous importance, e.g. the large variations in D/H already known among planetary atmospheres provide major constraints on atmospheric evolution. Even if there should be some elements for which no isotopic differences are found among solar and planetary materials, such null results are important because other elements (O, N, noble gases, C, Ti, Cr) are already known to show isotopic variations among bulk planetary materials. The highest priority is given to O isotopes, as widespread variations are already documented.
Measurement Objective Overview. The measurement objectives
in Table 1-2 are a good mix of surveys and focused studies that
address specific, important problems. The isotopic and elemental
surveys (9 and 17) are perhaps most likely to provide conceptual
breakthroughs by producing totally unanticipated results, but
they do not require discussion. Objectives (13) and (16) select
groups of elements whose relative abundances figure prominently
in a variety of cosmochemical applications. Objective (7) selects
specific nonvolatile elements for which there is the greatest
probability of finding isotopic differences
(Section 1.C.5.a below).
All of the other objectives represent focused studies whose importance
is summarized in the remainder of Section 1.C.
1.C.2 Volatile Elements; Implications for Planetary
Atmospheres. Volatile carbon compounds, nitrogen, and the noble
gases are the tracers of choice for tracking evolutionary processes
in planetary atmospheres. Knowledge of the initial isotopic
abundances of volatile species is imperative for identifying evolutionary
mechanisms, whether the modeling is forward, from presumed
primordial atmospheres, or backward from known compositions in
contemporary atmospheres. In many models, the initial compositions
of unevolved primordial atmospheres are assumed to be the same
as the composition of the present solar wind. With Genesis, it
is possible to address many aspects of the evolution of planetary
atmospheres; two examples are given here: (i) If as many believe,
the difference between the 20Ne/22Ne ratios
in the solar wind and in the terrestrial atmosphere resulted from
hydrodynamic escape of gases from our atmosphere, predictions
of escape models for other volatile species can be tested using
Genesis data. For example Pepin [1991, Figure 4] predicts that
the solar wind 36Ar/38Ar should be 9% higher
than the terrestrial atmosphere. (ii) Precise measurement of the
solar wind 129Xe abundance provides a needed initial
value for models of mantle outgassing of the terrestrial planets
based on 129I radioactive decay [e.g. Porcelli and
Wasserburg, 1995].
Box 1-2: The Importance of Oxygen Isotopes
One of the most remarkable results from the study of planetary
materials is that relatively large isotopic variations occur
in an abundant element -- oxygen. As illustrated in the
Figure,
different parts of the solar system have distinct proportions
of the three O isotopes: 16O,
17O, 18O. The cause of the variations
is unknown. An important goal planetary science is to discover
and interpret the solar-system-wide structure in O isotopes.
Understanding solar nebular evolution is inextricably linked to
understanding the O isotope variations. In fact it is widely
accepted that the origin of the solar system cannot be
understood without understanding the origins of the O isotope
variations.
As the major mass reservoir in the solar system, the Sun is obviously
important, and the solar O isotopic composition plays a key role
in the interpretation of planetary evolution. However,
as shown,
the error bars on present estimates of solar O isotopic composition
exceed the size of the figure. For these reasons measurement
of O isotopic composition of the Sun is the highest priority measurement
objective for Genesis.
At present, a popular model is that the variations are caused
by an inhomogeneous distribution of 16O-rich interstellar grains
at an early stage in the solar nebula [Clayton, 1993]. Planetary
materials today represent varying degrees of mixing of these grains
with a homogenized, relatively 16O-poor reservoir (nebular gas?).
If this model is correct, the O isotope variations provide a
map of the distribution of 16O-rich
interstellar materials in the solar nebula. Furthermore, the
precise position of the solar matter on the oxygen map constrains
the composition of the admixed interstellar materials. A general
prediction of this model is that the solar composition should
lie on the "mixing line" in the
figure. Data from the
Murchison meteorite [Clayton and Mayeda, 1984] give a specific
prediction of the solar composition (SM on the
figure). A second
possibility [Thiemens, 1996] is that the variations are a consequence
of solar system molecular processes. Specific molecular models
require knowledge of the solar O isotopic composition to make
quantitative predictions. A third model, based on differential
self-shielding effects in molecular photodissociation, predicts
solids to be rich in 17O
and 18O
compared to the nebular gas (opposite of the first model).
More detailed discussion
is available.
1.C.3 Lunar N Isotopes. Perhaps the major unsolved
mystery from Apollo is an observed variation in the 15N/14N
ratio with age for lunar surface samples. This trend might represent:
(1) a systematic change in the isotopic composition of solar wind
N [e.g. Kerridge, 1993], (2) the presence of nonsolar sources
of N on the lunar surface (lunar interior, Earth's early atmosphere,
etc.) early in the history of the Moon [e.g. Bochsler, 1994] or
(3) dominance of higher energy solar particles with low 15N/14N
in the older lunar samples [e.g. Bochsler and Kallenbach, 1994].
All 3 mechanisms have important implications, but none is completely
satisfactory. Mechanisms (2) and (3) predict that Genesis should
observe a 15N/14N ratio 10-20% higher than
in the terrestrial atmosphere. One model for mechanism (1) predicts
only a 2-4% enrichment [Kim et al., 1996]. Thus some, possibly
all, models will need revision. Terrestrial atmospheric hydrodynamic
escape models are only compatible with (1), but require a 10-20%
enrichment. Mechanism (3) can be directly tested by Genesis measurements
of higher energy ions (Objective 10). The possibility that mechanism
(1) represents solar surface nuclear processes will be tested
with data from Objectives (11), (12),
and (18) (Section 1.C.5).
If we are able to confirm mechanism (1), it would mean that evolutionary
processes can change solar isotopic compositions from those in
the solar nebula, complicating to varying degrees our ability
to infer initial solar system isotopic compositions depending
on details (which elements, etc.). However, confirmation of (1)
would be a conclusion of major importance because there is as
yet no known solar process which could cause the required evolution
of N isotopes. This is why the lunar N isotopic data have remained
a mystery.
1.C.4 Tests of Fundamental Assumptions. The solar
system formed 4.6x109 yr ago by isolation of an ~1
solar mass core within a larger molecular cloud. In the 17 orders
of magnitude density increase accompanying the collapse of the
Sun to main sequence, many events/processes occurred (loss of
magnetic fields, nebular disk formation, transfer of angular momentum,
bipolar jets, etc.), and it is far from obvious that these processes
were isochemical, as is implicitly assumed at present. Tests of
such early fractionation events are challenging but not impossible.
(Section 1.C.4.a) Even after the basic nebular structure was established,
more testable events are conceivable which could result in Sun-nebula
chemical fractionations (Sections 1.C.4.b and c).
Present knowledge
of elemental abundances precludes large (factor 2) b or c type
fractionations; consequently, at present we are forced to assume
that differences do not exist. However, it is not only the magnitude
of fractionations that is important. Quantitative knowledge of
which elements are fractionated and of the relative amounts of
fractionation can be used to define the events and processes that
caused the fractionation even if the residual fractionations are
percentagewise small because of later nebular mixing.
1.C.4.a Fractionations of Sun/Nebula Relative to Our Parent Molecular
Cloud. Such fractionations would be inherited by both solar and
planetary matter, and thus difficult to detect. The only basis
for detection is by quantitative comparisons between solar composition
and nucleosynthesis predictions. Except for the s-process, such
predictions do not now exist, but one can realistically expect
major progress as the result of increased observational knowledge
based on interstellar grains in meteorites and direct measurements
of interstellar grains from missions such as Stardust. The precision
of Genesis data and the ability to get improved data on curated
samples would permit observational accuracy to remain ahead of
theoretical uncertainties for the foreseeable future. A bigger
problem is that the only way to validate nucleosynthesis theory
is by comparison with solar abundances. Bootstrapping is required,
but as the Sun-nebula fractionation processes affect elemental
ratios, one general approach is to validate the nucleosynthesis
theory on the basis of predicted isotope ratios. Near term, the
only reasonably quantitative nucleosynthesis theory is the "main"
s-process for nuclei with mass greater than 100 [e.g. Kaeppler,
1989]. The most relevant chemical properties producing fractionations
are probably volatility and first ionization potential (FIP).
Consequently, one would look for differences between theoretical
and the Genesis relative abundances of key main s-process elements
(Ru, Te, Xe, Ba, Sm, Yb) which correlate with either FIP or volatility.
1.C.4.b Are Solar Photosphere and Solar Nebula Compositions Different?
No differences are expected based on the conventional assumption
that the Sun and planets formed from a common mixed reservoir,
but if this is not true, systematic differences in solar wind
isotopic compositions compared to those from planetary materials
would be observed (Objective 9). See also
[Web Document D].
i) Relation to Meteorite Isotope Anomalies. These anomalies are
complex and poorly understood. Oversimplified, they indicate (1)
a lack of total mixing of isotopically inhomogeneous presolar
material in the solar nebula (Box 1-1) and (2) isotope variations
ascribable to decay of "extinct" radioactive nuclei
(lifetimes << 4.5x109 yr, but comparable to the
time interval between formation of the Sun and the earliest nebular
materials). Type (2) anomalies might be accompanied by systematic
isotopic differences between solar and planetary matter if there
were late injections (supernovae?) of interstellar material. Although
controversial, it could be that variations in isotopic abundances
of extinct nuclei
(53Cr/52Cr,
26Al/27Al,
etc.) [e.g. Lugmair et al., 1996] point to such injections. Some
type (1) anomalies are only found in the first formed materials
(CAIs). Presumably, the additional mixing during subsequent nebular
evolution thoroughly eliminated most of the interstellar heterogeneities
in younger materials. (Oxygen is a glaring exception and, for
this reason, is given special status.)
It is presently not known whether the degree of mixing, and the
proportions of interstellar components mixed, applies only to
the 1-3 AU range from which presently-available planetary materials
come. Comparisons of solar and meteoritic isotopes test whether
the 1-3 AU mixing proportions correspond to average solar matter.
Measured differences can be used to identify the interstellar
sources of solar system matter. The magnitude of the differences
is of secondary importance; the important issues are whether they
are analytically significant and how precisely differences can
be measured. Based on meteorite anomalies [Web
Document A],
we give higher priority to the analyses of five non-volatile elements
(Objective 7) that appear to have the highest probability
of showing isotopic variations.
ii) Tests for Volatile Element Fractionations. Preferential
accretion of gas or of dust by the Sun can be tested. Preferential
here means relative to average initial solar system proportions.
Such differences would show up in a comparison of the abundances
of Kr and Xe estimated by interpolation from abundances of neighboring,
less-volatile elements (Se,Br,Rb,Sr,Y,Te,I,Cs,Ba) with the actual
measured Kr and Xe abundances [Wiens et al., 1991, 1992;
Web Document C].
(Interpolation is not accurate for other volatile
elements.) If the Sun formed from non-representative proportions
of volatile and non-volatile materials, compared to the average
for our parent molecular cloud, then there would be differences
between the interpolated and measured abundances. This is the
science focus of Objective 8. Corrections will be
required for first ionization potential differences between Kr
or Xe and the neighboring elements (Section 1.D), but tests for
fractionation at the 10% level should be possible.
1.C.4.c Do Photosphere and Initial bulk Sun Compositions Differ?
[See Web Document F for a less abreviated
discussion of the same material].
It is widely assumed that the average solar system elemental composition
is preserved by the material on the surface of the Sun. This is
because the present surface mixing zone (SMZ) and radiative interior
of the Sun are calculated to have formed very early, before thermonuclear
burning of H could produce abundance changes. Recent solar models
[e.g. Proffitt, 1994] allow for gravitational settling, thermal
gradient diffusion, and differential radiation pressure which
collectively produce compositional gradients beneath a well-mixed
SMZ. "Settling out" of heavier elements at the base
of the SMZ can occur in principle, but the turbulent nature of
the mixing, (e.g. convective overshoots) counters the settling.
Theoretical differences [e.g. Proffitt, 1994] between SMZ (photospheric)
and initial abundances are predicted to be small (order percents
for elements other than He), but it is important to make observational
tests to see if differences have been underestimated. The non-turbulent
parts of the calculation have large uncertainties while the turbulent
effects cannot be calculated. The abundances of 6Li,
7Li, 9Be, 10B, and 11B
(Objective 10) are sensitive
to these processes because of thermonuclear
destruction at the base of the SMZ, either during the early totally
convective (Hayashi) phase or over the time since the onset of
H burning. Although one needs real data to make interpretations,
suppose that the relative solar wind abundances of a few light
elements (X,Y,Z) closely match those of CI chondrites, then a
close comparison of Genesis elemental ratios, e.g. A/X, where
A is a heavy and X a light element, with the A/X ratio for CI
chondrites can test for heavy element depletions from the SMZ.
The best test is to select elements, e.g. Ni and Ir, differing
greatly in mass but with similar first ionization potentials and
cosmochemical properties.
1.C.5 Constraints on Solar Processes and History.
1.C.5.a History of the Sun from Lunar Samples. Lunar surface samples
have been directly exposed to fluxes of solar particles over the
last ~3x109 yr, in principle enabling the determination
of solar wind composition in the past. However, this record has
also been severely modified by impact processes. Non-solar inputs
(lunar interior outgassing, cometary, interstellar, etc.) are
probably also present; recognition and quantification of these
inputs is of major importance. Although significant progress has
been made in the last decade, it is clear that independent
knowledge of contemporary solar wind noble gas, nitrogen, and
carbon elemental and isotopic abundances is required in order
to interpret the lunar data. The lunar data and their relations
to Genesis are very complex, but as an example of one specific
issue for which Genesis provides a clean test, there is a consensus
that solar particles at greater depths (i.e. higher energy) have
different isotopic compositions than those released from shallower
depths (presumably solar wind) [Web
Document A]. For example the deeper ions have
20Ne/22Ne around 11, whereas more shallow
ions are consistent with the 13.7 ratio observed with the Apollo
foils. Based on fluences inferred from lunar samples, it should
be possible to measure the elemental and isotopic abundances of
noble gases at greater depths in the collector, corresponding
to ion energies greater than the solar wind (Objective 10). The
changes in isotopic and elemental noble gas ratios inferred from
the lunar data can be directly compared with the higher energy
ion compositions from Genesis.
1.C.5.b Solar Surface Nuclear Reactions. Key abundance ratios,
e.g., 19F/20Ne, 11B/14N
or the fluences of radioactive nuclei such as 14C or
10Be (Objectives 11, 12, and 18) are measures of integrated
solar surface nuclear processes on different time scales. There
is evidence for solar wind 14C in lunar samples [Jull
et al., 1994], with inferred fluxes measurable by Genesis
[Web Document E].
Accelerated protons reacting with even a very small
fraction of the 20Ne may produce a very large enhancement
of 19F because the overall 19F/20Ne
is very small. It is widely accepted that solar activity was higher
in the past, and the Genesis data can be compared with predictions
based on independent measurements of present-day solar surface
activity, giving quantitative measures of past solar activity.
1.D Are There Differences Between Solar Wind and Solar Composition?
1.D.1 Elemental Fractionation. The situation is different for
elements and isotopes. In situ spacecraft instruments observe
differences in element ratios between the photosphere and
the solar wind. From systematics and theoretical work [e.g. Marsch
et al., 1995], it is well-established that elemental fractionation
depends on: (1) first ionization time (FIT), i.e. the time required
for an atom in the solar atmosphere to become ionized. FIT is
a function of the atomís first ionization potential (FIP)
and solar physical conditions. The primary fractionation is a
relative enhancement of low-FIT (easily ionized) elements as a
group compared with high-FIT elements. (2) ion charge and mass.
Once an atom is ionized, it is subjected to Coulomb drag in the
flowing plasma, which depends on both charge and mass. This is
smaller than the FIT effect (both observationally and theoretically)
and (3) solar wind regime.
1.D.1.a Solar Wind Regimes. These refer to the different sources
of the solar-wind. There are two types of solar wind flow -- quasi-stationary
and transient. There are two major sources of quasi-stationary
wind -- fast wind from coronal holes (CH) and slow "interstream"
(IS) wind originating in or near coronal streamers. Transient
flows are produced by eruptions [coronal mass ejections (CME)]
associated with the disruption of magnetic field lines closed
above the solar surface. A CME can have either low or high speed.
The CH, IS, and CME constitute distinguishable solar wind regimes.
We estimate 30% CH, 65% IS, and 15% CME. The proportions vary
with solar cycle, but our overall sampling approach is not dependent
on when the mission occurs in the solar cycle. The strength
of high FIT depletions depends on regime, being less in coronal
hole flow than in interstream or CME flows. Further: (1) elements
with short ionization times show no relative fractionation within
a given solar wind regime, and (2) the fractionation of elements
with long ionization times tend to lie on a single curve, roughly
proportional to (FIT)-1/2, independent of regime [von
Steiger et al., 1995]. Increased understanding of FIT systematics
is expected as a result of the Ulysses, WIND, SOHO, and ACE (Section
1.E.1). See [Web Document H]
for additional discussion.
1.D.2 Correcting for FIT Elemental Fractionation. A boot-strap
process will be used assuming that a few spectroscopic photospheric
relative abundances (RP), e.g. Na/S and Ca/Si, are
known accurately (nominally around 10%). For a given element pair,
the double ratios, RG/RP (G=Genesis) provide
measured fractionations which can test FIT/FIP models proposed
from in-situ data and theory. The best models can then be used
to provide fractionation corrections for those elements whose
photospheric abundances are not well known. The reliability of
these corrections is enormously enhanced because Genesis will
obtain separate samples for each of the 3 solar wind regimes (see
Section 2.B for how we do this).
Because the amount of FIT fractionation
varies with regime, the amount of correction will also differ,
giving independent measurements of the corrected abundances. Reliability
will also be enhanced because models will be much better in 2005
than those today. The tests of these models provided by Genesis
data will be significant for solar physics. (See
Web Document G
for further discussion).
It should be emphasized that there is no observational evidence
that any corrections are necessary to determine the relative abundances
of the subset of low-FIT (also non-volatile) elements from which
the solid planets of the inner solar system material are made.
1.D.3 Isotopic Fractionation. [See also
Table 1-2).
There will be no ambiguity. If present, Coulomb drag isotope
fractionations would be easily recognizable from data systematics,
and the amounts of fractionation would be of considerable importance
to solar physics. The bottom line is that the solar wind is the
only plausible source of precise solar isotopic compositions.
If solar wind isotopes are fractionated, we need to know this.
Genesis, and perhaps only Genesis, can resolve the issue.
1.E Relation to Other Missions.
1.E.1 Complementarity of Sample Return and in-situ Measurements.
Some knowledge of solar wind composition can be obtained by ion
mass spectrometers on spacecraft [Web
Document G.2]. The Genesis
team understands the capabilities of the instruments on ISEE 3,
Ulysses, WIND, SOHO, and ACE. For major ion species, in-situ instruments
can now determine: (1) Velocity distributions (density, velocity,
temperature, anisotropy) as a function of time and solar wind
regime. (2) Charge state distributions. (3) Elemental abundances
for elements more abundant than Cl. (4) A few favorable isotopic
ratios e.g., 3He/4He, 24Mg/25Mg/26Mg,
20Ne/22Ne to within a few percent (1 sigma).
The solar wind samples returned by Genesis will extend the database
acquired by in-situ instruments by determining: (a) Elemental
abundances for much of the rest of the periodic table, including
important low-abundance light elements such as Li, Be, B, and
F and elements heavier than Ni to which the in-situ instruments
are not sensitive because of the lower abundances and inadequate
instrumental mass resolution. (b) Isotopic abundances at the precision
required for addressing planetary science objectives. For example,
for planetary issues, it is necessary to measure
17O/16O
to a precision better than the difference between 3.70x10-4
and 3.71x10-4. This requirement is set to match differences
measured in different types of meteorites, but such precision
is well beyond the capability of in situ instruments. In reality
the two types of measurements are highly complementary. As discussed
in Section 1.D, the results obtained by the in situ measurements
are essential to the interpretation of Genesis data. The objectives
of solar and heliospheric physics as well as planetary science
require both types of measurements [G. Gloeckler, private communication].
1.E.2 Can Genesis objectives be accomplished by Stardust? No.
The Stardust sample return capsule will be recovered, and some
materials will have been exposed to the solar wind. The solar-wind-exposed
surfaces are anodized Al and aerogel, both poor solar wind collectors.
Those surfaces will also be exposed to comet coma gases. We are
closely following the Stardust mission plans and are well-informed
on what is possible. By very careful analysis it might be feasible
to recover some solar wind data for noble gases, meeting only
1 of the 18 specific Genesis measurement objectives.
1.E.3 Relation to Galileo Probe Results. A long-standing major
issue is the extent to which the Jovian atmosphere is just a sample
of solar gas. We do not have space to discuss the pros and cons
of this important issue, but, in brief, although the precision
of the Probe isotope ratios is in doubt at present, some measured
isotope ratios (e.g. Ne and Ar) may be solar but elemental ratios
(e.g. Ne/Ar and C/H) may not. The only way to be sure is to have
precise independent data on solar composition.
