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Pluto, the Orange Frosty, served with a dash of Nitrogen, a pinch of Methane, and smidgen of Carbon Monoxide.

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Summary talk entries for the Pluto Science Conference continues. This is from the morning of July 24, 2013th on the topic of “Composition.”

Dale Cruikshank (NASA Ames) set the stage with a spectra-rich presentation and gave an overview talk about the “Surface Compositions of Pluto and Charon.” Putting it in context, even 45 years after Pluto was discovered, we did not know much about Pluto only where it was in the sky and its rotation period. That rapidly changed when Dale and colleagues saw strong evidence for solid methane on Pluto in 1976 (Cruikshank, Pilcher, Morrison, 1976 Science 194, 835), Jim Christy discovered the companion moon Charon in 1978, and repeated observations were made of Pluto and Charon in the 1980s.

Spectroscopy, the technique which spreads light into different wavelengths, has been a powerful diagnostic tool for the identification of molecular species, and therefore tells us the composition of the object. Low-resolution (R~100-500) spectra is sufficient to identify ice-solid features which are characterized by wide features, but higher resolution (R~1,000-10,000s) helps constrain models that determine temperature and also . New Horizons’ LEISA spectrometer covers the 1.25-2.5micron spectral band, with resolution R~240, and a mode of R~550 between 2.10-2.25 microns, making it ideal for identifying solid features. It’s proximity to Pluto during the July 2015 fly-by provides unprecedented spatial resolution. Compared to ground-based & Hubble spectral measurements which can only provide full-disk (~1500km/pix) measurements (because Pluto appears only in a few pixels), New Horizons’ LEISA will provide the true “first look” at the composition of Pluto at 6.0km/pix (global) with some patches at 2.7 km/pixel.

Images in this blog entry show flux (measure of amount of light) or albedo (measure of reflectance) versus wavelength.

Pluto Triton IR Spectra

Pluto’s near infrared spectrum (Grundy et al 2013) is rich in identifiable diagnostic solid materials, nitrogen (N2), methane (CH4) and carbon monoxide (CO). A comparison with Triton’s spectrum over the same wavelength is shown. Carbon dioxide (CO2) is suspiciously absent from Pluto’s atmosphere.

Pluto MIR Spectrum

Pluto’s mid-infrared (Protopapa et al 2008) show a series of methane bands. The gap at 4.2 microns is due to CO2 absorption from the Earth’s atmosphere.

Pluto UV Spectrum

Pluto’s UV Spectrum from HST (Stern et al 2012) also indirectly supports the presence of organics.

What do we know about the surface of Pluto? The major surface ice components are methane (CH4), nitrogen (N2) and carbon monoxide (CO). Some of the CH4 is pure, and some may be dissolved in N2. N2has been seen in two crystalline phases and the thickness should be at least a few centimeters. CO, may or may not be dissolved in N2. Ethene (C2H6) has also been detected (De Meo et al 2010). Suspected species, not yet detected, are Hydrogen Cyanide (HCN) and Carbon Dioxide (CO2). Predicted species include those from atmospheric chemistry, surface chemistry and other radicals.

There are tantalizing hints that HCN and other nitriles (where you have a carbon  with three bonds to a nitrogen molecule with the 4th bond to another atom or group) are potentially present (Protopapa et al 2008). If confirmed, the presence of HCN opens up a series of chemistry pathways that enable Pluto to be a pretty complex place.

HCN Gateway

HCN Chemistry pathways. HCN has not been confirmed to exist on Pluto, but suggested. If present, a whole set of possible chemistry becomes possible.

These ices are white but Pluto has a colored surface. It’s actually quite red. The coloring on Pluto is hypothesized to be due to the presence of tholins, a complex organic molecule formed by ultraviolet irradiation of simple organic compounds.

Pluto Color

Geometric albedo (measure of reflectivity) of Pluto as a function of wavelength. See how red it looks?

The Surface of Charon. Charon has an intriguing different kind of surface than Pluto. There is water (H2O) ice, perhaps crystalline ice, and ammonia (NH3) hydrate. But there are no CO, CO2, N2 or CH4, all which are present (or predicted) for Pluto. The nature and source of the ammonia is under debate. Could it come from below the surface and diffuse up or come from cryo-volcanism?

Charon IR Spectrum

Predictions for New Horizons. It will be hard to find HCN with LEISA due to its spectral resolution as there is a strong methane band nearby. Dale Cruikshank thinks it will be challenging as well to find alkenes.

The mystery of the missing CO2 on Pluto remains. Carbon dioxide is seen on Triton (see above), whose spectra is very similar to Pluto. Dale Cruikshank looks to NASA’s JWST (James Webb Space Telescope, a 6.5 m diameter visible infrared space telescope) as the proper tool to make this detection. New Horizons LEISA instrument has probably to low a resolution to detect CO2 features around 2 microns.

Will Grundy (Lowell Observatory) talked next on the “Distribution and Evolution of Pluto’s Volatile Ices from 0.8-2.4 micron spectra.” He reported on an IRTF (3.5 m telescope) SpeX (spectrometer) Pluto monitoring program spanning 10 years. The SpeX instrument provides R~1000 NIR spectroscopy over 0.8-2.4 microns. A recent paper on their findings can be found at http://adsabs.harvard.edu/abs/2013Icar..223..710G  (Grundy et al 2013 Icarus 223, 710-721).

They would obtain disk integrated hemisphere spectra because Pluto fills the SpeX slit, but during the course of this long monitoring they probed a variety of longitudes. Below are the longitudes on the Pluto that they probed. Each green point is the center of a particular pointing. This is overlaid on the best albedo (reflectance) vs. longitude surface map of Pluto from Marc Buie. In the coordinate system shown in this image, 0 deg longitude is facing Charon, with 180 deg longitude anti-Charon.

Spex Monitoring Program

http://adsabs.harvard.edu/abs/2013Icar..223..710G

Species abundance (measured as equivalent width) as a function of Pluto longitude. They have found that max CO amount is correlated with the 180 E region (anti-Charon), whereas largest amounts of CH4 is in the 270 E region. Equivalent width is a calculation of the depth of an absorption feature with respect to the absence of the feature at nearby wavelengths (continuum). When plotted against time, or in the case above, against spatial location (longitude location on Pluto), it can tell you something about the abundance variations of that molecular species.

In summary, they have found that ice distributions seem heterogeneous (mottled, not smooth). The NIR spectra show intriguing parallels between Pluto and Triton. From this 10 year period of observations they find that CH4 is increasing but CO and N2 is decreasing. They have also observed non-uniformities in both time and longitude.

With a 10 year Earth program they have only observed ~5% of a Pluto year, so perhaps they will start seeing seasonal changes? For a more lengthy discussion on suggested Pluto seasons, see this later blog post entry.

Predictions for New Horizons. Will Grundy is eagerly awaiting New Horizons LEISA’s infrared spectral data. The instrument will have much higher spatial resolution than these “global hemisphere” maps with the SpeX instrument. The spacecraft’s closest approach geometry will be the anti-Charon hemisphere (180 deg E). This will be ideal for probing the strongest CO signatures.

Noemi Pinilla-Alonso (University of Tennessee) provided a talk on “IRAC/Spitzer Photometry of the Pluto/Charon System.” With warm Spitzer/IRAC they took images in four bands probing the 3-5 micron range. Their intent was to look for the mid-infrared spectral signatures of N2, CO, CH4 ices and tholins, all which had discovered in the near-infrared (1-2.5 microns). They covered 8 longitudes with their observation set.

Results. Their data confirms the surface heterogeneity that was measured by HST (Marc Buie). They also found their “slopes in color with wavelength” do have a longitude dependence and fall into two groups 160-288 deg Longitude and 234-110 deg Longitude. Both N2 and CO are also found to be strong at 180 deg Longitude at mid-IR wavelengths. This agrees with Will Grundy’s measurements at shorter wavelengths from the IRTF (see above, this blog entry).

Jason Cook (SwRI) presented a talk on “Observations of Pluto’s Surface and Atmosphere at Low Resolution.” Intrigued by the ethane (C2H6) detection (De Meo et al 2010), he got the new idea to look for it this in old data he took in 2004 using the Gemini-N NIRI instrument, with R~700 (low resolution) spectroscopy. In his analysis, he had to include the C2H6 ice contribution to make a fit of ice abundances to the data. He was able to fit multiple methane bands and derive comparable amounts that agrees with other published methane detections at higher resolution.

Implications for New Horizons. The big take-away is that low resolution spectra with high signal precision are capable of detecting Pluto’s atmosphere. New Horizons LEISA spectra has R~500 so this data example is an excellent comparative data set. He is eager to talk with others who have low-resolution spectra of Pluto or Charon to apply the new analysis techniques.

Next, Emmanuel Lellouch (Observatoire de Paris, France) gave a talk on “Pluto’s Thermal LightCurves as seen by Herschel.” He ended his talk sharing tantalizing science on TNO temperatures from thermal measurements with Herschel and optical measurements used together to measure the diameter, albedo, and thermal inertia. They derive that TNOs have low thermal inertia (2.5 +/- 0.5 MKS), lower than Saturn’s satellites (5-20MKS), Pluto (20-30MKS), and Charon (10-20 MKS). More details can be found at http://meetingorganizer.copernicus.org/EPSC2012/EPSC2012-590-3.pdf.

Moving further out beyond the Spitzer/Herschel far infrared, into the sub-millimeter range, Bryan Butler (NRAO) talked about  “Observations of Pluto, Charon and other TNOs at long wavelengths.” As you go to longer wavelengths, you are less affected by solar reflection. You become dominated by the thermal emission from the body itself.  But the emission at these wavelengths will be weak such that building highly sensitivity instruments is key, such as ALMA (in Chile) or updated VLA, called the EVLA (in New Mexico). They have been using ALMA and EVLA to observe Pluto and Charon in 2010-2012 and they had to remove the background contribution as Pluto had been moving through the galactic plane in this period.

Pluto, Galactic Center

The path of Pluto is shown with the green line that appears to make loops. This is the path of Pluto projected against the sub-millimeter. The enhanced horizontal signal is strong submillimeter thermal emission from the plane of the Milky Way. This caused an undesired extra background signal that needed to be removed from data taken in the 2010-2012 time frame.

What’s Next? They wish to use ALMA to study Pluto & Charon and also attempt to detect Nix & Hydra, if they fall on the larger size. ALMA will be used to observe TNOs  and will have the capability to  resolve the largest TNOs like Eris (size ~2400 km diameter). They predict they can make high-SNR images of Pluto, but barely resolve Charon within a short observation time. To get high-SNR images of Charon would take more observatory time than they think would be awarded for a single object.

Pluto ALMA

Switching away from the infrared and sub-millimeter and moving back to the ultraviolet Eric Schindhelm (SwRI) gave the final talk in this session entitled  “FUV Studies of Pluto and its Satellites: From IUE to New Horizons.” IUE took the first UV spectra of Pluto in 1987-1988. This was confirmed with HST using the FOS (Faint Object Spectrograph) instrument in 1992. After 17 years, the HST COS (Cosmic Origins Spectrograph) instrument was used to observe two different longitudes, and they found some differences between the two data sets. The COS data indicated an absorption feature at 2000-2500 Angstroms (see Dale Cruikshank talk summary above), and it was suggested this is a hydrocarbon creating this feature.

HST FOS Pluto Spectra

Eric Schindhelm next described New Horizon’s Alice instrument measurements and predictions  for the Pluto and satellites during the New Horizons fly-by. He also summarized that more lab H2O, NH3 and CO2 ice FUV reflectance spectra is needed for interpretation of these data sets.

Predictions for New Horizons. Pluto’s UV reflectance spectra will be limited due to faint signal and atmosphere absorption. Nix and Hydra will be barely detectable in FUV. Charon’s albedo for wavelength longer than 1200 angstroms should be detectable and they expect to get albedo, color and composition. They also expect to distinguish between different mixing ratios of the ices (ratios of H2O to NH3, H2O to CO2, etc.) with the UV spectra obtained by New Horizons.

Although the predictions for detecting Pluto’s surface composition in the UV with New Horizons’ Alice instrument are expected to be limited, the Alice instrument will also be measuring Pluto Atmosphere (and searching for an atmosphere around Charon), which is its main purpose and directly addresses a prime Group 1 science goal.

Bring on the spectra!

 

 

 

 

More predictions about Pluto’s changing atmosphere. And Charon may have a few surprises of its own.

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Blog series continues. These are summaries of talks presented on July 23, 2013 at the Pluto Science Conference. The New Horizons mission will fly by the Pluto System on July 14, 2015, a place that has never been explored before by any other spacecraft. Many questions about the Pluto System remain unanswered. For more information about NASA’s intrepid explorer to the Solar Systems’ Third Zone go to http://pluto.jhuapl.edu/ and https://www.nasa.gov/mission_pages/newhorizons/main/index.html.

Richard French (Wellesley College) presented a talk on “A Comparison of Models of Tides in Pluto’s Atmosphere and Stellar Occultation Observations.”

We have come to understand that Pluto’s atmosphere is cold & tenuous, has a long radiative time constant, shows weak diurnal variations, indicates seasonal transport of volatiles with long term variations of atmospheric mass, and seems to be convectively stable. Current Pluto general circulation models (GCMs) predict smooth T(P) profiles reveal mean circulation and thermal structure. But there are problems. GCMs predictions (with these smooth T(P) profiles) are inconsistent with stellar occultation data, which imply much more complex T(P) profile. The other challenge to this mystery is that stellar occultations are spatially constrained (i.e., map across a particular lat/long swath of Pluto surface at the time of event).

Are there waves in Pluto’s atmosphere? This is one proposition to explain the structures (spikes) seen in the Pluto occultation data. Tidal models they have built make predictions for large scale and small-scale structures. Also they can predict temperature profiles with altitude. Next steps are to apply this model to other occultation geometries. Richard French showed a comparison of a tidal model (Toigo et al 2010) against occultation data from an event on Aug 21, 2002 and they showed qualitative agreement.

Richard French’s predictions for New Horizons fly-by: When New Horizons provides a true frost pattern, they can input this into their models and generate large-scale and small-scale structures for comparison with actual New Horizons atmosphere measurements. Their tidal models do generate regionally variable, latitude dependent thermal changes. If this is what New Horizons observers, their model can help constrain parameters.

Bruno Sicardy (Observatoire de Paris, France) next took us on a rich tour of “Pluto’s Atmospheric Pressure From Stellar Occultations: 2002-2012.”

Pluto Charon Dual Occultation

There was a dual Pluto & Charon occultation event on 4 June 2011. Pluto and Charon each pass in front of the star (at different times). Look at curve shapes. Charon’s curve sharply drops, indicative of no atmosphere, unlike Pluto’s curve, which has not-as-steep ingress/egress that indicates the presence of an atmosphere.

Using the light curve data, Sicardy and his team use a temperature vs. altitude model to fit the light curve depth, width and ingress/egress slope. Then with the temperature, they can derive a pressure. He presented results from the most recent Pluto occultation that was observed May 4, 2013. Good data and good fit. Next were shown the derived pressure (at 1215km) for occultation events observed from 1988 to 2013.

Pluto's Atmosphere 1988 to 2013

Occultation results show the Pluto atmosphere is increasing over the past few years. There is some notable evolution and implies a regular expansion. But a question from the audience stressed caution as we could be seeing just the northern pole facing the Sun with that contributing to the expansion but it could be a localized phenomena.

Bruno Sicardy’s predictions for New Horizons fly-by: Atmosphere will be present for the fly-by.

Michael Person (MIT) next described “Trends in Pluto’s Atmosphere From Stellar Occultations.” He started his talk with the advantages of occultation measurements:  you get spatial resolution (~1 km at Pluto) with direct measurements of atmospheres (temperature, pressure, number profiles). MIT has collected data sets from 1988 through 2013. Their group tends to separate the upper vs. lower atmosphere when they fit their data. He next showed a light curve comparison over. Are we seeing a gradually decrease lower atmosphere slope? Is there a gradual lowering of the separation boundary?

“Haze or No Haze? That is the question.” Best evidence of haze is from the occultation event of 2002, where there is a distinct change in brightness as a function of wavelength (Elliot et al 2003).  Attempt to look for haze in the 2011 occultation event with SOFIA in three bands was not successful. The main question is why does the haze come and go, and what is causing it?

What Mike Person is looking forward to: New Horizons will finally provide the size of Pluto! Knowing where the Pluto surface really is, or equivalently, the size of Pluto, is a key data point, as all these interpretations of occultation light curves and interpretations to atmosphere assumes a Pluto size.

Alex Dias de Oliveira (Observatoire de Paris, France).“Pluto’s Atmosphere from Jul 18, 2012 stellar occultation.” This is his PhD work and he provided an updated status of the steps taken from prediction of the event, the observation data collected, various calibration items, and first attempts to invert the light curve to get a temperature profile. He observed this Pluto occultation event with the ESO VLT (8m telescope in Chile) with the NACO instrument in the H band (1.65 microns). Comparison with the June 12, 2006 AAT event showed that spikes seen in the light curves were repeated in the July 18, 2012 event he observed wit the VLT in Chile.

Cathy Olkin (SwRI) presented results from “The May 4th, 2013 Stellar Occultation by Pluto and Implications for Pluto Atmosphere in 2015.” This was an event where Pluto passed in front of a R=14.4 mag star with a slow shadow velocity of 10.6 km/s. The event was observed from the southern hemisphere, from Cerro Tololo in Chile.

Erika Barth (SwRI). “Is Methane Supersaturation Consistent with the Presence of Haze Particles in Pluto’s Atmosphere?” She asked the question: If you put haze particles into Pluto’s atmosphere how do they interact with the methane in Pluto’s atmosphere?” She developed a model to ingest haze particles into a supersaturated environment and this predicts the growth of clouds and condensation of methane. Then when methane condenses out, that reduces the amount of observable methane. Her model requires the existence of a troposphere (which we learned earlier in Emmanuel Lellouch’s talk today that there is no evidence for this, but its existence could explain some phenomena, some observed to date, other predicted) and also predicts a thick troposphere as well. She created a Pluto version of CARMA, the Community Aerosol and Radiation Model for Atmospheres.

Jason Cook (SwRI) next spoke about his “Analysis of High Resolution Spectra of Pluto: A Search for Cold Gaseous Methane Layer, and Spatial Variation in Methane Column Abundance.” Occultations have told us that Pluto’s upper atmosphere (above 1195 km) is pretty warm (100 K). But 2.15 micron N2 ice measurements of Pluto’s surface tells us the surface is ~40 K. So this implies there needs to be a cold-layer in the atmosphere. To investigate a search for this “cold layer of air” they took NIR (near infrared) spectra with NIRSPEC on Keck with R=35,000 in 2011. They need to move to a two-temperature model to help constrain the observed data (i.e. measured methane line depths from the high-res NIR spectra), but the hot/cold ratio of the two temperatures is an unresolved topic (pun intended).

They also took spectra of Pluto over several nights to probe the different longitudes of Pluto (Pluto has a rotational period of 6.4 days) and they got a fairly consistent number except near 180 deg longitude where gaseous CH4 was not easily detected. They would like more data to probe this temporal measurement.

Methane Spectra on Pluto

Selection of high-resolution NIR spectra of Pluto obtained over several days. This series probes a range of Pluto rotations and show how methane lines (Q-branch) vary.

Eliot Young (SwRI) spoke about “Evidence for Recent Change in Pluto’s Haze Abundance.” Hazes have been observed on Titan (photolysis products from higher up in the atmosphere) and Triton (condensates from surface). The August 21, 2002 occultation showed evidence for haze (change in brightness with color, Elliot et al 2003), but 2007 (0.51 & 0.76 micron) and June & July 2011 occultation events in different bands (I & K bands) showed no change in color.

Occultations can only probe down to a certain depth, so they are limited. We don’t really how close you got to the Pluto surface. If you have a special case where you can have a central flash or sets of flash spikes, you can derive more information. By applying a new technique on the 2007 Mt John light curves, he proposed they can determine amount of haze by evaluating the attenuation in those parts of the light curve.

Central Flash Description:  A central-flash occultation is visible when the observer is located near the center of the shadow path of the object. It is here where the atmosphere near the edges of the occulting body (for Pluto occultations, this is Pluto) refracts extra star light (from the background star) directly opposite from the star, forming a “brightening” in the middle of the deep light curve.

Mark Gurwell (Harvard CfA) provided a talk entitled “Atmosphere CO on Pluto: Limits from Millimeter-Wave Spectroscopy.” Carbon monoxide (CO) is expected based upon the presence as an ice on its surface. The first direct detection of CO was done in the NIR with the VLT (Lellouch et al 2011). Then JCMT (Greaves et al 2011) revealed a CO(2-1) line in the submillimeter, but this line had not been there a few years back, leaving a mystery. There is still mystery in fitting the CO abundance based on the measured submillimeter width and strength of this line. He did show that Pluto had been in the fore-ground of a galactic emission during the JCMT observations. He supposes that they had contamination. They did their own observations using the SMA sub-millimeter telescope multiple times and did not detect the CO(2-1) line in the spectra (they have upper limits). So he is excited about using the ALMA array that has 30-50x SMA sensitivity to really address the CO, nitriles and isotopes.

And the final talk of the morning Atmosphere session just could not leave Charon out of it.

Alan Stern (SwRI) “Cometary Impact Produce Transient Atmospheres on Charon.” Most scientists have come to accept that Charon does not have an atmosphere (see earlier posting in Bruno Sicardy’s talk showing the dual occultation event for Pluto and Charon in Jun 2011.) But he postulates what about impacts? Coming from the Kuiper Belt, impactors (assuming cometary-level amounts of volatiles) could provide volatiles to the surface to Charon and therefore creating a “nanobar” atmosphere on Charon. Similar events could lead to atmospheres on Kuiper Belt objects. Their modeling (Trafton & Stern 2008) predicts presence of N2, CO, Ar, CH4, with CO dominating after impacts, and N2 being the dominate species (in terms of amount).

Predictions for New Horizons. Duty cycle would be short lived so it will be rare if New Horizons catches this event. However, with smaller impactor sizes, there could be a possibility that those events could have occurred within the “photoionization time” (before the molecule breaks down or escapes) or resulting implanted molecules by the time New Horizons gets there. Alan Stern coyly stated, “could be as much as a 25% chance” to see an nano-bar atmosphere on Charon.

A good question regarding surface volatiles that are revealed by impacts got the crowd excited. After all, when you describe an atmosphere you can categorize things as sources and sinks. Sources bring material to the system and here they could be not only the KBO impactor but also the materials that are revealed from the impacted-body after the impact.

On July 14, 2015 New Horizons will be doing a very sensitive experiment via the observations of the Charon occultation (Charon passing in front of our Sun as viewed from New Horizons).

Charon may indeed hold a few surprises of her own!